Pump Control System

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Thread Starter

Imran

Dear List Members, We had twelve number of pumps connected in parallel. Each pump motor has capacity from 110 to 200 horsepower. These pumps are used to control the supply to whole city. We have to mantain pressure of 3.2Bar in the line all the time. The problem is that in given instance of time if five or six pumps are running the line pressure is about 2.8Bar. On invoking another pump, pressure increases upto 3.8Bar which is not required. Can any one suggest solution so that pressure could be maintained exactly at 3.2Bar?? Thanks and best regards, A&D IMRAN SHABBIR Automation Engineer Automation & Drive P.O.Box 325 Dubai, U.A.E
 
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Steve Myres, PE

How about putting an AC drive or continuously variable transmission on one or two pumps and supplying some portion of the full 200hp available? Then if the demand calls for 6.5 pumps, you can run six pumps at full output and the variable one(s) at 50%. Alternatvely, you might put some kind of throttling valve on the output of the pump where variable output is desired. Either way the flow from the pumping system can be closely matched to the demand, and will give the desired system pressure when discharged to the main.
 
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Alan Rimmington

The best solution would be to convert some of the pumps to variable speed control. On a 12 drive system it would be advisable to have three drives converted to variable speed. The ideal pumping regime would be to use the required number of fixed speed drives to nearly reach the required pressure, and then run one or two variable speed drives to trim the pressure. By having three variable speed drives it allows for duty rotation and for maintenance periods. We have used this sort of system for our clients in the past. [email protected]
 
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Daniel_Rallo_Rub

Hello, Imran: Why don't try to put a couple of pumps with a variable speed drives. You could use these pumps to modulate in the higher band of pressure. You could have 6 pumps directly connected plus one in speed regulation. When the speed control goes to 100%, you can substitute the modulation pump by one direct and start the next with the variable speed drive. You could try also to interconnect pumps and variable speed drives alternatively. Daniel Rallo Rub Dept.d'Enginyeria EMTE Sistemas, S.A.
 
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We use a golden anderson pressure control valve tied to a AB 5/20E plc to control the line pressure from our off site fresh water pumping station. They been around a long time and has provided reliable control. Try www.gaindustries.com or call 1-(724)776-1524 Gary Drake Cargill Fertilizer Inc. 8813 Hwy 41 South Riverview, Fl 33569 phone 1-(813)671-6213 fax 1-(813)672-6438
 
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Adolfo Jimmy Saldivias

You may want to use VFD (Variable Frequency Drives) for this application. There are a lot of manufacturers of those equipment who come to mind. You can check the examples of applications in their websites. Yours is a pretty common application, so you should have no trouble finding those application files. Several names come to mind: www.abb.com www.yaskawa.com www.schneider.com www.siemens.com and so on. MBA Ing. Jimmy Saldivias TECSIM Phone: 591-4-523438 Fax: 591-4-523413 http://tecsim.trading.net
 
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Piotr Kowalski

Hello, I would consider two possibilities: 1. Try to balance the power of running pumps (assuming you have set of pumps with different horse power rating) by switching the appropriate pumps - disadvantage: frequency of the start&stopping will be higher in such a case with possible shortening of equipment lifetime. You could use the solution if you have rather slow changes in water consumption. 2. Install a frequency controller for one of the pumps. You will switch on or off the rest of the pumps to obtain a 'raw' regulation of the preassure, the precise controll would be done by frequency controller and a PID. Disadvanage - it is more costly solution. Kind Regards Piotr Kowalski P.S. Sorry for my poor english.
 
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PAS AHMAD NAZIR

Dear Imran, Is it possible to use Variable Speed Drives.This might solve the problem. Thanks & Best Regards _____________________ Ahmad Nazir Ahmad System Integrator 64 C-III, Gulberg III, Lahore Pakistan Process and Automation Solutions (ATD TD) Tel: +92-42-587 1010-16 Ext. 216 Industrial Projects & Technical Services (ATD) Fax: +92-42-571 7798 SIEMENS Pakistan Enginerring Co. Ltd. Email: [email protected]
 
Put a Variable Speed Drive on the two (2) largest pumps in the system. Let the system pressure control the Speed of the variable pumps and cycle the constant speed pumps on and off. my $0.02 worth Bradley G. Hite Intertech Incorporated mailto:[email protected] http://www.myplc.com Teaching Practical Skills for a Technological World
 
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I don't know if this is an option, but your best solution may be to install a variable freq. drive on even just a couple of the pumps. That way, 5 or 6 pumps can be run at full speed, and a 6'th or 7'th can be run on the VFD to bring the pressure to exactly where it is needed. There will be a few programming feats to overcome, but it should be fairly straightforward. Good luck. Jody Gallant CET Shadcomm Ltd.
 
