NPSH for Separator Water Stream
NPSH for Separator Water Stream
(OP)
I am working on a project to replace the pumps that are used to pump a water stream leaving a three-phase separator. The three phase separator operates at 60 deg C and 111kPaa. I have assumed that the water-rich stream being pumped is at equilibrium with the oil and vapor streams leaving the separator and as such the water stream should be saturated with small amounts of hydrocarbons.
For determining the NPSH available, I would like to know what value should I use for the water-rich stream vapor pressure. Is it reasoable to assume pure water (i.e. Pvp ~ 20kPaa) or is it better to take the vapor pressure equal to the separator operating pressure (i.e. 111kPaa)?
Many thanks
Gregq8
For determining the NPSH available, I would like to know what value should I use for the water-rich stream vapor pressure. Is it reasoable to assume pure water (i.e. Pvp ~ 20kPaa) or is it better to take the vapor pressure equal to the separator operating pressure (i.e. 111kPaa)?
Many thanks
Gregq8





RE: NPSH for Separator Water Stream
what is the source of the 3 phase stream?
did the existing pumps have operating problems?
can you recalculate the available NPSH of the existing pumps? at what level above the pumps suction the separator is located?
regards,
roker
RE: NPSH for Separator Water Stream
The feed to the separator is from several oil wells.
The existing pumps are positive displacement pumps that are fed using charge pumps. We are looking to replace these pumps with higher capacity centrifugal pumps.
The static head available is approx 4m of water whereas the NPSHR is 7.3m. If I assume that the vapor pressure of the water stream to be the surface pressure in the separator then there is insufficient NPSH available for the new pumps.
Since the amount of hydrocarbons dissolved in the water stream is small, it it reasonable to assume the vapor pressure to be that of pure water at the separator temperature?
Gregq8
RE: NPSH for Separator Water Stream
RE: NPSH for Separator Water Stream
RE: NPSH for Separator Water Stream
I agree to use the vapor pressure of water at pumping temperature, by the way a NPSHR of 7.3 m is quite high, did you try to work at lower rpm?
regards,
roker
RE: NPSH for Separator Water Stream
RE: NPSH for Separator Water Stream
The NPSHR does seem rather high, but it would depend on the pump head and flowrate (and hence pump speed) that is required.
RE: NPSH for Separator Water Stream
RE: NPSH for Separator Water Stream
The low low switch would most likely cut the flow alread when the water become more "emulsion" - so that pumping the emulsion wont happen.
I would assume the vapour pressure of water at 60 deg C!
Best regards
Morten
RE: NPSH for Separator Water Stream
RE: NPSH for Separator Water Stream
I found the following papers that describes how to address this issue:
1) Tsai, M.J. "Accounting for Dissolved Gases in Pump Design", Chemical Engineering, July 26, 1982, pp65-69
2) Chen C.C. et al, "Cope with Dissolved Gases in Pump Calculations", Chemical Engineering, October 01, 1993
3) Wood D.W. et al, "Pumping Liquids Loaded with Dissolved Gases", Chemical Engineering, July 01, 1998
The approach that is recommended is to use an 'effective vapor pressure' that lies between the process pressure and the the liquid vapor pressure. The 'effective vapor pressure' is defined as the pump-eye pressure at which the volume of flashed gas is 2.5% of the total volume.
At 60 deg C, the 'effective vapor pressure' calculated using the above approach is approx 42kPaa whereas the vapor pressure of water is ~20kPaa.
Best regards
Gregq8