Pipeline near Pig Trap station 2

[B.L.Smith]

Dear Friends,
I have two questions regarding pipeline design and supporting:
1- In most cases when a buried pipeline comes from UG to AG and connecting to pig trap, the pipeline supports between insulating joint and pig trap are hold down clamps/straps. I don't understand the reason for using these types of supports. When I analyze the UG pipeline up to the pig trap, it doesn't seem to need to use hold down supports. Although the first support of the pipeline is lift-off this issue does not cause any overstress in the pipeline. Why in most projects these types of supports are used?
2- Is there any reference for the pig trap station layout?

Regards,
B.L.Smith

 

[1503-44]

I suppose you have never seen a large liqiud and pipeline crud slug arrive ahead of a cleaning pig at maybe a little faster velocity than the designers thought possible. You'll wish there were a few more supports and straps.

BTW do not put isolation fittings on the goose necks between pipeline and traps. Put them on the lines running to/from stations, unless you like shorts, leaks and very difficult places to make repairs. Goose necks at launchers can be some of the highest stress locations you can find on a pipeline, esp if near anchors. Keep traps part of the pipeline CP system and isolate all connections to/from trap and pump, compressor, meter stations and the like.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[LittleInch]

B.L.Smith

1) I think "In most cases" is pushing it somewhat. sure, some do and some don't. As Ax1e says, this is the point where you can get some internal material changing direction including the pig itself, any crud in front of it and it can get alarming if the pipe starts to jump around. Hold down straps / supports cost very little.
Ditto any expansion forces might not lead to over stress but look strange and operators / managers don't like it.

If you're getting a lot of movement you might need to look at spring supports to allow for that but still provide support as the temperature changes.

2) NO real reference I know of, but many guides and company standards etc.

The locations and things around them change so much it's difficult to produce a standard layout.

There are a few "rules" worth following
1) Think about what is behind the end of the pig trap. Point it somewhere where a pig flying out won't lead to a catastrophic failure of the plant behind. Sometimes you might need a concrete wall or bund. It's rare, but has happened.
2) Think about the biggest possible intelligent pig and it's insertion / removal trolley that you need behind the door. Can be 8 to 10m needed
3) Install the pig traps flat. This slope nonsense is a joke and buggers up all the piing connections
4) Allow the pig trap to slide. If you anchor one end then all you do is break the support. Piping engineers will hate you, but it's not a vessel, it's still part of a moving pipeline.
5) Barred tees on anything more than 50% of the main pipeline
6) Don't let anyone change the ID of the pipeline all the way into the main barrel (i.e. it's still part of the pipeline)
7) Watch this quite decent simulator

https://www.ppsa-online.com/pig-launcher-simulators

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Different class location for gas so you need a different wall thickness from the typical main line's wall. I wouldn't want to roll a custom pipe for it.

Stamp the closures for BPV and don't forget the door lock-pressure release.

Slopes can be useful if you are designing an automatic pig launcher to hold 4 or 5 ball pigs at a time in the barrel behind a trigger mechanism to release one at a time on demand, but that requires a significant slope.


“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[LittleInch]

Agree with all that.

It's the 1% or 1 degree slope bit I hate - if you want to use spheres then yes you need about 15 degrees + or even vertical, but you very rarely see them nowadays.

Always remember the pig trap isn't a pressure vessel, only the end closure is stamped to ASME VIII.

And don't let the piping / plant guys design a pig trap station - they haven't got a clue.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[B.L.Smith]

Thank you very much, friends,
1- If I'm not mistaken according to ax1e explanation the main reason for using strap supports is due to the pigging force on bends. After reading your comment on this issue I searched several pipeline handbooks, but I couldn't find any handbook that addresses this issue. I think the point you mentioned is so important but I don't know why none of the handbooks don't mention that.

2- LittleInch, when using a concrete wall or bund behind the pig receiver or launcher is necessary? Is there any requirement for that?

 

[1503-44]

Occasionally plant people forget that pig traps are needed on pipelines and they route the pipeline right into the plant coming above ground right in front of their big beautiful golden process units. If a really large slug hits the traps at high velocity it is not unknown to do some significant damage, including launching a pig right into the middle of the processes. Pig traps need to be located on plant boundaries, as far away from process units as possible, allowing space for construction of anchor blocks if necessary, or at least to allow room to bring in smaller inlet/discharge lines with curves and bends allowing flexibility to reduce stress and pipeline movement in the direction of critical equipment.

There is no requirement, but at times it might be a useful method of protecting people and plant property. But I see it as a bad solution to a big routing mistake.

Handbooks only cover the basic stuff and people that write them sometimes don't have much field and operational experience.

Flying pigs clocked at 200+ kph

https://pubmed.ncbi.nlm.nih.gov/15553280/

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[LittleInch]

Also agree with ax1e.

Plenty of designers have never built or operated a pipeline system, far less doing pigging operations. So it doesn't get fed back into the design.

The requirement is that you design something to be as safe as possible. Pigs do fly out of traps from time to time and hence anything behind it could be damaged. So either build a wall or point it somewhere it won't do any damage.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

and .... always open the door while standing on the opposite side of the hinge.

I think operations guys don't have time to write books.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[B.L.Smith]

Thank you very much for your responses. I have three more questions about pig trap systems:
1- As you know pipeline thickness changes depending on location class. What is the allowable value for changing in the pipeline ID which does not affect pigging?
2- What is the proper distance between pig signalers and a pig trap?
3- Is it necessary to place a pedestal/foundation directly under large bore valves(20" and above) for supporting them?

 

[1503-44]

I'm thinking it would be a good idea for Pig traps & launchers to have a warning sign to avoid open them while standing in front, on the hinged side, or within the closure's swing path.

“What I told you was true ... from a certain point of view.” - Obi-Wan Kenobi, "Return of the Jedi"

 

[LittleInch]

I missed the previous post..

1) Allowable change is a pig dependant thing. Typically changes of around 5% in ID are Ok, anything more than that starts to cause problems. Some vendors limit it to a certain no of mm, but this depends on the size of the pipe.

2) Again varies by pig etc, but I would go for min 5D from the pig trap reducer or barred tee to make sure the pig is actually travelling

3) Usually nowadays you don't support the valve itself directly, but the pipe either side. Supporting the valve makes it the support point and changes in pipe movement can cause the valve itself to flex and jam.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.