SEAL APPLICATION, LOOKING AT THE WHOLE SUBJECT SA001
Seal application is divided into three parts:
- Classifying the fluid you will be sealing into its proper categories.
- Specifying the correct seal materials
- When necessary, applying sensible environmental controls
CLASSIFYING THE FLUIDS WE WILL BE SEALING SA001-5
To be able to seal the wide variety of chemicals used in the process industry you need a method of classifying chemicals that puts them into neat, logical categories. These categories can be handled by the use of an off the shelf seal, a special seal design or by controlling the environment in the stuffing box and outside the seal faces.
Any fluid can be classified as either a liquid or a gas and placed into seven sealing categorizes.
- Fluids sensitive to small changes in temperature and/or pressure. SA007
- Fluids that require two mechanical seals. SA008
- Non lubricating liquids, gases and solids. SA009
- Slurries classified as solids in liquid. The solids may or may not be abrasive. SA10
- Liquids sensitive to agitation. SA011
- Liquids that react with each other to form a solid. SA012
- Lubricating liquids. SA013
Now that leakage is no longer tolerable and product dilution is no longer desirable, you must have knowledge of these chemical categories to approach the job of effective sealing.
In most cases the fluid you are sealing will fall into several of the above mentioned categories. Using heat transfer oil as an example we note that it falls into the following five categories:
- Hot. This oil is pumped at 600 -700 Fahrenheit (315 -370 C); the fluid is too hot for available elastomers.
- Film Building. The product cokes at these temperatures.
- Dangerous. You do not need this high temperature oil leaking out. It is not only a fire hazard, but a personnel hazard as well. Recent information indicates that some of these oils are also classified as carcinogens.
- Costly. Most of these transfer oils cost between $12.00 to $20.00 per gallon (3,8 Ltrs.)
- Slurry. Because of the coking, solids are always present.
To successfully seal heat transfer oil you would have to address all of these problems at the same time. As is the case with all slurry applications, you would also have to recognize the problems with vibration (impeller imbalance), thermal growth, and frequent impeller adjustments if you were using open impellers.
In addition to handling various chemicals we are often faced with extreme or severe operating conditions. These conditions fall into seven categories also:
- Hot products – Defined as too hot for one of the seal components, or hot enough to cause the fluid to change from a liquid to a gas or solid. Heat transfer oil is a good example of a fluid that will “coke” at elevated temperature.
- Cryogenic fluids – They present a problem for elastomers and some carbon faces. Liquid nitrogen or oxygen would be an example.
- High Pressure – Defined as stuffing box, (not discharge) pressure in excess of 400 psi. (28 bar). Pipe line and boiler-circulating pumps can have stuffing box pressures of this magnitude.
- Hard Vacuum – Defined as 10-2 Torr or below. This number is well below most condenser or evaporator applications, but does come up every once in a while.
- High Speed – Defined as the seal faces moving greater than 5000 feet per minute (fpm.) or 25 meters per second. Most process pumps do not approach this speed. The Sundstrand “Sundyne” pump is typical of a high-speed application.
- Excessive motion – defined as more than 0.005 inches (0,15 mm.) in a radial or axial direction. Mixers, agitators and specialized equipment have shaft movements up to 1/8 inch (3 mm). Long shaft vertical pumps and pumps equipped with sleeve or journal bearings, are another application for excessive motion.
- Excessive vibration – Unfortunately there are no reliable numbers for the vibration limits of mechanical seals. Most vibration studies have addressed only the bearings. It is important to know that excessive vibration can:
- Open the lapped seal faces.
- Chip the outside diameter of the carbon face.
- Break the metal bellows used in some seal designs.
- Wear the driving mechanism used to transmit torque from the set-screws to the seal faces.
- Loosen drive screws.
- Shorten bearing life
- Most seal designs can damage (frett) expensive sleeves and shafts.
- Some, but not all designs have built in vibration dampers to relieve some of these problems
Now we will look at choosing the materials for the individual seal components. There are usually three materials to choose:
THE ENVIRONMENTAL CONTROLS SA006
For any given seal application problem there are three generally accepted solutions:
- Put in a standard or “off the shelf” seal and hope it works.
- Build a special seal that can compensate for the problem once it occurs.
- Control the environment surrounding the seal to prevent the problem from occurring in the first place. If you control the seal environment you will avoid the inventory and delivery problems associated with special seals.
In the following paragraphs I will:
- Address the subject of environmental controls in detail.
- Show you how to seal each of the categories.
- Show you how to seal the special operating conditions.
- Discuss some special seals
It turns out there are only a few things you can do in the stuffing box area to control the environment around the mechanical seal. As an example you can:
- Control the temperature in and around the stuffing box. You can raise the temperature, lower it or keep it within certain limits
- You can control the pressure in the stuffing box. You might want to raise it to prevent a product from vaporizing or you might want to lower it to save the expense of going to a high pressure seal.
- You can control the pressure between dual seals. There are occasions when you will have to raise this pressure, lower it or keep it within narrow limits.
- You can replace the fluid in the stuffing box. The replacement fluid may be less dangerous, a good lubricant or just easier to seal.
- You can keep atmosphere away from the outside of the seal because the moisture in atmosphere can cause problems with some seal applications.
Here are some ways to control the temperature in the stuffing box area.
- Flush the stuffing box with a compatible cool clean liquid. Many seal glands have this connection available in a more convenient location than the stuffing box lantern ring connection.
- Flush is a misunderstood term. It describes six very different functions. Please look at the following illustrations and note the connections.
|Discharge recirculation. In this arrangement a line is connected from the discharge side of the pump to the lantern ring connection in the stuffing box (A) or an appropriate connection in the gland.
The fluid flows from the discharge side of the pump through the stuffing box to the back of the impeller.
Suction recirculation. This time the recirculation line is connected from the bottom of the stuffing box to the suction side of the pump or some other low pressure point in the system.
It uses the same connection (A) but on the bottom side of the stuffing box. The bushing in the bottom of the stuffing box must be locked into place with a snap ring or it could move with the differential pressure.
|Jacketing fluid. The cooling or heating fluid flows through a jacket (B) that is surrounding the stuffing box.
Be sure to go in the bottom and out the top of the jacket to prevent an air pocket
Barrier or buffer fluid. The fluid is circulated between two seals (E) either by convection, a seal pumping ring, or by a separate circulation system.
If the circulating fluid is at a higher pressure than the stuffing box it is called barrier fluid. If it is at a lower pressure it is called buffer fluid.
Quench. Please look at connection (D). The fluid (usually low-pressure steam) is passed between the seal and a disaster bushing that has been installed in the rear of the seal gland.
This is also called an API (American Petroleum Institute) gland
|Flush. Please look at connection (C). A liquid, from an outside source is injected into the stuffing box at one atmosphere above stuffing box pressure and dilutes the product you are pumping.|
- Use two seals with a cool liquid circulating between them. A two way balanced cartridge seal would be an excellent choice. This arrangement provides cooling at the seal faces where it will often do the most good.
- Use the jacketed stuffing box that came installed on the pump (connection “B”) or install one if it is missing. These jackets are available as a replacement part for the back plate on most popular pumps or as an after market bolt on accessory. To use the jacket properly:
- Dead end the fluid you are trying to control. This means no lines in or out of the stuffing box except those used to circulate the jacketing fluid.
- Install a thermal bushing in the bottom of the stuffing box. Carbon is a good choice because it is a poor conductor of heat compared to the metal pump components. A typical clearance over the shaft would be 0.002 inches per inch of shaft diameter (0,01 mm/mm of shaft diameter).
- Circulate the heating or cooling fluid through the jacket to control the temperature. Six to eight gpm. (25 to 30 liters /min.) is typical of the amount of cool water needed to cool down heat transfer fluid to the point where it will stop “coking” and viton O-rings will be acceptable. If your water is too hard you should substitute condensate or low pressure steam.
- An API (American Petroleum Institute) gland is available for most mechanical seals (connections C & D). The gland has several features to provide various functions. It can be used as:
- A quench connection (D) to provide heating or cooling outboard of the seal or to remove any liquid or vapors that might escape between the seal faces. Steam can be injected to lower the seal temperature in the event of a fire. In the event of a major seal failure this quench connection can be used in conjunction with the gland disaster bushing to direct seal fluid leakage to point where it can be collected. Be careful of using too much steam pressure because the steam will leak through the disaster bushing and blow through the lip seal trying to protect the bearings.
- A flush connection (C) to provide clean fluid to the stuffing box, or it can be used to vent air out of the stuffing box in a vertical pump application.
- A close fitting, non sparking disaster bushing to provide shaft support in the event of a bearing failure or to protect personnel in the event of a massive seal failure. The bushing will direct most of the leakage to a drain or tank where it can be collected.
- Heat tape or tracing lines can be installed around the stuffing box to provide a limited amount of temperature control.
- Install a cooler in the line between the pump discharge and the stuffing box. Keep in mind that this system only works while the pump is operating so it would be of no value if the application problem occurs during pump shut down or when the pump is used in a “standby mode”.
- Use only balanced seals in these applications to avoid the heat problems associated with unbalanced seal designs. Elastomers positioned close to the lapped faces or the use of two hard faces should also be avoided for the same reason.
Controlling the pressure in the stuffing box area
- Increase stuffing box pressure by installing a recirculation line from the pump discharge back to the stuffing box (connection A) with a close fitting bushing in the bottom of the stuffing box. Try to avoid positioning the recirculation line so that it aimed at the lapped seal faces or thin bellows seal plate materials. Many fluids contain solids that will abrade these parts. Be sure the close fitting bushing is positively retained in the bottom of the stuffing box. A snap ring is generally good enough to hold the bushing against the bottom of the scuffing box.
