Crude Vac Bottoms Sealing 1

[crowsell]

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? Any help would be greatly appreciated.

 

[Milkboy]

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.

HTH

-
Milkboy

 

[stopsunset]

questions:
what is causing the blockage of the barrier fluid you referred to?
what do you mean by " improper operation of the circulator system?"

 

[crowsell]

The circulator unit has a pressure regulator with a very small orifice which tends to plug if any foreign particle finds its way into the system. By "improper operation" I mean Operators not ensuring the circulator is running prior to starting the serviced pump, not ensuring cooling water is open to the reservoir cooling coil, not adjusting the pressure properly, basically human error.

 

[longeron]

I pretty much agree with Milkboy.

I'd skip the Plan 53A for the very reason you mention above. I'd be weary of Plan 53B because if something should happen to the heat exchanger you have to find an accumulator bladder material that would handle some serious heat. There is another Plan 53- 53C that uses a "piston pot" that will automatically track the pressure in the pump- it relys upon F=PA, tubing from the discharge of the pump goes into the top of the pot pressurizing it up- there is a a volume below the piston full of your barrier fluid. The trick is that there is a rod on the side pressurized by the discharge of the pump decreasing the area that pressure acts upon so the barrier fluid is pressurized to a point say 10% below discharge pressure. That should be sufficient enough to be the 30 psig above seal chamber pressure you need. The rest of the system is a deconstructed Plan 53A as you'd normally see- a heat exchanger in the thermosyphon loop and the tubing...

If it were my system- I'd buy the Plan 54 skids. Optimally a Plan 54 is set up for one seal only. That way the location of a leak is quite obvious. If you decide upon a Plan 54 with multiple seals, work with your vendor to design a method to locate an inboard seal failure.

These systems are quite reliable. Most are designed with filters in the return side before the control valve. Most are also designed with some sort of interlock system so that if the pumps should stop, pressure would be maintained in the seals. With proper training of your operators and millwrights things should be quite smooth.

With these kinds of temperatures (think ULSG, ULSD, Cat, and Coker units) and the new emissions requirements you will see much more of the Plan 54.

 

[JJPellin]

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.