What is recommended practice for venting the SC in a gas seal design?

[HoustonMechEngr]

I know there is a common problem with nitrogen accumulation, particularly in vertical stand-by pumps with gas seals using nitrogen as the barrier gas. Over time the nitrogen accumulation can lead to liquid displacement from the eye of the impeller and a rough start-up that often requires operators to block in the nitrogen gas supply to the dual gas seal assembly (resulting in premture failure).

Is the recommended mode of venting simply taking a line from the seal chamber back to the suction riser or vapor space in the suction vessel? I know some people choose to simply run a simple plan 13 but how effective is it in ensuring all gas vapors are evacuated from the impeller area? On the line back to the suction riser/vessel, I would think a simple block valve is required and is left open while in standby mode correct?

 

[bk19702]

You are correct in that the preferred method is to take a line from the seal chamber back to a suction riser or vapor space. I have run gas seals on vertical in-line caustic pumps where each seal chamber had a circ line back to the caustic tank for N2 venting. In that case I took things one step further by maintaining the standby pump with the suction valve closed and discharge check-valve bypass line open so that a slip-stream of product was constantly fed through the standby pump from the pump in service, subsequently forcing any N2 vapors out of the cavity as well.

Vapor space of the suction is optimal, but if you set up a vent line with isolation valve as you described, taken from the seal chamber to a point outside of the suction valve of the pump, you should be fine. As stated, this is more of an issue on in-line vertical pumps, but can occur on horizontally mounted pumps at low flow rates.

 

[HoustonMechEngr]

bk19702,

When you say "point outside of the suction valve of the pump", are you suggesting a tie-in point upstream or downstream of the suction valve?

 

[bk19702]

Preferably upstream - you want to take the vent as far away from the pump as you can. It depends on how you piping is installed and what would be accessible for tie in.