Vapor pressure of Vacuum Tower bottoms for NPSHa calculations 1

[chemebabak]

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.

 

[JJPellin]

It is difficult to define a vapor pressure for VTB. If it was pure asphalt, it would crack before it would boil. So, vapor pressure would be undefined. But, the vacuum tower is not perfect. There is some amount of slop wax and HVGO in with the tar. The conservative approach would be to define the vapor pressure as the pressure above the liquid level in the tower. Just like most fractionation services, the vapor pressure and the static pressure cancel each other out and all you have left is elevation head less line losses. I checked two out of three of our VTB services and that is how we calculated NPSHa. We did not take any credit for static pressure above vapor pressure. The field for vapor pressure is blank in all of the datasheets for our VTB pumps.

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

 

[chemebabak]

JJ, thank you for your response. The tower and pumps are both existing. The scope of this project is to replace the pumps with new ones since the existing pumps require continuous maintenance. The liquid level is based on historized data, so yes it is 9-12 ft above the pump.

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.

 

[SeanB]

I just wanted to say that I agree with Johnny Pellins comments. I just took a look at a VTB pump that I specified a while back and that was the approach taken - vapor pressure is equal to operating pressure of the tower.

 

[25362]

IMHO a bottom quench, by circulating back some of the cooled vacuum bottoms' stream, could help to ensure proper pump operation.