- Eliminate the pressure drop between seal faces by using two seals with a higher-pressure barrier fluid circulating between them. This is very important in the sealing of chemicals such as ethylene oxide that will penetrate into the dynamic elastomer, expand and blow out the other side causing severe damage to the elastomer and unwanted leakage.
- Flush the stuffing box with a higher-pressure liquid. This is the best solution if the fluid contains solid particles that could interfere with the seal movement. If you are using balanced mechanical seals designed with the springs out of the fluid you will need only a small amount of flushing.
- The only reason you would want to lower stuffing box pressure is because your seal does not have high pressure sealing capability. It is possible to lower stuffing box pressure by the use of environmental controls, but a high-pressure seal would be a much better choice. In an emergency you could lower the pressure by one of the following environmental controls:
- Equalize the pressure in the stuffing boxes of a double ended pump by connecting the stuffing boxes together to get even seal wear. This is a common application for a double ended centrifugal pump.
- It is possible to lower stuffing box pressure by installing a close fitting bushing in the bottom of the stuffing box and recirculate to the suction side of the pump. Be sure to “lock in” the position of this bushing with either a snap ring or some other retaining device to prevent it from moving towards the seal. Be careful of using this control on a vertical turbine pump because the high velocity liquid recirculating to the suction can heat up the line to the point where it can become “red hot”.
- Lower the sealing pressure differential on the inside seal of a dual seal application by utilizing an intermediate fluid pressure between two tandem seals. Be sure the inner seal is balanced in both directions.” Balancing a seal in two directions is sometimes called “two way balance“.
Replacing the fluid, or provide a lubricant if the sealing product is a non-lubricant (non-lubricants have a film thickness less than one micron)
- Use two seals with a higher-pressure lubricant as the barrier fluid. This is an excellent choice in most gas applications or liquids that have little to no lubricating properties. This form of lubrication will often solve the problems associated with seal “slipstick” and some other types of vibration. Some new seal designs have hydrodynamic or hydrostatic faces that allow you to seal gases with a small amount of controlled gas leakage into the product.
- Flush the stuffing box with a liquid lubricant.
- Cooling the product will sometimes turn a non-lubricant such as hot water into a lubricating liquid.
- For some vacuum applications it makes sense to install a discharge recirculation line to help destroy the vacuum in the stuffing box area. This works well with mechanical seals, but does not work as well with conventional packing.
NOTE. If an open impeller has been adjusted too close to the back plate, the “pump out vanes” behind the impeller can cause a vacuum to occur in the stuffing box. The problem exists with those open impeller designs that adjust towards the volute (Goulds is an example) and the mechanic is used to adjusting the impeller to the backplate (Duriron as an example). Someone must inform the mechanic that Goulds and Duriron impellers adjust in opposite directions.
Decreasing the amount of liquid agitation in the stuffing box.
This becomes very important if you have to seal a liquid that increases its viscosity with agitation. We call these liquids “dilatants”. Connect the bottom of the stuffing box to the suction side of the pump to allow a single pass of the liquid through the stuffing box. Make sure the connection is very close to the seal faces. You will be better off using the seal gland flush connection rather than the stuffing box lantern ring connection.
Some liquids decrease their viscosity with agitation. We call these liquids “thixotrophic”. In some instances the thinner liquid film can cause more face wear and seal “slip stick“. If this problem exists use one of the environmental controls mentioned above.
You will recall that there were seven categories of liquids to seal.
- Fluids sensitive to small changes in temperature and/or pressure.
- Fluids that require two mechanical seals.
- Non lubricating liquids, gases and solids.
- Slurries classified as solids in liquid. The solids may or may not be abrasive.
- Liquids sensitive to agitation.
- Liquids that react with each other to form a solid.
- Lubricating liquids.
In the next section we will look at each of these categories in detail and I will recommend various environmental controls to help you seal these fluids reliably.
SEALING FLUIDS SENSITIVE TO A SMALL CHANGE IN TEMPERATURE OR PRESSURE SA007
Liquids and gases are both called fluids and a fluid can shorten the life of a seal in only two ways:
- It can cause the seal faces to open allowing solids to penetrate.
- It can damage one of the seal materials.
In this discussion we will be considering how small changes in either temperature or pressure will cause one or both of these failures to occur and learn how to prevent these changes especially when the pump is stopped and often subject to both temperature and pressure fluctuations.
A change in temperature can:
- Cause a fluid to crystallize. The crystals will make the seal stick to the shaft and open the faces when the shaft moves. Caustic and sugar solutions are examples of this.
- Cause a liquid to vaporize; blowing the lapped seal faces apart, letting solids penetrate between the faces or causing damage as the faces bounce open and shut. This happens any time water flashes to steam.
- Cause some liquids to become viscous, preventing the seal faces from staying in contact. Bunker fuel oil becomes very thick when it gets cold.
- Cause some liquids to solidify, either sticking the seal to the shaft, preventing the flexible seal parts from moving, or causing the seal faces to stick together. Sugar syrups do this when they get hot. Some fluids do it when they get cold.
- Cause a film to build on the seal sliding components or between the faces. Oil varnish or “coking” is as typical example of this problem. Hard water will build a film on the seal sliding components as the water temperature increases. If the system is new and has not been passivated (protective oxide film on the metal surface) Ferric oxide or a similar oxide can build up on the sealing components. This build up will accelerate with temperature.
- Cause a liquid to become a non-lubricant. Water becomes less of a lubricant as its temperature increases. This lack of lubrication can cause “slip stick” problems between the lapped faces.
- The corrosion rate of most corrosives increases with a rise in temperature. A general rule of thumb says that the corrosion rate of an acid will double with an 18°F (10°C) rise in temperature. This is the reason we avoid the use of packing in acid pumps. You will recall the packing generates almost six times the heat of a balanced mechanical seal
If you are not using a dual seal with a pressurized barrier fluid between the seals, then you will get some sort of a pressure drop across the seal face. A pressure drop could:
- Cause the fluid to vaporize and blow open the lapped faces. If this happens several problems might occur:
- Solids penetrate between the faces, imbed themselves into the softer carbon and destroy the lapped hard face.
- As the product passes across the faces a cooling occurs, causing the faces to close. When the faces close the cycle repeats its self and the alternating closing and opening will probably crack the carbon as it bangs against the drive lugs or you will chip the carbon face on its outside diameter.
- If the product freezes when it evaporates, it could freeze any oil or grease that was put on the seal face causing damage to the carbon. This vaporization will also freeze the moisture on the outboard side of the seal causing ice that can restrict the movement of the seal. You can see the ice on the shaft outboard of the mechanical seal.
- Cause the liquid to solidify. Paint is a mixture of a solid and a solvent. If the solvent evaporates the paint will solidify between the faces. This can also occur if the suction of the pump is under a vacuum (negative suction head) because the pump is trying to lift the fluid.
If the temperature or pressure of the pumping fluid never changed we would seldom have any application problems. Since pumpage pressure and temperature changes are normal (especially at shut down) we are going to have to become skillful in controlling the temperature and pressure in the stuffing box area to prevent a premature seal failure.
In the next few paragraphs we will look at various methods of controlling temperature and pressure in the stuffing box area. We will begin with the jacketed pump.
If your pump is not equipped with a jacket (B), one is probably available from the pump manufacturer or an after market supplier
A carbon thermal bushing is installed in the end of the stuffing box to reduce the heat transfer between the product you are pumping and the fluid in the stuffing box.
When you use this technique be sure to check:
- The cooling jacket must be free from scale and calcium build-up. There are many cleaning products on the market you can flush through the jacket to insure that it is clean with out having to disassemble the pump.
- Dead-end the fluid; no recirculation lines either into or out of the stuffing box. Check carefully because some of these lines can be hidden by insulation. We are trying to trap a small amount of liquid in the stuffing box that will be easy to either heat or cool.
- The best fluids to circulate through this jacket are steam or condensate. Shop, river water, or city water is generally too hard and will form a calcium film on the inside of the jacket.
- Remember that steam will act as a coolant with hot oil applications.
- The steam temperature can be controlled by the use of a regulator on the outboard side of the jacket. The temperature of steam is directly related to its pressure.
- You can use a mixer valve that will blend the steam and some condensate to give you a very precise control over the stuffing box temperature.
- The main advantage of this environmental control is that it lets you regulate the stuffing box temperature when the pump is shut down. That far outweighs the disadvantage of having to provide circulation to the jacket.
- Be sure to bring the coolant into the bottom of the jacket and out the top. This will insure that there are no bubbles trapped to restrict heat transfer.
- Because you are “dead ending” the fluid, centrifugal force will throw the solids away from the seal components and very soon the seal will be positioned in a clean environment at exactly the right temperature.
The quench and drain connection is next
|Steam or water can be injected into port (D) and drained out the drain port on the other side of the gland (not shown)
A non sparking disaster bushing is placed in the end of the gland with a small clearance (0.025″) over the rotating shaft
This connection is used to heat or cool the outboard side of a single seal and wash away any product the might leak across the faces or build up outboard of the seal.
- Use only low-pressure steam or water. You do not want these products to penetrate through the disaster bushing and get into the bearings. This is another reason to replace those bearing grease or lip seals with either a labyrinth or a positive face seal.
- The non sparking disaster bushing has two functions:
- To direct most of the seal leakage to a drain where it can be collected, or a flare where it can be burned.
