Fire Case for Flammable Refrigerants? 1

[jrthomas]

I am currently working with a client to document and review existing pressure-relieving devices per API standards. This particular plant manufactures a variety of both flammable and non-flammable refrigerants, which are stored in 10,000+ gallon tanks.

My question is this: Should the pressure safety valve on a tank containing flammable refrigerants, such as R-142b (1-Chloro-1,1-Difluoroethane), R-32 (Difluoromethane), or R-1132A (1,1-Difluoroethylene), be sized for fire? Of course, the contents are flammable, but would these refrigerants qualify as “pooling liquids” per API? The normal boiling points for these refrigerants are approximately +14°F, -61°F, and -122°F for R-142b, R-32, and R-1132A, respectively. I have looked through API 521, but get the feeling that Section 5.15 “External Pool Fires” was written mainly for flammable hydrocarbons with relatively high boiling points that easily remain liquid, even at high ambient temperatures.

 

[RaRo]

Is there a chance for having a (self-)burning leak with those refrigerants?
Is there any other plausible source for a pool fire (within a 12m radius for example)?
Does your weather give <14°F temperatures (with a limit, e.g. for more than 10h a year)?
If so, I would study the pressure relief including an external fire scenario.

 

[zdas04]

The analysis needs to be done for the R-142B, but I don't think that it is really credible that a pool of liquid with a 14F boiling point will remain liquid long enough to be a factor in the relief analysis. I could be wrong, but the thermodynamics feels wrong.

The fire case for the others is just not credible.

David

 

[jrthomas]

Thanks for the replies RaRo and zdas04. We have been assuming that there is a chance for a flammable refrigerant leak to occur with subsequent ignition; the main contention has been with the pooling issue. Just to clarify, each refrigerant is stored separately, in nearly pure form. The lowest ambient temperature for the area in the last 10 years was -5°F. It seems safe to go ahead and assume that the R-142b would remain liquid, but as David says, the thermodynamics for R-32 and R-1132A does not feel right. So should the fire case be dismissed for R-32 and R-1132A and protection from a jet fire be considered (prevention of leaks, fireproofing, depressuring, etc.), as in Section 5.16 of API 521, 5th Ed.?

 

[Chance17]

No one takes issue with Fire analysis for butane or propane tanks.
Why are the flammable freons chemicals any different?

 

[jrthomas]

Yes, sizing the PSV on a propane tank for fire is the exact same example that a fellow engineer used when I asked him about this issue-and it makes sense.

At the same time, in this plant, the PSVs on a couple of R-32 tanks (similar to propane in that it is flammable and has a similar boiling point) were sized only a few years ago by a reputable firm for liquid relief due to thermal expansion. Fire case was dismissed due to R-32 not being able to form pooling liquids.

Quick question: If we determine that thermal expansion is the only valid scenario for one of these tanks, is it OK to have an extremely oversized (say, "N" orifice) vapor service valve up there? Would money need to be spent just to take out this "N" valve and put up a 3/4 x 1? Again, it just doesn't "feel right" assigning "thermal expansion" as the only credible scenario for these large tanks.

Ultimately, the client and myself want to do what is right, and want to make sure lives and equipment are protected.

 

[zdas04]

The only time I consider the fire case credible on a propane tank is if there is another source of flamable liquid (e.g., if a gasoline tank could rupture and flow around the propane tank). I don't find it credible that propane could pool outside of containment.

David

 

[jrthomas]

In order to make consistent recommendations to the client concerning required PSV size, would it be good to establish a criteria for performing fire analysis based on the difference between refrigerant boiling point and lowest expected ambient temperature?

Something like: If (Min. Expected Temp.)-(Refrig. BP) < 40°F, Then pooling is likely, so fire analysis should be performed.

If this type of analysis "told" us not to consider fire case for a flammable refrigerant, then maybe we could recommend to the client some of the fire mitigation methods outlined in API 521.

 

[NUJeff]

I would suggest checking out ASHRAE 15 Section 9.7.5(American Society of Heating, Refrigerating and Air-Conditioning Engineers - Saftey Standard for Refrigeration Systems)

Below is an excerpt, but I'd suggest getting a copy of the standard and reading through it.

9.7.5 The minimum required discharge capacity of the pressure-relief device or fusible plug for each pressure vessel shall be determined by the following:

C = fDL

where

C = minimum required discharge capacity of the relief device expressed as mass flow of air, lb/min (kg/s)
D = outside diameter of vessel, ft (m)
L = length of vessel, ft (m)
f = factor dependent upon type of refrigerant (see Table 2 of this standard)(f = 1.0 (0.082) for R-142b, 1.0 (0.082)for R-32, and Not Listed for R-1132A)

Note:
1. When combustible materials are used within 20 ft (6.1 m) of a pressure vessel, multiply the value of f by 2.5.
2. The formula is based on fire conditions. Other heat sources shall be calculated separately.

http://www.inglenookeng.com

 

[zdas04]

NUJeff,
The OP really doesn't have a problem with calculating the PSV capacity, he seems to have that. He's trying to determine credible scenarios so that he can determine the required releif capacity. Specifically, if it is credible for any of these fluids to pool around a pressure vessel at anticipated ambient temperatures. If they can pool, then the pool can catch fire and you have to include a fire case as credible.

