SUBJECT : Heat, how it affects the pump and mechanical seal. 1-4
Every day salesmen call on customers and make claims that their pump, or mechanical seal can take more heat than the other guys. Before we rush out to purchase these wonder products, we should take a closer look at the heat problem.
The heat comes from several sources:
- Generated at the seal faces or by packing rubbing against the sleeve.
- Friction of the pump rotating parts, especially if the discharge is throttled.
- Ambient conditions. The weather or atmosphere surrounding the pump.
- The product contains a certain amount of heat
- Two parts rubbing together, that are not supposed to be rubbing, can generate a lot of local heat.
- Grease,or lip seals rub against the shaft very close to the bearings.
- Running to the left of the best efficiency point (B.E.P.) means that the discharge is restricted.
The heat will affect you in several ways. It can :
- Increase the corrosion rate of any corrosive liquid.
- Change critical tolerances.
- Destroy some seal faces
- Shorten the life of any elastomer (Rubber part) in the system.
- Change the state (ie. liquid to a gas) of the product you are pumping.
- Increase pipe strain.
- Waste valuable energy
- Change the viscosity of the bearing oil and eventually cause bearing failure
- Heat on the suction side of the pump can cause cavitation.
We’ll look at each of these areas in detail, and at the end of this paper make some recommendations to improve both your pump and seal life.
The following numbers are typical of the conditions in a stuffing box when you are sealing with a conventional original equipment, unbalanced seal.
|Stuffing box pressure|
|Seal face diameter|
|Seal face area|
|Seal spring load||2,0 kg/cm2|
|Face load from the spring|
|Stuffing box volume|
|Face coefficient of friction|
We will make the first calculation in the inch size:
Hydraulic closing force = 100 lbs/in2 * 1 in2 = 100 lbs
Hydraulic Opening force = An average of 50 psi on the faces * 1 in2 = 50 lbs.
100 lbs closing + 30 lbs Spring force – 50 lbs opening = 80 lbs closing
80 lbs * 0.2 * 1885 F.P.M. = 30160 Ft lbs./ min
778 ft lbs. / min. = 1 Btu..
30160 / 778 = 38.8 Btu../min.
38.8 Btu../ min would raise 1 pint of water 38.8 degrees Fahrenheit each minute, so we would have to flush in 38.8 pints (4.84 gallons per minute) of cooling water if we did not want the product to get hot.
Metric looks like this :
A Newton Meter is a Joule so we have 690 Joules/ sec.
690 Joules/Sec.* 60 Sec./Min. = 41,400 Joules per minute.
41,400 * 0.239 joules per calorie = 9,895 calories (9,9 Kilo Calories) per minute.
9.9 Kilo calories per minute would raise 9,9 liters of water one degree Centigrade per minute.
Since we have only one half a liter (500 cc ) in the stuffing box, we would have to flush in 9,9 * 2 or 19,8 liters / minute to prevent a temperature rise in the stuffing box.
The amount of heat generated by a properly installed balanced mechanical seal is insignificant.
The amount of heat generated by packing varies with the type of packing and the individual packing the pump. On the average you will find that packing generates six times the heat of a balanced mechanical seal.
HEAT GENERATED BY FRICTION WITHIN THE PUMP
No pump is 100% efficient. If a pump is rated 60% efficient, that means that 40% of the power is being converted to heat. In a normal temperature stabilized pump, running at its best efficiency point, (B.E.P.) the temperature rise within the pump is calculated from the following formulas :
A temperature rise of 18° F across the pump or 10° Centigrade is considered excessive. This can occur if the pump is run with a shut or excessively throttled discharged valve.
If you would like to calculate the temperature rise of the liquid in a running pump when the discharge is shut, use the following formula:
- If pipes, pumps, valves and other equipment are placed next to hot boilers or exposed to extreme changes in weather we’ll have to consider this addition or removal of heat in troubleshooting temperature related problems.
- All fluids are processed within some temperature range. It’s this heat that we will be adding to, or subtracting from. Many fluids are pumped close to the temperature at which they’ll vaporize, solidify, coke, crystallize etc.
- It’s critical that you determine the desired operating range for your fluid before you make any attempt to alter it.
- Rotating parts rub against stationary parts when the pump shaft experiences deflection. L3/D4 explains this problem in great detail.
- These seals add heat at the worst possible location. Grease or lip seals will also cause shaft wear at the point where the seal material rubs the rotating shaft.
- A general rule of thumb is that all chemical reactions double with a eighteen degree Fahrenheit rise in temperature (10 degrees Celsius). Corrosion is a chemical reaction and therefore corrosion increases with temperature. This is the best reason for converting any acid pump from packing to a mechanical seal.
- Critical tolerances include: Wear ring clearance, seal face loading, throttle/ thermal bushing clearance, bearing interference, impeller/ case clearance, pump/motor alignment, etc.
