Vapor pressure of Vacuum Tower bottoms for NPSHa calculations
Vapor pressure of Vacuum Tower bottoms for NPSHa calculations
(OP)
Engineers,
we are trying to calculate NPSHa for vacuum tower bottoms pumps at 600°F. We have very limited process data. I have been given vapor pressure as 5 psia and 2.5 psia by the client. This is a difference between 14 and 7 ft of head for NPSHa calculations. According to the client, the vacuum tower is operating at full vacuum and 9-12 ft liquid level above the pump. There is a 1 ft line loss. The vapor pressure will indicate a cavitation or not of the pump.
I would like to someone to offer a typical VTB vapor pressure at 600°F or indicate a table that I can reference.
we are trying to calculate NPSHa for vacuum tower bottoms pumps at 600°F. We have very limited process data. I have been given vapor pressure as 5 psia and 2.5 psia by the client. This is a difference between 14 and 7 ft of head for NPSHa calculations. According to the client, the vacuum tower is operating at full vacuum and 9-12 ft liquid level above the pump. There is a 1 ft line loss. The vapor pressure will indicate a cavitation or not of the pump.
I would like to someone to offer a typical VTB vapor pressure at 600°F or indicate a table that I can reference.





RE: Vapor pressure of Vacuum Tower bottoms for NPSHa calculations
It seems unlikely that your tower could be so low to the ground to have only 9-12 feet of elevation at normal liquid level. Are you defining the elevation at Low-Low level in the tower, bottom tangent, bottom nozzle or some other level? I would suggest that you not design for a normal conservative lowest possible level in the tower. Look at normal liquid level and balance that with the true variability of level. If you are too conservative with the assumed minimum level, you are going to end up with a pump that is extremely large and slow or has a very, very high suction energy.
Johnny Pellin
RE: Vapor pressure of Vacuum Tower bottoms for NPSHa calculations
As I think about it, I do agree with your company's practice to not consider the tower pressure versus vapor pressure in NPSHa calculation. After all, since it is a very low pressure the fluid will be above the bubble point in the tower. Any vaporization will be in the tower just like it is designed.
NPSHa is calculated as as absolute tower pressure - vapor pressure + static head - line loss. Therefore, 0 - 0 + 9 - 1 = 8 ft NPSHa. The existing pumps have 6 ft NPSHr and that must be why they are not cavitating. The new pumps that the vendor quoted have an NPSHr of 9 ft. I need to find another pump vendor or reconsider the pump.
If anyone else has any comments, I am happy to read and consider them.
RE: Vapor pressure of Vacuum Tower bottoms for NPSHa calculations
RE: Vapor pressure of Vacuum Tower bottoms for NPSHa calculations
IMHO a bottom quench, by circulating back some of the cooled vacuum bottoms' stream, could help to ensure proper pump operation.