PSV on a Produced Gas Cooler

[valene]

Hi
By code I need a PSV to protect my Air Cooled heat exchanger where Produced Gas flows in the tubes. I did a Fire Gas Expansion case sizing & determine the results are not credible at Pdesign=1,200 kPag for gas composition 98% methane.
I decided to select a 3/4" x 1" thermal relief PSV with Pset=1,200 kPag & Trelief=100 degC with a 0.11 in2 orifice. I believe this would suffice. Am I correct?

 

[Latexman]

So, if the line to and from the cooler had valves closed due to a shutdown and a fire broke out, the MAWP would not be exceeded?

Good luck,
Latexman

 

[dcasto]

If there is a PSV on the compressor you MAY not need a seperate PSV for the cooler. If there is a direct path or a path that can be CSO or locked, then you do not need a seperate PSV. If you have valves that cannot be locked, then you need to look at the fire case. My experience says that it does not take a very large valve to protect a vessel that has only vapors in it.

 

[valene]

Thanks Latexman & dcasto for your reply.
That is correct. The MAWP for the heat exchanger is 1200 kPag. I checked for the scenario where valves are blocked and a fire broke out. In this scenario the tubes will melt before it reaches the MAWP. So I decided to spec only a 3/4"x1" thermal to satisfy code requirement.
There are currently block valves upstream & downstream of the PG Cooler but they are not CSO.

 

[Latexman]

Cool! What's the tube material of construction and what's the fins material of construction?

Good luck,
Latexman

 

[valene]

The tube is made of SA-179 & the Air Cooler fins`are Aluminium.
Cheers

 

[Latexman]

ASTM A 179-88a / ASME SA179 is a seamless, cold-drawn, low-carbon steel. While it is good engineering practice to assume the aluminum fins will melt in a fire, it is NOT good engineering practice to assume carbon steel will melt in a fire. You have a credible fire scenario. You do not have to use the finned area for fire exposure, but you do have to use the tube area.

Good luck,
Latexman

 

[valene]

Thanks Latexman. I did a fire gas expansion earlier with a tube surface area of 32m2 but @ 1,200 kPag the Trelief is like 1990 deg C which led me to conclude Fire case is not a credible scenario.

 

[dcasto]

Wait, you have to assume that the tubes are already at 1200kPag, OR at some elevated pressure that is plausable when the tubes are shutin. If the temperature goes from 30 C (300K) to 600K, the pressure just doubled for a gas in the tubes.

 

[Latexman]

dcasto,

There's the rub. We don't know what the operators are going to do in the future.

Is it reasonable the HX gets blocked in while vented and at atmospheric pressure? Yes.

Is it reasonable the HX gets blocked in while at the normal operating pressure? Probably.

Is it reasonable the HX gets blocked in while at the maximum allowable working pressure? Probably.

Is it reasonable the HX gets blocked in while at the MAWP? Hmmmmmmm . . . not so sure.

Good luck,
Latexman

 

[Latexman]

I meant the 3rd one to say:

Is it reasonable the HX gets blocked in while at the maximum allowable operating pressure? Probably.

Good luck,
Latexman

 

[Latexman]

Starting conditions of the highest anticipated pressure and lowest temperature will result in the lowest sizing temperature and the largest relief requirement, i.e. conservative.

Good luck,
Latexman

 

[dcasto]

I've used an admistrative control on an item like a pig launcher. We installed a carseal and a placard that noted the CSO valve was for thermal protection, operator must open or blowdown pig trap to below 50 psig if CSO valve is to be closed. The risk of a PSV failing at an unattended area that could cause a vapor cloud was higher than an operator breaking a seal and closing off the thermal/fire equalizing line.

 

[CJKruger]

valene,

Refer to Appendix M in ASME to see if you really need a PSV.

In some areas you have to apply to the local authority to use Appendix M. So sometimes it may just be easier to put a 3/4" PSV on the cooler.

My preference is to use Appendix M as much as possible.

If it is an air cooler you may be above 25 ft, in which case you don't have a fire case. If you do have a fire case, Latexman is correct, the fact that the tubes will melt or rupture before relief doesn't change anything (it's in one of the code cases).

 

[valene]

Thanks everyone for your contribution! Much appreciated!
Just to conclude I'll work backwards to see what is the highest operating pressure that the Air Cooler can see in the tubes with the HX block in. Then I'll do a Fire Gas Expansion based on this highest operating pressure although I have to be careful I am not protecting for a double jeopardy situation. My current Produce Gas operating pressure is 150 kPag @ 85C.
Our local provincial Boiler's Branch unfortunately requires us to install a PSV on this Air Cooler.

 

[SeanB]

Valene is there any chance that you are in Wild Rose Country? I remember having this issue before on S & T exchangers where ABSA forced us to install thermal relief valves even though there wasn't a credible situation.

 

[valene]

Good guess SeanB! Yes, this is a ALberta, Canada issue where ABSA wants us to intall a PSV on my Aerial Cooler therefore my inclination to spec a 3/4"x1" thermal on outlet tube side of the Produce Gas.