I have 2 pumps in Crude Vac Bottoms service (750 F) which currently have dual metal bellows seal arrangement with Plan 54 barrier fluid circulation.  We have numerous failures due to either blockage in the seal system causing lack of barrier fluid circulation, or improper operation of the circulator system.  We are considering keeping the Plan 54 with a new circulation unit and shorter, neater tubing runs from unit to seal, or a Plan 53A proposed by another seal manufacturer which would have seal oil pot pressurized with nitrogen blanket.  Problem with the 53A is that I have read articles which talk about the issues of nitrogen absorption into barrier fluid and then release at high temperatures causing foaming.  Can anyone provide some insight into an operator-friendly seal system for this application which has been used successfully elsewhere?

Nitrogen will absorb into the fluid at any pressure but in sealing terms you may use a bladder accumulator at pressures over, say 30 BarG, to avoid a significant amount of absorbtion.

Here you may want to use a Plan 53B at all pressures.
Closed Loop with an Accumulator and Air Fin Coolers.

You will of course be reliant on the integral pumping ring in the seal rather than the forced circulation you have in Plan 54.

Unfortunately I cannot give you a working ref for this system
on your application.

I personally like P54s. Controllable Pressure and Flow and water cooling to boot!
Again, personally I would try to prevent the failures you are having as the P53B would also have it operating issues with misinformed users and the pipe work would have to be spot on as your are relying on thermosyphon.

We have 6 pumps in Vacuum Bottom service. All of them use a pressurized double seal that uses the unit-wide gland oil supply.  In all of our units the gland oil system uses heavy vacuum gas oil.  We take a stream of this gland oil and run it between the double seal.  We regulate the flow with an orifice on the inlet side.  A flow meter and pressure gauge on the outlet side are used to regulate the internal pressure between the seals using a needle valve.  The outlet flow is directed to a reflux line that dumps back into the vacuum tower.  If the pressure is maintained at 30 psi above seal chamber pressure, the initial flow indicated on the flow meter should correspond to the design flow used for the orifice.  If the outlet flow drops, that would indicate a leak in the inner seal and gland oil flowing into the process.  As with any double seal, it is vitally important that the gland oil pressure is always maintained above the seal chamber pressure.  The inner seal is not reverse balanced and will open up on reverse pressure, pushing tar into the gland oil piping.  This assumes that the pump maintain possitive pressure on the sesal when running, which ours do.  An additional flow meter on the inlet flow can be helpful to be certain you detect inner seal leakage.  Loosing gas oil into asphalt is very expensive.

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