API Fire Sizing for PSV on S&T HEX Tubeside

[KernOily]

Hi guys. A question here, and your opinion/thoughts/nasty comments/rude gestures wanted. How would you handle fire sizing for a tubeside PSV on a shell and tube HEX? The HEX in question has steam on the shell side and a 50-50 mixture of water and heavy crude oil in the tubes. This PSV is for the tubeside.

It seems to me the API fire sizing equation will not be valid because the shell contains vapor, so the heat transfer from a plant fire will be much delayed in reaching the tubes while the process is online. In my opinion to do this rigorously would require a full-on heat transfer analysis.

What say ye? Thanks! Pete

 

[MortenA]

In the first instance - would a fire cause the fluid in the tubes to be blocked in (closure of ESDV)? Then the worst case sizing may actually be continued steam supply and blocked in tubes.

Bestregards

Morten

 

[pleckner]

This topic has been beaten to death in this and other chemical engineering forums alike. My question to you is why do you need a PSV for the tubes? There is nothing in any Code that requires this. I will use a PSV for thermal relief due to liquid full and blocked-in only as insurance against having to worry about replacing a tube bundle. A small 3/4" x 1" PSV is a lot cheaper than the tubes and there is no downtime.

 

[lizking]

Also, if the steam pressure is high enough, you may need to analyze the tubeside protection for a pinhole leak.

 

[ggordil]

For the tube side of heat exchangers I have always used Shell standards, which are based off of API 521. According to them, heat exchangers what have floating rear end heads are self relieving for vaporization during external fire. Types E, M, and N, are not self relieving. If you can not take credit for floating tube sheet or you have an E, M, or N type exchanger, I suggest that you use the wetted area of your channel to find the heat transferred to the tube side of the exchanger.

A note on blocked in cold side (oil & water) while the hot side (steam) is still flowing i.e. thermal relief. I always recommend checking if the hot side temperature is above the boiling point of the cold side at the MAWP. For example, if the steam is at 350 deg f and the cold side MAWP is 100 psig, in a cooling water exchanger, there is a possibility that vaporization will occur.

 

[KernOily]

Thanks for the replies guys.

pleckner - ASME Sec VIII requires all Coded vessels to be protected from overpressure. The shellside of this HX is already protected by a PSV located back upstream. Due to the configuration of the process, the steam side PSV is unable to protect the tubeside. Therefore, I need a separate tubeside PSV. Sorry for asking a question that has been beat to death here and elsewhere but I DID do a search of the archives and did not find my answer. BTW, I myself have answered a sh*tload of questions posted here in other forums by others, such questions themselves having been 'beat to death'. So please bear with dumb questions from a dumb-diddle ME playing a process engineer on TV.

ggordil - This HX is a type AEU so it does not have a floating head. I will look at using the channel as the wetted area. Thanks for the note re: blocked-in tubeside; I will consider this case. Thanks! Pete

 

[pleckner]

You miss read my intentions. No question is dumb.

And no, the tubes are NOT part of the Code. Heat exchanger tubes are not pressure vessels. You don't stamp the tubes, you stamp the shell and the tubes are a separate entity. I have and will continue to argue this point. You will see that ASME Section VIII (and API) says the protection is from internal failure; I don't know of any other way to interpret this other than the shell is to be protected from some failure in the internals...tubes perhaps?

For other discussions on this topic check out the message board at

http://www.cheresources.com,

Pressure Relief Devices Forum.

 

[CJKruger]

KernOily,

A fairly good question, even if it has been beaten to death. My opinion is:

- You have to look at all the cases in API 520/521, and layer in what ASME wants from you.

- You often have a thermal relief valve on the cold side of your exchanger, except if it is protected by another PSV with no intermediate valve.

- In your case you may also have a tube rupture case.

- If truely your only case on the tube side is the fire case, then I believe a PSV is of no use (partly because the wetted external surface area is very small).

- But to meet ASME requirements, you have to show that for the fire case it is protected by another relief valve somehwere else. However, in this case you can have block valves (that are not locked open) in the relief path (per Appendix M).

 

[Arnieg]

According to the engineering department at API Heat Trasnfer the tubes / tube sheets on the code stamped tube and shell heat exchangers that they manufacture are code. The U-1 form that they provide with the heat exchangers requires the owner to provide relief valve protection on both the shell and tube side. Also all tube repairs must comply with code requirements.

 

[lizking]

Arnieg, that's always been my understanding: both sides of the exchanger are Code stamped and treated as two separate "pressure vessels." i.e. both sides require relief protection per Code.

 

[JoeWong88]

I do agree with Phil explanation (We have discussed this issue in message board at

http://www.cheresources.com,

Pressure Relief Devices Forum...), PSV is not required for tube side IF it is NOT designed and fabricated according to pressure vessel code.

Nevertheless, if your plant is design to comply to API 14C (section A-10), you may have to provide PSV on tube side (subject to safety analysis in API 14C check list).

From practical point of view, although the heat transfer rate is low, do we dare to conclude that there is NO overpressure in tube side ? More over HX leakage is one of the event happen without knowing it occur and you will only know when you shutdown for maintenance ?

Next point would be the inventory associated risk. Although the code does not call for it but from risk point of view and insurance premium perspective, provision of PSV may be a more practical way of handling it. Read more in "Requirement of overpressure protection devices on system design to PIPING code".