6 replies to this topic

[KGPTE]

Hello All,

We are having cavitation issues with our sour water stripper reflux pump. The available NPSH is sufficiently (12 feet) higher than the required NPSH (5 feet) at 180°F. However we are currently operating at a higher temperature of 225°F and I am trying to determine the vapour pressure of the pumped liquid for the NPSH calculations. Can anyone help me determine this value? Not having readily available tables, I thought about using PROII with GPSA thermodynamic model, but not sure if it is accurately taking into account the H2S + NH3 equilibrium - can I use a simple flash vessel at bubble point, and vary inlet pressure until the outlet liquid comes out at 225°F?

Composition of the fluid is: 1.5% H2S, 2% NH3, 96.5% water; all mol%.

Appreciate the help.

[ankur2061]

Hello All,

We are having cavitation issues with our sour water stripper reflux pump. The available NPSH is sufficiently (12 feet) higher than the required NPSH (5 feet) at 180°F. However we are currently operating at a higher temperature of 225°F and I am trying to determine the vapour pressure of the pumped liquid for the NPSH calculations. Can anyone help me determine this value? Not having readily available tables, I thought about using PROII with GPSA thermodynamic model, but not sure if it is accurately taking into account the H2S + NH3 equilibrium - can I use a simple flash vessel at bubble point, and vary inlet pressure until the outlet liquid comes out at 225°F?

Composition of the fluid is: 1.5% H2S, 2% NH3, 96.5% water; all mol%.

Appreciate the help.

KGTPE,

Considering Sour-PR package in HYSYS (recommended package for simulation of sour water stripper), the vapor pressure corresponding to 225 F for the given composition is 4.13 bar (abs).

Hope this helps.

Regards,
Ankur.

[Zauberberg]

Why do you operate the reflux drum at such a high temperature?

[KGPTE]

Thanks Ankur, I got the same result using PROII so looks like that works.

Zauberberg, not by intent, we're having issues with our overhead cooler.

[agoragb]

Being 96.5% water, at 215F, it is already boiling. Ammonia & H2S are much lighter. A pressure gauge at the pump suction will help identify the actual NPSHA.

[Skyline]

You can use HYSYS simulation program to determine the vapor pressure. Set the vapor fraction as 0 at your operating temperature, with the composition as you mentioned before. Hope it helps.

[joerd]

This all being said, the vapor pressure cancels out for the NPSH calculation, or am I missing something? The pressure in the reflux drum is the bubble point pressure, and the NPSHa at the pump is Pdrum + Pstatic - Pdrop - Pvap, where
Pdrum = Pvap = vapor pressure of the reflux stream
Pstatic = static head
Pdrop = friction loss in the suction pipe
So, the NPSHa = Pstatic - Pdrop and your cavitation issues likely result from excessive inlet line pressure drop. Either you are having a lot more flow than designed, or a restriction in the inlet line. I am not sure what the temperature does to NPSHr for the pump, that may also play a role.