David

 

[NUJeff]

zdas04,

Sorry for any confusion. The ASHRAE is not completely clear, but the section referenced is telling the user what the minimum capacity (required capacity)of a relief device must be for the different types of refrigerant based on vessel size. This is not an uncommon approach to sizing fire cases as 29CFR.1910.119 (NGL Storage)outlines a minimum PSV requirement for fire as well based on surface area and these are vessels containing propane).

At the end of the day, if you believe the ASHRAE standard applies (my experience tells me it does), then you don't have to decide if fire is credible, the standard tells you it is credible (See note 2 in original post) and this is your relief rate.

- Jeff

Jeff

 

[zdas04]

From the size of the operation, it looks bulk-industrial rather than end-user to me. The ASHRAE stuff is end-user (even if the end user is a company). For bulk-industrial you have to develop possible scenarios, determine which are credible, and estimate required flow rates for the credible scenarios. Once you have a maximum credible flow rate, then you can use whatever PSV calculation is appropriate for the device and imposed backpressure the valve will see. The OP is trying to do the second step, not the fourth. The ASHRAE calc may be the right one for step 4, I don't know, but he isn't there yet.

David

 

[jrthomas]

I must admit that I am not familiar with ASHRAE standards but, as zdas04 points out, they seem to be geared toward end-users of heating and refrigeration equipment and especially those designing such equipment.

In our case, the client is manufacturing and storing refrigerants on-site. Due to the nature of this process, we are sizing all PSVs based on equations found in API 520 and API 521. API 521 describes itself as an "International Standard," and goes on to say that "Although intended for use primarily in oil refineries, it is also applicable to petrochemical facilities, gas plants, LNG facilities and oil and gas production facilities." I doubt that ASHRAE standards are meant to be applied to the process side of these types of facilities, but I could be wrong. In a similar way, API 520 is not meant to be applied to fired vessels (boilers), but leaves them to ASME.

On a side note, the client does have several "packaged" refrigeration units in use within the plant, and I would not be surprised if the designers of these units sized the PSVs based on ASHRAE, and not API, standards.

 

[MortenA]

I know that you can disregard fire if your vessel is elevated a certain distance above the nearest structure. But is it so that if you can argue that a fire could "never" occur at the location of the vessel - then you can disregard the fire scenario? And do you only have to consider the liquids that are stored in the vessels you are considering?

I mean e.g. a truck parked next to the vesels could catch fire. Or some barrels temporarely stored near the vessels containing lube oil or whatever needed somewhere? This has happened before.

Best regards

Morten

 

[jrthomas]

Right, fire can generally be disregarded for wetted surfaces > 25 ft above any structure that can support a pool fire.

And YES, I would say that the fire scenario can be disregarded if you can argue that fire could "never" occur. In the case of especially volatile flammable refrigerants, I believe that the pool fire case can be disregarded, but not the jet fire case. API 521 admits that "a relief device might not prevent vessel failure from jet fire impingement." So I wonder: should the PSV installed on a vessel that is at risk for a jet fire be sized like it would for an "ordinary" pool fire case, or should the next credible scenario in line (e.g. thermal expansion) be used for sizing?

And finally, NO, I believe that you should consider more than just the liquid being stored in the tank (maybe follow a rule similar to that suggested by RaRo in the 2nd post). I guess how far you take this rule is up to you (or maybe your insurance company). A gasoline tank located 10 ft from my extremely volatile, non-pool forming, even non-flammable refrigerant, would mean that both should be sized for pool fire exposure. But, assuming a fuel truck driving through the plant is going to spill and ignite its contents around a tank might be too extreme. Or maybe not. It depends on the situation.

 

[Duwe6]

"It depends on the situation"

And the consequences. A seemingly insignificant 1-in-a-million risk gets pretty significant if the consequences are a dozen or more body bags and over 100 hospitalised. BP proved this point at Texas City.

Get a pro from the company writing insurance on the tank farm. And plan on installing a PRV after the evaluation. Or maybe not -Risk * Consequences may not be deemed sufficiently sufficient to warrant PSV's. Gotta have a pro look over the ENTIRE area.

 

[don1980]

To answer your question, first we must know what type storage tank you're usings. Is it a low-pressure API tank (< 15 psig) or is it a pressure vessel (15 psig or greater). If it's an API tank, and it's located in the US, then OSHA 1910.106 mandates a pressure relief device sized for fire exposure.

