VFD Pump Control

As I see it.

1. Too much is a relative term, as is small. It seems reasonable for protection and control of more than 1000HP worth of pumps.

2. The pumps do not want anything, the organization that is responsible should want to be able to sense a problem before it becomes a catistrophic failure.

3. Lead - Lag programs can accomplish this.

4. Scada would be my preference, it is good for many control applications.

5. If you want to control the pressure you should measure it.

Hi Nourbe

Seems you have a big project... what type of pumps are you using, how are they mounted, skid type or something else and the flow rate required? That would be my first set of questions.

However, as I have installed and commissioned many surface and downhole pumps I can tell you the following

Your designer is correct... you need the valves, flow switches and vibration sensors for the following reasons.

Valves: if a pump stops and others are running, the the back pressure will feed back through the tripped pump causing it to "backspin"(you make no mention of non-return valves). If the PPLC then tries to start the pump as it is back spinning it WILL break the motor or pump shaft. Back spin sensors attached to the motor might help here and a GOOD motor controller will help.

Flow Switches: Yep you need them as if you have a required flow rate, then you need to automatically start another pump if the flow rate drops or a pump trips out. You need to know if each duty pump is running or you are producing your required flow rate. A simple controller will monitor this situation and switch in another pump if required

Vibration Detection: Do not operate without these..... if you do and your installation is not alined correctly there is a real good change that you will destroy it and the pumps. Furthermore if something mechanically happens to the pumps/motor, then the vibration switches will detect it and trip out the damaged pump. Better safe than sorry. I have further info on vib switches if you require. The acid test is when your pumps are running you can stand a coin on the pump or pipework.. it should NOT fall over.

Pressure Transmitters: If you have VFD/VSD then providing you have the PT is set up correctly to your discharge pressure, then your flow rate will be constant PER PUMP. You cannot have ONE PT controlling ALL your VFD's. The beauty of having a PT connected to a VFD is that as the pump wears it's efficiency decrease (discharge pressure drops) and the VFD will speed up to correct the losses, a good way of telling when you need to replace a pump, that and a motor amp chart. NOTE OF CAUTION: ensure that the max Hz output from your VFD does not make the pump exceed the OR of the pump. With VFD's you can set the max Hz

SCADA..if you want 100% automatic or you just need to monitor the system, to monitor would be the easier solution, having it control automatically would be complexed, however SCADA systems I've worked with give you monitoring AND remote control of each pump and valve

I suggest you work with your designer as I suspect you are now trying to second guess him, mainly because the system he has suggested might be expensive.. would I be right?

If you want a cheaper system you will have to get your operators to work harder, spend more time with the pumps, monitoring flow rates, duty cycles and pump efficiency, spend more time open and closing valves, maintenance staff spending more time checking the motors.... and the HUGE possibility of operators NOT getting something right, which will cost you in the long term.