Fire sizing RDs for a non-flammable 1

[Korichnevijgigant]

Got a quick question.

Where I work we have large quantities of Nitrogen Tetroxide in different vessels. In the past we had been required to have fire protection on these vessels.
But we have a new Code engineer and Pressure systems manager, both of whom are pushing to have these relief devices removed because NTO is not flammable. A few engineers have been trying to push back since the boiling point of NTO is 70.1 F (294.35K) and we think that because of this low boiling point the vessel could be over-pressurized easily.

Any thoughts?
Thanks

 

[Duwe6]

Is the vessel rated for significant pressure? If so, will it withstand the pressure of 250-450°F NTO vapor pressure?

Is it located where there would be significant property/equipment damage if it catastrophically ruptured?

If you can keep the vessel cool during the largest creditable fire case, and if the vessel can withstand the pressure of a bulk liquid temperature of whatever that fire heat input calc's out to be, maybe the R/D is unnecessary. By 'cool' I mean a water curtain/fog that envelops the tank. Think automatic sprinkler system with the proper backups for the firewater pump.

But since you posted in API Storage Tanks, I suspect that the vessel won't take the pressure.

Why the heartburn over a R/D? You only need to change them out every 5 years, so it is a minor expense that provides a lot of safety. As I've mentioned elsewhere, get your insurance underwriter involved if your Eng tries to get rid of the R/D without a compelling engineering basis that fully addresses the above. When your Boardroom sees the 'bump' in insurance $$ this safety degradation should cost, a R/D every 5 years will look cheap.

 

[Korichnevijgigant]

The vessel is rated for 300psi and can hold up to 6000 gallons. It is located where it could cause significant equipment damage and possible personnel injury/death. Its about 100ft from a technician building.

The vessel has a "sprinkler" system, but it is not automatic, it must be turned on by our fire department if they see the warning lights.

The problem is that the PSM has the final say, and almost all the managers blindly follow what the Pressure systems group says. And in most cases the RDs here are never changed unless they burst. BUT we have maybe 20 cycles per year.

 

[don1980]

This is usually a pretty easy decision to make. If the vessel contents are not a potential source of fire, then check to see if there's anything in the area that could spill and cause a pool fire. If a conservative assessment concludes there's no credible risk of pool fire, then size the relief device for something other than fire.

BTW, I assume these guys are suggesting you stop sizing the relief device for fire, rather than suggesting you remove all relief protection. If so, their suggestion is perfectly reasonable.

Before doing anything however, consider this. It's unusual to install a rupture disk on a pressure vessel like this - it's normally much better to use a safety valve. I'm not familiar with this fluid, so I can't say for sure, but this looks like a fluid that might have a risk of auto-reactivity (always be very careful and suspicious of nitrated compounds). Rupture disk are frequently used when there's a risk of reactivity. That's because they open big and open fast, and they can easily handle the 2-phase flow that often results from reactive relief cases. So, check to see if those disks were installed to protect this vessel from a reactive relief scenario. Usually, reactive relief scenarios are initiated by fire, but that not always the case.

 

[rutherford703]

all pressure vessel designed to the ASME Code Section VIII must be protected by pressure relieve devices unless the design pressure can never be reached.

Equipment in a plant area handling flamlable or combustible fluids is subject to potential exposure from an external fire, which may lead to overpressure resulting from vaporization of contained liquids. The hazard may exist even in terms of equipment containing nonflammable materials. The equipment must be protected with pressure relief valve.

The question is "Is your plant area a flammable environment?". If the answer is yes, the pressure relief valve shall not be removed.

 

[Korichnevijgigant]

@don1980
"BTW, I assume these guys are suggesting you stop sizing the relief device for fire, rather than suggesting you remove all relief protection. If so, their suggestion is perfectly reasonable."

No they are wanting us to remove all relief protection, the GN2 and GHe source protection relief valves are upstream of the check valves, so basically the only relief valves in the NTO system are for liquid lock.

As for the reason they use an RD and not a relief valve is because of the boiling point of the fluid (70f) and that it has an OSHA Ceiling PEL of 5PPM. So when we remove the RV for servicing it basically fogs the area with vapors.

@rutherford703
I wouldnt think the area where this propellant tanks are as flammable, HOWEVER they are coated with lots of old insulation that nobody knows exactly what it is, the technicians regularly drive their trucks in front of the vessel and a mere 60ft away is an equal size container full of Monomethylhydrazine (which is hypergolic with NTO) so if you are thinking conservatively or worst case I would say most definitely there is some heat sources

 

[TD2K]

To reach your design pressure, the nitrogen tetraoxide has to get just over 100C. Is a fire the only way of getting this fluid this hot?

http://encyclopedia.airliquide.com/...sureGraph/Nitrogen_dioxide_Vapor_Pressure.GIF

< BUT we have maybe 20 cycles per year >

What do you mean by this?

