Pump Motor of Inertia-Calculation

[stanier]

How does one calculate the moment of inertia for a pump, motor and the rotating fluid? OK I know you can get it from themanufacturer but on some occassions the fluid rotational mass, coupling etc is missing. Yes I guess the motor manufacturers are prety good at having the information.

Is it done on a test bed?

 

[Bjegovic]

well, a good guess would be to find moment of inertia for the motor at hand (or simillar to that one) add to 15%, to account for the pump impeller and coupling.

 

[stanier]

Unfortunately I am not in the guessing business.

I need the data for transient analysis purposes.

 

[electricpete]

Do you have the simple formula's for the moment of inertia for solid and hollow steel cylinders? If not I can dig them up.

In theory you could calculate rotating inertias from observing the way the rotating elements accelerate under a known torque, but that sounds like a lot of work.

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[stanier]

I was hoping for a pump designer to come up with a procedure for calculating the mOI for all the rotating elements even the impeller which is an irregular shape, the water in the voulute etc.

How much engineering judgement goes into the procedure? Once the MOI is known for one size of pump in a family of pumps can one use a correlation to determine the rest?

MOI for cylinders I can look up in the texts, thanks anyway.

 

[Bjegovic]

Calculating the water hammer effects is exactly why I had to search for the pump+motor inertia.
The software i used to work with needs MoI as an input necessary for transients calculation.

I am not an expert in this field but I have done more than a few calculations, and I think that the error will be negligible in all respects if you add cca 15% to the known motor MoI and use this MoI as an input to your simulation.

If you want to stay on a safe side, use known motor MoI and neglect the pump+coupling MoI.

On the other side, the MoI is relevant only until non-return valve closes. Don't forget to take that into account.

 

[pmover]

actually, the MOI is determined by the pump mfg. short of obtaining that, there are other techniques available to determine MOI. although i do not have the book with me, but a source is "a torsional handbook" by E.J. Nestorides (spelling?) of the B.I.C.E.R.A. (british internal combustion engine research association).

good luck!
-pmover

 

[stanier]

Hi Bjegovic

I specialise in surge analysis. The software i use allows the input of momoent of inertia. I also have models to estimate the MOI based upon Streeter & Wylie anbd their industry referenced database.

I really want to know how the pump manufacturer goes about it. Is it by testing or some theoretical calculation based upon the pump series and then authenticated on the test bed?

Any pump designers out there?

 

[meintsi]

We calculate initally off a very, very accurate solid model and let SolidWorks do the rest.

Remember...
[navy]"If you don't use your head,[/navy]
[navy]your going to have to use your feet."[/navy]

 

[Bjegovic]

hey Stanier,

what is the name of the software you use?

in my ex-company, we used an old custom-made program with primitive interface, but it was giving very good results.

We also wanted to have a comercial software, as to double-check the results. What commercial software (with cost less than 5-6000$) would you recommend?

 

[vanstoja]

Regarding how the pump designer goes about determining moment of inertia, our primary designer used testing of the complete rotor assembly (with pump impeller)in air principally though he extended results of one design's test results to estimate MOI for a new, similar pumprotor design which was subsequently tested for verification when hardware was available. Testing involved pendular or torsional vibration of the wire-suspended rotor to determine period of vibration relatable to MOI. Koptev,N.N.(1969), "Experimental Methods of Determining Moments of Inertias of Masses", Russian Engineering Journal, Vol.9, pg.22 provides 15 examples of MOI test setups and equations for complex mass shapes. To test a pumprotor with water mass in the pump impeller, I suppose that thin metal shrouds to close off inlet and discharge openings could be welded on temporarily to contain a volume of water with an acceptable error in MOI results.

 

[stanier]

I use AFT Impulse for surge analysis.

http://www.aft.com.

I have used it for five years now. The support is great. Engineers not software gurus.

I am not sure of the US price as I am in Australia and only pay for upgrades these days. The price is on the website. You can also download demos.

Its a great program because its drag and drop functionality to build models quickly. Flexible databases allow you to build up pump profiles etc. They also have all the standard mitigation devices as junctions that you can configure.