HEAT GENERATION, TECHNIQUES WE USE TO REDUCE IT H011
Here are some recommendations to lower the amount of heat being generated within the pump and the mechanical seal.
The pump is equipped with shaft packing
- With the development of the split mechanical seal in the early nineteen eighties pump packing has become almost obsolete. Packing a pump shaft is like driving your automobile with the emergency brake engaged. A balanced mechanical seal will generate six times less heat than a good set of packing. This saving in electricity, or whatever form of energy you are purchasing will more than pay for the seal in less than two years. A 50% return on investment should get the attention of any accountant.
The pump has a mechanical seal.
- Use only the balanced type seal with low friction faces. Be sure to set the face load properly and remember this has to be done when the pump is at its operating temperature. A cartridge or split seal is the only way to set face load. Back pull out pumps (ANSI. or ISO.) present a special problem because the seal is installed in the shop and the initial open impeller setting is almost always made at the piping. Those designs that adjust to the back plate are the exception.
- Do not isolate the seal faces with a gasket or rubber boot. These elastomers and plastic components are not very good conductors of heat.
- Open impellers have to be adjusted to keep the pump running efficiently. The seal must be repositioned each time the impeller is moved. Again, cartridge or split seals are your only option.
- Be sure to vent vertical stuffing boxes to prevent air from being trapped in the stuffing box. Good seals have this vent located in the seal gland.
- Make sure dual seals have the buffer or barrier fluid circulating either by convection, a pumping ring, or through a forced circulating system.
- Try to avoid oil as a buffer or barrier fluid between dual mechanical seals. Oil has too low a specific heat number and it is not a very good conductor of heat
- Check that the environmental controls are functioning properly. Cooling jackets stop functioning when calcium builds up on the jacket wall. Condensate or steam are good coolant alternatives if you have problems with hard water.
- Make sure that the stationary face is centered around the shaft to prevent rubbing if the shaft is displaced because of run out, whip, wobble, unbalance, vibration, bending, misalignment etc.
- Check the oil-level and change the oil on a regular basis. A pump running at 1750 rpm is almost the same as running your car at 50 miles per hour. This means that every 2000 hours your pump shaft travels about one hundred thousand miles. If the pump runs twenty four hours a day it will run 2000 hours in 83.3 days or just under three months. Imagine that your pump bearings go 100 thousand miles every three months. At 1500 rpm the pump bearings travel 150,000 kilometers every 90 days. Check the oil level with a properly installed oil level gauge or sight glass. The dip stick we find installed on some pumps is better than nothing, but it is hard to tell if the level is in the center of the lower bearing ball..
- If the bearings are not fit properly they will generate excessive heat. Refer to a bearing chart during your next installation to insure you have the proper dimensions. The internal clearance in a properly installed bearing is just a few ten thousands of an inch (thousands of a millimeter). To do this properly you will need an induction coil and a shaft that has been ground to the proper tolerances. Avoid cooling the outside diameter of the bearing because it will shrink and generate still more heat. Cool the bearing oil, never the bearing or the housing holding it.
- The bearings should be lasting for many years. Most ball bearing failures are caused by lubrication contamination or high heat as a result of over-lubrication. Try to do the installation job carefully.
- The grease or bearing lip seals should be thrown away and replaced with labyrinth seals or positive face seals that will not add heat to the bearing oil or let contaminates into the oil reservoir. The labyrinth, or positive face seals will not cut or wear the expensive shaft and, as you know, this is a serious problem with all grease seals.
- Nothing beats insulation for keeping ambient temperature away from your pumping fluid.
- More than one maintenance man has built a dog-house over his pump and controlled the temperature within the dog house.
Other heat sources
- Watch out for bypass lines and re-circulating lines adding heat to the suction side of a pump.
- With some parallel pump installations one of the check valves can see a higher back pressure causing the pump to run with a throttled discharge and generating more heat.
- A recirculation line from the discharge of the pump back to the stuffing box will not only add additional heat to the fluid, but will also increase the amount of solids in the stuffing box. In almost every case you will be better off connecting the line from the bottom of the stuffing box back to the suction side of the pump. Caution: do not use suction recirculation if you are pumping a fluid close to its vapor point.
- Check the wear ring or impeller clearance on a regular basis. As the pump looses efficiency the heat and vibration will increase.
- Pipe strain can cause wear ring contact.
- Some metal bellows seal salesmen tell the customer to shut off the cooling jacket because it will not be needed with that type of mechanical seal. This is never true with petroleum products because of “coking” problems, and in those instances where the seal could survive the higher temperature, the bearings will be affected because of the excessive heat being conducted down the shaft.
Pump modifications that will either lower the amount of heat being generated or lessen the affect of this heat.
- Use a larger stuffing box for mechanical seal applications. You can use the jacketed type if you need extra cooling. If you find there is not enough material to bore out the present box you can purchase the larger bore box from your distributor or manufacturer as a spare part.
- If the pumping temperature exceeds 200 F ( 95 C) convert the wet end of your pump to a “centerline design” to avoid pipe strain at the suction side of the pump.
- Convert to a solid stainless steel shaft to lessen the amount of heat that will be transferred to the bearings.
- Add oil cooling to the bearing case if you are going to see higher temperatures.
- Convert to a “C” or “D” frame adapter to avoid misalignment problems.
- Use mechanical seal designs that work better at these elevated temperatures. Desirable features would include:
- Balanced for low heat generation.
- A split or cartridge design for easier installation.
- Carbon/metal composite for better heat dissipation.
- High temperature elastomers or “no elastomer” designs
- Solid rather than a coated hard face.
- Springs positioned out of the fluid.
- The use of an unfilled carbon for good face density.
Excessive heat causes seal and bearing problems. Since the heat can increase corrosion, destroy seal faces, vaporize the fluid, coke the oil, solidify some liquids and crystallize others, change critical tolerances, attack the elastomers, increase the bearing squeeze, cause misalignment and pipe strain, etc. it would be ridiculous to try to build a mechanical seal or bearing capable of operating in excessive heat.
Most claims for high temperature seals address the problem of elastomers and ignore those other factors that we have discussed in detail. This explains the popularity of the high temperature bellows seal that must be cooled in all high temperature petroleum applications.
There is no magic, but there is a sensible approach.
Do as many of those things we have discussed in the above paragraphs and if you find that you still have trouble try to find some logical method of getting additional cooling to the seal and bearing oil. We discussed a lot of those options in the above paragraphs.