Q & A > Question Details
A 8m3/hr & 35m H pump is available with us and we are using for pumping organic residues which is the regular operation carried out in day today operation. Now we are planning to use the same pump for recirculation purpose for about 3 to 4 hrs/shift.But the recirculation back to the storage vessel pipe static head is only ~ 6 - 8 meters, but the original design of the pump is 35 m head. I have checked the pump curve & found that by reducing the actual head from 35 m to ~ 8 m , flowrate will increase. So accordingly I have sized the recirculation pipe size to the recommended velocity. Should this solve the problem? What other consequences might it have?
28/10/2011 A: MAHA RAI, HINDALCO IND. LTD. RENUKOOT, mahaprasad.rai@rediffmail.com
It is always advisable to select correct specification of the pump. By throttling the discharge valve or by using orifice in discharge line will cause huge power loss which is recurring in nature. Better you buy and install separate pump for the purpose illustrated by you.
05/08/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
I agree completely with Mr. Vetters. However, it is not clear to me whether you want the high flow or the low flow. If you want the low flow, you can simply use a throttling discharge valve. Alternatively, you can “run the head up the curve” in order to have less flow by undersizing the discharge piping. Yes, this will work, too, if you do not want to use a valve to regulate the flow. Possible problem with a high velocity in an undersized pipe: depending on the material being pumped, high velocity could result in erosion of the pipe. If you use a throttling discharge valve, it should be a globe valve, as a control valve would be. Or, use a restriction orifice, as was suggested. A globe valve is a variable restriction orifice, isn't it?
05/08/2011 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Depending on the pump the increase in flow will increase the pressure drop in the suction line to the pump, which could cause NPSH problems on the pump. You also want to check that your pump driver is designed for end of curve flow to make sure that the driver is good for the increased horsepower likely needed for the increased flow. Additionally you want to look at whether the new conditions move the pump away from its BEP (Best Efficiency Point). The further you get away from BEP, the greater your maintenance cost for the pump is likely to be.
An alternative is to put globe valve or restriction orifice in the recycle line to take some additional delta P. By sizing the valve or orifice properly you can make the pump operate wherever you want on the pump curve and no need to increase the size of the pipe.