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You need at lease one VFD controlled with a pressure loop that will always be on controlling to the pressure you desire. For example 11 of your pumps could be constant speed and 1 would be on a VFD. As demand increased the VFD would speed up to meet that until it is at top speed then slow it down and bring on a constant speed pump at the same time. If demand decreased to the reverse. This is a very simplistic way of describing the process and I know there will be a bit of logic to get it done but this is the basic idea. Garth Gaddy City of Fresno, Water Div.
 
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Johan Bengtsson

A more detailed version of what others are already suggesting: have one PI (or PID) controller for the pressure, the output from this is the demand in number of pumps, but not as an integer. Take the integer part of this and make that number of pumps run at full speed. take the fraction and use that for a VFD, as someone suggested you should perhaps have more than one VFD for maintenance purposes, but only use one at a time since that will simplify control. You should measure at what speed a pump don't produce a flow (at that speed it don't add to the pressure it just barely keep the water from not flowing backwards) If you want, for example 6.1 pumps you turn on 6 pumps at full speed and the VFD should be 10% up at the scale between the speed producing 0 flow and full speed. In order to not turn on and off pumps too often when you are controlling around X.0 pumps you should be able to drive the VFD either a little bit below the speed producing 0 flow (yes the water flows backwards at this time, check with pump manufacturer I don't think this will be a problem) or if you can run it a little bit above the speed a directly driven pump. This effectively means you should not turn off one pump as as soon as you go below for example 6.0 but rather wait until 5.9 or whatever you can accept as min speed of the VFD. I have been involved in controlling two pumps in a similar fashion, it was a flow controlling process and that added some other things to consider (a pump produces different flow at the same speed if the pressure is different, this will not be an issue here since you want the same pressure) /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------
 
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Darold Woodward

I have built similar systems using a PID control algorithm. You must do some testing to determine the output of the pumps and "tune" the system to your specific requirements. You can turn pumps on based on pressure, but usually you need to turn pumps off based on flow to keep the pumps at high efficiency rather than running more pumps than required all at a low speed. Darold Woodward PE SEL Inc. [email protected]inc.com
 
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Heavner, Lou [FRS/AUS]

Looks like everybody is recommending VFD on some of the pumps and that would be my first choice. As an alternative, if VFD is not feasible, you could consider a recycle or pump around loop with a control valve. You would always run with one more pump than required and use the spillback valve to control the pressure as required. This may be more expensive to operate, but may also be much less expensive to add. Regards, Lou Heavner Emerson Performance Solutions
 
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gerald beaudoin

Check out the Cutler Hammer SV9000 series of VSD's. They are perfect for pump applications as they even have built logic for cascading pumps and also have a P and I controller built in so you just have to feed in the 4-20 ma signal! Not sure though if they go up into the HP range you need. Gerald Beaudoin Leahy Orchards Inc.
 
The best way I seen work is to use the following devices to control the flow and pressure: 1. Variable speed drives on at least the first pump. More would be helpful. 2. A return line with a pressure control valve set at a slightly higher value than you want. This would help eliminate surges. Place a flow meter in this line and use it to help determine when to shut down a pump. 3. A slow pressure controller (PLC or Single loop). It would not even have to be a real PID control loop. I have seen it used where you just set you setpoint with a deadband and slowly increase the speed or decrease the speed of the VFD based on your process variable. When demand decreases water will begin to flow back through the return line. You can measure this and using it, along with some kind of demand algorithm, determine when to shut of a pump. Start a pump in the normal way. (demand) I have used this type of setup in potable water supplies before. It is a good safe way to do it.
 
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