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[Learn Syan]

As per Appendix E - Net Positive Suction Head Versus Net Positive Inlet Pressure of API Std 675 - 1994 (page 37), NPIP (Net Positive Inlet Pressure) Available is determined as follows:

NPIP Available = Pa + Pz - Pf - Pvp - Pamax

Where:

Pa = Absolute pressure at surface of liquid in kPa absolute (psia)
Pz = Static Head (+) or Static Lift (-) in kPa (psi)
Pf = Inlet line, valve & fitting friction losses at maximum viscosity in kPa (psi)
Pvp = Fluid vapor pressure or gas dissolution pressure in kPa absolute (psia)
Pamax = Pressure loss due to acceleration head in kPa (psi) (Note 1)

Note:
1. Applicable for controlled volume pump to account for effects of system acceleration head.

However, based on article "Difficulty of applying traditional NPSH concept to metering pumps" written by Patrick Deniau (Business Development Manager, Milton Roy Europe), "Vapour pressure is not to be considered here (in the context of metering pump), as the pump does not differentiate if the pressure is coming from a liquid or a gas." Refer to page 3 of attached pdf file.

Based on basic chemical engineering knowledge, this statement contradicts to my understanding of NPSHA calculation wherein vapour pressure has to be considered either for centrifugal pump or reciprocating pump, except with the additional requirement of acceleration head loss calculation when dealing with reciprocating pump NPSHA calculation. This understanding is also in line with the NPIP Available formula from API Std 675 as mentioned earlier.

I wonder if anyone can elaborate the validity of above statement made by Patrick Deniau (July 2009).

Appreciate if anyone can shed some light on any references for which vapour pressure can be excluded in NPSHA calculation for metering pump applications.

Attached Files

•  difficulty-of-applying-traditional-npsh-concept-to-metering-pumps1.pdf   271.23KB   234 downloads