Common Discharge Pumps
Common Discharge Pumps
(OP)
Hi,
I am analyzing a system that consists of three identical centrifugal pumps. I am debating weather the pumps are in parallel because the literature indicates that for a series of pumps to be considered in parallel they would have to share a common suction and a common discharge. This is not my case, the three pumps share the discharge. From the design point of view two pumps should be enough in order to move water from an open tank, however all of them are constantly running in order to keep the level in the tank constant even when the impeller size was increased. Now, I suspect the limiting factor in this case is the discharge pipe. Has anybody ever seen a problem like this, and if so, what's the best way to try and approach a solution. I should add that we are about to expand our operations and thus we would like to try and add an extra pump, but find if it's the line what's causing the restriction.
I've attached a drawing for better undestanding.
I am analyzing a system that consists of three identical centrifugal pumps. I am debating weather the pumps are in parallel because the literature indicates that for a series of pumps to be considered in parallel they would have to share a common suction and a common discharge. This is not my case, the three pumps share the discharge. From the design point of view two pumps should be enough in order to move water from an open tank, however all of them are constantly running in order to keep the level in the tank constant even when the impeller size was increased. Now, I suspect the limiting factor in this case is the discharge pipe. Has anybody ever seen a problem like this, and if so, what's the best way to try and approach a solution. I should add that we are about to expand our operations and thus we would like to try and add an extra pump, but find if it's the line what's causing the restriction.
I've attached a drawing for better undestanding.





RE: Common Discharge Pumps
More info on flow and pressure would be helpful. Also, what's controlling the level in the tank? The control valve?
RE: Common Discharge Pumps
Can you plot a system curve? Plot pressure at the header converted to head in feet vs the total flowrate of all running pumps? Find that pressure for as many flowrates as you can. If you want to add another similar pump, you will have to extend that curve to however far it goes plus the added flowrate of another pump. Compare that "system" curve with one pump curve. If the pump curve shows sufficient head is still available (head where the pump curve crosses the system curve) at a flowrate equal to 4 pumps, you might be able to get away easy (without increasing the size of the pipe).
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Common Discharge Pumps
The level in the tank has to be kept at 60% in order to maintain the level in the priming chamber (the tank on the suction of the pumps). The control valve maintains the level and is currently 45% open. The flow going into the pit is approximately 4500 GPM. TDH is 120 ft, NPSHA is 18 ft and NPSHR is 16.93. The pump capacity is 2640 GPM.
The pressure on the common discharge header is 60 psig.
RE: Common Discharge Pumps
In addition to the excellent advice given by BigInch, I would suggest a pressure survey of your system. If you have the taps available, check the pressure at:
* The pump suction
* The pump discharge
* The common 18" header
* The inlet to the control valve
* The outlet from the control valve
This should make it clear if the problem is an excess pressure drop in the piping or not. I would also verify that you have adequate submergence on the suction line. Even if you have enough NPSH available, it is still possible to have vortexing that could allow gas entrainment in the suction of the pumps.
Johnny Pellin
RE: Common Discharge Pumps
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Common Discharge Pumps
During previous analysis it was determined that the conditions previously mentioned (pit level and valve position) are optimum to maintain a constant level in the priming chamber and thus avoid cavitation, the data I included previously is from the data sheet. You are right about the condition of the valve if one of the pump goes down, it indeed opens up 100% to try to maintain the level down.
Thanks for the suggestions, there are several places where I can get the pressure including the discharge of the pumps and upstream of the discharge point from the second pump. No luck with the suction side and so I will have to rely on calculations.
BigInch,
One more question, I am not very clear as to how to vary the flowrate in order to plot pressure vs. flowrate. These are very good suggestions and I will try to implement them. Thank you for the assistance.
RE: Common Discharge Pumps
RE: Common Discharge Pumps
RE: Common Discharge Pumps
"The flow going into the pit is approximately 4500 GPM. TDH is 120 ft, NPSHA is 18 ft and NPSHR is 16.93. The pump capacity is 2640 GPM.