- To prevent the rotating shaft from hitting the stationary seal face if you have a bearing failure. If the product you are pumping ignites, this could cause a fire or an explosion. In any case the damage would be severe without this non-sparking disaster bushing.
- A steam line hooked up to this connection can be used to put out a fire in the stuffing box area. All you need is a solenoid valve and a melt switch that will open the solenoid when it senses high temperature (same as a fire sprinkler system).
Discharge recirculation is the next environmental control
|A line is connected from the discharge side of the pump to the stuffing box through the stuffing box lantern ring connection (A).
This line can be used to pressurize the stuffing box area with the discharge pressure available at the pump.
- Do not aim this connection at the seal faces or sliding components. The abrasive action of entrained solids can injure the lapped faces or destroy a seal component. Thin wall metal bellows seals are very sensitive to this abrasive action.
- The high velocity fluid can also interfere with the seal movement so be very careful how you make the connection.
- Use a restrictive bushing in the end of the stuffing box to assist in keeping a higher pressure at the seal faces. You can see this bushing in the above illustration.
The dual seal is another option:
|In this illustration the dual seals are connected in a tandem configuration.
Either low pressure buffer fluid or high pressure barrier fluid is circulated between the seals
Dual seals are another way to control either temperature or pressure at the seal faces. You can:
- Circulate a fluid at the correct temperature between the seals. You can cool the area, heat the area or hold the temperature at precise limits if that is desirable. Be sure to bring the fluid in the bottom and out the top of the gland to avoid air pockets.
- You can pressurize between the dual seals to prevent a pressure drop across the seal faces. If you use the two way balanced version of a dual seal you can choose either a higher barrier or lower pressure buffer fluid between the seals.
- Fill the system and convection tank with anti-freeze and you will prevent ice from forming out board the inner seal. This can happen any time you seal a product that can freeze moisture in the atmosphere. CAUTION: Do not use automotive anti-freeze because some brands contain a chemical used to plug up leaks in the radiator and other parts of the system.
Here are a few more considerations about controlling pressure and temperature in the seal area:
- A cooler in the line between the pump discharge and the stuffing box is not a good method of controlling stuffing box temperature because it functions only when the pump is running, and many problems with crystallization, solidifying, becoming viscous, etc. occur when the pump is shut down or in a “standby mode.”
- Flushing the system between batches seldom cleans the stuffing box area and the mechanical seal.
- Flushing the stuffing box with an outside fluid is the universal environmental control. You can always replace the fluid that is giving you trouble by flushing in a clean liquid at the right temperature and pressure. It will cause product dilution, but maybe you can flush in finished product or a fluid that is compatible with the fluid you are trying to seal.
- Heat tracer lines are often used in piping systems, but are seldom placed on the stuffing box. Maybe you will find it practical to trace and insulate the stuffing box for your application.
There is little need to lower the pressure in the stuffing box area. If you find that the stuffing box pressure is to high for your mechanical seal, you are better off purchasing a high pressure mechanical seal that will satisfy your application.
SEALING FLUIDS THAT REQUIRE TWO SEALS SA008
We can use dual seals to:
- Control the temperature at a seal face.
- Prevent a pressure drop across a seal face.
- Eliminate atmospheric conditions outboard of a mechanical seal.
- To break down the pressure in a high-pressure application, by inserting an intermediate pressure between the seals. Two lower pressure seals can then be used to seal a high-pressure fluid that would normally require a very expensive high-pressure mechanical seal.
- To provide a lubricant if one is needed to prevent “slip stick” between lapped seal faces. This is always a problem when you are sealing a gas or non-lubricating liquid.
- As a back up if the first seal fails.
Certain products require the use of two mechanical seals. The list would include
- Radioactive material.
- High temperature heat transfer fluids that can start a fire if they leak to the atmosphere or any high temperature fluid that would present a danger to personnel in the area.
- Many products are considered to be toxic to humans. Hydrogen sulfide is a good example.
- Cryogenic fluids. Products like liquid nitrogen, oxygen etc.
- High-pressure fluids. Many boiler feed pumps and pipeline applications fall into this category.
- Carcinogens (cancer producing chemicals)
- Bacteria laden fluids.
- Expensive fluids.
- Non lubricating gases.
- Hard vacuum applications.
Dual seals can be of either the rotating or stationary version and can be installed in four different configurations.
- Back to back, facing in opposite directions
- Tandem, facing in the same direction
- Face to face, facing towards each other
- Concentric, one inside the other.
The fluid that circulates between the seals is called barrier fluid if it is higher than stuffing box pressure. It is called buffer fluid if it is lower than stuffing box pressure. It can be circulated between the two seals by:
- Natural convection using a convection tank. Insulated piping coming from the top of the gland to the convection tank and finned piping coming out of the tank will aid convection if heat removal is a problem.
- A pumping ring can be installed between the seals for those instances where natural convection is not sufficient to remove the heat being generated between the faces. This is very necessary when oil is used as the barrier fluid. Oil has a low specific heat and poor conductivity, making it a poor choice as a barrier fluid. Most of the newer cartridge dual seals come equipped with a built in pumping ring.
- When you are introducing the fluid between the seals from an external source be sure to bring the fluid in at the bottom of the dual seal gland and out the top to prevent an air pocket from forming in the gland.
The following illustrations describe the rotating version (the spring or springs rotate with the shaft) of these dual seal configurations. You should be aware that a stationary version is also available from any of the major seal companies. You should also consider:
- Use only the hydraulically balanced version of these seals to prevent the generation of excessive heat between the seal faces.
- Two way balance is always desirable in any dual seal application to allow you the option of using either a high or low-pressure barrier fluid, and to prevent the seal faces from opening if either the system or the barrier fluid pressure fluctuates. o Select seal faces with good thermal conductivity.
- Try to locate any elastomers away from the seal faces. Elastomers are very sensitive to heat.
The first configuration we will look at is the “back to back” version of a rotating seal.
|The two rotating faces can be separated by a single spring, multiple springs, or two metal or rubber bellows.
Many versions of this seal use two separate seals independently attached to the shaft.
The rotating back to back version would be your worst possible choice. Here are some of the reasons:
- This configuration requires a higher barrier fluid pressure between the seals. This means that an inner seal leak will cause a dilution of your product. There will be no visible evidence of this happening unless someone notices a change in the product concentration or tank level.
- In operation the outboard seal is carrying the higher differential pressure and should be the first seal to wear out or fail. When this occurs the barrier fluid pressure will drop and the inner seal can blow open. In other words, if the seal works as designed, both seals will fail at the same time.
- High barrier fluid pressures are hard to maintain because of pressure fluctuations and varying system pressures. Water hammer and pressure surges are not that uncommon.
- A reversing pressure can blow the inner seal open. Seals should shut with pressure. They should not “blow open” when something goes wrong.
- If a connection in the barrier fluid system is ruptured the inner seal can blow open, dumping the pump contents to the environment. The second seal would be of no use.
- Note the snap ring holding the inner stationary face against the end of the stuffing box. This part is missing in just about every application I have ever seen. Without this snap ring, higher process fluid pressure can over compress the inner seal spring force moving the stationary face into the rotating face, causing massive face wear and very high rubbing temperatures.
- A common version of this seal utilizes spring loaded dynamic O-rings. O-rings should be placed in O-ring grooves; they should not be spring-loaded. The Durametallic CRO seal is typical of that configuration.
- This version is known as the “double fretter” in the sealing industry. It will groove the shaft in two places just beneath the O-rings.
- This seal is often used in slurry applications. Centrifugal force will throw the slurry into the inner faces causing excessive carbon wear. The slurry will then pack in front of the moveable face preventing it from moving as it tries to slide forward to compensate for normal face wear, thermal growth, most impeller adjustment and shaft end play.
Tandem is the next version. This is the configuration you find in most oil refinery applications.
|The seals are connected in series with a low pressure buffer fluid circulating between them.
Here are a few comments about the tandem version:
- It takes the most axial space of the four types, and as a result is seldom found in the process industry, although newer designs are being made shorter as a result of computer finite analysis programs.
- You need two glands and this adds to the cost as well as the axial space required.
- A low pressure buffer fluid is circulated between the seals, eliminating the possibility of product dilution.
- A loss of buffer fluid will not cause the seal faces to open.
The face to face version is next. In some designs the two rotating faces run against a single stationary face that has circulating holes drilled through the face.
|This face to face version is a compromise between the “back to back” and the tandem version:
- You normally run a lower pressure buffer fluid between the faces. If you lose this pressure the seal faces do not open.
- Half the seal is in the stuffing box with the other half outside. This means the seal does not take an excessive amount of either axial or radial space.
- Many versions of this seal specify a common stationary unit with holes drilled in the stationary for circulation. I do not like this configuration because if you break the stationary face you could lose both seals. There should be two separate faces specified for maximum safety.
The concentric version is next:
- This configuration takes the least amount of axial space, but requires the most amount of radial space. You will sometimes find them used on a top entering mixer application, but you will seldom find them in a pump application because of the radial space required.
- Because of the common stationary used there is the danger of losing both seals if you break the stationary face.
Here is a drawing of the convection system I mentioned a few paragraphs back.
|The convection tank is a unit you can either purchase or manufacture your self. When a manufacturer supplies this unit, it requires a Boilermaker Stamp and a 600 psi (40 bar) rating, making it very expensive to purchase.
You can probably manufacture one for your purposes at a much lower cost.
