Environmental controls and special seals 3-2
For any given seal application problem there are two generally accepted solutions :
- Build a special seal that can compensate for the problem.
- Control the environment surrounding the seal to prevent the problem from occurring in the first place. If you control the seal environment you’ll avoid the inventory and delivery problems associated with special seals.
In the following paragraphs I’ll be covering each of these environmental controls in detail.
- Flush the stuffing box (Port “C” in the illustrations at the end of this paper) with a compatible, cool, clean liquid. Many seal glands have this connection available in a more convenient location than the lantern ring connection. Flush is a misunderstood term. It describes six very different functions:
- Discharge recirculation, where 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.
- Suction recirculation, The recirculation line is connected from the bottom of the stuffing box to the suction side of the pump (A).
- Jacketing fluid, The cooling or heating fluid flows through a jacket that is surrounding the stuffing box (B).
- Barrier or buffer fluid, The fluid is circulated between two seals either by convection, or by a separate circulation system (E).
- Quench, The fluid is passed between the seal and a disaster bushing that has been installed in the rear of the seal gland (D).
- Flush, A liquid, from an outside source, is injected into the stuffing box at one atmosphere above stuffing box pressure and dilutes the product a small amount (C).
- Use two seals with a cool liquid circulating between them (E). 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 (figure “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’re 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 can substitute condensate or low pressure steam.
- An A.P.I. Gland is available for most mechanical seals. The gland has several features to provide various functions. It can be used as:
- A quench connection (Q) to provide heating, cooling, or to remove any 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.
- A flush connection (F) to provide clean fluid to the stuffing box or it can be used to vent the stuffing box and seal in a vertical pump application.
- A close fitting, non sparking, disaster bushing (DB) to provide shaft support in the event of a bearing failure or to protect personnel in the event of a massive seal failure.
- Heat tape or tracing lines can be installed around the stuffing box to provide a small 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 problem occurs during pump shut down
- Use only balanced seals in these applications to avoid the heat problems associated with unbalanced seal designs. Elastomers in the faces and two hard faces should also be avoided for the same reason.
- Increase stuffing box pressure by installing a recirculation line from the pump discharge back to the stuffing box (figure “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.
- 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 elastomer, expand and blow out the other side causing severe damage to the elastomer.
- Flush the stuffing box with a high pressure liquid. This is the best solution if the fluid contains solid particles that could interfere with the seal movement.
- The only reason to lower stuffing box pressure is because your seal does not have high pressure sealing capability. It’s 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. This is a common application for a double ended centrifugal pump.
- It’s 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 by utilizing an intermediate fluid pressure between two tandem or “two way balanced seals”
- 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.
- 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 destroy the vacuum in the stuffing box area.
NOTE. If the impeller has been adjusted too close to the back plate the “pump out vanes” can cause a vacuum to occur in the stuffing box. This often happens if the impeller adjustment has been made backwards (as is the case with Duriron pumps). The problem exists with those open impeller designs that adjust towards the volute (Goulds is an example).
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 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.
- Flush with a clean liquid. Check with your seal manufacture for the minimum amount of flush that is needed. Balanced seal designs with the springs located outside of the fluid and most metal bellows designs require only one to two gallons (4 to 8 liters) per hour.
- Note: this is per hour not per minute. At shaft speeds below 2900 rpm. filling the seal cavity with a compatible grease is usually satisfactory.
- Increase the seal clearance in the stuffing box area. Replacement back covers with extra large stuffing box designs are available for most pumps. Bolt on, large diameter stuffing boxes are also available in the after market. If your product is cool you can probably run the fluid “dead ended” with no connections coming into or out of the stuffing box.
- If you product contains sub-micron particles, as is the case with Kaoline (china clay), you’ll have to circulate a higher pressure clean liquid between two seals to prevent solids penetration between the faces. In some cases two hard faces also helps. Almost any dual seal design is acceptable with the exception of the “back to back” rotating design which is never acceptable in any application.
- Recirculate to the suction side of the pump when possible. This will circulate cleaner fluid from behind the impeller, through the stuffing box, and then back to the suction side of the pump. Original equipment manufacturers do just the opposite by having the stuffing box fitting connected to the discharge side of the pump. CAUTION! Do not connect to the suction side of the pump if :
- The fluid is being pumped at or near its vapor point as this could cause flashing in the stuffing box location.
- If the solids have a low specific gravity (they float on the liquid) you may have to go to a clean liquid flush because centrifugal force will work against you.
- Durco or Flowserve pumps use a stuffing box that is pressurized close to suction pressure. so recirculaion cannot occur.
- Single stage, double ended pumps position the stuffing boxes at the pump suction. You will not be able to recirculae the stuffing boxes to the suctionand get flow.
- Any time that you deal with a slurry application you’re going to have a couple of other problems as well ,so be prepared for them:
- Frequent impeller adjustment and excessive wear ring wear. You’ll need a cartridge seal or a sleeve mounted split seal to compensate for the impeller adjustment. Cartridge seals can generally be reused if the pump has been disassembled to replace the wear rings.
- Vibration will increase as the impeller goes out of balance due to abrasive wear. This can cause drive lug wear and carbon face chipping. Vibration damping will become very important. Seal designs that incorporate o-rings have a built in natural vibration damper. Metal bellows seals require a separate solution.
- Wear of the rotating components. This is especially true if the seal rotates in the fluid. Better seals are designed to cause rotation of the fluid in the seal chamber.
If you prefer to solve the application problem by using a special seal. the following thoughts might help in deciding your selection.
- Metal bellows seals. An excellent choice for cryogenic and high temperature, non petroleum liquids. Petroleum products “coke” in the presence of high heat so cooling is necessary in these applications.
- Carbon / metal composites to conduct heat away from the seal faces. Do not use “glued in” versions.
- Elastomers located some distance from the seal face to protect the elastomer (rubber part) from the additional heat generated at the seal faces
- Low friction face combinations. Carbon / tungsten carbide or Carbon/ silicone Carbide are among the best. Some duplex material faces are showing good results in these applications. Carbon impregnated silicone carbide is an example of such a material.
- Elastomers that have a wide range of operating temperature. Kalrez® is a good example.
- Low expansion metals such as Carpenter 42 and Invar 36 that will still retain the carbon or hard face in the holder even though the temperature changes greatly. Be aware that low expansion metals have poor chemical resistance so be careful in using them.
- Stationary seal designs are subject to a differential temperature across the seal face and body if a recirculation line or flush is being used. This differential temperature can cause the face to go out of flat. You will be better off with a rotating design in this instance.
If you elect to solve only the sealing problem you must keep in mind that the extremes in heat and cold will also affect the bearing seals as well as the bearing oil. Unless you address these problems separately you will be better off controlling the temperature in the stuffing box area and solving most of the bearing area problems at the same time.
- Springs out of the fluid, the most common place to clog a seal.
- Vibration damping because wear causes the rotating assembly to go out of balance.
- Be sure the dynamic elastomer moves to a clean surface as the seal carbon face wears.
- Use centrifugal force to clean the sliding seal components. Rotating seals (the spring loaded face rotates) should be your first choice.
- Non stick coatings on the metal parts to prevent a build up of solids on the sliding components. These coatings are porous so don’t use them for corrosion resistance.
- If possible, rotate the slurry to reduce seal component wear.
®Dupont Dow elastomer