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Pump startup/stop time 1

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skoutso

Petroleum
Mar 3, 2003
18
Surge analysis (water hammer) of liquid pumping systems requires knowledge of pump startup and stop time. In old existing systems such information is not generally available from pump manufacturer. I wonder if there is some rule of thumb for quick estimation of those parameters for centrifugal pumps (electric and diesel engine driven).
 
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I think its very important not to introduce rules of thumb into transient analysis. Most of the time we're talking about fractions of a second, so a small error in one parameter can produce large errors in results. Start time is also dependent on the amount of liquid that you are effectively accelerating in the pipeline itself too, therefore a rule of thumb couldn't be based on typical driver size alone. Same a running a vehicle up to speed. A 100 horse in a sport car might get you up to speed quickly, but put it in a heavy Mercedes and you get nowhere. I believe you would be far better off to estimate the moment of inertias of the pump and apply the driver's torque curves to find the start time. Since torque is usually more standard for electric motors than IC engines, you can make some rules of thumb there.

Electric drivers are quite a bit different than diesel, as diesels starting cold require some run to idle for warm up, after which you could procede to ramping up flow. I've used start times for a diesel run to idle with these parameters,

D PUMPSTRT F(T) 5
0.0 0.25 0.50 0.75 1.00 /* RPM
0.0 0.02 0.25 0.30 0.35 /* TIME MINUTES

After reaching idle its hard to say in general how long it might take to reach normal running rpm. The physics behind each driver type is pretty much the same too. It depends of course on engine torque output at its instantaneous rpm, engine and gearbox inertia and pump wet inertia plus the mass of liquid in the system being accelarated. The mass of liquid (Enter specific gravity of the product) being accelerated can have a significant effect on time and how the pump is started. The big reason that some pumps are started against a closed valve or into open system. Put a low powered/torque output pump or a high powered/hi torque pump on the same system and you would experience a large difference between them in the start time taken to reach normal flow velocity. Not to mention the variation that could occur as to what the "normal" velocity might be in any given system. Since velocity is a function of time at acceleration squared, errors get squared too.

About all you can really generalize is that a typical electric pump drive would have about 150% of its rated torque available for starting rotation and running up of the motor, coupling, gearing or belts if any, wet pump and line fluid. In some systems, might be a second to five, in others 15 seconds to up to a minute. The transient would be significantly different. Stopping, that's another thing entirely.

Its only fluid produced torque acting on the inertia and friction of the rotating parts and flowing friction on the fluid mass being decellerated in the pipe.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)
 
Thanks for your info BigInch.

1. What's the meaning of "D PUMPSTRT F(T) 5"?
2. RPM are in thousands?
 
Just a header line for data contained on the next line, curve name, time function spec and the number of data points on that curve. Your program probably has a different function call.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)
 
The 0 to 1 above is actually a factor X the idle rpm (around 800 rpm or so), that effectively makes 1.00 = 800 rpm

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)
 
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