Question on Parallel Pumps to Increase Flow

[spudlake]

I have a double suction centrigual pump in a papermaking application. Goulds3420 30x36-42 Lo pulse impeller. Powered by two 1000hp DC motors. Motors controled by a drive at a speed to meet a downstream pressure setpoint. (Pump TDH=150-200ft. Q= is 25k-40k gpm) Needing more pressure/flow at times but limited due to motor overload. Bigger motors are getting very expensive.

Would like to consider adding a smaller 2nd pump in parallel. I know different sized pumps in parallel can be a bad idea. Could go to two identical pumps. Although, the pump supplier say's they have not seen it either way in my application. I've read about many successful parallel pump applications. I suspect the accuracy of the head required by my process may be why they have not heard of it.

I beleive the best chance would be pressure transmitters on each discharge pipe just before their intersection.? Pumps then run to a speed to keep these balanced while also maintaining the downstream pressure transmitter setpoint.

Any advice on this application or how best to control this? Is there any reason a parallel pump setup can not control just as precisely as a single pump?

Thanks for any help.

 

[BigInch]

But what keeps the pressures at the transmitters constant? The pressure at those PT points are functions of not only what's going on with the pumps, but what's also going on downstream, which is also a function of what's going on at the pumps.

In theory, if there is reasonable steady state, closely matched pumps should work. In practice, no two identical pumps stay identical for long.

Whether two nonidentical parallel pumps work or not often depends on where the system curve intersects the pump curves and the angle of that intersection(s). If the intersection is in a flat area of the pump curves, its not likely to work well, if at all. If the system curve intersects more curved regions of the pump curves, it will tend to work better, because you get a more pronounced change in head and pressure for a little change in speed. In the flat areas of the pump curves, a small change in pressure brings on a large change in speed, making it more difficult for the pumps to zero in on the set points. The system curve has the same effect. If the system curve is flat or intersects the pump curves at a good angle (right angle is best), it will return a good pressure variation with a change in flow. If its a relatively flat system curve at the pump curve intersection, large changes in flows will accompany small changes in pressure, so control once again becomes difficult. The more closely matched the two pumps are, the more each will have the same effect on the system curve for any variation in PT readings.

Whether you use two PTs or one, doesn't really matter because the physical requirements still necessitate a balance of pressures wherever the two streams come together. If one pump produces a lesser pressure at that point than the other pump, flow will tend to enter into the lower pressure area and back up the pump on its curve to where it can finally equalize that pressure, albeit at a lower flowrate. If that lower flowrate affects the system curve drastically, such as reducing pressure significantly which drops both PTs, both pumps may tend to overrun as they then try to respond to that lower pressure signal.

With wide flow variations that take you from regions where both the pump curves and the system curves are flat, forget it. You'll have to go manual until you get into better territory.


**********************
"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)

http://virtualpipeline.spaces.live.com/

 

[spudlake]

Thanks BigInch.

My pump and system curves are fairly perpindicular with the existing pump.

So forget my idea of balancing the two pumps with a pair of PT's. You're right, physics dictates they will balance at the intersection and it still comes back to the system & pump curves.

To clarify, my PT setpoint only changes with a few operator controlled parameters. So for the most part(for hours at a time) the pump drive varies the rpm slightly (say +/-2 rpm out of 600rpm)trying to hold the PT to a constant setpoint. This PT is at the far end of the system piping just before the flow squirts out of a giant nozzle.(a given PT setpoint produces a given nozzle jet velocity)

I guess the control strategy could be to use a constant to keep the two pump's speeds proportional. Maybe its 1.00 if the pumps perform identical. Then together they are tuned to vary their speed in order to hold the PT setpoint.

 

[Artisi]

You say these are low pulse impellers - are they fan pumps on the head box? If they are you know very well that pressure pulsations are critical and just adding an additional pump could result in problems - think you need to involve Goulds in any discussions re increasing flows -/ adding pumps.

 

[BigInch]

Sounds like a very very flat system curve.

**********************
"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)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

spudlake- you also need to involve the head box manufacturere if this is the application.

 

[spudlake]

Thanks Artisi. They are headbox fan pumps. I have involved both the pump and headbox supplier. Neither recomended it or have seen it on a single layer headbox. But pressure from management spurred me to explore the concept more. I chose this forum just to hear from some pump experts in other industries. Thanks for the input.

 

[Artisi]

Sounds like management should also discuss this with both suppliers, not from a pump / head box consideration but a finished product quality point of view.