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To VFD or not to VFD: Centrifugal pump flow control

Jun 08, 2021
to VFD or not to vft that is the question that will be answered today Hello, I am Kevin Dorma, welcome back to my board, we often use

centrifugal

pump

s to transfer a liquid from one place to another and we always need a way to

control

the

flow

rate . We can use a valve to throttle the

flow

or we could use a variable frequency drive or VFD to change the speed of the

pump

. Today we'll look at where a VFD is a good way to

control

flow and where it's not, let's say. We want to transfer water from one pond to another and we will use a pump with a variable frequency drive for this purpose controlling the flow with the variable frequency drive requires less pumping power compared to a fixed speed pump and a control valve and this This It is because the control valve requires a pressure drop and this wastes energy, but the benefits of a variable frequency drive are only available if we can actually change the flow rate by changing the speed of the pump.
to vfd or not to vfd centrifugal pump flow control
We need to observe how the pump behaves with a change in speed. and how the entire system behaves with a change in flow rate, we need an analysis of the pump and system curve, let's start by looking at how the behavior of a

centrifugal

pump changes when we change the speed of the pump, let's look at a pump curve Typical, the pump curve is the relationship between the discharge head of the pump H and the performance or flow rate of the pump Q 100% of the pump speed there will be a certain relationship between the discharge head and the flow rate there will be a minimum flow rate below which we should not operate for prolonged periods of time and the maximum flow rate which we should not exceed at ninety percent of the maximum speed the pump curve moves as it approaches the origin the minimum flow rate is reduced Maximum flow rate also reduces and the entire curve shifts closer to the origin as we reduce speed and we can see the same pattern if we go down to 80% of maximum speed.
to vfd or not to vfd centrifugal pump flow control

More Interesting Facts About,

to vfd or not to vfd centrifugal pump flow control...

Okay, now let's look at the system curve. The system curve tells us how the pipe inlet pressure changes with the flow rate at zero flow. Qualify the Pipe The inlet pressure is the same as the delivery pressure of the pipe and as the flow rate increases, the pressure required at the pipe inlet also increases and because the hydraulics of the pipe are generally quadratic, this curve has the shape of a quadratic, this is the relationship between the required pipe supply pressure as we change the flow rate through the pipe. Ok, now let's draw the pump curves at 80, 90 and 100% speed over the top of the system curve.
to vfd or not to vfd centrifugal pump flow control
The intersection between the system, the pump curve and the system curve defines the 100 percent flow rate. The flow rate is right here with the Associated discharge head and as we reduce the pump speed to 90 percent we reduce the flow rate and we reduce the pump speed further to 80 percent and obtain a similar reduction in flow rate. This is an example of a system dominated by friction and a steep pump curve. There is a uniform relationship between pump speed and flow rate. This is a very good application for a variable frequency drive. Now let's consider how a flat pump curve will behave in this friction. dominated system our flat pump curve will look something like that 100% speed and then 90%, it gets good, the curve gets closer to the origin, then we reduce the pump speed even more to 80% and get closer to the origin even more.
to vfd or not to vfd centrifugal pump flow control
The point of intersection between the pump curve and the system curve defines the required flow rate and discharge pressure. In this case, with the flat pump curve we still have a pretty nice reduction in flow rate as we reduce the pump speed. This is also a good application for a variable frequency drive to control flow. Let's change this to a system with high gain and static head and very little friction. This could be lifting water to a high altitude or pumping liquid to the top of a distillation tower and again we will see in a pump curve with a fairly steep pump curve at 100% speed, there is a good flow rate through the pump and when we reduce the flow rate to 90% we get a good reduction in flow rate, but we see a potential problem that we are now encroaching on the minimum continuous flow required, if we try to reduce the pump speed to a level less than 80%, we will definitely go down below the minimum continuous flow required for the pump, which can cause damage and in this case the pump probably cannot generate enough head to lift the water to this elevation, the flow through the pump stops and there is a There is a good chance that we could damage the pump by operating it in this mode for an extended period of time.
Now let's look at a system dominated by static height. and a flat pump curve we now see the problem that at 90% of the pumps speed there is a stable controllable flow rate through the pump and we operate at this flow rate but a small increase in pump speed to 100% will push the flow further. At the upper limit, there is a good chance that we could damage the bearings and, likewise, if we reduce the flow just a little bit, the pump cannot develop enough static head to push into this system and in In this case, the pump stops and we have no flow. through the pump this is not an operable control strategy a VFD will not work in this service.
We can summarize our findings with a table. The two rows are for a friction dominated system and for a static load dominated system. The two columns are for a steep pump curve. and a flat pump curve our explanation by hand has shown that a variable frequency drive can be a great option for a friction dominated system with little static head rise typically this is the pipeline application, a static head system and A steel pump curve may be acceptable but a static head system and a flat pump curve will fail. This guide should help you choose a reliable VFD control strategy for centrifugal pumps.
Thanks for watching, check out my other videos in the whiteboard series, take care.

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