VFD pressure control 2

[Infomechatronic]

I'm new to the world of engineering, finished my degree about 6 months ago, so please pardon the relative stupidity.

I'm trying to set up a pump for a water feature. The jets on the pressure side of the pump will be controlled via air operated valves via a plc, bla bla bla.

Long story short, I want the pressure in the line to remain constant despite the number of open valves on the outlet. I was going to use a variable frequency drive to control the pump motor, based on a pressure transmitter on the pressure side of the pump. I was going to use a Gem Sensors 2200 series 4-20mA transmitter, and connect this to the VFD unit directly.

Problem is I can't find much information on these with my limited knowledge, my understanding is that these result in 4mA at min pressure, 20mA at max. What I need to know is that is this a direct connection to the VFD (i haven't chosen a VFD type, I'm presuming the majority have the same inputs/outputs) or is something else needed to suit this application? I'm aware most modern VFD's have internal modifiers to choose the input range etc, but do they have inverters too, so as the current increases the frequency decreases?

 

[Warpspeed]

Yes it can work, but you will need to be able to control the dynamics of the feedback with a PID controller.

Some better VFDs have this feature inbuilt, but not all.

PID stands for "proportional, integral, derivative".

Basically you need to be able control how fast, and how far the VFD corrects for detected fluctuations in line pressure. Too much and it will over correct and be unstable (and probably oscillate). Too little and it will not control pressure very well. By adjusting the constants of "P" and "I" the system can be tuned to give satisfactory results. "D" is rarely used, and you probably will not need it.

If the VFD has an inbuilt PID control feature, the instructions with the VFD will probably tell you how to set it up and tune it.

 

[GeorgeFletcher]

I would suggest that you visit the Grundfos web site where you can download the pump curves and details of their pressure controllers. The beauty of their system is that it all fits together. Simply wire the control to the module in the pump.
We use this system for pressure and secondary temperature control because it works.

 

[zdas04]

I always like cheep and reliable for this sort of problem (it is very much like a downhole jet pump problem, you want to supply constant pressure to the jet, but the flow demand changes rapidily with no schedule). I use a "constant pressure valve" for that problem. This valve always bypasses some amount of flow (usually 3-5% when demand is greatest, up to 20% when the demand is minimal). You never have to deal with the very bottom or very top of the valve throttling curve. When demand for pressure suddenly decreases, the constant pressure valve takes more flow immediately, when the transient is over the valve takes less flow.

I've found these valves to work over a very wide range of demand without the complexity and expense of a VFD (I guess I'll always just be a Flangehead). Nothing is free and that 3-5% minimum flow takes energy, but the numbers work out that that is a small price to pay.

David

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[freefallingbody]

For an IC engine test bed, where the number of engines to be tested varied from 10 to 30, a control system as desired by infomechatronic was considered. That is, a variable freqnecy drive controling speed of the motor and solenoid valves on the water line to each engine. A pressure transmitter of 4-20mA was wired into the system giving signal to the VFD. Theoretically, one would expect good control and some energy savings too.

One point which I overlooked was, how flat is the pump curve? I mean, the pump pressure was almost the same when flow was at 100% of the end uses operating as well as at 50%, due to the flatness of pump's head flow curve.

Finally, they removed the VFD and opted for simple 2 way control valves (no bypass. The system is now controlling the water line solenoid based on an electrical interlock from the test bed panel.

A.V.Dinesh
Devki Energy Consultancy P. Ltd.,
Vadodara, India