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#1

# Is heat gain in centrifugal pumps the result of bearing friction alone?

Is heat gain in centrifugal pumps the result of bearing friction alone?
02-24-2014 03:22 AM
Top #2
It is a rather curiously phrased question.

If the pump is running dry, the motor power will drop significantly since the pump is not being asked to do useful work. What remaining power is demanded of the motor will be to overcome any bearing friction and mechanical seal friction.

The motor will generate heat of course due to inefficiency and since the impeller is close coupled to the motor shaft some of that heat (together with any frictional heat generated from the bearings/seal) may transfer through to the impeller.

No math required really.
02-24-2014 05:37 AM
Top #3
If the pump is running dry, some of heat will generate in charning of air in and arround Impeller
02-24-2014 07:43 AM
Top #4
if pump is running on no load then it could be sever
02-24-2014 10:13 AM
Top #5
Yes Simon, I am of the same opinion too that something feels ackward. Anyway, the answer is all friction, why only bearing. The driver shafts imparts kinetic energy to the pump shaft which is connected to the rotor by a key. The rotor transfers the energy to he fluid and pressurizes it, as the pumped fluid flows along the path provided and is then discharged into the delivery line.
Heat will be produced at all these points of energy transfer, but bearing heat generation will be maximum. The bottomline is that mainly efficiency loss is equal to the energy loss which will be mainly heat and inertial.
02-24-2014 12:19 PM
Top #6
Almost all comments that I have read have assumed that the pump is running dry. If the application is a repriming pump on a suction lift, the suction leg could have been lost leaving the pump with a partial casing of liquid. If the pump continues to run, or is called to run when the level sensor requires it to and it can't reprime for some reason, the turning impeller can impart enough energy to the liquid to make it boil. A casing temperature sensor can keep this from happening by dropping out the motor starter at a certain preset temperature.
02-24-2014 02:45 PM
Top #7
Heat can be generated by shearing of the liquid and friction between moving parts. It would seem this question was asked because of an unstated operating problem. Systems are designed with a set of design parameters. When you deviate from these parameters problems can occur. For example a pump running dry, but also many others! A control valve or system component may malfunction. If a valve sticks half open upstream or down stream, an actuator fails and personnel think they can operate it by hand as needed, cooling water on jacketed seal chamber is shut off or scales up and looses its ability to remove heat, etc....I would caution folks that temperatures should be closely monitored and when a severe temperature out of ordinary occurs, YOU BE VERY CAUTIOUS AS TO HOW YOU RESPOND AND TIME MAY BE OF THE ESSENCE. I am aware of a pump in an acid application which was run dry inadvertantly. One of the operators then opened a valve in the suction line and the pump exploded! Excess heat can be a minor problem, can damage 0-rings and seal faces in mechanical seals, can change the state and or properties of the material being pumped. Not everyone pumps water, ie blood, sewerage, paint, glue, lubricants, salts solutions, etc......Be safe. Look at a problem from all angles! Always be alert and ask why when you see something that grabs your attention. DON'T PRACTICE YOUR ACCIDENT UNTIL YOU GET IT RIGHT!
02-24-2014 04:57 PM
Top #8
Jamie, you make a good point. I think the OP's question was worded such that it results in a lot of fuzziness as to exactly what the system setup is.

Certainly some residual liquid will absorb power and if the system has a high static pressure, the pump might never make enough head and hence run at shutoff, boiling as you describe.
02-24-2014 07:02 PM
Top #9
Hi
For the last 30years or so I have been using the heat gained in the pumped fluid to determine the efficiency of the pump.
There are several factors which determine the heat gain but by far the most significant factors are the efficiency of the head generated by the pump.
For instance a pump running at 70% efficiency will raise the temperature of the water by approximately 1mk/metre head.
Unfortunately there is a great deal of mathematics behind this theory which is based on thermodynamics.
In the early 80's I developed the Yatesmeter which in the UK is the standard on site method of testing all the parameters of a running pump.
It measures Flow Power head and efficiency.
MauriceYates@aol.com
02-24-2014 09:26 PM
Top #10
Simply put, if a pump is running at 70% eff., then the balance of the output power is being absorbed by the pumped medium as heat.
If flow is stopped through either a closed head or loss of prime situation, then the pumped medium will continue to rise in temperature which can then become explosive depending on the pressure.
02-24-2014 11:27 PM
Top #11
Just a comment to support all of the above.
In any system you have to consider that you never get 100% of the energy you put in to the system out as "useful" energy, the 2nd law of thermodynamics. So you are always going to convert (waste) a % of the input energy via one of three routes:
Tribologically generated heat (Plainar bear/wearings in the pump on shafts and impellers and to a lesser extent rolling element bearings outside the pump, Mech seal faces.
Elastic deformation of the shaft if it is not supported by the fluid and starts to whip the shaft heats up through hysterisis.
Viscous Heating of the media in the pump through constant shearing caused by reduced flow.
If these are not "brought under control" by re-establishing an effective flow through the pump then you run the risk of the energy within the media reaching levels which result in chemical reactions taking place, yes even resulting in explosions as the system tried to dissipate the energy being put into it.
This really shouldn't be any surprise for an engineer who has studdied thermodynamis and material science. The fact that few of us ever had the desire to complete the chemistry element of our schooling (Mech engineers focus on physics right?) means we often don't see the finale of this situation coming.

Great discussion!