STUFFING BOX PRESSURE CONTROL S098
We have very little control over the products that we must seal, but we have a great deal of control over how we elect to seal them. Special seals are seldom the answer. In most cases satisfactory seal life can be obtained by carefully choosing the seal materials for temperature and chemical compatibility, selecting a balanced O- ring seal design and then controlling the environment in the stuffing box to prevent a change in the fluid characteristics from affecting the seal performance.
Controlling stuffing box pressure is one of these controls and is extremely important in many seal applications. In the following paragraphs we will discuss several methods of controlling pressure, but first we will learn where the environmental control is necessary.
We can raise the pressure in the stuffing box to:
- Prevent a fluid from vaporizing in the stuffing box or across the seal faces. If the product vaporizes across the seal faces it can open the faces and possibly do some damage as the faces rapidly open and then close because of the cooling affect of the fluid expansion. In many cases solids will be left between the faces as the fluid vaporizes.
- Destroy a vacuum in the stuffing box. A balanced O-ring seal can seal either vacuum or a positive pressure. Vacuum often implies higher heat at the seal faces and that is never good for a mechanical seal because heat can cause the product to change state.
- Many split seal designs can accommodate either vacuum or a positive pressure application, but not one that alternates between them (the reverse pressure is forcing the splits open). Raising the stuffing box pressure will keep it positive so that a split seal can be applied.
We can raise the pressure between dual seals to:
- Stop a pressure differential across a dynamic elastomer from failing the elastomer. This is a serious problem when we seal ethylene oxide or any fluid capable of penetrating the elastomer and blowing out on the low-pressure side.
- To prevent sub-micron solids from penetrating between the lapped faces. Kaoline is a good example of this type of product. Sub-micron products have no problem penetrating lapped seal faces when the pressure drop is from the outside diameter to the inside diameter of the seal face.
- To take the load off the inboard face that is sealing a non-lubricating fluid and transfer the load to the outboard seal. This can make a dramatic difference in the life of the inboard seal
- To prevent a pressure drop across the faces that could cause a product to solidify. Many solids are dissolved in a liquid that will vaporize at atmospheric pressure. Paint is a good example of this application.
- To prevent a liquid from vaporizing between the inboard faces and blowing them open.
We can lower the pressure in the stuffing box:
- If it is too high for a standard balanced O-ring seal
- To vent air from a vertical pump.
Now, to the actual techniques:
In this application we connect a recirculation line from the discharge side of the pump to the stuffing box or a flush connection in the gland. You should install a close fitting bushing into the bottom of the stuffing box with a clearance of about 0.002 inch/ inch (0.002 mm/mm) of shaft diameter. The bushing can be manufactured from any material compatible with the fluid you are sealing. Carbon is often selected as a first choice.
A properly installed balanced O-ring seal will not generate enough heat to flash a liquid between the seal faces as long as the stuffing box pressure is at least one atmosphere (one bar or 15 psi.) higher than the product vapor point. You should have no problem in getting this additional pressure if you have installed the restriction bushing.
Please look at the illustration below. This is a cross section of an API (American Petroleum Institute) gland as used in oil refineries.
The technique is the same as discharge recirculation, but in this application we connect the stuffing box or seal gland to the suction side of the pump or a low-pressure sump instead. We do not use a restriction bushing in this application because the differential pressure can cause the bushing to move and contact the mechanical seal. Suction recirculation works best with the proper gland connection but it can be used with the lantern ring connection if necessary. Be sure the connection is on the bottom or as close to the bottom as possible of the stuffing box or gland and as close to the seal faces as possible.
Caution: Lowering the pressure in the stuffing box is sometimes a bad idea because of the danger of flashing the sealing fluid. The technique is commonly used, however, to remove air that might be trapped in the stuffing box of a vertical pump or to provide normal circulation through the stuffing box when the sealing fluid contains solids. By connecting to the suction side you will be pulling fluid from behind the impeller, through the stuffing box and then to the lower pressure on the suction side of the pump. Fluid behind the impeller usually contains fewer solids than fluid coming from the pump discharge side. It should be mentioned that this technique works very well on most closed impeller pumps and those open impeller designs that adjust to the volute of the pump.
There are four conditions when we would not use suction recirculation:
- If the solids in the fluid have a low specific gravity and float on he liquid.
- Any open impeller pump that adjust to the back plate. These pumps have a flow through impeller that causes the stuffing box to be at just about suction pressure, so no flow will occur. In the United States the Duriron Pump Company builds many of these designs.
- If the liquid in the stuffing box is close to its vapor point. Any lowering of the stuffing box pressure could cause the liquid to vaporize.
- Single stage double-ended pumps have the stuffing box at suction pressure.
Another common application for this connection is to cross connect the stuffing boxes of a multi-stage, pump to equalize the pressures and balance the seal face wear.
Using two or more seals with a lower pressure between them.
- Some seal companies use this as a technique to stage the pressure in a high-pressure application. I do not approve of this method because the operator is lead to believe that there are multiple seals in the pump when in fact the multiple seals are acting as one and a failure in any one of the seals will fail them all. I believe you would be better off purchasing a high pressure seal for this application.
Using two seals with a convection tank installed between them.
- This is done to take the load off the seal that is sealing a non-lubricant or to prevent a pressure drop across the seal faces. The convection tank is filled with a lubricating liquid and the tank pressure is adjusted to provide the necessary pressure differential. With the proper instrumentation you will be able to tell which seal wears out or fails first. The second seal will act as a back up until you can shut the valves and start the repair.
- As an example: In the pump stuffing box you have a 75-psi (5-bar) non-lubricating liquid. You cannot afford product dilution so you install two seals with a lubricating buffer fluid of 75 psi. (5 bar) between them. This will take the load off of the inboard seal that is sealing the non lubricant and shift the load to the outer seal that is containing the lubricating barrier fluid.
- The convection tank can be purchased or manufactured from an appropriate corrosion resistant material. Some companies (Coca-Cola as an example) ship their product in a similar tank and then scrap the tank because of sanitary or safety regulations. Many of these tanks can be purchased at a low price and modified for your needs. The air connection on the top will allow you to regulate the tank.
- Keep in mind that convection tank applications are limited by the combination of seal size, face combination, barrier fluid pressure and shaft speed. Check with your seal supplier for a specific recommendation.
- When ever possible avoid selecting petroleum base liquids for the barrier fluid circulating in the tank and between the two seals. Petroleum fluids have a very low specific heat that will cause overheating and coking problems. If you have the choice, water is the ideal heat transfer fluid because of its good conductivity and high specific heat number. If water is not acceptable choose any compatible fluid with a high conductivity and high specific heat value.