You have many choices when it comes to your choice of the buffer or barrier fluid:
- Anti-freeze is popular in northern climates. Do not use the commercial brand that contains a chemical used to plug leaks in radiator hoses.
- Finished product is always acceptable.
- A fluid compatible with your product is often used.
- You might consider a cleaner or solvent that will be used to clean the system.
- If a flush is being used in the system, you can always use that as a barrier fluid.
- Once again, try to avoid using any type of mineral, petroleum or vegetable oil as a barrier fluid. Oil has a low specific heat and poor conductivity that can cause varnishing and coking problems between the seals. Some of the newer synthetic and heat transfer oils can be used if the temperate is not too high.
Now that you have all of the basics under control, we will use a dual seal to solve some of our common application problems:
Use a pressure higher than stuffing box pressure (barrier) between the seals to:
- Prevent kaolin (china clay) or other micron size solids from penetrating between the faces.
- To provide face lubrication if the product you are sealing is a non-lubricant. This will prevent excessive wear and “slip-stick” problems.
- To prevent a pressure drop across the faces that could:
- Cause a product to vaporize and open the lapped faces
- Cause a fluid to solidify, paint is a good example
- Cause Ethylene Oxide to attack the dynamic O-ring in the inboard seal. Ethylene Oxide can penetrate the elastomer and blow out the other side if there is a differential pressure across the O-ring.
Use a lower pressure (buffer) between the seals to:
- Create an intermediate pressure in a high-pressure application.
- Use a low pressure, with an anti-freeze as barrier fluid, to prevent ice from forming outboard of the seal when sealing products that freeze at atmospheric pressure. You will see ice on the outside of the pump if you trying to seal one of these applications.
- To be able to identify which seal has failed.
CAUTION: do not put one half the stuffing box pressure between the dual seals. This will put an equal hydraulic load on both seals and they will wear out at the same rate.
Circulate the correct temperature fluid between the seals especially when the pump is shut down. You are going to have to make the decision as to what barrier fluid temperature is needed. You can increase the temperature, decrease it, or hold it within narrow limits:
- To prevent a product from crystallizing.
- To prevent a product from vaporizing.
- To prevent a product from becoming viscous.
- To prevent a product from solidifying.
- To prevent a product from building a film on the seal faces.
- To prevent the product corrosion rate from increasing with heat.
Here are a couple more considerations:
- With the proper selection of barrier or buffer fluid pressure you can transfer the hydraulic load to either the inboard or outboard seal. It is never wise to use the barrier fluid at a pressure of half the stuffing box pressure because this would cause both seals to share the load and they would be expected to wear out at the same time. It is always better to select one of the seals to carry the load.
- A transmitter can be installed on the convection tank that will send a signal to a control panel informing the operator which seal has failed or worn out.
- Some people are tempted to run the outboard seal dry. With the correct selection of the seal type and the proper materials this is sometimes possible at the lower shaft speeds, but not recommended. You should use a convection tank under some fluid pressure to be able to tell if you have had a seal failure, and which seal has failed.
- Use the cartridge version of these seals to ease the installation problem and allow you to make impeller adjustments and compensate for thermal shaft growth. Be sure the cartridge sleeve is sealed to the shaft inside the stuffing box, or fluid will flow between the cartridge and the shaft making cartridge removal difficult. Some manufacturers seal on the outboard end, but this is not desirable.
- If you use the stationary version of these seals:
- Back to back is acceptable; the fluid will be at the seal O.D.
- Tandem is acceptable if the stationary units are positioned in the glands.
- Face to face is not acceptable. The fluid will be at the I.D. of the inner faces and centrifugal force will work against you.
- Concentric is OK. if you can locate a concentric version of a dual seal.
If you are going to use a cartridge version of the stationary dual seal be sure it has some type of self-aligning feature to prevent excessive movement as a result of “cocking” when the cartridge sleeve is attached to the shaft.
SEALING NON LUBRICANTS SA009
When we are discussing mechanical seals a lubricant is defined as a fluid that has a film thickness of at least one micron (0.000039 inches) at its operating temperature and load. If the product we are sealing is not a lubricant we are forced to use the self-lubricating characteristics of the carbon/ graphite mixture in the seal face.
The key to this self-lubrication is that carbon can form strong chemical bonds with gases such as water vapor. The adsorbed gas then weaken the interlacing bonding forces, releasing the graphite, which in turn reduces the rubbing friction. Many other types of vapors and gases can be readily adsorbed by carbon/ graphite and in some instances inorganic compounds can be added to the carbon/ graphite if adsorbable gases are not present or in short supply. Graphitizing of the carbon (heating it to 4000 degrees Fahrenheit or 2200 degree Centigrade) is another approach to self-lubrication.
In the seal business we are faced with the challenge of sealing three types of non-lubricants. I will address the problems in order of their difficulty, starting with:
The non-lubricating liquid. Hot water and many solvents fit into this category. The lack of lubrication at the seal faces causes more rapid wear of the carbon face.
This carbon face is really a combination of carbon and graphite with the graphite being a good dry lubricant. As the seal face wears the graphite is deposited on the hard face (you can see the black ring) leaving the carbon behind. The function of the hard face is to give the graphite a place to deposit.
Testing has shown that when we seal a lubricating fluid the lubricant becomes trapped between these asperities (the peaks the graphite leaves when it deposits on the hard face) and in many cases becomes a vapor, separating the two running surfaces.
A lack of lubrication between the seal faces can also cause a destructive form of vibration called slipstick. Without proper lubrication the lapped seal faces try to stick together, but “slip” when the seal drive mechanism engages the drive lugs and inertia accelerates the faces off of these lugs. The faces then slow down as a result of the poor lubrication. This alternating “slipping” and “sticking” causes severe vibration with a resultant “chipping” at the out side diameter of the carbon face along with drive lug and slot wear.
The amount of wear experienced by the carbon /graphite mixture is affected by:
- The surface speed of the seal faces. (a combination of shaft rpm. and seal face diameter). PV (pressure x velocity) numbers are not really valid because the carbon is sensitive to “P” but not to “V”
- The spring load on the seal faces and the area of the seal faces.
- The stuffing box pressure. Keep in mind that this number can vary during pump operation.
- The quality and grade of the carbon/ graphite face.
- The surface finish and hardness of the hard face.
- The cleanliness of the sealing fluid.
- The accuracy of the initial installation dimension.
- The hydraulic balance designed into the face.
- The hardness of the carbon.
- The thickness of the lubricating film.
- The affect of centrifugal and hydrodynamic forces on the face loading.
There is little chance of excessive heat developing between the seal faces and in the stuffing box area because the generated heat can be carried away by the conductivity of the non-lubricating liquid surrounding the seal.
All of the above means that the extra heat will probably not affect the elastomer (O-ring) generated between the seal faces, as a result of the poor or no lubricating properties of the fluid you are sealing.
The non-lubricating gas is next:
This application has all of the problems associated with the sealing of non lubricating liquids, but now you have the additional problem of heat, because gases are for the most part good insulators and do not let the heat generated between the faces dissipate to the surrounding product and metal stuffing box. Heat can affect a seal several ways:
- Filled carbon faces can be damaged depending on the filler or binder that was selected. There are special filled carbons manufactured if the gas can not be adsorbed into the carbon/ graphite releasing the graphite to provide dry lubrication.
- The elastomer (rubber part) is probably the most sensitive to an increase in heat. Its proximity to the seal faces is very important in dry running applications. Heat can cause an initial compression set off the elastomer and eventual complete destruction. Each elastomer compound has a temperature limit as well as sensitivity to certain chemicals and compounds.
- Most fluids are affected by an increase in heat. They can crystallize, solidify, lose their viscosity, vaporize, or build a film. In each of these cases, seal life will be affected.
- The corrosion rate of most corrosive fluids will double with an 18° Fahrenheit (10°C) increase in temperature.
- Seal flatness, face load, carbon squeeze, elastomer interference and many other tolerances can be affected by a change in stuffing box temperature.
Sealing a dry solid is the worst of the lot.
You now have all of the problems associated with the sealing of a gas, with the additional problem of a bunch of solids thrown into the mix. This application is seldom associated with pumps but is commonly found in mixer applications. The application is very similar to sealing a slurry so you should try to select those seal designs that have non-clogging features. These features would include:
- Springs out of the fluid.
- Rotate the seal in the powder to take advantage of centrifugal force to throw the solids away from the sliding components.
- The elastomer must move to a clean surface as the seal face wears.
- Select non-fretting designs. They are especially important in dry solids applications.
- Teflon® coating of the rotating parts helps to prevent the solids from sticking to the moving components.
The majority of mixers designed with bottom entering stuffing boxes are especially sensitive to this problem. Try to locate the seal inside of the mixer and out of the narrow stuffing box or you will have trouble with the solids packing around the outside diameter of the mechanical seal.
A clean flush with air or a suitable gas seldom works in this application because the air channels through the dry solids, or the vessel pressure will equalize with the incoming air pressure stopping the flow.
Most of these applications are slow speed (less than 500 rpm.) so a non-clogging type seal works well. A non-metallic, outside seal can be used if you are prepared to clean it out with air or some other gas between batches.
A split seal with air introduced into the bottom of the gland is getting good results in many applications.
In some applications it is acceptable to use a compatible grease in the stuffing box to prevent the ingress of solids. A balanced O-ring type seal, running at lower motor speeds should not generate enough heat to affect the lubricating qualities of the grease.
SEALING SLURRIES SA010
A slurry is defined as solids suspended in liquid that cannot be dissolved by controlling the temperature and / or pressure of the liquid. The solids may or may not be abrasive.
