Selecting a metering pump

  • Thread starter James Bond>>>>>>>>>>
  • Start date
J

Thread Starter

James Bond>>>>>>>>>>

i am working on a project Water Blending & Treatment Plant and new to this process. I have queries as>>>

1. What is metering pump.

2. For metering pump there are two parameters.

a. Speed Reference.
b. Stroke Length.

Speed reference is for controlling Flow rate & Stroke Length is used for control PH level.

* How to use these two parameters for controlling the metering pump output.
Means which PID controller two use>>>
Cascaded or Feed Forward , Ratio Controller.

I am a bit confused plz help me out on this>>>>

Any sample program would be very much beneficial.

Thanks in advance.........
 
Also, the point which I guess I didn't make clear in the original post is that this is a METERING pump and the whole point is that you don't need a PID control loop for the pump itself. You directly calculate the speed and stroke length required to give you the flow rate you need.

Note that while some metering pumps may be supplied with flow meters I would not usually recommend you control using these. For especially low flows or low pump speeds the flow meter is next to useless because the metered chemical flow is not constant and the flow meter simply isn't as accurate as the metering pump itself. Trying to do feedback control like this defeats the whole reason for having a metering pump.

You might choose to use a PID loop or other control to calculate the desired flow target for a metering pump (eg controlling colour with dyes), but equally often, the required flow target is simply a ratio of another setpoint in the plant so there is no PID loop at all (eg adding a fixed ratio of chemical to a product).

Rob
www[.]lymac.co.nz
 
R
Its common to use a metering pump for pH control.
The pulse rate is controlled by an analog signal. The stroke length is usually set locally

The stroke length sets the quantity of product per pulse so you set that to give the maximum requirement at maximum frequency. i.e. if your controller is wound out to 100% and you can't get enough product to correct the pH you would increase the stroke length. You want to end up with a pulse rate somewhere around 50%

One thing to keep in mind, you need considerable volume downstream of the addition point otherwise the pH will spike at each pump pulse i.e. you wouldn't put the pH probe in the pipe just downstream but into a mix tank or re-circulation line.

Roy
 
W
Actually, Roy, that's only true for higher end and larger metering pumps. Most small metering pumps are driven by a scaled pulse input.

One of the best ways to measure flow out of a metering pump is to mechanically measure (at calibration) the actual physical output per stroke of the metering pump. The pump manufacturer sells kits to do this. Then all you have to do is to do the math.

Walt Boyes
Editor in Chief
Control and ControlGlobal.com
555 W. Pierce Rd Suite 301
Itasca, IL 60143

[email protected]
www.controlglobal.com
630-467-1300
 
R
Walt,

I am thinking LMI or Prominent pumps that accepts a 4-20 mA signal and generates a pulse internally. I think that would be the easiest type to implement.

pH transmitter --4-20---PLC---4-20---Metering pump

I have used a few of the pulse input types you mention. I don't think you need to measure the flow in this case, it's pH control you are trying to achieve.

Regards
Roy
 
W
I've used them too. Industrial water treatment is full of very cheap people...and very cheap products. You can find much less costly metering pumps (even from LMI or Prominent) if you don't demand a 4-20...so what you buy is a very inexpensive pulse rate converter (4-20 to pulse). Frankly, I'd prefer to do it your way, but it often isn't competitive. There's a reason that some industrial water treatment professionals are called "water doctors" or even "water criminals."

Walt

Walt Boyes
Editor in Chief
Control and ControlGlobal.com
555 W. Pierce Rd Suite 301
Itasca, IL 60143

[email protected]
www.controlglobal.com
 
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