Is it necessary to assembly a PSV at the pig launcher&receiver

[PipingX]

Hi guys:
There is a natural gas transmitting pipeline,with the design pressure 10Mp and 26" in diametre.Is it necessary to assembly a PSV at the pig launcher&receiver.
On looking for some project,i found some with PSV and the other without.i dont make sense why.

yours
pipingX

 

[Latexman]

I bet it's because some folks think a pig launcher/receiver is pipe-ular, and some folks think a pig launcher/receiver is vessel-ular.

Good luck,
Latexman

 

[superdaigle]

Latex is correct. It may also be due to the fact that some launcher/receivers are stamped, and some not.

 

[dcasto]

remove the psv and use a check valve around the block valve that isolates the pig launcher. Or just leave the kicker line open all the time.

 

[SNORGY]

I haven't read the American Pipeline Code, but for Canadian installations, the PSV is not required provided that you have overpressure protection in the form of a pressure-limiting system that is completely independent of the pressure control system.

Some folks still do design the barrels to ASME Section VIII Division 1, particularly for automated pig launchers. ASME would probably force the overpressure protection in the form of pressure relief.

Regards,

SNORGY.

 

[rneill]

In Canada, pig barrels are considered to be just part of the overall pipeline system and provided the overall system is adequately protected (as described by Snorgy) then you do not require any separate protection on the pig barrel.

I've seen pig barrels designed to ASME VIII Div 1 (or more usually the closures) but I haven't myself seen one to which the ASME Code stamp has been applied (which would then necessitate a PSV).

 

[PipingX]

Thanks all.the psv should not be adopted.

 

[zdas04]

Whoa there pardner. Before you throw the baby out with the bath water you have to think through whether regardless of code requirements your application needs overpressure protection. If you don't have adequate procedures in place to ensure that you will never isolate a launcher/receiver barrel that is full of liquid then you need thermal relief.

I recently saw a situation where a pig arrived (with a full load of water) about quitting time. The operator didn't want to deal with it that late so he opened the side valve, and shut the barrel isolation and bypass valves. In the morning there was a production meeting, followed by a team lunch. By early afternoon the receiver had drained itself through a 10 inch long split in the barrel. A vessel full of liquid will increase pressure about 100 psig for every 1F increase. Doesn't take much sun to cause a real mess. A tiny PSV would have prevented that. A procedure (which was followed) that prohibited leaving the receiver full and pressurized would have prevented it. One or the other was required even if the code calls it a "pipeline accessory" and doesn't require a code stamp--we're Engineers for god sake, when did we discard our analytical sense in favor of a discussion of which code to blindly follow?

David

 

[SNORGY]

Fair comment, zdas04.

If thermal only, perhaps then the relief valve discharge disposition could be a 1" o-let on the pipeline side of the main barrel isolating valve (routing expanded liquids back into the pipeline). That would get around the need to install a lit flare (for a sour site) or a pop tank near the barrel. What have you done in similar situations?

Regards,

SNORGY.

 

[zdas04]

Exactly that. The concern is the line freezing, but if you make sure the PSV outlet slopes down you can minimize the amount of trapped liquid.

I've also used procedures that prohibited leaving a barrel isolated. I like leaving the kicker/bypass open when not doing pigging operations, but that bothers some folks too. Other people like to leave the barrel vented. This works, but fugitive emissions from a weeping isolation valve can be big.

For the folks who don't trust procedures, a small PSV that exhausts to the process seems to work. The required relieving capacity is teaspoons.

David

 

[rneill]

Typical pigging procedures shouldn't allow leaving a pig barrel isolated, full of liquids and under pressure so this should be a pretty rare incident. Concur that it's not a bad idea to have a thermal relief and that this should be tied back into the pipeline to prevent atmospheric discharges. A small 3/4" TRV is pretty cheap even with a little bit of piping.

To be honest, I don't think I've seen a pig trap with a thermal relief on it before and none of the standard pig trap drawings that I've seen used have shown this detail. Doesn't mean it's right though. Perhaps it hasn't been an issue as with the climate around here, the contents would probably tend to cool rather than heat up.

I believe the Canadian Association of Petroleum Producers is currently working on a guidance document around pigging and pigging procedures. No idea when it might be published though.

I took a quick look at the Norsok Specifications (the Process Systems document) as it has design requirements for pig traps and while it addresses interlocks, vents, drains, pig detectors, etc, it doesn't talk about relief so I'm guessing they are relying on the pigging procedures as well. I also looked up the Oliver Compact Pigging Systems (CPS), a complete packaged solution, and it isn't equipped with thermal relief either.

On balance, I think most people rely on procedures.

 

[msquared48]

With a pig launcher in the syste, I'm assuming this is for a rendering plant?

Mike McCann
MMC Engineering

 

[zdas04]

And I thought you Structual guys were Civil.

 

[BigInch]

A thermal relief is more than a good idea. Its required for any pipe segment that can be blocked in. If you can close that kicker line valve, its blocked in... so you need it.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[SNORGY]

I suppose there are a number of places you could route the thermal PSV to. Presumably, if you're pigging at all for process reasons, there is an inlet separator someplace; otherwise a tank to collect the batch chemical / fluid. There is also, I assume, some form of spill collection basin under the pig closure. Of all the options, I would be inclined to prefer just routing it back into the pipeline to keep the rest of the downstream equipment completely removed from the equation in order to avoid other environmental, operations and maintenance issues.

Regards,

SNORGY.

 

[zdas04]

I was assuming we were talking about a Gas Gathering System pig. The system operates on blood, guts, and feathers so putting the teaspoons of water from a thermal releif back into the line is no big deal. The purpose of running a pig like that is to remove bulk pools of condensed liquid to reduce corrosion risks and/or to improve system pressure drop. The gas gets a lot of processing after the pig receiver (including a plant inlet separator somewhere down the line).

I regularly design systems where all the liquid from a pig run is routed into the downstream piping to be subsequently pigged out of that line. This daisy chain goes on until you reach a line drip or a plant inlet scrubber. The scheme works well.

If we're talking about a liquids batching pig then all the decisions are different.

David

 

[dcasto]

as I said, route the thermal relief to the buried inlet line. We removed all our thermal reliefs on our C2+ lines in the late 1980's because of a failure on one of them laid a vapour cloud across a hiway at 2:00am. The US regulators loved it. Exxon adopted our use of 3/8" check valves too.

 

[BigInch]

Even for gas it can be a good idea in some fire scenarios.

Either a thermal relief, or as dcasto says, a check (with a locked open backup block valve for when the trap is open), going back into the pipeline would be fine, if is a liquid line and there is no oily water drain around, otherwise a drip pan might have to do.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/