It does no good to try to identify the number of solids or their size because no one knows how these numbers relate to slurry related seal problems. Whenever you deal with slurries there are several points you must consider:
- The slurry can clog the flexing parts of a mechanical seal causing the lapped faces to open as a result of both shaft and seal movement.
- If the slurry is abrasive it can wear and damage the rotating components. This can be a serious problem with thin plate metal bellows seals.
- The pump rotating assembly will go out of balance as the slurry wears the impeller and other rotating components. This will cause excessive moving of the seal components.
- The pump will lose its efficiency as critical tolerances wear rapidly. This can cause vibration and internal recirculation problems. The wear will also cause the need for frequent impeller adjustments that will cause problems with mechanical seal face loading unless you are using cartridge seals.
It is generally believed that the main problem with slurries is that they penetrate between the lapped seal faces and cause damage. Although this is true, it is also true that they cannot penetrate until the seal faces open.
Seal faces should be lapped to within three helium light bands. That is a distance just a little bit shy of one micron. Compare this to the fact that the smallest object that can be seen with the human eye is forty microns in size and you will appreciate the technology used in the manufacture of mechanical seals. As a matter of comparison, look at a common coffee filter. It filters out particles larger than ten to fifteen microns.
All of this means that the seal is in fact a superior filter and as long as you can keep the two lapped faces in contact there little chance for solids to penetrate the faces and do any type of damage.
There are three approaches to the sealing of slurry:
- Design a seal with non-clogging features.
- Create a clean sealing environment for the mechanical seal.
- Do a combination of both
Let’s look at each of the approaches and in the process learn a sensible method of sealing any slurry:
You can build a seal with non-clogging features.
- Take the springs out of the sealing fluid. They cannot clog if they are not in the slurry.
- Make sure the sliding or flexing components move towards a clean surface as the seal faces wear.
- Take advantage of centrifugal force to throw the solids away from the sliding/flexing components and lapped seal faces.
- Use a non-stick coating like Teflon® to prevent the slurry from sticking to the sliding components.
- Use only balanced seal designs. The additional heat generated at the seal faces can cause many products to solidify, coke, and crystallize creating an additional solids problem.
- Metal bellows designs can be used but they must have extra thick plates to resist excessive wear. Extra convolutions will have to be provided to compensate for the higher spring rate caused by these additional plates. Rotating the abrasive fluid with the bellows can be a big asset. Some commercial designs have this feature.
Another approach is to create a clean sealing environment.
Give the seal as much radial room as possible. You can either bore out the packing chamber or install a large bore-sealing chamber. Try to give yourself at least 1-inch (25 mm) radial space if possible. The more room you can provide for the seal the better off you are going to be.
Try to remove the solids from the sealing area. There are a number of techniques for doing this. Some work and some do not. First we will look at the solutions that do not work very well and comment on their problems:
- Bad Solution #1 Connect a filter in the line installed from the pump discharge to the stuffing box. Since the discharge is a higher pressure, the flow of liquid through the filter will clean up the fluid and then there will be clean liquid flowing to the stuffing box.
- Comment: The problem with this idea is that the filter will clog and no one will clean it.
- Bad Solution #2 Install a cyclone separator into the line instead of a filter. Connect it between the pump discharge and suction with the third (the center) port connected to the stuffing box.
This idea is just as bad. The cyclone separator was never intended to be a single pass device.
They work well if used in a bank of several filters but there is not enough pressure differential between the suction side of a pump and the stuffing box for them to be effective.
- Bad Solution #3 Install the seal outside the stuffing box so the springs will not be located in the dirty fluid.
The problem with this idea is that as the seal faces wear they must move forward and in doing so they will move into the dirty fluid.
The result will be that the movable face will hang up in the solids and the faces will open. Another problem with this approach is that centrifugal force throws the solids into the seal faces and not away from them.
- Bad Solution #4. Install a double rotating seal in the “Back to Back” configuration with a higher pressure; clean liquid barrier between the seals.
This is a very common approach to the problem and has all of the problems associated with installing the seal outside the stuffing box.
In addition to a rapid failure you will also experience product dilution as the barrier fluid leaks into the pump.
- Bad Solution #5. Since we are discussing things that don’t work we might as well try two hard faces. Needless to say they will not prevent the faces from opening and when they do open experience shows that you are going to destroy both hard faces. Some seal salesmen may even try to convince you that the seal faces are designed to “grind up” the solid particles into a fine powder. In other words the seal is designed as some type of a “quasi- milling machine”
Now we will look at some methods that do work:
Good Solution #1.
- Flushing with a clean liquid is a good method of cleaning up the pumping fluid. The amount of flushing you will need depends upon the design of your seal. If the design has multiple small springs in the fluid, then more flushing will be required. There are various sources for the flushing liquid :
- Finished, clean product or one of the mixture’s clean ingredients
- A liquid compatible with what you are sealing.
- A solvent.
- An additive that is going to be added down stream and could be injected into the stuffing box location.
- Clean water.
- A compatible grease can be used with most balanced seals running at lower speeds
- Be sure to start with a flushing pressure that is at least one atmosphere (15 psi or 1 bar) higher than the stuffing box pressure. You can use a pressure gauge to determine stuffing box pressure. You can then use a flow meter to regulate the amount of flushing fluid.
- With intermittent service pumps it is a good idea to have an electrician install a solenoid valve with a delay switch that would allow the flushing fluid to come on thirty seconds prior to the pump starting and to leave the flushing valve open for a few minutes after the pump has stopped.
- NOTE: Never introduce live steam into the stuffing box as it could cause the product to flash and the pump to cavitate.
Good Solution #2.
- Install an oversize jacketed sealing chamber and “dead end” the fluid. Dead ending means that there are no circulation lines coming in or going out of the sealing chamber.
- You can use the cooling jacket to remove the heat being generated by the seal faces as centrifugal force cleans up the solids that are present in the small amount of fluid, trapped in the seal chamber. This solution works exceptionally well with fluids where temperature control is important. Heat transfer oil is a prime example.
- If the fluid you are sealing is not hot the cooling jacket will not be necessary. Sometimes one filling of clean liquid into this oversize, dead ended stuffing box is all that is necessary to seal even a severe slurry. Needless to say this application works best on a continuous running pump.
- If the specific gravity of the solids is less than the liquid they are suspended in, centrifugal force will not work for you. A clean flush will be necessary in this instance, or you might try filling up the dead ended stuffing box with a higher specific gravity compatible liquid.
Good Solution #3.
- If the solid particles are sub micron in size, two seals with a higher pressure barrier fluid become necessary. In some instances you might want to use two hard faces on the inner seal. Kaoline and some dyes are a good example of products with sub micron size particles.
- Tandem seals with a high-pressure barrier fluid circulating between the seals are ideal. Make sure the inner seal is balanced in both directions or you may have trouble with it opening up during pressure reversals. The tandem configuration keeps the abrasive solids at the outside diameter of the seal so that centrifugal force works for you.
Good Solution #4.
- Install a large seal chamber on the pump and connect a recirculation line from the bottom of the stuffing box back to the suction side of the pump. The size and number of solids that you are trying to remove will determine the size of this line.
- This arrangement will cause liquid to flow from behind the impeller to the stuffing box and then on to the suction of the pump. Fluid entering the stuffing box from behind the impeller has been centrifuged and should be a lot cleaner than the fluid you are pumping. This solution works well with closed impeller pumps and those open impeller designs that adjust to the front of the pump volute. If your open impeller adjusts to the back plate (as is the case with the Duriron pump) this method is not as effective.
- Do not use this technique if:
- You are pumping close to the vapor point of the fluid because lowering the stuffing box pressure could cause the pumping fluid to vaporize in the stuffing box and in some cases between the seal faces.
- You are sealing a Duriron pump where the impeller adjusts to the back plate. There is very little difference between suction pressure and stuffing box pressure in most Duriron pump applications.
- If you are sealing single stage double ended pumps where the stuffing boxes are at suction pressure.
- If the solids have a low specific gravity or density, and float on the liquid
Be sure to compensate for the fact that the rotating unit will go out of dynamic balance.
- The seal faces have to be vibration dampened. O-ring type seals are equipped with a natural vibration damper because of the dynamic elastomer that has been installed. Metal bellows seals have to be provided with some other method. Letting the seal face holder rub and vibrate along the shaft is a normal approach used by most metal bellows seal manufacturers. The logic is questionable.
- Give the seal room to move. Shaft run out and vibration can cause the seal rotating components to contact the inside of the stuffing box unless you have installed an oversized sealing chamber.
- Use motion seals if the run out or vibration is excessive. Unlike pump seals, these seals have much wider hard faces and extra internal clearances. Most popular designs can compensate for plus or minus 1/8″ (3 mm) in a radial direction and 1/8″ (3 mm) in an axial direction.
- Move the seal closer to the bearings. Split seal designs are a logical choice because most of them come with a stuffing box extension gland that positions them next to the bearings. A support bushing or sleeve can be installed in the end of the stuffing box to minimize the affects of unbalance, vibration and shaft whip or wobble. A variety of materials are available for these support sleeves. Check with your supplier for availability in your area.
The pump will lose its efficiency and experience more shaft movement as close tolerances wear.
- If you are using open impellers it will mean frequent impeller adjustment. In this case a cartridge seal is your best approach as impeller adjustments can be made without disturbing the seal face loading. Split seals can compensate for the initial impeller setting and split seals mounted on a split sleeve will easily compensate for movement caused by temperature growth or impeller adjustment.
