Plate & Frame HX rupture disk sizing 1

[mjcook]

I'm trying to size a rupture disk for the hot side of a plate and frame heat exchanger. The fluid can potentially be trapped in the heat exchanger by actuated valves on the inlet and outlet. What should I use as the wetted surface area for the fire sizing case? I was considering treating the exchanger as a simple block and using the entire area to be conservative, but since the surface is irregular, the effective surface area may be even larger than that, right? Anyone have a suggestion? Thanks.

 

[Montemayor]

If this deals with a true plate & frame heat exchanger, this can have two answers:

1) In the case of a 100% welded plate unit (no gaskets used - the unit is essentially hermetic): the fire case relates to the EXPOSED SURFACE to the fire - which is a simple case of multiplying the length of the unit by the perimete around the rectangular cross-section.

2) In the case of the gasketed unit (the more common case): you are relying on the wrong safety procedure, in my opinion. I maintain the Fire Case (a pool fire under and around the unit) is an immediate danger not because it will heat up the outside surface and cause thermal expansion of the trapped liquid, but rather the immediate danger is the Achille's Tendon of this type of exchanger (and the reason a process engineer should always be wary of): the normal rubber or plastic gaskets will melt, fuse and fail sooner than the mass of trapped liquid will expand due to heat input. Therefore, the idea that you could expect to have the trapped liquid expand and subsequently vaporize is, at best, a flawed and erroneous supposition. You must design for the worst case scenario - and that is that the gaskets will fuse or melt, spilling the contained liquid to the outside impinging flames. This, then makes the relief valve a superfluous piece of hardware for this case.

I believe you still need a relief for fluid expansion (probably piped away from the area), but what you require for the Fire Case is a Depressurization System - fully automatic or manual upon detection of a fire in the area. I would not put too much faith in the automatic because it can be triggered accidently and cause a unit shut down. The manual system puts a lot of faith on administrative control. This dilemma explains why the gasketed plate & frame unit is never chosen by experienced engineers for a site location where it may undergo a Fire Case. This type of exchanger is simply specified wrong for this type of area. You need a robust and proper type of unit to resist the external flame impingement - a shell & tube, double pipe, or spiral.

I don't understand your concern with the "irregular surface". Are you alluding to the internal heat transfer surface? The heat transfer surface has nothing to do with the fire-exposed surface. The two are totally different.

Art Montemayor

 

[mjcook]

Thanks for your reply Art. I need to clarify my original post. This plate and frame HX is a gasketed unit.

The hot side I am trying to protect from overpressure contains distilled water. The cold side contains a 30% Ethylene Glycol, balance water solution. It is not possible to overpressure the cold side of the unit.

So, even though the contained fluid is non-flammable, and even though the gaskets will likely fail before overpressure, as I understand it I still have to provide overpressure protection for the fire case to meet ASME code.

My irregular surface comment was concerning how the plates extend past the gaskets. I'm wondering if the exchanger surface will act more like a finned surface and provide more heat transfer during a fire than a flat surface would. I didn't want to assume a flat surface and underestimate the amount of heat transfer that would occur in the fire scenario.

 

[Montemayor]

MJ:

Thanks for the basic data. Now I've got the picture. Your application, from the latest info, seems to be rather benign but I agree with your idea of putting in a rupture disc. Today, I would opt for a buckling pin instead of a rupture disc. This device is much more accurate, cheaper, standardized, and can be replaced with more ease and safety. ProtectoSeal of Bensenville, IL has excellent service and know-how.

Because the fluid involved is water, I'd pipe it with only one elbow pointing downward, and into a potential pool fire.

You are sizing for the Fire Case and trying to "size" your relief during a blocked-in scenario. I still maintain that you have a case where thermal expansion is going to dominate - or your built-in "rupture disc" will activate and relieve -- your gaskets on the plate exchanger! But I would protect the exchanger with the bucking pin anyway - it's the proper, engineered solution. Water has tremendous latent heat requirements - probably one of the highest (if not the highest) in your plant environment. Before it starts to vaporize and generate excessive pressure, it will expand and blow your gaskets (literally!).

I do not believe you will ever generate steam in your plate xchanger before the relief device opens under the Fire Case. Of course, even with the buckling pin open, the unit will generate steam if the fire continues around it. All you need do is install a nominal sized buckling pin such that it's choked flow capacity exceeds the steam generation during the post-relief liquid expansion. The area you use should reflect some factor that compensates for the excess metal exposed. This is not going to be a large device; I would double the calculated area. Don't forget, in these type of scenarios the attempt is to be conservative in estimating the worst case. This is not an exact science, but it can be a safe science. Don't fail to document all your calcs, assumptions, and basic data in your PSM procedure.

I hope this helps.
Art Montemayor