Controlling the temperature in the stuffing box 4-6
Many fluids are adversely affected by a change in their temperature, and when this change takes place, seal failure is almost sure to follow. The failure can take several forms:
- One of the seal components can be destroyed. The elastomer, seal faces, or metal parts will almost always be altered at some degree of change in ambient temperature.
- Coated hard faces can “heat check” (crack).
- Carbon fillers can melt and pits can form in carbon/ graphite faces as trapped air expands and blows out pieces of the carbon.
- Hydrocarbons can solidify (coke) between the seal faces and pull out pieces of carbon also, causing small pits that will prevent you from conforming to fugitive emission standards.
- Carbon graphite faces can lose their lubricating ability at cryogenic temperature and chip on the outside diameter as “slip stick” vibration takes place.
- Elastomers can take a “compression set” and crack at elevated temperature
- Cold temperatures can cause elastomers to harden.
- The liquid can crystallize, restricting seal movement and open the faces.
- The liquid can vaporize between the faces forcing them open.
- The viscosity of the fluid can change either restricting seal movement, or making the fluid less of a lubricant.
- The liquid can solidify causing the seal to become inoperative.
- The liquid’s corrosion rate will double with an 18° Fahrenheit (10° C) rise in temperature.
- The liquid can convert to a film between the sliding seal components, restricting their movements. The magnetite that forms in hot water is a good example of this.
- A film can form on the seal faces causing them to separate.
- Lapped seal faces can distort and go out of flat at elevated or cryogenic temperatures.
By keeping the stuffing box temperature within specified limits you can prevent all of the above from happening. These limits vary with each fluid, but they can be obtained from any one knowledgeable about the fluid that has to be sealed.
A balanced mechanical seal incorporating the following features and installed at the proper compression, is your best insurance against a significant rise in stuffing box temperature:
- Proper face balance. 70/ 30 is the most common to 5000 fpm. (25 Meters per sec.)
- Low friction face materials. Carbon/ graphite vs. a silicon carbide hard face is the best.
- The correct spring compression to control face loading.
- Faces with good heat conductivity. Tungsten carbide and silicon carbide have excellent thermal conductivity compared to most other hard face materials.
- A small cross section carbon/ graphite face press fitted into a metal holder is better than solid carbon/ graphite for removing heat from between the lapped faces.
Sometimes, that is not good enough, so occasionally you’ll have to come up with some additional method of controlling the temperature in the stuffing box area and between the lapped seal faces.
THE HEATING / COOLING JACKET.
If your pump does not have a heating/ cooling jacket installed, one is usually available from the pump distributor. If possible try to select an oversized stuffing box with a cooling/ heating jacket cast around it. This jacket can be used to heat a product, cool a product, or keep the product within close temperature limits. When using the jacket there are several important things to keep in mind:
- Install a carbon bushing into the bottom of the stuffing box to act as a thermal barrier. The clearance over the shaft should be about 0.002″ per inch diameter of shaft (0,002 mm/ mm of shaft diameter). The length should be at least 3/8 inch (10 mm).
- Dead end the stuffing box. In other words no discharge, flush or suction recirculation lines connected to the stuffing box. Many hot fluids contain lots of solids; the dead ending feature will allow you to centrifuge the fluid in the stugging box and clean it up.
- If you elect to use water as the jacketing fluid make sure that it’s not hard water because it will form a layer of calcium on the walls of the jacket restricting the heat transfer.
- This jacket is also used to cool the rotating shaft in hot applications. If the cooling is lost the heat will conduct back to the bearings causing their premature failure.
- Steam is an excellent medium to control heating or cooling. A regulating valve can be installed on the discharge side of the jacket for precision pressure control which will, in turn, control the stuffing box temperature within narrow limits. A mixing valve proportioning steam and water is another method of controlling temperature within precise limits.
THE QUENCH OR DRAIN CONNECTION
Quench and drain connections are available in American Petroleum Institute (API ) type glands. The quench and drain connection (Q) is used in conjunction with a close fitting, non sparking disaster bushing (DB)
- When using the quench for temperature control keep in mind:
- Excessive fluid will be directed towards the bearing case. Be sure to use only small amounts of steam or water. It would be wise to replace the existing grease seals with mechanical bearing seals if you elect to use quench as your primary temperature control method.
- The drain connection should be connected to a suitable tank to save the condensed steam.
- The quench connection can be used to vent a volatile product to a flare where it can be burned.
- The quench fluid can also remove any solids that have built up outboard of the seal, as well as remove any vapors that might leak across the seal faces. This is an important feature when sealing products that can crystallize at the seal faces and on the outboard side of the seal.
- The drain connection is used to direct the major amount of failed seal leakage away from the bearings or any personnel in the area. It should be connected to an appropriate tank for retention.
THE FLUSH CONNECTION
In temperature control applications we flush in cooled product to control the stuffing box temperature. If you use the pump fluid cooled, or cooled finished product you’ll have no problem with product dilution.
