Sealing fluids sensitive to a small change in temperature or pressure
SEALING FLUIDS SENSITIVE TO A SMALL CHANGE IN TEMPERATURE OR PRESSURE SA007
Liquids and gases are both called fluids that can shorten the life of a seal in 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 and 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 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
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.
- 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.
- On February 17, 2018