- Closed impeller pumps will have to be disassembled and the wear rings changed when the clearances become excessive. If you are fortunate enough to have adjustable wear rings on your pump then only an outside adjustment will be needed and the pump will not have to be put out of service. Cartridge seals can almost always be reused in these applications because the seal faces were not separated as the pump was disassembled.
- Remember that with closed impeller pumps the wear rings will have to be replaced when the normal clearance doubles. A typical normal clearance would be 0.008″ to 0.015″ (0,2 to 0,4 mm). A good rule of thumb is that the pump will lose 1% of its capacity for each .001 inch (0,025 mm.) of wear ring wear.
A few more thoughts about the sealing of slurries:
- Kaoline (china clay) is a product that is used in many industries including paper and pharmaceutical. Its abrasive particles are less than one micron in size and as a result will penetrate lapped seal faces, causing rapid carbon and hard face wear. In this application it is necessary to use tandem mechanical seals with a higher clean barrier pressure between the faces to prevent most of the penetration.
- In addition to one of the recommended solutions mentioned above, two hard seal faces can also be used because some particles will always penetrate the faces.
- Using a combination of packing and a split mechanical seal is proving to be an ideal solution in many applications. With the seal installed there is no pressure differential across the packing and therefore the solids do not try to penetrate. Move the packing flushing line to the bottom of the split seal housing and flush the packing through this connection instead of the lantern ring or seal cage. The flushing is necessary to remove the additional heat being generated by the packing.
- You should be able to cut the flushing fluid volume down to about one third of the amount you had been using. Since the packing is not being forced to the shaft only a small amount of cooling is necessary. CAUTION! It is important that the flushing fluid be kept at a higher pressure than the stuffing box pressure. If this pressure differential fails it could force the packing into the rear of the mechanical seal. A split adapter plate installed between the split seal and the stuffing box face can prevent the packing from blowing out if the flushing pressure is lost.
- If you elect to use a rotating metal bellows in a slurry application, remember that the bellows should rotate the fluid in the sealing chamber. Most bellows designs allow the thin bellows plates to cut through the abrasive slurry and experience severe wear and breakage in a short period of time.
SEALING LIQUIDS SENSITIVE TO AGITATION SA011
This category of sealing is the one that is the least understood by most of the people that are involved in the process industry. It is easy to understand how temperature can change an “easy to seal liquid” into a difficult to seal crystallized product, a solid, or a gas, but it is hard to see how agitation alone can have much of an affect because pumped liquids are continually being agitated.
Whether or not you are going to have a problem often depends upon how long the fluid is going to be agitated, and how fast the agitation takes place. We all know that cream becomes butter with agitation and if you beat it fast enough and long enough an egg white (a fluid) will become a solid.
The fluid we find in a pump stuffing box seldom gets the proper circulation. The stuffing box lantern ring connection (A) is commonly used for this purpose and if you will look at the area closely you will see that the fluid is trapped in the seal face area where it is exposed to long periods of high-speed agitation.
If the fluid is not affected by agitation or mixing we say it is a Newtonian fluid (you remember, the apple fell on his head and he discovered gravity). These fluids are not considered a sealing problem for us unless they are sensitive to temperature or pressure changes, or contain lots of solids. The fluids we are concerned about are the non Newtonian fluids, and the problem ones fall into three neat categories:
Dilatants. The more you agitate them the more viscous they become and in many cases they can solidify. Any time a fluid becomes viscous it can interfere with the ability of the mechanical seal to follow shaft “run out” or vibration. This hysteresis or delay will allow solids to penetrate between the lapped faces or allow fugitive emissions to escape to the atmosphere.
- Dilatants are commonly used in industries that manufacture cleaners. You need this increased viscosity to hold the cleaner on a vertical surface. Many sugar syrups and clay slurries fall into the same category. In the paper industry the product “Kaoline” or china clay is a common example.
- To insure proper sealing you must insure that the product circulates through the stuffing box only one time. This would be the case if you used a suction recirculation line connected close to the face of the seal, at the bottom of the stuffing box, to the suction side of the pump, or some other low pressure point in the system. In this application it is important to use either seals that have no spring or springs in the fluid or metal bellows seals.
Thixothrophic fluids are the opposite of dilatants. Their viscosity decreases with agitation.
- Non drip paint is a good example of this type of application, along with automobile wax or most of the very viscous hand cleaners you find available on store shelves.
- The decreased viscosity can cause the product to become a non-lubricant as the film thickness diminishes to less than one micron between the lapped seal faces. This will cause an increase in face wear and in the case of carbon/graphite seal faces, create a potential color contamination problem with some color sensitive products.
Plastic materials release their viscosity suddenly and present the very same problems as thixotrophic fluids.
- Ketchup or the tomato sauce product you find in restaurant bottles is a good example of a plastic fluid.
When dealing with any of these problems be sure to keep the agitation in the stuffing box to a minimum. In some isolated cases the seal hydraulic balance diameter could be lowered and/or the spring face load reduced to lower the amount of shear. If you are running at higher than conventional motor speeds this can be a real problem.
The use of two seals with a pressurized lubricant as a barrier fluid can keep a lubricant between the faces and diminish the color contamination problem. If color contamination is a real problem, the use of two hard faces is recommended.
As is the case with just about any fluid sealing problem, the use of a reliable, clean, compatible, liquid flush is the universal solution. It is often the only solution if you find that none of the above suggestions are practical in your application.
SEALING LIQUIDS THAT COMBINE TOGETHER TO FORM A SOLID COMPOUND SA012
We want to be sure that we do not confuse this application with fluids that set-up or harden because of:
- A change in temperature. Chocolate solidifies when it cools
- A change in pressure. Paint solidifies when you lower the pressure enough to let the solvents evaporate.
- Agitation. Cream becomes butter.
In this application we are talking about liquids that are combined together and then the hardening takes place. If you mix a resin and hardener together you get epoxy glue. Some of the newer coatings and many plastics are made this way
Most of the time the liquids are combined outside the pump to form the compound, so sealing never becomes a problem. We only have to seal the individual fluids and they fall into the convenient categories we list under “Seal application”.
Occasionally we run into a batch mixer application where two or more liquids are mixed and then pumped out of the mixer before the setting-up or hardening takes place. The mixer is then flushed with a solvent of some type to remove the resultant compound and the solvent is then pumped through the system to clean the piping.
The problem occurs with the pump emptying the mixer, because the stuffing box area never gets the proper flushing necessary to prevent the compound from setting up and restricting the seal movement.
The time element takes over and the compound solidifies in the seal components restricting their movement and sometimes it solidifies between the lapped seal faces causing them to break when the pump shaft rotates.
The solution to the problem is obvious. We need a more thorough cleaning of the pump stuffing box area.
Please look at the following illustration:
- Fluids “A” an “B” are added to the mixer and blended together.
- The pump empties the mixer, but some of the compound is left inside and it will solidify unless it is flushed away.
- The solvent flush valve is opened and solvent “C” is added to the mixer. The mixer is filled, agitated and then emptied by the same pump. This action also flushes the compound from the lines.
- There is always some of the compound trapped in the pump stuffing box. Flushing the mixer and lines will not remove it.
The illustration also describes the solution to the problem.
- An inlet line is connected from the solvent flush to the bottom of the pump stuffing box and an outlet line is connected from the top of the stuffing box to the pump discharge.
- The solenoid valve opens when the solvent flush valve opens and mixer flushing begins. This flow provides a constant cleaning of the stuffing while the mixer is filling. Bringing the fluid into the bottom of the stuffing box and out the top is critical to the success of this application. Sometimes it is difficult to install a fitting at the bottom of the box, so get it as close as you can.
- When the solvent flush valve closes, the solenoid valve is wired to close also.
I have some recommendations for the seal that you will be using in this application:
- Installing an oversize stuffing box makes sense.
- This is an instance where using a seal with two hard faces is a sensible choice.
- Because most solvents attack popular O-ring materials, you will probably be using Chemraz or Kalrez® as the dynamic elastomer.
- Select a design with the springs out of the fluid. A metal bellows seal without a dynamic elastomer is another choice that is logical.
- Be sure the fluid in the stuffing box is at the seal outside diameter. It will be a lot easier to flush away.
- If you prefer a dual seal in this application, be sure to use a tandem design with the compound and solvent at the outside diameter of the inner seal.
SEALING LUBRICATING LIQUIDS SA013
This is the sealing application everyone wants. Unless the fluid also falls into any of the categories mentioned above, it should be just a matter of picking the correct materials, installing the seal correctly and then stand back and watch the seal work beautifully.
Now that you understand the basics, we will take a look at a couple of typical hot applications that cause a lot of seal problems:
- Hot water
- Hot oil
SEALING HOT WATER SA014
Water is normally considered a good lubricant and can do an adequate job of providing lubrication between the lapped faces of a mechanical seal, but there are a few problems:
- At a temperature above 180°F (80°C) the water lubricating film is not thick enough to separate the sliding surfaces of the seal faces. Cold water has a film thickness of about one micron which will keep lapped seal faces separated most of the time. Hot water has a film thickness of only one third to one half of that amount depending upon the temperature.
- At some combination of temperature and pressure the water will vaporize, expand and open up the lapped seal faces. When this occurs:
- The carbon outside diameter can become chipped and damaged as the constant vaporizing and subsequent cooling vibrates the seal faces causing them to bang together. Drive lugs will wear, metal bellows can break and lug driven hard or soft, faces can crack.
