independent vfd of 3 pumps feeding system at separate locations 11

[electricpete]

I am asked to provide input on the following project.

We have three 75HP 1800 rpm well water pumps. They pump into a common header, except that the tie-in points for the three pumps are widely separated (up to a mile apart I think).

The plant load varies widely. Up until now these machines have been fixed speed. Also the pumps are far from the operator’s stations so often the wrong number of pumps is running for the load (far to the left). We have had very low reliability. Presumably we are also wasting a lot in energy costs.

What has been proposed is to install VFD for each motor. Each motor will be controlled independently by sensing it’s discharge pressure and varying speed to match a target.

Also there is some kind of lead/lag feature where one pump would be designated to turn off during periods of extremely low load. There would be a time delay before the pump turned off and minimum off-time before it turns back on.

I am not very familiar with control schemes for this setup. I asked the question how do we control the load sharing and ensure the pumps are operating near BEP. I was told that all of the above parameters (pressure setpoints and time delays) will be adjustable. We will watch and see what it does and tweak as necessary.

The well water system is deemed relatively low priority to the plant and the project is expected low-budget and quick turnaround. We have quotes for the VFD system installed to feed existing motors at somewhere around $12,000 per pump. (Outdoor installation with cabinets).

My two questions:
1 – Is individual pressure control suitable for this type application?
2 – Is a detailed study of the system operating parameters required first, or is it ok to trust we can tweak it later.
3 – What do you think about the 3rd pump shutoff feature? I am worried that under certain conditions we are doomed to have constant cycling as soon as the time delays expire.
Thx.

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[ozmosis]

This might be a daft point so I make my apologies now..
Would it not be worth investigating putting a tank (header) close to where you need it and just run the pumps to top up rather than on demand?

 

[JerryLavoie]

Your application sounds very similar to our plant well-water system. We are a steel mill in South Carolina. We use 3 deepwell pumps which each operate from vfd's. The vfd's are not linked to eachother in any supervisory manner. Each drive runs off a local pressure sensor to a controller which provides a pid reference to maintain a pressure setpoint by varying frequency of the vfd.

I can not speak to the electrical efficiency of this design, as we have never studied it. But we have high reliability with little or no maintenance.

I do not see the need for the pump on/off feature you mentioned. This may be intended to save electricity under low load conditions, which is not a concern for our application. I would advise you to determine roughly what you are paying now for electrical power under the constant speed operation, and I am sure you can save much of that power by controlling the pressure, as once pressure is established in the piping system, it requires much less power to maintain it under low load conditions.

You are welcome to visit my site and see the system we have installed. You could then speak with our water systems operators and maintenance technician and get a feel for how satisfied we are with this system. We did ours in-house, using off-the-shelf drives and typical pressure process controllers and sensors.

- Jerry Lavoie
Automation Engineer
Nucor Steel Berkeley
lavoiej@nucorsteel.com

 

[rmw]

Pete,

I didn't see this thread until now. What it looks to me like is the analogy that I use to non utility type folks to try to describe how a variety of power plants put power on a grid. (I usually describe a lake with contributors and users with level control being the voltage without the complications of current flow or pressure waves, of course.)

Wouldn't your control of these pumps mimic that type of control? After all, your well water header is a "grid" of sorts, and the individual contributions of each pump affect how the header acts, as does with each control valve that uses the well water.

Now, unfortunately, I still haven't met one of my life's goals which is to visit the brain center of my local utility to see how they dispatch units.

But, based on activities that I have had over the years with power plants, I do know that the dispatchers know exactly what each unit is doing and what that is doing to the grid. Me and a plant operator got our hands slapped pretty good once when he delayed getting to where they told him to be while I completed getting some test data.

I see your situation as analogous, for whatever this comment is worth. Maybe someone who has a feel for that type of control could help.

Your problem is real, and your question is a good one.

rmw

 

[HCBFlash]

- yes, you'll be safe to tweak it into better and better operating characteristics.
- yes, individual pressure controls may be suitable, but when you start, you'll be best to only vary your speed (continuously) to pressure on your primary pump. -- Using a single pressure sensor, and controller input, will simplify your system and improve reliability, and that's how I'd approach it.
- yes, the 3rd pump on and off delay will help, and if you're using a PLC (and IT's) analog P / PI / PID control, you'll easily be able to rotate your primary, secondary, intermittant pump order to suit your pumps to equalize the load share (and pump/motor/drive wear), or accomodate PM or breakdown occurrances, or even to reduce demand on a certain aquifer.

I've basically dealt with very similar setups on hydraulic pumping systems and large multi compressor air systems. I'm no wellguy though if your wells themselves are inconsistant.

- Whose VFDs to use is perhaps a question of politics. Whichever local vendor gives you the best service and support is often the best choice. I've had the situation where the supplier that gave the best service by far, was the supplier that had the "second class" product lines available, and that's why they knocked themselves out to be everybody's best buddy. Allen Bradley used to have very stuck-up distributors to match their own corporate attitude, and I used to do mostly Square D. That was some time ago though. AB has come a long way in recent years toward better customer service, but are still overpriced for a lot of stuff. All that said, they do make great drives, and to me that means durability AND features, AND availability.

remember: An opinion is only as good as the one who gives it!

 

[Marke]

Hi Electricpete
Lots of good data here!!
There is no doubt that the preferable way, is to use common control for all the pumps. Many of the drives available today have multi pump controls to ensure that all pumps operate forthe same period of time etc. If you want to operate these pumps totally independantly, this is commonly done in water reticulations systems, there should be no problems provided that the pumps are sufficiently separated. The piping provides a hydraulic impedance that gives sufficient isolation to prevent instability.
If the pumps are close enough to interact togather hydraulically, I would use slightly different set points, and different integration factors in the PID control. This will tend to create a natural master slave situation with no electrical interconnection.
Most drives also include a sleep function. If you use a pressure wakeup rather than the usual PID wakeup, you will have less problems with drives starting and stopping all the time. This will also help to improve the stability of the system.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[akselj]

I have built several multi pump controllers using Danfoss VFC with cascade option.
One pump is always frequency controlled, and the rest can be single or variable speed, the VFC is controlled by a pressure gauge keeping the pressure constant.

In simple setups the main pump, frequency controlled, is trying to keep the pressure constant, by some 110% speed it is slowed down, and one single speed pump is started, ans the speed of the first pump is adjusted to keep the pressure. The single speed pumps should have soft-start, but I have not seen problems by direct or Y/D start.

Up to 4 single speed pumps can run on the system, controlled by relay outputs from the VFC.

Systems with up to 5 frequency controlled pumps can also be powered by Danfoss VFC.
The starting sequence of the "slave" pumps can alternate to ensure use of all wells and to spread the wear of the pumps.

http://www.us.water.danfoss.com/products/cascade/

Bruchure

http://www.us.water.danfoss.com/literature/cascadebro.pdf

Aksel