Need metering pump control advice.

[ToddM 0]

Hi folks....For the past couple of weeks I've been working on a metering pump control system using a ML1200 and an inverter to control the speed of a small pump. I'm monitoring the level of chlorine coming into our plant in PPM and that is what I've based my control on....until today....THEY are going to add a flow meter on the incoming water line and want me to incorporate the flow meter in the logic. The way I have it now is like this. The PPM of clorine goes up the pump speeds up. I don't think I really need the flow meter but I'm going to add it anyway, I just need some Ideas as to how to use this extra input to help control the pump. Of course I could be wrong and I really do need this extra control but none the less I got to use it and I figured I might learn something new. Any help would be greatly appreaceated. Thanks and later Todd. P.S. Heres a copy of my code so far. This is not in service yet so there may be a few errors. CLORINE.RSS Edited 31 May 2007 by ToddM

[Bill Linne 0]

I'm confused by this part. Seems like you'd slow the pump if the Cl2 goes up (in order to maintain a constant Cl2 value). Or, are you KILLING Cl2 with the product from the pump? What you are contemplating is called Flow Pacing, and it involves controlling pump speed proportionate to water flow (through the new meter). The typical advantage would be faster response. You need not wait for the Cl2 analyzer to take and analyze it's sample and output the result to the pump controller. You just vary pump speed based upon flow through the new meter. Regards, Bill

[ToddM 0]

Bill... If I used this method would I need the input from my chlorine sensor? Looks to me like I would have to know how much sodium thiosulphate to add according to the amount of flow {which I should find out tommorow when I talk to one of the chemical engineers} But I think I would still need that input to tell me if it's right or not. Anyway thanks for the help and if you could post an example of flow pacing I sure would appreaceate it. Thanks and later...Todd Edited 1 Jun 2007 by ToddM

[Bill Linne 0]

Todd: The plant for which I am responsible is one of those that puts the Cl2 into the water, and I am not a fan of flow pacing so we don't use that technique here. My only experience with sodium thiosulphate is in tablet form, and we use it if we need to "blow-off" (flush) a pipe and water will go into the storm drain. All that is to explain I know little about dosing the stuff . . . so, other than wasting chemical, what is the down-side of too much sodium thiosulphate? If the chemical guys tell you you need xx volume of sodium thiosulphate per unit flow, then I'd say all you need the Cl2 analyzer for is to be sure your metering pump is working properly. I have no example logic for you. If you Google "flow pacing" you'll get several good explanations of the good, the bad, and the ugly. One of the typical down sides of flow pacing is that you must setup for worst-case scenario. In your case, if influent Cl2 is typically .5 ppm, but can reach .75, you'd need to dose sodium thiosulphate adequate to neutralize the higher value. Anyway, I hope that helps a little bit. Let us know what you decide and how it goes, eh? Bill

[paulengr 44]

There are really two constants here in a perfect world. First, you have the dosage per gallon. Second, you have the scale factor to deal with the flow rates (the strokes/min to gallons/min conversion), which should be relatively constant. The first one changes as the concentration changes. The other changes as the physical parameters (pumps wearing out, tank head pressure drops) change. In a real system, a single PID will correct for both. You can simply set a coarse estimate of the flow rate factor and let the PID loop do the fine tuning. You are controlling to ppm but you have two factors: flow rate, and concentration. Chemical reagent pumps are usually very linear as long as you set the stroke length within their linear range. And you normally control them either with pulse-frequency signals (pulse rate = flow rate), or 4-20mA signals which get converted to stroke rates. Do not attempt to control stroke length too or you will have problems (from experience). If it was just a flow rate thing, you'd be doing ratio control and that would be your loop. But you're dealing with concentrations changing, too. My suggestion then is that you leave your existing PID control alone and let it adjust for concentration (ppm). If flow was steady then, you wouldn't need the flow meter. BUT, take the output value and multiply it by the flow rate. This provides feed forward control based on the incoming flow rate. As to what scale factor to use, try a small one at first and slowly increase it until you see good response when you make changes to the flow rate. This is identical to a sodium dithionate control loop which works very well under those circumstances. However, do NOT try to do the same thing with pH. pH is a log scale, which most people don't realize. Also, how a particular chemical reacts in terms of pH is highly nonlinear (pK factors). So sometimes linear PID works. Sometimes not.

[michael G 2]

I have called it feedforward look at http://www.controlguru.com/wp/p41.html - Feedforward for more information If implemented correctly for your process it improves the control Your existing PID is required plus the feedforward PS the controlguru site is a great resource

[ToddM 0]

Paulengr and Michael G. Thanks for your input Thanks for the info , looks like another great site to bookmark for further reasearch Thanks again for all the help, later....Todd Edited 2 Jun 2007 by ToddM