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.
There are circumstances where suction recirculation doesn’t make sense. See: S104
|Jacketing fluid. The cooling or heating fluid flows through a jacket (B) that is surrounding the stuffing box.
Be sure to pipe into 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.
- The API provides drawings or plans describing piping arrangements that have historically been used in the petroleum industry. Not all of the plans are recommended or referenced in the API Standard.
- 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
- Use only balanced seals in these applications to avoid the heat problems associated with unbalanced seal designs. Elastomers designed in the lapped faces and 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.
- 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 re circulate 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 re circulating 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 leakage.
- 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 back plate (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’ll 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 “thixotropic”. 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.