- A general rule to remember is that each inch of stainless steel will grow 0.001″ of an inch for each 100 degrees Fahrenheit temperature rise. In the metric system it grows 0,001 mm. per millimeter for each 100 degree Celsius rise.
- Open impellers must be set to a specified clearance from the pump case or back plate. A 0.015″ ( 0,5 mm.) clearance would be typical. If you increase this clearance 0.002″ (0,05 mm.) the pump will lose 1% of its pumping capacity.
- In closed impeller applications, the general rule is that each additional 0.001″ (0,03 mm) of wear ring clearance will decrease pump capacity by one percent.
- Unfortunately all materials do not grow at the same rate and in the same direction. As an example: steel grows about 60% to 70% less than stainless steel and most mechanical seal faces grow at about one third the rate of stainless steel. This is important to remember when you set critical settings and interferences. It’s also one of the main reasons we should do everything we can to keep down excessive temperature rises within the system.
- This also explains why we have less trouble with mechanical seals and bearings in equipment that runs continuously, as opposed to intermittent service equipment that goes through many temperature cycles.
- Many of the popular carbon/ graphite seal faces have binders and impregnates that can be melted or otherwise destroyed by excessive heat. Some of the lower cost carbons will blister when sub surface air expands because of elevated temperature. This is the main reason I have advocated unfilled carbon/ graphite seal faces at all of my Rotating Equipment Seminars.
- Plated and coated hard faces are subject to heat checking and cracking if improper bonding methods have been used. I do not recommend plasma spray processes for this reason.
- Some of the cheaper ceramic faces can be cracked with as little as a 100 degree Fahrenheit (55° C.) temperature differential across the seal face.
- Pressed in carbons and hard faces can become loose in their holders. This has caused some seal manufacturers to glue in seal faces and as you can imagine, not a very satisfactory solution.
- Some seal face designs can go out of flat with very little temperature differential. This is very critical in cryogenic (cold) applications and we often have to lap the seal faces at cryogenic temperatures to prevent them from distorting in operation.
- Heat will cause elastomers to take a compression set and if enough heat is added the elastomer will probably become very hard and crack. All elastomer compounds have a rated operating temperature range
- Water becomes steam. Glue, paint and all kinds of polymers with odd sounding names can solidify. Oil changes its viscosity, caustic and sugar syrups crystallize and the list goes on and on. Centrifugal pumps and mechanical seals can handle liquids, they have problems with vapors and solids.
- If a Cryogenic fluid evaporates across a mechanical seal face it can freeze any installation lubricant that might have been put on the face and either tear up the carbon, or break the hard face.
- The easiest product to pump or seal, is a cool, clean, lubricating liquid. Heat can cause that liquid to vaporize, crystallize, solidify, carbonize, build a film on surfaces, become dangerous etc.
- The finest lubricating oils will not work when the oil breaks down to form first varnish then coke. The bearing oil will start to do this if the oil gets above 240 F.(115 C.). Remember that a properly installed bearing is running about 10 degrees F. (5 C) hotter than the oil temperature. You can only guess what kind of temperature rise we get in improperly installed bearings. You should also remember that lubricating oil and grease have a useful life of thirty years at 30°C. and the life of the lubricant is cut in half for each 10°C. rise in temperature above that number
- Pipe strain causes the shaft to be displaced from the center of the pump assembly. Rubbing, premature seal / bearing failure and misalignment are always the result of this problem.
THE WASTING OF COSTLY ENERGY.
- The energy we pay for can be used to move fluid in your process or heat it up. The pump’s job is to move fluid not generate heat. If you want to add heat to a liquid there are far more economical and efficient methods of doing so.
- Cavitation is defined as cavities or bubbles in the liquid. A major cause of cavitation is caused by heating the incoming liquid beyond its vapor/ pressure point.
CHANGING THE VISCOSITY OF THE BEARING OIL
- Heat lowers the viscosity of the bearing oil causing increasing wear. As the oil heats up it will change state, first forming a varnish coating and then turning into a black coke solid.
- 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 what ever 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 MECHANICAL SEAL.
- Use only the balanced type 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 (A.N.S.I. or I.S.O. ) 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.
- 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 barrier fluid circulating either by convection, a pumping ring, or through a forced circulating system.
- 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 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, not the dip stick we find installed on some pumps.
- If the bearings are not fit properly they’ll 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’ll 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 from twelve to fifteen years. Most failures are caused by lubrication contamination or high heat. Improper installation is a major source of high heat problems, Try to do the 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 high 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 do this 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.
- 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’re going to see higher temperatures. Be sure to cool the oil, never the bearing outside diameter.
- 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.
- Split or cartridge for correct installation.
- Carbon/metal composite for better heat dissipation.
- High temperature elastomers or “no elastomer” designs
- Solid rather than a coated hard face.
- Springs out of the fluid.
- Unfilled carbon for 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.
Heat is always a problem, but now you have the tools to fight back.