9 replies to this topic

[asaklsak]

I have a tank of 90m3 capacity storing water at atmospheric pressure. Water from this tank is fed to a pump of rated capacity 21.5 m3/h through a 3" pipeline. The suction nozzle of the pump is of dia. 2" and a reducer is attached at the inlet of the pump.

 

During running operation, flowrate through the pump is unable to exceed 12-15 m3/h.

 

I wish to check the adequacy of this pipesize for the rated flow of the pump? Is there a way to calculate the maximum flow through the pipe segment using tank level? Is a 3" line followed by a 2" segment adequate to flow 21.5 m3/h directly from the tank to the pump ?

 

 

[Santoshp9]

Dear Asaklsak.

Your Suction pipe Size 3" is quite Ok. & also suction Nozzle of pump 2" is ok.

You will have to check  your down stream pressure drop and draw a system resistance curve over your characteristic curve to check the respective flow of pump.

 

Regards,

Santosh

[shan]

The flow corresponding to NPHSA (Net Positive Suction Head Available) = NPSHR (Net Positive Suction Head Required) is the max flow.

[fallah]

asaklsak,

 

What you are trying to do is hydraulic calculation of the piping around the pump in order to specify the pump characteristics to be delivered to pump vendor in a data sheet to select proper one. This activity has to be done before pump selection while you are doing vice versa.

 

Anyway, as santosh mentioned, to check if the pump can handle its rated flow with existing piping route/design you should draw the system curve over the pump curve and evaluate the flow corresponding to the intersection point.

[Art Montemayor]

Asakisak:

 

For your interest, I am attaching a copy of Jacques Chaurette's Chapter 4 from his excellent and practical book on pumps.  In this chapter you will find paragraph 4.4 where The System Curve is discussed.

 

I hope this helps you out.  You can download Jacques' book for free from the Internet.  I have uploaded it on our Forums before so that all our members could profit from it.

 

 chapter4.pdf   1.43MB   154 downloads

[asaklsak]

Thanks everybody for you replies....The pump in question has an NPSHR of 2m. I had already done an NPSH check and the said diameter of pipeline is OK.

 

What i am worried about is that for a flowrate of 21.5 m3/h, the velocity of water in the 2" pipeline comes out be close to 3 m/s. Is it really possible to achive this much velocity in a pipe segment when the said fluid is flowing directly through a tank without any pumping??

 

As per everyone's suggestion, I will definitely draw the system curve over the pump curve and come back if that solves the problem??

[Santoshp9]

Dear Asaklask,

 

Yes,3m/s velocity is really possible through 2" pipe.

In centrifugal pump during pumping of fluid, pressure at impeller eye is always less & It causes pressure differential from available source to eye of impeller.

Always energy flows from High to low and due to this fluid have velocity high during pumping.

 

If you think your fluid is stored in atmospheric tank having drain of 2" & sufficient static head of fluid over it ,then its possible to have 3m/s of velocity flow through it.

You can refer Crane fluid flow problems hof master excel sheet available at Cheresources for better understanding.

[Santoshp9]

Dear Art Sir,

Every time I see 0 warning points below my nos. of posts ,what does it mean?.

 

Regards,

Santosh

[Raj Mehta]

You see here that the diameter is kept constant throughout, but now the flow that you getting actually is not the desired one. And we also know that the pressure drop in the line is directly related to the velocity of fluid in the pipe.

 

Your velocity, as seen above, is decreasing and hence the flow rate. So, According to me it definitely has something to do with the pressure drop in your line (which is not as per calculation done during the designing). I advice you to check that also along with the plotting of the system curve.   

 

Kindly see the attached sheet where I have calculated velocities for both rated & actual flow of your system using Q=Av.

 

Correct me if I am wrong. 

 

Thanks. 

Attached Files

•  Max. flow through pipe.xls   6.5KB   62 downloads

Edited by Raj Mehta, 31 March 2013 - 11:43 PM.

[acer_asd]

Dear  Asaklask,

 

There doesn't seam any problem in suction side of the pump.

 

I would advise you to follow these steps for identifying root cause of the problem provided it is a centrifugal pump.

1. Take the pump curve see what is shut-off head and compare it with pump curve. For this, you need to have pressure gauge in suction and discharge of the pump. If shut-off head is matching with pump curve. Then you can almost be certain that there is no problem with the pump.

2. You open discharge valve slightly and note down flow rate and differential head. You open the valve and take another reading of flow rate & differential head. Take 4-5 reading till opening discharge valve is fully open. Compare these reading with the pump curve. If these reading matching with the curve, you are certain there is no problem with pump.

3. Now Calculate theoretical pressure drop in discharge piping and draw the system curve (pressure drop curve in discharge piping vs flow) and match it with the pump curve. if flow is not as per this intersection point of system curve and pump curve then there is problem in piping.

 

I believe if you follow above steps you will certainly identify the root cause of the problem.

 

I hope this solves your problem

 

have a great day

Reagrds