Mechanical Seals (Pressure Vessel Seal Support Systems)

[shotcalla]

Hello all,

I was wondering how these systems work? Specifically, what heat transfer issues arise when employing such systems?

Many thanks in advance

Mabasa

 

[longeron]

There are 2 basic systems that use reservoirs.

API Plan 52: Unpressurized dual mechanical seals use a reservoir and piping to provide a circuit for fluid between two seals. The reservoir is often connected to a vent or flare header in such a way that leakage from the 1st seal can collect in the reservoir or percolate through the fluid in the reservoir without building significant pressure in the vessel. There is a restriction in the piping that allows some pressure to build so that a pressure gauge or transmitter can indicate leakage.

API Plan 53A/B/C: Pressurized dual mechanical seals use the same circuit. In some cases a reservoir is connected to a Nitrogen header used to pressurized the reservoir above the pressure the 1st seal sees inside the pump. Variations of this system use an accumulator or "piston pot" instead of the reservoir and N2 header.

Heat is always an issue to consider with seals. Especially with dual seals. Many of these systems utilize a heat exchanger within the reservoir or an exchanger piped into the fluid circuit to keep the fluid between the seals cool.

To simplify things- one must consider how hot the pump operates and the heat generated by each of the mechanical seals. Estimates of heat soak and generation can be provided by your vendor.

Keeping the fluid between the seals cool is essential to long seal life.

 

[longeron]

Just noticed something that might be confusing:

In a Plan 52 the restriction I mentioned is between the reservoir and vent or flare header not in the circuit between the reservoir and seal.

More often that not, major seal vendors can also supply these seal support systems and are happy to design one to meet your requirements.

 

[bk19702]

The total heat load needs to be considered, which for a dual pressurized seal is inboard seal face generated heat + outboard seal face generated heat + heat soak; you can incorporate churning effects too if you have a larger seal, but typically churning heat can be neglected. On the unpressurized arrangement, the total heat load is usually the sum of the inboard and outboard seal face generated loads; the amount of soak, if any, will be minimal. Ultimately the magnitude of the heat load and subsequent cooling flow rate required comes down to the amount of heat soak - seals in higher temperature processes with dual pressurized seals will require a fairly substantial flow rate for cooling. This gets triggered by whether or not your reservoir or system is being cooled, which is when the heat soak becomes a factor - especially in a dual pressurized arrangement.

 

[shotcalla]

Thank you for the replies. I have another question:

>How would one model/simulate the heat transfer phenomena occurring, are there any examples of models/simulations you know of?
>I have come across a proprietary numerical coding (C'Stedy), by John Crane Inc.

I have used Fluent CFD code before, but this type of simulation is new to me.

Once again thank you very much in advance

 

[stanier]

http://www.mcnallyinstitute.com

for all about seals you would ever want to know

 

[shotcalla]

Thank you for the link.