- Solids dissolved or suspended in the water will be left between the seal faces when the water vaporizes. They will imbed into the softer face causing severe wear and damage to the hard face.
- A phonograph finish can form on the carbon if a large particle of scale or any foreign matter is blown across the two faces. The seal will leak through this damaged face.
- Slip stick can occur because the faces are trying to stick together due to a lack of lubrication between them. The alternating sticking and slipping will produce a vibration that will chip carbon, break bellows and crack lug driven faces unless some form of vibration damping has been installed.
- In many piping systems magnetite (Fe304) forms on the inside surfaces as a corrosion resistant covering. This magnetite breaks loose from the piping walls and often collects on the seal components. It can be recognized by its black color and attraction to a magnet. The magnetite affects the seal a couple of ways:
- Being an abrasive material it will mechanically attack the seal sliding elastomer by penetrating into it. This will cause “hang up” and eventual leakage.
- It will wear the sliding elastomer sealing surface.
- Loose magnetite is very common in new water systems. The problem wills eventually clear its self up after the system has been in use for about a year and the ferric oxide has formed into a stable layer.
- Hot water is dangerous. The leakage will be invisible as it flashes to steam. If the hot water is part of a condensate system it may have to be sealed under vacuum conditions.
In order to seal this product effectively, you must address all five problems at the same time. We will begin by learning how to pick the correct materials for the seal components, then we will choose a seal design and finally apply the correct environmental controls to insure that the above problems are being addressed.
Picking the materials:
- The seal face combination should be unfilled carbon graphite, or graphite impregnated silicon carbide running against either solid silicon carbide or tungsten carbide as your first choice. Plated or coated faces should not be used in this application.
- The elastomer. Use ethylene propylene to 275 degrees Fahrenheit (135 C.) If you seal at a higher temperature, either Kalrez® or an equivalent will be necessary. In most cases you should be trying to cool the water to increase the face life. If the water is cooled, a high temperature elastomer is not necessary.
- The metal components, 316-grade stainless steel is preferred. Metal bellows or springs should not be manufactured from stainless steel to avoid chloride stress corrosion problems. Hastelloy “C” is your best choice for the springs or metal bellows.
Choosing the mechanical seal
- A balanced, O-ring mechanical seal should be used. Both rotating and stationary versions are acceptable although stationary is preferred. The O-ring will allow sealing in both directions if the application alternates between vacuum and pressure.
- A cartridge seal should be used for ease of installation and in the case of open impeller pumps, to allow for impeller adjustment as the pump cycles between operating and ambient temperature. Do not use cartridge mounted stationary seals unless they have been fitted with some type of self-aligning feature.
- A motion seal should be specified if the pump is equipped with sleeve or journal bearings. This is a very common arrangement with multiple stage boiler feed pumps.
- A high-pressure seal should be used if the seal chamber pressure (not the pump discharge pressure) exceeds 350 psi. (24 bar). High-pressure seals are of a more rugged construction that prevents face distortion and elastomer extrusion.
- Split seals can be used in some of these applications, but a few of the commercial designs have trouble when the stuffing box pressure alternates between a positive pressure and vacuum. Sleeve mounting the split seal helps with impeller adjustment, or in the case of vacuum applications the seal can be installed backwards, or with a discharge recirculation line installed to keep a positive pressure in the stuffing box. Note: many hot water applications are dangerous so dual seals are recommended. Care must be exercised if you use a stationary metal bellows seal design. Flow through the normal flush or recirculation connection can cause a substantial temperature differential across the seal face that can cause the lapped seal faces to become distorted.
The environmental controls you will need to seal hot water:
To insure the longest possible seal life, the water should be cooled as close to ambient temperature as possible. The cooler the water the better it will lubricate the faces.
- Install a carbon bushing into the bottom of the stuffing box to act as a thermal barrier. Utilize the jacketed stuffing box on the pump to cool down the stuffing box fluid. Be sure there are no recirculation or flush lines coming into or out of the stuffing box. If there is no jacket installed on the stuffing box one can be purchased from the pump manufacturer or an outside vendor. If you purchase the jacket from an outside vendor be sure to order the enlarged, jacketed seal chamber or replacement back plate with the large, jacketed seal chamber cast into it.
- NOTE: Be sure the cooling jacket is functioning. If you are in an area that has hard water, calcium can coat the jacket surfaces interfering with the heat transfer. In that instance you must provide for jacket cleaning on a regular basis or substitute condensate as the cooling medium. The cooling jacket is also necessary to prevent heat transfer to the bearing case. Each 18 degree Fahrenheit (10 C.) rise in oil temperature will cut the life of the oil in half.
- If cooling is not at all possible another alternative is to pressurize the stuffing box to at least one atmosphere above the water vaporization pressure. Installing a close fitting bushing into the bottom of the stuffing box and using a recirculation line from the pump discharge to pressurize the box can do this. As noted above be careful of leaks in the fittings. This could be dangerous in some high-pressure boiler feed pump or boiler circulating pump applications. Depending upon the pressures involved you may be better off with a special high-pressure seal design.
- NOTE: You are going to have trouble when the heat transfers back to the bearing oil. Many pumps have a bearing oil cooler available to provide the necessary cooling. Check with the manufacturer for this accessory. At 200° Fahrenheit (100° C.) non-contaminated oil has a useful life of only three months. The lip or grease seals used in these applications have a useful life of only three months also, even when the temperature is closely controlled. These seals should be replaced with labyrinth or positive face seals.
- It is not wise to install a cooler between the pump discharge and a pump stuffing box. Although this arrangement will provide adequate cooling, in most cases it is too dangerous at elevated temperatures because of possible leaks in the additional piping and fittings.
- Tandem seals, with a pumping ring and cooler installed between the seals is another alternative, but this application takes a great deal of axial room.
- An API (American Petroleum Institute) type gland with a cool quench connection is not a good choice for this application.
- The quench water will vaporize when it hits the hot surfaces under the seal, causing solids to form that will restrict the seal movement and contribute to the corrosion of the seal sleeve and other components.
- Those designs that have the springs out of the sealing fluid can easily clog the springs in this solution.
- Excess quenching water can leak back into the bearings through the grease or lip seal.
SEALING HOT OIL SA015
- The largest user of hot oil pumps is the heat transfer oil customer. Many consumers use these products with oil temperatures exceeding 500° Fahrenheit (260° C) and 600° to 700° F (315° to 370° C) becoming common. Some hotels have recently installed these systems in their laundry to dry clothing.
- Heat transfer oils have many advantages over the steam that was formally used in these applications.
- The product does not flash.
- No boiler blow-down.
- No deaeration heat loss.
- No high-pressure. This means it is not only safer but also tends to leak less.
- No licensed boiler operator needed.
- The temperature can be kept uniform over a large processing area.
- You can heat and cool with the same system.
- These oils are excellent in systems that are water/ steam sensitive.
- The product is kept in a closed system. This means that all leakage can be stopped.
- There is less corrosion in the system.
In addition to these heat transfer oils you will encounter hot petroleum oil applications in refineries and hot organic oil applications in various other industries. There are several problems associated with sealing these hot oil products and each of them has to be solved if satisfactory seal life is ever to be obtained.
- High temperature oil is generally too hot for most commercially available elastomers. (the rubber parts)
- The product “cokes”.
- These coke particles form at the elevated temperatures and coat them selves inside the system piping, hardware and on the mechanical seal working parts.
- The “coke” particles restrict the movement of sliding and flexing seal components causing the lapped seal faces to open.
- The amount of coke that forms is a function of time and temperature. In other words coking will be a more severe problem in a closed loop system than it will be in the oil refining business.
- Contrary to popular opinion, testing has shown that air or oxygen is not needed for the formation of coke. This means that seal designs that try to eliminate oxygen by quenching or some other method will not work. The use of steam quenching is limited to its cooling effect only.
- The product is always a fire hazard and depending upon the type and brand you purchase there could be toxicological problems. Keep in mind that the seal is going to wear out or fail at some time and the product is going to leak out to the atmosphere.
- Thermal growth of the pump parts will cause problems in maintaining proper pump “wear ring” and impeller clearances as well as the correct seal compression.
- Misalignment between the driver and the pump and between the piping and the pump suction is a serious problem at elevated temperatures.
- The product is costly. Leakage represents large monetary losses and personnel danger as well as environmental problems.
- Heat tracing must be provided throughout the system to prevent the product from becoming too viscous during periods of prolonged shut down. Unfortunately no one ever heat traces the stuffing box.
- Vibration is always a problem with hot oil pumps because the coke attaches to rotating components interfering with the dynamic balance.
- You always end up pumping slurry, which means frequent impeller adjustments or wear ring replacement and excessive vibration due to the imbalance caused by wear of the rotating parts.
- As the coke builds up on the inside of the discharge piping the pump will operate further off of its best efficiency point (BEP) causing shaft deflection, vibration, and excessive seal movement. Coking on the inside of the suction piping can also cause cavitation problems
Although there are many techniques available to address each of these problems, the combination of these problems eliminates most of the common techniques and leaves the customer with very few options to get good seal life. Regardless of the seal selected you must address all of the problems or the seal life will be shortened.
Oil refineries pump hot oil with closed impeller pumps and as a result have to put up with the additional problems associated with replacing “closed impeller” wear rings. Unlike the chemical industry they cannot take advantage of the features of an open impeller design that can be easily adjusted to maintain maximum efficiency. There are two reasons why oil refineries chose closed impeller designs with mechanical seals and API (American Petroleum Institute) glands:
- Fear of a bearing failure that could cause sparking as the metal impeller contacted the metal volute. The soft non sparking metal wear ring on one end of the shaft and the carbon disaster bushing installed in the API (American Petroleum Institute) gland on the other would insure no hard metal contact if a bearing failed as the shaft was turning.
