Dual/Tandem Seal Piping Plans 1

[milltir]

The API 682 plan 52/53 arrangements for tandem and dual seals respectively show the buffer/barrier fluid inlet (BI)& outlet (BO) connections on the outboard seal with nothing connected to the flush (F) connection on the inboard seal.

Will the buffer/barrier fluid always be able to remove the heat generated from the inboard seal faces or might you expect to see say a plan 11/13 on the inboard seal?

 

[JJPellin]

We have 52/53 piping plans with and without a separate piping plan for the primary seal. It is probably most common to use a plan 11, 21 or 23 on the inner seal. Just as you suggested, if the vapor pressure margin is tight, you still need to carry away as much heat as possible. In other services, there are reasons why we would not want to put flush on the inner seal. In some of those we use a cooling coil in the seal pot to help with extra cooling. It all depends on the process temperature, the vapor pressure margin and even dimensional limitations. In some pumps, there just isn't room to put another flush connection on the seal gland or head.

 

[Milkboy]

..you can also consider using the 'Plan 11' connection to prevent sedimentation of solids (and the like) in the stuffing box if required.
The Plan 52/53 would generally take care of the cooling.
In my experience I have seen over 95% of Plan 52/53 systems *without* the Plan 11.
As JJPellin says, some have it, some dont depandant on the application.



-
Milkboy

 

[longeron]

There are more and more hot services where a standard 52/53 reservoir cannot be sized to deal with the heat load.

In cases like these a Plan 11 will not remove enough heat to help, generally beacuse the process conditions are part of the problem. And again in most of these cases the process fluid precludes the use of a Plan 21 or 23 because the product sets up when cooled significantly. Think Coker or Atmospheric Tower Bottoms pumps.

To effectively deal with these applications your best bet is a Plan 32 with a cool, clean, compatible flush fluid and your Plan 52/3, or a Plan 54 (imagine a lube oil skid to support the seal with filters, pumps, and heat exchangers instead of relying upon a pumping ring within the seal to circulate the barrier fluid)with a heat exchanger and large reservoir sized for the service.

Then you also have to worry about your buffer/barrier fluid. Is it capable of removing enough heat withtout degrading? Many users have resorted to heat transfer fluids because they are some of the few fluids available rated for such temperatures. There are others that use specialty barrier fluids from ExxonMobil, Chevron, or Royal Purple(to name a few big names in barrier fluids), or those who take chances with fluids rated for lower temperatures.

I'd be intested to hear if anyone has had experience with Royal Purple BF 910 or Dowtherm HT.

Then again, you could go old school with a single metal bellows seal with a steam quench. Environmental and Safety concerns are driving more people to double or tandem seals here.

 

[JJPellin]

We use an Exxon product called SYNTURION 6 for high temp barrier fluid. We have had good results. I don't care for a plan 32 in services like coker charge. When we analyzed the flush in our 6 coker charge pumps, we found it was costing us well over $1 million per year to flush those seals. We go to great pains to get all the gas oil out of our coker charge and it is very expensive to put it back in. We plan to convert them to a Plan 54 system as you describe. For a crude bottoms or similar service plan 32 is a better choice since the flush will be recovered at the next step in the process.