If it's a pressure vessel, then the user has to decide whether or not it justifies protection from fire exposure. ASME, API and other coses/standards don't mandate when a vessel must be protected from fire exposure - that's a user decision. I think most engineers don't understand that. That is, they incorrectly think there are mandatory requirements defining when fire exposure must be used as a design basis. Codes/standards provide lots of guidance on how to proceed after the user has decided to consider fire as a sizing scenario, but that initial decision is left entirely to the user. General practice is to protect pressure vessels from fire exposure if they can be exposed to pool fire from a burning liquid that has a flash point of 200F or less, or from any liquid that is operated at a temperature above it's flash point.

Ultimately, the decision is up to the user. I'd advise you to be conservative in evaluating fluids with low BP's. It's true that a fluid with a sub-ambient BP will evaporate quickly,and ordinarily not create a liquid pool, but consider to possibility of a sustained leak. In such a case the cement slab will be greatly cooled, causing an accumulation (and spreading) of liquid. It's true that liquid won't remain a liquid for very long, but if it's burning the net result will be a fire that very closely approximates that of a pool fire.

The fact that a fluid has a sub-ambient BP can be used to justify not sizing the relief device for fire. I've done that in cases where the inventory was low, but I wouldn't make that assumption when the potential inventory is high, such as it is in your case (10000 gal).

In all cases, evaluate the risks and make a decision based on good engineering judgment. Understand that the user is empowered to decide when to consider fire exposure and when not to consider it. Except for storage tanks in the US, that is a user decision. You won't find any statements in ASME, API or other codes/standards that say you must include fire exposure as a sizing basis for your relief device. That decision is intentionally left to the user because each cases is different, and there's no way to make one-size-fit-all statements that apply to all situations.

 

[zdas04]

There is a lot of uber conservative discussion above, and that is not all bad. Every company and every engineer has to evaluate risk tolerance prior to starting a PSV evaluation. I have evaluated scenarios for BP for many years (both as an employee and as a contractor) and I've seen enough evolution in their risk tolerance over the years that I always start the next evaluation with a discussion about how much risk they are willing to accept and document the discussion prior to starting scenario analysis.

As I recall (and I don't have my copies of API 520 and API 521 with me), a jet fire is not a credible PSV scenario since it is reasonable to assume that the jet would burn through a vessel before any fluid changes could reasonably be expected to significantly raise the pressure. In short, pressure relieving devices would not provide any protection in a jet fire. That has always looked like a very reasonable common-sense approach that is often lacking in over-pressure analysis.

David

 

[don1980]

A pool fire can be created by a sudden dump of a vessel's contents, or a leak that can't be isolated. When the fluid is a low BP liquid, the worst case fire risk is a sustained leak that can't be isolated. In the case of say propane, which has a NBP of -45F, it's easy to think that it'll automatically be a jet fire. That's true if the leak is in the vessel's vapor space, but if the leak occurs at the bottom of the tank, or in some section of the liquid-out piping, then what you'll get is a 2-phase flashing stream. For example, when propane is flashed from 75F & 130 psig to the atmosphere, the result is 2-phase stream that is 60% liquid by mass. Of course, that initial liquid will quickly vaporize when it hits the warm cement or ground. But the cement will quickly be cooled by the -45F leaking fluid, and unless the leak is very small, the liquid phase will start to spread. It's still vaporizing quickly, but if the leak is say 5 gpm or more, then there will be continuous layer of liquid propane on the ground. The pool won't spread like that of a gasoline spill, but the net effect is that you can have a fire that looks very much like a pool fire, and a leak from a 10000 gal vessel can continue for a long time.

I'm not saying you should always conservatively assume that leaks from the bottom of a propane vessel will result in pool fire. If the vessel is small, then it's unlikely a 2-phase leak will last long enough to cause overpressure, even if the liquid phase does form a pool. But, I'd suggest not jumping to that conclusion when the inventory is high. Just be careful when you're dealing with vessels that can potentially leak flammable liquid for hours.

A fluid with a +14F NBP will flash to a very low fraction of vapor and high fraction of liquid, and unless the inventory is small, this fluid creates a clear risk of pool fire in my opinion.

The bottom line is that each case should be evaluated based on the details of that specific case.

 

[jrthomas]

Wow, there's some good discussion going on above. If I had to summarize it, I would say, "It's up to the user to decide whether to size a PSV on a pressure vessel for the fire case."

Thankfully, the above posts go beyond just saying this, and give "the user" some things to consider when dealing with flammable refrigerants, such as:
-Refrigerant properties (especially boiling point), ambient conditions (especially temperature), size of inventory, and plot layout, and how these factors influence the likelihood of a pool fire or jet fire.
-Equipment type (API tank, pressure vessel, end-user refrigeration equipment, etc.), and how this influences the specific relief device standard that is used.
-Level of risk deemed acceptable by the user.

I really appreciate all the replies I have gotten concerning this issue, and will move forward with the client with all these things in mind.