The inlet of a centrifugal water pump is 7ft above the free surface from which i

Question

The inlet of a centrifugal water pump is 7ft above the free surface from which it draws. The suction point is a submerged pipe, (entrance equivalent length. 8.5ft). The supply line consists of 12ft of 2m schedule 40 commercial steel pipe and contains one long radius elbow, (equivalent length 3.6ft), and one check valve, (equivalent length 19ft). The discharge line is 2in schedule 40 commercial steel pipe and includes 2 long radius elbows, (equivalent length 3.6ft each), and an 80ft run. The discharge of the pipe is 20ft above the free surface and is open to atmosphere. The water is at 70'F. The following 3 points are from the pump curve. Flow Rate, GPM head, ft 90 50 60 87 30 102 Generate the curve fit equation for the pump curve. Generate the equation for the systems operating curve. What is the flow rate through this pump? Is this pump a good choice for this application?

Explanation / Answer

For the pump curve the equation is a 2nd order polynomial so with your 3 points you can determine this curve mathmatically (you can use excel if you dont know how to do the math). You end up with the equation: y = -(11/900)x^2 +.6x + 95 To find the system operating curve you need to know what the head loss vs. flow rate is. The losses for a 2in Sch. 40 pipe can be found online or most likley in your text book. If you are given a PSI/ft loss you just mutliply this number by .434 to obtain your Head/ft of pipe loss. Here is what I found for 2in Sch. 40 pipe: 10 GPM - .19 PSI (.0817 ft Head) 20 GPM - .68 PSI (.292 ft Head) 30 GPH - 1.43PSI (.615 ft Head) Again the equaiton for pressure loss vs flow is based on the Hazen–Williams equation which is a 2nd order polynomial so you only need 3 points to define it. Using basic algebra (or excel) you will find the equaiton is: y = .073x^2 +.56x + 22.76 The point at which your pump and system curvs intersect will be the opperating point of the pump. This occurs at 29.4 GPM. Without having the BEp (best efficency point) of the pump it is hard to say with 100% confidence that this pump is a good choice. But, based on the curve the intersection is near the point where the pump curve begins to flatten and this is usually at the upper point of the recommended operating range. My guess would be that in a pinch this pump would do well but I would choose a different one if the resources were available. I hope this helps

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