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Low Npsh Pumps

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#1 Abdu


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Posted 02 December 2008 - 03:52 AM

Dear all,
I am facing problem for selection of hydrocarbon condensate transfer pumps because of low available NPSH( around 1 m).The flow rate is around 80m3/hr @ 6 barg.Please help in selection of pump for this kind of low available NPSH.
Thanks & regards,

#2 Majid-Process


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Posted 02 December 2008 - 04:23 AM

You can used can type multistage pumps which are very commom for condensate transfer. They use a bowl assembly which increases liquid pressure gradually in several stages and resul in lower NPSHR.


#3 Andrei


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Posted 02 December 2008 - 09:07 AM

There is always a problem to ensure enough npsh for the a tower or separator pumps. And that is because the products are close to the liquid-vapour equilibrium point.
I know only two ways of increasing the npsh available:
1. Increase the elevation difference between the source vessel and the pump axle;
2. Subcool the liquid.
The first one is detrimental to the construction costs, the second to the operating costs.
Most people prefer to raise the elevation of the source vessel. In your case you have to raise the source vessel with at least 2 meters. If your vessel is already built at low elevation, you really are in trouble...
Anyways I do not think you will find a centrifugal pump to require only 1m npsh, the manufacturers are usually comfortable with more than 5 m, but they will live with 3 or 4. Go for the low speed pumps, the higher the speed the higher the npsh required.

Good Luck

#4 KR


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Posted 02 December 2008 - 09:44 AM


In case of low NPSHa, air pump (air operated diaphragm pump) should be okay.

Good luck

#5 latexman


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Posted 02 December 2008 - 12:30 PM


What is the temperature?

#6 Abdu


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Posted 02 December 2008 - 10:21 PM

Dear sir,
Thanks for your responses.
The temperature is about 85 deg celcius.Here the vessel is elevated as possible according to the site,evethough the required NPSH not available.Please suggest the solution.
Thanks & Regards,

#7 Andrei


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Posted 03 December 2008 - 08:57 AM


In my activity usually the equipment elevation is set by the process conditions, and not the other way around. In very special cases, like existing and/or very big vessels the elevation is not set from process requirement considerations.
You may consider installing positive displacement type pumps but probably it will cost you more than a centrifugal pump.

#8 Rama


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Posted 03 December 2008 - 01:34 PM

Dear Abdul,

I had come across a similar problem a couple of years ago, where the lay out which had been finalised long before the “minor” condensate vessel location was decided. (Fluid was Steam condensate at 100o C.)

The NPSHa was very limited. Hence we asked the manufacturers to quote for pumps with the lowest available NPSHr. As rightly pointed out, lower RPM pump would give a better NPSHr, but is also larger and more expensive. The selection of the pump was mainly on the NPSH quoted rather than the price – though the price was not very high.

We got a pump with 1.1 m NPSHR. We also decided to raise the condensate vessel by about 0.5 m, became stingy with the suction line and reduced the fittings by thinking “out of the box” and finally raised the LL (Low level) indication, while keeping the same LL level (Low Low level.) This did reduce the capacity a little, but it was a compromise all had to agree.

A better solution would have been to use a steam pump which uses the stem itself (or air pressure ) to push the condensate out from the vessel. (It uses about 3 kg of steam per 1000 kg of condensate pumped) Please see: http://www.spiraxsar.../TI/p135_04.pdf for one such. There are other manufacturers too. However the client was not quite confident initially to use this “new” technology (for them). Later this was tried out in their old unit and they were coming round to accept it.

I am not sure of the largest capacity that this type of pump can handle, which you might have to investigate.

#9 djack77494


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Posted 04 December 2008 - 02:44 PM

Why not just google search or check technical websites for low NPSH pumps. There are any number of pump manufacturers who offer pumps with low NPSH requirements, including the "big guys" like Goulds, Sulzer, and Durco. There are also more specialized manufacturers like Corken or Blackmer that manufacture pumps needing little NPSH. Their websites would almost surely lead you to pump selections meeting your requirements.

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