Parallel Pumps - Pressure / Flow control

[jonelv]

Hello

Looking at a pumping system comprising 3 VSD centrif pumps discharging into a rising main. Pumps can run either on their own or any 2 in parallel. I want to ensure that in parallel the load is shared and pumps run at same speed. To maintain a dedicated pressure in the main, is it best to just use pressure set-point to increase / decrease pump speed (or pump run current) or would control be better if flow set point was also used ie if actual pressure higher than set point, reduce flow set point and reduce pump speed to match? Control will be via PLC so PID loops etc can be programmed in to suit.

Cheers

Jo

 

[hydrae]

Jo
It depends on the shape of the pump curve, if the curves are steep the pressure control will work, if the curves are flat then incorporating the flow will be needed as you describe.
One other thing to keep in mind, even though the pumps are the same make, model, and trim (I assume) they will perform differently and you will see this difference in the flows, so in order to perfectly balance the flows, each pump may run at different speed.
Hydrae

 

[jonelv]

Hydrae

Thanks for this.

Accepting that the pumps may run at slightly different speeds to achieve balance, do you think I'm therefore likely to get best balance by controlling pumps against current setpoint (ie target amps) rather than speed?

Jo

 

[hydrae]

Jo
Depends upon how precise you need to be, your customer is looking for a stable pressure (I assume). Is there a need for equal flows from each pump? if so, why? If not, then keeping the PLC code simple is better.
If during startup testing the pumps operate close to each other have them run at the same speed, your target is pressure.
But if the curves are flat there may be a case where a small difference in performance will result in the lower performing pump may operate at below a minimum flow, or the better performing pump operating above maximum flow (I do not know the system curve). If this is the case, the additional complexity of the code is required to provide for this unique application. Whether you base it on flow or amps depends upon what you are trying to protect, the pump or the motor. I would protect the least available componet. If you base it upon amps how likely is a locked rotor condition? Most drives are smart enough now to figure out if the rotor is locked but that may be disabled if the amps are the input control.
Hydrae

 

[Macbeth]

OK here is my take on this. Your pump curves and System design are what determine the possibilities of system performance as stated in all above. From your post I will assume you have three centrifugal pumps plumed in parallel with common discharge.

Affinity Laws:
1. Flow changes directly proportional to Speed.
2. Head Pressure changes by the square of Speed.
3. Horsepower required changes by the cube of Speed.

Looking at Law #3 the slower a pump runs the less it cost. Using this Law with the pump curve to sequence the pumps can save Big$ depending on the size of your pumps.

Looking at a simplified model:
Pump at 100% speed = 100% Flow, 100% Head pressure, 100%Hp
Pump at 50% speed = 50% Flow, 25% Head pressure Uses 12.5%Hp

As you can see if the system needs 100% Pump Flow and only needs 25% of available head pressure to operate it cost much less to run two. So the main factor should be how low can your head go.

Use pump curve to determine BEP, once the system reaches this point start the second or third pump. BEP is almost never 100% but usually about 80%. By doing this you will achieve the Best COP.

Use system pressure to satisfy any field equipment needs and a flow transmitter to determine the required number of pumps to run. Some times it helps to have remote equipment automatically adjust the system pressure set point.

Also if you have three pumps use all three pumps.

Run all pumps at the same load not all VFD are equal some will allow more slip than others. Bit this is not a major problem a load difference of 2-3% is not important.

Well that’s enough rambling.