Variable Frequency Drives

[sam01]

I need information on putting together a booster pump system utilizing effluent water for a municipal plant. Initially the minimum demand is about 3300 gpm. The final maximum demands are about 12000 GPM. The system needs to supply effluent water at a pressure of 60 psi. The total number of pumps needs to include one standby and one out of service pump. Can someone put me on the right track to determine the number and limits of each pump, VFD operation in regards to pump speed and flow and the control logic for complete pump operation.

 

[ravisankar]

flow rates in your system are varying from 25 to 100%. in this case if you go for one pump only, the speed of the motor at low capacity condition i.e at 3300 gpm should be reduced to 1/4 th of the full motor speed. Because flow is directly proportional to the speed. but head capacity of the pump at this speed will reduce by 1/16 th as speed is proportional to the square of discharge head.

in your case pressure of 60 psi is a little bit confusing, for centrifugal pumps develop head equal to the resistance in the system.

however coming to your question you can split your pumping system into two and both pumps should be controlled by a vfd. you have to take a pressure signal at the end user point of your system. (not in the pump discharge-this will lead to a wastage of energy). a simple logic can operate both your pumps at optimum efficiency levels independant of flow rate requirement. this is already a proven system and widely used.

(Note: M/S Danfos did a good job in pumping system and are having specialised drives for each system. you can get better knowledge from their website.)

regards,


ravisankar

 

[Slagathor]

Sam01,

A couple of comments/questions....

1. Step #1: Have you developed a system head curve? It sounds from your description that your delivery point is not far from the pump station (in other words you have a flat system head curve, and static head plus required delivery pressure make up your head). If this is not the case, you should give us some more info.

System head data is important to make proper pump selections. A system with lots of static head (flat curve) will require more pumps than a system with little static ("curvy" curve) head and lots of pipe friction.

This is because as you slow a pump down, the head drops twice as fast as the flow (affinity laws). This means that with a flat curve, you ride up the pump curve fast. Pretty quickly you can get too far to the left of the allowable operating range of the pump. (This is not as much of a problem with a multi stage Vertical Turbine, but with a lower specific speed pump, like a Horiz Split, it will not be happy running too far to the left. This does not sound like a HSC app, so I will assume you are talking about a VTP)

My gut feel is that a 3+1 system will work well. Four pumps installed, three running, one spare. 4000 GPM each. Since it sounds like you do not need 100% cap now, your could install 2+1 (or 1+1), and design in a spot for the future pump(s).

You should check out Sanks book on pump station design. Most everything you would ever need is in there....

 

[sprintcar]

The easiest way to size pumps is to contact the manufacturers and let them do it! Most have charts or software taking your gpm, head and pipe size and converting it to pump size.
Try these guys - they're one of the best

http://www.giwindustries.com

 

[sam01]

Thanks for everyone's help. I would like to maintain a system pressure of 60 psi at the plant. The effluent water will be utilized at various locations. My suction header is drawing off of an effluent channel. A positive suction head of 14 feet plus atmospheric. I want to set up a pipe network system to minimize friction losses. The system curve is flat which starts at 60 psi and 10 to 20 feet of pipe losses. The initial system demands vary from 3300 gpm to 5500 then to 8300 and finally to about 12000 gpm. How do I maintain the system head pressure and cover all the flow requirements utilizing VFD's? Thanks again for everyone's help.