2 replies to this topic

[SSH]

Dears,

 

In Glycol dehydration units the circulation pump is normally far from it's fluid's boiling point, but because of existing dissolved gases in rich TEG, even with very low content, e.g. methane, ethane, etc. (totally 0.5 wt%) it's vapor pressure is high enough to come up with a low available NPSH (< 2 m).  Interesting is that by deleting dissolved gases from composition, the NPSHa goes higher than 6m.  I have seen this issue in some major designers documents as well. 

My question is that "are dissolved gases an exception to NPSHa calculations"?

[Art Montemayor]

SSH:

The answer is no, dissolved gases in the suction fluid of a pump are not an exception to the NPSHa value.  There is a negative effect that has to be taken into consideration.

However, fortunately some of the things you are stating are not true for a TEG dehydration unit:

• The dissolved gases to be found in a TEG unit are in the Rich TEG - as you correctly state.  However, Rich TEG is not subject to being pumped in a normal TEG unit.  It is nearly always under very high pressure and needs no pump to travel from the high pressure contactor to the nearly atmospheric pressure stripping column.
• Essentially all dissolved gases in the TEG are removed in the reboiler section - including the stripping gas added downstream of the reboiler.  This is so because of the 400 ^oF reboiler temperature and the insolubility of gases in the TEG at that temperature.

How do you calculate the available NPSHa of the Lean TEG (the fluid being pumped) to result in less than 2 meters?  Please furnish your calculations so we can see how you do this.  I have pumped TEG with piston, gear, and Kimray pumps and never had any problems.  In fact, I never found a need to calculate the NPSHa in TEG units.  All pumps used worked very well and never gave any signs of NPSHa problems.  The Lean TEG has a very low vapor pressure because it is far removed from its boiling point (545 ^oF).

 

[Propacket]

SSH
I guess you are referring to results of Hysys. Yes it shows some methane and ethane in the composition of lean TEG at pump suction. I believe it is a software glitch. If you want to calculate the lean TEG vapor pressure, you should remove the vapors from the TEG composition. As mentioned by Mr.Art, no NPSH problems have been seen in TEG pumps. The company i work for has supplied hundreds of TEG plants and never seen any problems in TEG pumps.