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Effect Of Density Of Fluid On Pumping


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#1

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Posted 26 September 2009 - 10:53 AM

We have multistage centrifugal pump at Dispatch terminal for boosting pressure of LPG to nest pumping station in interstate LPG pipeline.We are operationg this pump since last seven years,& we were getting density of LPG in range of 0545-0.575 MT/m3. But Now due process change in upstream plant LPG components have been changed(propane components increased0.So we would be getting LPG density in range of 0.500-520MT/m3.As per OEM specification density range shall be 0.515 to 0.585MT/m3.
Now if we run this pump with this lower density LPG than specified,what are adverse effect on pump?(operation and mechanical point of view)
what is your advice on our direction of thinking.
we are thinking on following points
(1) There may be metal to metal contact at close clearance parts in pump.(vapor pressure of propane(maximum content in LPG) is high, so at pumping temperature(45-50degC),vapor may form at interstage bushes, this vapor has low lubrication property)
(2) There may damage in seal faces.
(30 Problem in NPSHa due high vapor pressure of propane in LPG, this problem may not appear as suction pr. available is 22kg/cm2.
Pump is multistage(8 stages ,back to back impellers), Pumping temperature is around 45-50DegC.Suction Pr=22bar,discharge Pr=65 bar.

#2 breizh

breizh

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Posted 26 September 2009 - 09:46 PM

You may find answers to your query :
http://www.gouldspum...at_pf_0001.html
regards
Breizh

#3

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Posted 30 September 2009 - 06:12 AM

thanks breizh!!For your inputs
I have gone through all literature available at gouldspump, I could not find specific answer to my problem.Our pumps are 8 stage horizontal pumps (balancing by back to back impeller).Mechanical seal double(tandem) with API plan 52 + plan 11.( buffer fluid is mineral oil for seal). operation parameter are as: Fluid LPG(propane+butane),density=0.500-0.510MT/m3,Suction pressure=20-24Kg/cm2,Discharge pressure=64-67Kg/cm2.Pumping temperature=45-50 DegC,Now propane content in LPG has been increased after upstream process change, so density of LPG decreased to 0.500 MT/m3 which is less than OEM specification.Vapor pressure of propane at pumping temperature is about 13-16 Kg/cm2. So NPSHa=22(P suction)-16=6Kg/cm2, so NPSH should not be problem in this case.I have read in article (cheresources)that after entering in pump pressure further reduces and velocity increases at eye of impeller. At our pump suction gauge pressure shows 22 kg/cm2,then at impeller eye pressure may be less than this, may equal vapor pressure of LPG at pumping temperature.
Second thing in our mechanical seal suction side and discharge side seal box pressure is around 25-30 kg/cm2, changes as per flow rate changes of pipeline(with help of VFD).In seal box pressure may equal vapor pressure of LPG(13-15 kg/cm2 at 50 degC) at temperature of pumping & seal faces may run dry...so there is possibility of seal running dry.
Third is metal to contact at interstage bushes, throttle bushes...reason is same at high temperature LPG vapor pressure equals to pressure at particular part in pump.This may not happen in high pressure stages but chances are there in stating stages.
Forth,due to low density,work done by pump will be low which in turn results to low efficiency of pump.
any advice input are highly appreciated.




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