This looks like fairly nasty stuff

http://www.praxair.com/praxair.nsf/0/21dc98a7edd0bcbf85256a860081e83e/$FILE/p4633e.pdf

< The vessel is rated for 300psi and can hold up to 6000 gallons. It is located where it could cause significant equipment damage and possible personnel injury/death. Its about 100ft from a technician building. >


Don makes a good point if there are no sources of combustible or flammeable liquids that could cause a pool fire around the vessel you can make a case for not sizing the relief valves for that case. But I wouldn't want to have NO pressure relief on this vessel. What happens if there is a fire in an adjacent building? You wouldn't have the heat input of a pool fire but I could see the fluid getting up to design pressure. I wouldn't want to be the engineer trying to explain why the pressure relief protection was removed. Yes you have the deluge system but it's manual operation, most of the companies I've done work for would not allow me to take credit for that in terms of overpressure protection.

Maybe a LOPA (layer of protection analysis) might be justified to quantify the risk this tank presents.

 

[Korichnevijgigant]

@TD2K

by the 20 cycles, I mean that the vessel is pressurized for operation. This does not include environmentally induced cycles (in the summer the dT here is about 20-30F)

< But I wouldn't want to have NO pressure relief on this vessel. What happens if there is a fire in an adjacent building? You wouldn't have the heat input of a pool fire but I could see the fluid getting up to design pressure. I wouldn't want to be the engineer trying to explain why the pressure relief protection was removed. Yes you have the deluge system but it's manual operation, most of the companies I've done work for would not allow me to take credit for that in terms of overpressure protection.>

I completely agree, however these guys dont believe in "engineering judgement" and want my team to prove why we need it, and usually when they say that they mean provide wording from some Code. I have shown them it could be overpressured easily due to the low boiling point, but they believe that since they have moved the relief valves upstream to the inert gas pressure sources that they are 100% safe.

 

[Duwe6]

Put a 3-way ball valve on the R/D nozzle. with an L-shaped porting. Now you are either 'looking' at the "A" R/D, partially "looking at both "A" & "B", or only "looking" at "B". Standard relief device setup for items that will not be taken out-of-service.

I currently have this setup on Acetaldehyde, Propionaldehyde, and Liquid Ammonia. The guys can safely remove one device, and the other ["B"] device is on-stream. Safe for them, safe for the vessel. And since it is not impossible to overpressure your NTO vessel, ASME Sect VIII requires an overpressure dsevice. Period.

 

[don1980]

@ Korichnevijgigant - this sounds like a possible misunderstanding. I'm guessing the code engineer is fully aware that you can't just remove a vessel's only relief device just because there are no identifiable causes for overpressure. The sizing basis is up to the owner, but code says to install a relief device on all pressure vessels. There's a special exemption spelled out in paragraph UG-140, which allows the option of protecting a vessel by system design. A hazard analysis review might possibly conclude that this application is an acceptable for UG-140. That's a risk assessment decision for the owner to make, but I'd strongly advise against using UG-140 for a vessel this large. There are some legitimate applications for UG-140, but by default, install a relief device unless there's a real good reason for not doing so. The lack of any credible causes of overpressure, by itself, isn't a good reason to use UG-140.

 

[GreenHawk]

All,

Can you guide me whether pressure safety valves are needed on steam/condensate drums or on steam side of heat exchangers while there are no flammable liquid vessels in the vicinity? Alternatively, does this requirement change when there is a flammable liquid vessel nearby? What is the RAGAGEP on this matter?

Thanks,

 

[don1980]

ASME code requires overpressure protection for all ASME pressure vessels. That's RAGAGEP and it applies regardless of whether there are any credible causes of overpressure. The relief device sizing basis is up to the user. If flammable liquids are in the area, and it's not a vapor-filled vessel, then there's justification to size a PSV for fire. If not, then sizing for fire exposure isn't justified, but you still need a relief device to satisfy code.

 

[786392]

With any significant Radiant external heat exposure,

Probably we also have BELVE(Boiling Expanding Liquid Vapor Explosion) potential existing

Definitely there seems to be a need to further explore for the nearby habitats,humans& assets safety.

Best Regards
Qalander(Chem)

 

[psafety]

Now in the States ALL vessels are protected. MAWP only works in the the new code that was previously CC2211.