THE DUAL SEAL
Another method of providing temperature control is to utilize two seals with the correct temperature liquid circulating between them as a barrier or buffer fluid. Look at the following illustration:
- When using a dual seal for heating/ cooling be sure to bring the fluid into the bottom of the seal gland and out the top to insure that the void between the seals is full of fluid. This is an excellent method of controlling the temperature at the seal faces if you are experiencing an over-heating problem.
- A convection tank can be installed between the seals, but it will seldom do an adequate job of lowering or raising the barrier fluid temperature. In almost every instance forced circulation will be necessary if you need any degree of heating or cooling. Convection tanks are satisfactory for removing the heat generated by balanced seal faces, but that’s about all.
- A convection tank can been used with an installed cooling coil and a pumping ring built into the mechanical seal.
- The amount of barrier fluid circulation needed will be determined by the seal size, speed, and stuffing box pressure. Your seal supplier will gladly supply this information.
- Water should be selected for the barrier fluid whenever possible. Oil is a poor choice because of its low specific heat and poor conductivity.
THE HEAT EXCHANGER
The normal procedure is to install the heat exchanger in the discharge recirculation line connected between the pump discharge and the stuffing box. If you elect to use this method be careful of the following:
- This can be dangerous in hot water applications because a leak in any of the fittings will direct high pressure, hot water into the atmosphere and some one may be standing close by and become injured.
- Many hot fluids also contain solids that will clog up the heat exchanger.
- The temperature control is effective only while the pump is running. Many seal failures occur at start up because of lack of proper temperature control while the pump was idle.
- If you want to use this method, and only a small amount of cooling is necessary, a commercial automotive, automatic transmission cooler can be used effectively in many applications.
- A heat exchanger can be used with a pumping ring. In this application low pressure fluid is circulated out of the top of the stuffing box, to the heat exchanger, and then back to the seal through the bottom connection on the seal gland.
- Vertical pumps require venting or the seal will trap air in the stuffing box, causing high heat at the seal faces. To vent the stuffing box properly, connect a suction recirculation line between the seal flush connection and the pump suction. Vertical pump applications also present a problem for dual seal applications. You’ll need to provide some method of venting air trapped at the outside seal.
- Carbon/ metal composites are a good choice for heat dissipation across the carbon seal face.
- Try to avoid seal faces that are thermally isolated by elastomers.
- Silicon carbide is a good choice for the hard face because of its excellent thermal conductivity feature. Use the alpha sintered type to avoid chemical compatibility problems
- If you elect to use anti-freeze as a barrier fluid between two seals, do not use the automotive brands as many of them contain an anti-leak chemical that will clog up the mechanical seal. Water is the best barrier fluid because of its high specific heat (1.0) and good conductivity. Oil is a bad choice because of its low specific heat (0.25/0.30)
- If you must use oil, try to select a heat transfer oil.
- Heat pipes should have application in stuffing box cooling, but their application experience is very limited.
- Try to select seal designs that have the elastomer positioned away from the seal faces. The elastomer is the one seal component that is very sensitive to temperature change. Because elastomers usually have poor thermal conductivity, cooling one side of the elastomer has a minimal affect on the other side.
- Unfilled carbon/ graphite seal faces are absolutely necessary in higher temperature applications. Less dense seal faces experience trouble when air trapped below the surface of the carbon, expands and blows out pieces of carbon from the center of the seal face. The exception to this is high temperature oil that will coke at the seal faces and pull pieces of carbon away. These resultant pits will cause problems if you are trying to meet fugitive emission standards.
- In those pump designs where the open impeller is adjusted back against the back plate (Flowserve), any impeller adjustment tends to over-compress the seal faces, causing high heat. Proper face load is essential to long seal life so cartridge designs should be specified any time you use open impellers in high heat applications.
- Keep in mind that the pump cooling jacket is also used to cool the shaft that is conducting heat back to the bearings. If you have a high heat application, you might consider a stainless steel shaft because of its poor heat conductivity compared to steel. Some bellows seal manufacturers tell customers that they no longer need stuffing box cooling and the result is premature bearing failure.
- A centerline design pump is always desirable in hot applications to prevent pipe strain at the pump suction and damage to the close clearance wear rings. Instead of supporting the volute at the bottom this design bolts the pump feet to the sides of the volute, allowing the volute to expand both up and down. The wet end of your pump can be modified to this configuration, or a new wet end can be purchased.
- If the seal is going to be used in a hot oil application, do not hydrostatically test the seal with water or a water based fluid. Moisture trapped in gaskets, elastomer clearances, and other small crevices will flash when it comes into contact with the hot oil, causing a potential damage to the equipment, seal and/ or the people in the area.
- In cryogenic applications it is not practical to heat the seal area to protect the elastomer. A non-elastomer seal with a special self lubricating cryogenic carbon is your best solution to this application. Be aware that the moisture laden atmosphere can freeze on the out board side of the seal restricting the seal movement as the faces wear. In most cases a dual, non elastomer seal with a non freezing barrier fluid between the two seals is going to be your best choice.
- On February 13, 2018