B31.8 1

[Makun]

Hi, I've a Scraper trap designed by B31.8. I´ve searched in the document if it requires a relief system, but I didn't find it. Is it necessary?

 

[zdas04]

Depends on where you are. Some governments have specifically said that launchers and receivers (which is the more common term for "scrapper trap") must be built to the BPVC, therefore need overpressure protection. Most jurisdictions leave it up to engineering judgement.

In the U.S. I never build launchers and receivers to the BPVC, but engineering judgement says that there is a reasonable chance that the barrels could be isolated while full of liquid. That says to me that I have to install thermal relief.

David

 

[gr2vessels]

You are right David, it is prudent to include a thermal relief valve. Unfortunately, some of the manufacturers take short cuts using piping codes for the design of these pig launchers/receivers for competitive advantages and tend to ignore the fact that the trap exposed to sun can overheat very quickly and the pressure rise rate is staggering. Only the traps built to BPVC have provision (nozzles) for pressure relieving! The ones built to B31.8 don't have spare nozzles for pressure relief, that's scary.
Also, the receiving of the pig is not always followed by depressuring of the trap, you know 'the pig can wait' mentality. Luckily that most of the closure fabricators have protective latches in place to protect the careless operators (I'm thinking of O&G applications, not food or farmaceutical applications).
Cheers,
gr2vessels

 

[zdas04]

You know, I've never seen a manufactured launcher/receiver in the wild. I've designed and built dozens of them, and the ones I design have an excess of nozzles to address the real world problems I've seen. For example, I put a vent on a launcher throat because once I had a leaking barrel isolation valve and every time we shoved the pig into the throat the pressure would build up and spit it back into the barrel section. Now when that happens, we can open the throat vent until the barrel is pressurized, problem solved. When you're putting in 3 extra nozzles, using one for a thermal relief is no problem.

David

 

[chemg]

Hi Guys

I've been checking too the 31.8, but i'm not sure if it's only for gas pig traps (or scraper traps...).

We have a diameter specification for the pipes around the trap, quoting that the kicking pipe must be 4" less than the piping size and the recieving pipe 2" less, but i cannot find any standard that rules it (for hidrocarbon pipes).

The pipe is 12", so the reciever is 10" and the kicking pipe will be 8". The trap will launch/ reciever smart pigs (the longer ones).

Anyone have some idea where to look?

Regards

 

[zdas04]

cheng,
You really should start a new thread for a new question, but your question was nearly close enough to the original question and I'm avoiding work this afternoon.

Design standards for launchers/receivers are a bit thin on the ground, things like kicker/bypass size etc. vary widely from designer to designer.

One thing that you really need to work on is terminology. If you start at the tee and work backwards then:
- The "side valve" comes off the branch of the tee
- The "barrel isolation valve" comes off one of the tee runs (the other goes to the pipeline)
- The "throat" comes off the barrel isolation valve
- The small side of the "reducer" comes off the throat
- The "barrel" comes off the large side of the reducer
- The "closure" comes off the barrel
- The "kicker" (for launchers) or "bypass" (for receivers) enters the barrel (hopefully fairly close to the closure) and connects to the pipeline outside of the side valve.

OK, with terms defined, the side valve, barrel isolation valve, and throat are all the same size as the pipeline (not smaller, think about it, that would be stupid how would the pig get in?). The barrel and closure are the next readily available pipe size larger than the pipeline (e.g., for a 10-inch launcher, the barrel will generally be 12-inch).

With your example, the barrel isolation valve, side valve, throat are 12". Barrel and closure are 16" (they can be 14", but 14" pipe is not generally available). There are a lot of theories as to size of the kicker/bypass, I'm not going to go into all of them. My approach is to stay under 200 ft/sec at normal operating pressure and design flow rate. For a 12-inch with a normal operating pressure of 100 psig, I generally end up with a 6-inch kicker/bypass. At 600 psig (if I'm confident that this pressure will be normal for a long time) then I'll drop that down to 4-inch.

David