Will VFD AND MINIMUM FLOW FEEDBACK CONTROL WORK

[ratash]

I have a water sump with a sump pump. The flowrate into the sump is variable. The pump has a VFD. There is a feed back loop with a FCV in it(this is to maintain min flow for the pump).
There are 3 signals going to the FIC, which controls the FCV. They are the sump LCV (tied to the VFD), the pressure transmitter and the flow meter in the discharge line upstream of the feed back loop.

Do you see any problem with this design as far as control is concerned?

Thanks.

 

[BigInch]

doesn't seem to be a need for FCV. Is the pump hard to turn on or something, so you never want it to be stopped, or do you have something that solidifies at some low flow limit? If not, just let the level control close the valve and put the VFD to sleep (turning the pump off). When the level comes back up, it can wake the VFD up again. Set the VFD output on wake up equal to the pump's max speed, adjusting to reach minimum speed required to generate the head you need to get into the downstream piping at min flow. You'd also put the VFD to sleep when the FIC says less than min flow is going into the downstream piping, otherwise the FIC can pace the VFD between those two control points.

If you have molten sulfur or something similar, ok then use a minimum flow.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[BigInch]

Sorry, read my "min level" as a max sump level.

So then you come on with max speed at highest sump level. As the sump level drops, maybe you'll get less flow and can back down on rpm and head needed until flow goes to some minmum, then turn off and let the sump fill again.

There, that sounds better.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[ratash]

Thanks. The pump will be running 100% of the time (requirement). The level controller controls the VFD and if the level goes way below the min level, then the recycle valve opens on flow control.

 

[BigInch]

Well OK, then you need the recycle valve.
So, now its when flow into the downstream goes to zero the rpms are down to whatever you need for min flow and the recycle valve's opening up.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[ratash]

Thanks for the tip. Do we need the pressure transmitter in the control logic?

 

[BigInch]

Nice info if you have somewhere to display it, but not necessary for the above control system.

I use this next one, since many times I don't have FI;

Alternate methods would have the VFD vary the sig from HI rpm to LO rpm based on hi to lo suction pressure (NPSHR). It provides some measure of cavitation protection. If you have a discharge PI, that could act as a high pressure override in to the discharge line. With the LC opening the recycle valve, or the FI opening the recycle valve. Either one could work. Would you rather control off of the level or the downstream flow?

Always another way to skin the cat.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization

 

[ratash]

I trust your judgment and feel good about the control scheme. Thanks you are the best!

 

[BigInch]

Its nice trying to decide which indicator not to use for a change.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization