HEAT GENERATION, ITS AFFECT H010
The heat generated in the pump stuffing box, between the seal faces, and other parts of the system will affect you in multiple ways. It can:
- Increase the corrosion rate of any corrosive liquid.
- Change critical tolerances.
- Destroy some mechanical seal faces.
- Shorten the life of any elastomer in the system including grease seals.
- Change the state of the product you are pumping from a liquid to a gas or solid.
- Increase pipe strain.
- Waste valuable energy
- Change the viscosity of the bearing oil and eventually cause bearing failure
- On the suction side of the pump it can cause cavitation.
We will look at each of these areas in detail, and at the end of this discussion make some recommendations to improve both the pump and seal life. We will start with where the heat comes from:
Heat generated at the seal faces
- This number can easily be calculated: Please see heat generated at the seal faces.
The heat from the ambient conditions is another source.
- If pipes, pumps, valves and other equipment are placed next to hot boilers or exposed to extreme changes in weather we will have to consider this addition or removal of heat in troubleshooting temperature related problems.
The heat in the product its self.
- All fluids are processed at some temperature range. It is this heat that we will be adding to, or subtracting from. Many fluids are pumped close to the temperature at which they will vaporize, solidify, coke, crystallize etc.
- It is critical that you determine the desired operating range for the fluid before you make any attempt to alter it.
The heat generated by parts rubbing together.
- Rotating parts rub against stationary parts when the pump shaft experiences deflection.
The heat generated by the bearing seal.
- These seals add heat at the worst possible location. Grease or lip seals will also cause shaft wear at the point the seal material touches the rotating shaft.
What affect can additional heat have on the liquid in the pump?
The corrosion rate of the liquid will increase:
- 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 will change.
- 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 50 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 make critical settings and interferences and 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.
Some mechanical seal faces can be destroyed.
- 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.
The elastomer (rubber part) life can be drastically shortened
- 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 that can found in another section of this series
The product can change from a liquid to either a solid or a gas.
- 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 evaporates across a mechanical seal face it can freeze any 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 has a useful life of thirty years at 30°C. (86°F.) and the life of the lubricant is cut in half for each 10°C. (18°F.) rise in temperature
- 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. See another section of this manual for a detailed explanation of the various types of cavitation.
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.