Plan 72/76 Support For Dry-Running Secondary Seal in Hydrocarbon Service

[HoustonMechEngr]

We have a pipeline application that is utilizing a primary wetted seal design (hydropadded) with a dry-running spiral grooved secondary (non-contacting design). What is a typical set pressure for the Plan 72 N2 source? I would think I have to have a minimum of 5-10 psig above normal flare pressure to ensure flow; however, should we actually be setting our system based on pressure or flow?

Let's say I set the plan 72 @ 20 psig and it translates to a corresponding flow measured is SCFM. Won't this create some back-pressure in my plan 76 line due to the restriction orifice? I wouldn't think the spiral grooved pumping action would be enough to relieve the seal cavity of all pressure so I should have some pressure build-up during normal seal operation right? Of course an upwards spike in pressure would indicate a possible primary leaker.

 

[bk19702]

A typical starting point for the N2 purge rate is 0.5 to 0.6 SCFM, which relates to a flow rate for a 5 PSIG differential pressure across a 0.062" orifice. You can adjust the rate up or down depending on application specifics. Typically, Plan 72 is more flow based as it is being introduced to insure the HC leakage past the primary seal is diluted and routed to containment (flare) in an attempt to mitigate emissions.

You should only see an increase in pressure on the Plan 76 side if the primary seal begins to leak, in which case the pressure isn't relieving fast enough across the Plan 76 orifice / outboard seal faces. I've run a ton of wet / dry tandems in the same configuration as your describing with similar supply pressures on the Plan 72 with no issue. How are you controlling the flow on the panel? I'm assuming with a needle valve, as that was the typical method used. I believe the newer edition panels for API 682 4th edition have an orifice on the Plan 72 panel as the flow control method.

 

[HoustonMechEngr]

Control method is with a needle valve. I've never liked the flow orifice as you are unable to vary the flow rate should you require a higher amount of flow to reduce emissions at the secondary sealing cavity. I guess you could adjust pressure higher to induce more flow, but some secondary dry-running seals (contacting designs like the 48SC from JC) obviously have pressure limitations in the dynamic mode.