Sealing NGL in a Reciprocating Pump
Sealing NGL in a Reciprocating Pump
(OP)
We are evaluating reciprocating pumps in NGL service. However, there is an inherent issue with sealing these pumps especially when we have a suction pressure in the 300psi range. Basically, we cannot perfectly seal them (as far as I know).
Interestingly, the pump vendor is recommending fabrication of a "distance piece" that will cover and seal the cradle area in order to capture natural gas that gets past the pumps' packing.
For me, this threw up red flags. Why would someone recommend capturing an explosive gas in an enclosed area? What if something came loose from the pumps' push rods, and caused a small spark in the cradle area? Further, if we contain more than a few pounds of pressure in the cradle area, wouldn't it push in the pump's crank case as well? Are there any regulations against this?
Unfortunately, I do not have nitrogen available to flush the area to keep it clear of natural gas. Does anyone have an experience with a better solution?
I appreciate your time.
Interestingly, the pump vendor is recommending fabrication of a "distance piece" that will cover and seal the cradle area in order to capture natural gas that gets past the pumps' packing.
For me, this threw up red flags. Why would someone recommend capturing an explosive gas in an enclosed area? What if something came loose from the pumps' push rods, and caused a small spark in the cradle area? Further, if we contain more than a few pounds of pressure in the cradle area, wouldn't it push in the pump's crank case as well? Are there any regulations against this?
Unfortunately, I do not have nitrogen available to flush the area to keep it clear of natural gas. Does anyone have an experience with a better solution?
I appreciate your time.





RE: Sealing NGL in a Reciprocating Pump
What can be done? Liquid gas leaking over the plunger seals must be avoided under any circumstance unless you can live with that - and I guess you can't. The manufacturer can provide a plunger seal flushing system which is pressurized. But that means that you must find a liquid which does not "contaminate" you NGL. The amount of flushing fluid that contaminates the NGL would be very, very small but it would be there.
The other idea is a diaphragm pump which is hermetically sealed but it will be difficult to find one for that application. LEWA might be worth a try.
RE: Sealing NGL in a Reciprocating Pump
and 2) install an automated method of detecting a packing rupture (to shutdown and vent the unit) such as a pressure switch set at a low pressure (say 1 to 2 psi), or a flow switch in the vent line.
RE: Sealing NGL in a Reciprocating Pump
RE: Sealing NGL in a Reciprocating Pump
Eventually they were fixed by replacing them with API BB3 centrifugal pumps.
The unfortunate part about recip pumps is that they are so good at a wide range of flows in a fixed head situation. Process engineers forget that packing is a terrible sealing solution and spec them out for projects all the time, to my frustration.
We have used vapor recovery systems, but you have to design them to be statically sealed or vented to an appropriate place. Other ideas like Plan 53 style barrier systems have never had much success in our machines.
RE: Sealing NGL in a Reciprocating Pump