Fire PSV for PHE' s

[bth1]

Does anyone have any thoughts on the following...???

For a new offshore project we are providing gasketted plate type heat exchangers for the oil train.

My thoughts are that installing a PSV on the hot side for fire (the cold side is provided with a PSV for thermal relief) is pointless as the gaskets are likely to disintegrate/leak, and thus 'feed the fire', before the relief valve has even had a chance to lift.

Secondly, we are also required to provide SAFE charts as per API 14C. The HX section in 14C is primarily concerned with S&T HX's. If I leave the PSV out on the PHE's hot side, how can I justify this in my SAFE chart??

 

[gedomin]

As far as I know, the contingency of a fire is not commonly the governing case for pressure relief analysis in heat exchangers, as is the case for vessels with important holdup volume, specially vaporizing liquid.
Normal flow piping can provide an escape route for some contingencies, including fire. Check for delta P and overpressure potential.

You have to specify HE design pressure based on the worst process condition, like pump shutoff pressure, etc. Possibly no PR device is needed.

For the case of the cold side, of course you must consider liquid expansion as a contingency. Preventive measures for overpressure could be as simple as avoid unnecessary block valves, or specify car-seal open. Every particular case must be analyzed in detail.

Also, code requirement (for S&Ts) says about 2/3 rule. Design pressure of the low pressure side must be no lower than 2/3 of the high pressure side.

Regarding fire contingency, this has much to do with your fire fighting facilities, rather than mechanical design. Fireproofing might be considered.

Sorry not having API 14C with me right now... not sure what is it.

 

[Latexman]

How do you design/specify the gasket so it will disintegrate/leak at a rate large enough so the PHE does not explode and hurt/kill someone or cause further catastrophe and damage? I have missed that technology in my career. Can you fill us in? Can you give us some credible references? I imagine that is what goes in the SAFE chart. Otherwise, I will stick with sizing a relief device that will do that.

Good luck,
Latexman

 

[gedomin]

Gaskets are specified depending upon process conditions. Catastrophes shall be focused separately.
PR devices have nothing to do with gasket selection.

To mention: Important consideration in plant design is equipment spacing. It has to do with fire events.
Oil industries have their standards regarding equipment spacing, stack arrangement, and so on.
I would invite you to learn more about NFPA (National Fire Protection Agency) regulations.

 

[bth1]

My reference is ICI's 'Process SHE Guide No. 8 Pressure Relief' which states "Plate Exchangers: Fire relief is not usually needed as fluid under pressure will leak through the gaskets (and "feed the fire")".

 

[DavidG14]

The purpose of a relief device is to prevent a pressure vessel from catastrophically rupturing. I don't think a Plate & Frame Heat Exhanger is even a pressure vessel. Does it have an ASME Stamp/Design?

As the plates will leak and prevent a catastrophe, I see no reason to add relief protection for fire. There may be other cases that might require a safety device and you will undoubtedly uncover these duiring a HAZOP.

 

[Latexman]

My company did actual fire testing of gasketed plate and frame heat exchangers and advised they do NOT leak nearly as much as people ass[red]u[/red]me. They mandated relief devices on them.

My first post was a spoof, playing on this knowledge. Why would one rely on a gasket to blow out at the right moment by chance if they could scientifically calculate a relief device when someone's life may depend on it? I just don't understand the logic.

Good luck,
Latexman

 

[Latexman]

DavidG14,

Plate and frame heat exchangers with MAWP > 15 psig have ASME stamps.

Good luck,
Latexman