- Shaft expansion or impeller adjustment could cause the rotating, open impeller to contact the stationary volute. To prevent sparking, the impeller or volute would have to be manufactured from a soft non-sparking metal such as aluminum or bronze and this would not be very practical. Hence the closed impeller with the soft wear rings
To insure long seal life you must do the following:
The product has to be cooled in the seal chamber:
- The oil must be cooled to stop the coking. Coke is a function of heat. Many years ago it was believed that oxygen had to be present for coking to occur, but testing has shown that this is not true. You can coke any petroleum product in an inert atmosphere as long as the temperature is high enough. The finest lubricating oil available will start to coke at 300° F (150° C). The oil temperature and time determine the amount of coking that you get.
- The oil must be cooled to prevent damage to any elastomers that might be installed in the seal or shaft sleeve. Elastomers that are subjected to high heat will first take a compression set and then shrink in volume. They will eventually grow hard, crack and leak excessively.
- The oil must be cooled to reduce the amount of heat that will be transferred through the shaft to the bearing oil or grease. This heat will reduce the viscosity of the lubricating oil or grease and eventually cause premature bearing failure. The SKF bearing company states in their lubrication literature, that the life of bearing oil is cut in half for each ten degrees Centigrade (18° F) increase in bearing oil temperature. They recommend 60° C to 70° C (140° F to 158° F) as an ideal oil temperature.
- The grease or lip seals are sensitive to any increase in shaft temperature. A stainless steel shaft is a good choice in these applications because stainless steel is a poor conductor of heat compared to carbon steel. This is the reason there are no stainless steel frying pans unless they are clad with either aluminum or copper.
You must install a back up seal for the following reasons:
- The product is dangerous. Leaking hot oil can start a fire or injure any personnel in the area. Many brands are toxic and some have been identified as possibly carcinogenic.
- The product is too costly to tolerate even small amounts of leakage.
- Back up cooling is necessary if the primary cooling method fails. A back up seal, with a cool barrier fluid system, can provide this cooling
- If you elect not to use a back up seal, then be sure to install an American Petroleum Institute (API.) type gland.
Take a look at the illustration below. This is an API (American Petroleum Institute) gland that can perform several functions:
- The disaster bushing (DB) can provide shaft support if you lose a bearing.
- The leakage will be directed to the quench and drain connection (Q) when the seal wears out or fails.
- The quench connection (Q) will allow you to use steam for product cooling, but do not use too much because it could penetrate into the bearing case.
- You can connect steam to the quench connection and use it to put out a fire, should it occur on the outboard side of the seal.
- In this application the flush connection (F) is not used. The stuffing box is “dead ended” to take full advantage of the heating/ cooling jacket.
Whenever possible a large diameter cooled sealing chamber should be installed on the pump:
- To allow room for centrifugal force to throw solid coke particles away from the seal faces and sliding, or flexing components
- Misalignment is always a problem in these pumps. This shaft displacement can cause the rotating seal to rub against stationary parts in a conventional stuffing box.
- Vibration means movement. The seal must be free to move within the seal chamber.
- When the pump stops gravity will pull solid particles to the bottom of the stuffing box. A large seal chamber will almost guarantee that the particles will not collect around the seal at this time.
A cartridge seal is necessary in most applications.
- Thermal growth will cause volute, casing and shaft expansion. Only a cartridge seal can compensate for this movement and allow for the impeller adjustment that will be necessary.
- The wear caused by the slurry will cause frequent impeller adjustments. The average pump used in these applications has almost 0.250 inches (6 mm) of adjustment possible.
To compensate for misalignment you will have to:
- Use a “C” or “D” fame adapter to compensate for misalignment between the pump and its driver.
- These adapters are available from all good pump companies and will compensate for misalignment as the pump goes through its temperature transients.
- No other method of alignment works anywhere near as well. If you are going to do a conventional alignment with dual indicators or a laser aligner be sure your calculations compensate for thermal growth.
- Use a “centerline” wet end to prevent excessive wear ring wear and pipe strain at the pump suction. If your pump did not come equipped with this type of wet end it can easily be installed in the maintenance shop. Look at the following illustration:
|The centerline wet end has the feet attached to the sides of the pump at the centerline instead of the bottom of the pump
The centerline design allows the hot volute to expand up and down and eliminates a lot of pipe strain due to thermal expansion.
Now that we have discussed these important points let’s take a look at some solutions that are often offered, but we should not adopt as our solution. Here are the things that do not work well:
Bad solution #1.
- Use a metal bellows seal to eliminate the need for cooling in the seal area. Although the metal bellows does not have rubber parts that are sensitive to high temperature cooling is still needed for the coking. Most bellow suppliers offer an A.P.I. type gland to provide low-pressure steam behind the seal for cooling purposes and thereby eliminate the option of backup sealing. This quenching should be limited to only a back up cooling status. If quenching is done with water rather than steam, watch out for a calcium build up outboard of the seal. This “hard water” build up can restrict the movement of the flexing portion of the seal as it tries to compensate for face wear.
- If you substitute condensate for the quenching fluid the build up can be eliminated almost entirely.
Bad solution #2.
- Run a line from the discharge of the pump through a cooler and filter to cool down and clean up the oil going into the stuffing box. The problems with this solution are obvious. The filter will clog and the cooler will become inoperative as coke builds up on the tubes.
Bad solution #3.
- Use two seals and run cool oil between them. You have addressed the cooling problem but you have not addressed the problem of the slurry with this solution.
What then is the best solution that addresses all of the problems?
You should install a large jacketed sealing chamber. These bolt-on accessories are available from your local pump or seal supplier.
- Be sure to dead end the stuffing box. In other words no lines coming into or away from the inner seal chamber. Do not worry about the heat. With a six to eight gallon per minute (20 to 30 liters/ minute) flow through the cooling chamber the cooling jacket can keep the temperature down to 200° to 250° Fahrenheit (95° to 120° C) without any trouble. If you have hard water in your area condensate may be the best choice to use as the cooling medium. In some cases low pressure stream is satisfactory. If you anticipate long periods of shut down, low-pressure steam will be your best choice because it will keep the heat transfer oil at the proper low viscosity during these shut down periods.
- You should install a cartridge dual seal that has built in slurry features with the inner seal balanced in both directions. If the pump does not have precision bearings a dual motion seal with the same features will work just as well. “Two way” balance is necessary because the system and barrier fluid pressure can and will vary.
- The dual seal is necessary to conserve the expensive product and to provide a safety feature when the inboard seal wears out or fails. It will also allow you time to schedule a seal replacement.
- Install a convection tank between the two seals and use cool heat transfer oil as the barrier or buffer fluid. A lower pressure or buffer fluid is preferred. A slight pressure on the tank will allow you determine which seal has worn out or failed first. A pumping ring or forced lubrication between the seals is necessary
- Install a carbon restrictive bushing into the bottom of the stuffing box to act as a thermal barrier. Applications have worked without this bushing but not as well as with it. Any materials that have poor heat conductivity will work as well as carbon as long as they are non-sparking and dimensionally stable.
That is all there is to the application. Centrifugal force will clean up the small amount of fluid in the sealing chamber while the cooling jacket holds the temperature low enough to prevent coking and damaging the seal elastomer.
The only problem with this system is that it works so well we often forget to clean the cooling jacket on the pump. A small layer of calcium inside this jacket will act as an insulation and destroy the cooling affect of the jacket. Be sure to keep this jacket clean or substitute steam or condensate for the cooling water, and then don’t worry about it.
Here are a few additional thoughts:
- A cartridge dual bellows seal can be substituted as long as adequate vibration damping has been provided to prevent breakage of the bellows. With metal bellows seals try to rotate the fluid in the sealing chamber to prevent excessive wear of the thin bellows plates. In the past, heat treated AM350 stainless steel was the bellows material of choice. In recent years Inconel 718 is becoming popular.
- The bearing grease or lip seals should be replaced with labyrinth or positive face seals. The original equipment manufacturer (OEM) lip seals have a design life of about two thousand hours (84 days) and they will cause costly shaft fretting damage. These grease or lip seals will also allow moisture to penetrate into the bearing case dramatically reducing bearing life.
- If you eliminate these lip seals, you will be able to convert to a solid shaft and improve the “stiffness ratio” enough to prevent some of the shaft bending and vibration that is experienced at start up, and as the pump runs off of its’ best efficiency point.
- Cool oil flush with a restriction bushing installed into the bottom of the stuffing box, is another choice. Be sure that the flushing pressure remains at least one atmosphere (15 psi. or 1 bar) higher than the stuffing box pressure.
- Do not hydrostatically test the seal with water. Any moisture left in the seal or trapped in a gasket will flash to steam when the hot oil enters the seal. This could be dangerous.
- When using an API (American Petroleum Institute) type gland be sure to check that the quench and drain ports have not been confused with the flush ports. If these ports are connected incorrectly it could be very dangerous.
- If you are using stationary bellows seals with a cool oil flush be careful to direct the flushing fluid away from the seal face. Since the bellows is not rotating the cooling on one side and the hot system temperature on the other can cause the bellows seal face to go “out of flat”.
- Recent tests show that carbon faces always experience some pitting in hot oil applications. In the past these pits were ignored, but fugitive emission standards dictate that two hard faces should be use in all hot oil applications.