Low capacity pumps for regenerated TEG 1

[poli60]

Dear Experts,
I'd like to discuss about the choice of a very low capacity pump in the oil & gas sector.
The pump service is the circulation of Lean TEG: from the bottom of the Regenerator, but already cooled by 35°C below its boiling point. So, the NPSH is not a problem.
The head requested to this pump is 60 m.
Now, the two items that seem challenging:
- the client asks for an API 610 pump;
- the design flow of the pump is 10 lpm (liters per minute) - because the plant has a very low design capacity.
After a first investigation, assuming the API 610 request as not fully applicable:
- the process thinks of a Sundyne pump;
- the machinery specialist proposes a rotary (lobe) pump;
- the operation expert don't like the above solution and proposes a canned (hermetic) pump.
Based on the above, I'm asking for the following:
- expert comments and experiences (pros & cons);
- relative costs of the 3 choices (order of magnitude, of course).
Thank you in advance for any hints you can provide.

 

[georgeverghese]

The lean TEG recirc pump in a typical TEG regen circuit has the lean TEG surge drum as its source. "Lean TEG" in reality has small amounts of volatile condensate contaminant, since TEG is a good solvent for aromatics extracted out of the gas stream at the absorber. So you dont have much by way of NPSHa at this pump, since this lean TEG / condensate mix at the surge drum has a vapor pressure equal to the operating pressure of the surge drum.

Would suggest you find adequate static head to get the required NPSHr for the pump you wish to operate. I wouldnt go anywhere near these notorious high speed Sundynes if you ask me.

 

[poli60]

Thank you for your comment.
Between the surge drum and the pump there is a heat exchanger to cool down the TEG.
Moreover, the surge drum is elevated to take in consideration the effect of the dissolved hydrocarbons.

 

[georgeverghese]

Well, then it is possible the lean TEG /condensate mix would be subcooled at the pump. So if you have sufficient NPSHa by way of subcooling and the static head ( less the frictional losses in piping and HX), my preference would be the helical rotary gear pump for 10 lpm. Less preferred option would be a metering type multihead plunger pump with pulsation dampeners.

There is a risk that you may not be able to take all the credit for the subcooling effect at this HX, if there were to be one or more tube to tubesheet joint leaks in the HX (rich HP TEG leaking into lean LP TEG), especially if the rich TEG is corrosive and the joints are just plain roller expanded. This risk can be possibly be lowered be asking for seal welded tube to tube sheet joints at this HX and with better materials of construction.

 

[poli60]

Thank you for the additional considerations.
I wouldn't like to bother anyone but... I have, again, a couple of questions:
a) You propose a helical gear pump instead of a lobe pump. Could you kindly discuss this preference?
b) The (very expert) operator was against the proposed rotary pump (and in favour of the canned one) because "it could suck down the structured packing" (contained in the above regeneration tower). Could it be a realistic scenario?

 

[georgeverghese]

Its only because I'm more familiar with helical gear pumps in continuous operating service. I dont have any references to lobe pumps in such services. Maybe your rotating machinery engineer does.
Dont see how this rotary pd pump can suck the structured packing in the regen column - the surge drum is floating on the regen column overheads through a pressure balance line, which in turn is connected to the flare gas recovery header or direct to LP flare. So the worst that can happen is reverse flow of LP flare gas into the pump when there is a loss of level in the surge drum. All block valves in the balance line and the flare gas outlet from the regen column would be locked open in normal operation.

 

[poli60]

Thank you, georgeverghese!
I appreciate your helpful comments.
Kind regards.