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Pumping High Vapor Pressure Hydrocarbon CondensatedHelpful Member! 

CamiloReyes (Mechanical) (OP)
26 Apr 07 13:48
I am having troubles trying to find a pump with a very low NPSH/NPSP that will transfer (or be used as a booster) hydrocarbon condensate with a vapor pressure of 14.46 psi-a, being the local atmospheric pressure 14.5 psi; from an atmospheric tank to a 100 psig transfer line.

Does any one have a good tip?

thanks

Helpful Member!  Scipio (Mechanical)
26 Apr 07 14:11
Hydrocarbon condy is always a pain to pump - need a lot more info, just how low is low NPSHR?  I know your vapour pressure has zero margin, but what's the static head & line losses between the tank low level and assumed elevation of the pump suction?

Also, what's your flowrate?  The higher the flowrate, the higher the NPSHR is likely to be of any pump you put into service.
dcasto (Chemical)
26 Apr 07 17:34
You may be mixing pressures here (reid and true).  Draw it out.  You have an atmospheric tank with HC condensate.  So the HC condensate is at vapor pressure in the tank (or it will be as soon as all the light ends boil off) OR is it a Stabilized condensate in a tank in a sub cooled condition?

In any case, You may need a verticle can pump if nothing else to get the head.

check on Afton pumps
CamiloReyes (Mechanical) (OP)
26 Apr 07 18:46
Thanks, this is the missing info:

The flow rate is aprox 5gpm (batch, so it can be defined a flow of 4 or 6gpm), the NPSHa=0.3ft, keeping a LLL in the tank of 1ft (taking out losses, adding the differential head due to the elevation of the tank)

The Condensate has this properties:
Sg: 0.6641
Density: 0.6987 g/cm3 @ 60F
Vapor Press: 14.46 psi-a
it condensates on the line that goes to the flare, and drain to the tank (with water). When there is water on the tank the pump (centrifugal) can work, but as soon as the water level goes down and we start to pump the fluid it flash on the pump suction line.
Scipio (Mechanical)
27 Apr 07 9:28
Yep, that's low NPSH alright.  0.3 feet is nothing, there isn't a low NPSHR pump on the market that'll operate under those conditions.  For one thing I generally wouldn't recommend a pump with a margin less than 0.6 to 1 meter between NPSHA and NPSHR.  In your case that'd mean a vertical canned pump like dcasto suggested, but good luck finding one that can hit that flowrate.  I'm guessing your rated point would be below the minimum flow of the smallest models on the market, but it's worth a try.  In addition to Afton, there's also Goulds Pumps, Sulzer, Flowserve and Weir Floway.

Failing that, if the liquid level can't be raised to give you at least 3' NPSHA, you may need to build some sort of underground well to locate a pump low enough to produce the suction head you need.  You may also want to look at a very slow hydraulic diaphragm pump, I've used them with good results in service similiar to yours (drawing liquids out of hydrocarbon condensate knockouts on natural gas compressor battery inlets), though I had about 3' - 4' of NPSHA to work with in most cases.
CamiloReyes (Mechanical) (OP)
27 Apr 07 10:11
Scipio,

That's right, maybe I have to give them first more NPSH and try again with WILDEN, MRoy, Boc Edwards/Hick Hargreaves, etc. Which brand has you used the most?

Thank you
Scipio (Mechanical)
30 Apr 07 10:46
I've used Wilden and Milton Roy quite a bit, though not in this service.  I wouldn't put Wilden into high vapour pressure service, but the Milton Roy's might work.  I have used hydraulic diaphragm pumps successfully in this kind of service, the last was a Pulsafeeder 7440 that was running close to minimum speed in order to minimize the NPIPR.  It also ran continuously recycling back into the suction tank when not actually dumping to prevent vacuum-locking.  Bear in mind if you're looking at a reciprocating positive displacement pump you'll need to carefully address acceleration head effects on the suction line losses.  Oversize your suction piping, keep the pumps as close to the separator as possible, look at using a suction stabilizer, use full-port block valves, avoid suction strainers if you can, use duplex or triplexed liquid ends to minimize pulsation amplitudes, etc.  

On the centrifugal end, I've also used Dickow Pumpen and Sihi and pumps in low NPSH, low flow service, they have models with NPSHR down as low as 0.5 meters, which is still more than you're looking at right now.  The trick there is their designs normally work of very tight clearance, so they don't work well in a service with any kind of solid contaminants.
timbones (Mechanical)
30 Apr 07 10:58
How big a tank is it? Maybe you could put the tank on a nearby pipe rack? Or is this an already built thing?

You could also look at raising the LLL. Another foot may make all the difference.

Canned suction centrifugal may not be an option given your flow rate is so low.

Another option might be to look at a low speed progressive cavity pump. Something by Moyno.

Tim

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