The pressure on the common discharge header is 60 psig."
Total 120 ft --- discharge header 60PSI ?????????.
Think you neeed to sit back and rationalise the data as pointed out by Johnny Pellin - plus establish the flow rate from each pump.
RE: Common Discharge Pumps
The duty point on the curve suggests that two pumps should operate to achieve required flow. (4500USGPM/2 is 2250USGPM each). this operating point is at a lower discharge (header) pressure therefore the control valve must open to allow this.
If infact the system pressure has increased to the 170 feet predicted by the information supplied, a better solution would be to install a 14" impellor in each pump and increase the motor size accordingly.
To verify the problem and therefore validate the solution the pump suction and discharge pressures are required.
Mark Hutton
RE: Common Discharge Pumps
Right now we only have one point on the system curve, the blue dot at 138 ft (I added 18 ft suction head to the 120 TDH to get the system head equivalent plot on the pump curve chart) and 2650 gpm.
I took a guess at what the system curve with the control valve in the present position MIGHT look like. Open the attachment. What we really want to know is what the system curve looks like with the control valve full open. Maybe thats something like the purple line. To get the system curve without the control valve effect, we need to change the position of the control valve, measure the discharge pressure at the header, convert to head, and then subtract the head burned off by the control valve to arrive at one point on the true system curve (w/o any CV effect) at your tested flowrate. If you can adjust the position of the CV and read or estimate the head burned by the CV for a couple of points you can get several tested flowrates and a good relative handle for drawing in the system curve (purple line) where its supposed to be by plotting the discharge header Head - CV head value with each flowrate you check.
If you can do that, then you'll know if you can add another pump just by opening the CV, rather than by increasing the discharge pipe diameter. I extended the pump curve green line to some other higher flowrates, so wherever that green line intersects the purple line will be the running point at any other flowrates. The individual flow through any and all identical pumps will always be that flow read under that intersection point of the green and purple lines. To get total flow, just multiply by the number of pumps in parallel.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Common Discharge Pumps
John
RE: Common Discharge Pumps
Get 1 accurate flow and pressure (at the pump discharge or corrected to the discharge point) together with the actual static head and an estimated resistance curve can be calculated.
RE: Common Discharge Pumps
BigInch,
I have to take exception to a few of your comments. First, the system curve with and without the control valve cannot be parallel as you have them drawn. They have to intersect at zero flow since the static head is unaffected by the presence of the control valve. Second, the flow estimate for a single pump cannot be multiplied by the number of pumps. Since the system curve always has slope, you will always get less flow from the second pump than you got from the first and less flow from the third pump than you got from the second. Each successive pump added will contribute less and less.
Johnny Pellin
RE: Common Discharge Pumps
And your second comment about N x Flow, yes I realized that when I said it, but without knowing very much about what exactly is out there in relation to the piping config and how it affects the system curve, I thought it wasn't going to be too very far away from what we can reasonably estimate right now, but yes it will of course be less than N x.
Thanks for clarifying.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Common Discharge Pumps
RE: Common Discharge Pumps
RE: Common Discharge Pumps
You are absolutely right. As a matter of fact according to operations (before my time) those pumps would cavitate quite often. It seems the previous engineer worked on this system but unfortunately the data left with him so it's difficult to say how the system was analyzed in the past.
We are about to expand our operations and so we need to investigate the root cause of this issue to avoid the same predicament. Additionally, even with our current flow conditions it is always desired to keep one pump on stand by.
Next step will be to get a pressure survey on the system, as Johnny suggested, and research the control valve so we can find an optimum point at which two pumps can handle the present flow without cavitating.
Again thank you all for the valuable recommendations as they have clarified much of the confusion I had.
RE: Common Discharge Pumps
RE: Common Discharge Pumps
Seems to me you've already found it....each pump operates at 1500 gpm.
To increase this...increase your NPSH available and you'll be able to get more out of each pump.