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Decision to have VFD by Process

Decision to have VFD by Process

Decision to have VFD by Process

(OP)
I hear from my design office that Pump selection/calculation always done by mechanical/process guys and then pump manufacturer selects the suitable motor, even if there is a requirement of VFD, it comes from the process/mechanical though VFD being a electrical equipment.

My question is, What drives the process guys' decision to use a VFD?
Replies continue below

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RE: Decision to have VFD by Process

Maybe because the Process and Mechanical Engineer normally involves in the early stage of the design with Project team and specify the requirement of the equipment, including the pump and driver, etc. Of course, the final decision needs to be confirmed by other related Disciplines, such as Electrical, as well as the Project/Client

RE: Decision to have VFD by Process

Correct process/mechanical decides on VFD. Usually based on wide range of flows and pressures required by the process system. So the decisions that go into choosing a VFD are prcess/mechanical engineering considerations - not electrical.

RE: Decision to have VFD by Process

In process plant design the use of a VFD should be based on flow rate range requirements and piping - equipment system curves, neither of which is within electrical scope of work. Electrical's job is thus normally limited in most cases to supply properly configured power.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

RE: Decision to have VFD by Process

(OP)
Hi 1503-44,

As I read, VFD’s are mostly suitable for applications where the pump duty is NOT expected to be constant.
Add on question: If we are going to alter the pump flow rate by changing the pump speed, then why install a discharge control valve? Either we use a VFD or a control valve. Not both of them together; but I have seen both of them installed. Any reason?

RE: Decision to have VFD by Process

I'll presume that we are dealing with centrifugal pumps.
If you need to control flow but the pressure doesn't change much, then use a control valve.
If the pressure will cover a wide range, then a VFD is very useful.
Because in a Centrifugal pump the flow is linear with speed but pressure is square of speed.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

RE: Decision to have VFD by Process

Nick, good question. IMO many dual installations are an attempt to fix, I use that term loosely, a bad pump selection. Having both is usually not a sign of an efficient system design. In 95% of all cases there will be a more efficient solution than using both vfd and control valve, Personally I struggle to fabricate any specific example of the 5%, but I don't know everything yet, so I leave that allowance. It could easily be 2%, or 0%. There are also practical control problems. It is very difficult to control both flow and pressure. In fact, you can not. You set one, you get whatever the other must be. A certain flow into a fixed system will require and result in a certain pressure. Any pressure applied to a fixed system results in a certain flow. Its not a mix and match thing. As you know, a voltage applied to a resistor, results in a given current and a given current applied to that resistor requires a certain voltage. Can you seperate the effect of V, same as pressure, and I, same as flow? If you control V across a fixed resistance you get X amps. If you then tweek the amps, then the Voltage starts moving, but a voltage controller wouldn't let it go where it wants to and holding it fixed, results in no change in amps. So, then you start playing with R, a control valve actually changes system resistance to get the flow (if its FCV) you want, or the pressure you want (if PCV), the voltage controller hangs on and keeps V constant, but now current starts going off.

Any change in pump speed has a corresponding change in pump flow rate. Speeding up a pump increases both pump flow and pump discharge pressure. If you do that with the idea to control flow, you (your piping system) must accept the resulting pressure. If you do that to control pressure, you must accept the resulting flow. That reality creates a control problem.

Trying to control both creates a short circuit in the control logic.

Vfd pressure control, with flow control valve.
Say your system pressure is running high and you want to reduce it by using vfd. Your pressure transmitter signal is high, so it must slow the pump speed to reduce pressure. Well, that reduces pressure, but also reduces flow. Now your flow rate transmitter is telling the flow control valve to open, which it does, and that causes the system pressure to rise. But how can it rise? Your pump is slowing down. Opening the flow valve also wants to increases flow. But your vfd is slowing down the pump while the flow control valve is opening even more. So, where does that lead? A pump running at 0 rpm with the flow control valve at full open, or anywhere inbetween?

Say your system pressure is running low and you want to increase it by using vfd. Your pressure transmitter signal is low, so it must increase the pump speed to increase pressure. Well, that increases pressure, but also increases flow. Now your flow rate transmitter is telling the flow control valve to close, which it does, and that also causes the system pressure to drop. But how can it drop? Your pump is running faster. Closing the flow valve also wants to decrease pressure. But your vfd is speeding up, pressure is increasing, while the flow control valve is closing even more. So, where does that lead? A pump running full speed with a closed flow control valve, or anywhere inbetween.

Vfd flow control, with pressure control valve.

Say your system flow is running high and you want to reduce it by using vfd. Your flow transmitter signal is high, so it must slow the pump speed to reduce it. Well, that reduces flow, but also reduces pressure. Now your pressure transmitter is telling the pressure control valve to open, which it does, and that causes the system pressure to rise. But how can pressure rise while the vfd slows the pump down with the pressure valve opening even more. So, where does that lead? A pump running at 0 rpm, producing no pressure, or flow with the pressure control valve at full open, but with no flow, or anywhere inbetween?

I'll let you think about the remaining cases. They all produce unstable results. Then they start adding limit switches, trying to keep the instabilities from running away too far off the map.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

RE: Decision to have VFD by Process

In some cases, there may be a min setpoint discharge PIC and PCV on a VFD modulated pump, and this may be to keep the flow of liquid from discharge through the shaft seals for primary seal cooling. When pumping throughput is low, pump speed would have dropped and so would discharge pressure. And this discharge pressure may not be enough to keep the minimum seal coolant flow going. Hence a discharge min setpoint PIC - PCV to throttle the discharge and maintain the required dP to keep the primary seal cool.

RE: Decision to have VFD by Process

@1503-44 - why would have a VFD and a control valve? That makes no sense. The VFD main purpose is acting as the flow/pressure controller?

--- Best regards, Morten Andersen

RE: Decision to have VFD by Process

I agree and I can't think of any reason or practicle application for using both vfd and CV. As you say, It makes no sense to me either. Adjusting one makes a corresponding change in the other. No advantage, no gain. Great only for creating instabilities and wasting money.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

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