Mobile Pig Traps & Key Interlock 3

[jonjose123]

Hi All,

Do we need to provide key interlock for Mobile Pig Traps...

Pig trap Size is 10" x 6".. As per Company only the main line valve and kicker line valve to key interlocked.

This mobile pig trap will be used for 10 flow lines...and there are existing flow lines in the field another 15 not for which a mobile pig trap already in place without Interlocks.

We proposed to client not use interlock for the current 10 traps too, as in case if they want to use current Mobile trap to pig previous lines , will not be in a position to do so as this will have interlocks and the previous ones doesn't have.

Is there any other justification we can present to client to not to provide key interlock for current Mobile traps.

Regards,
Jon

 

[LittleInch]

There are no code or, AFAIK, regulatory requirements for pig trap interlocks.

Therefore the requirement for each system is a company dependent issue.

If the interlock system between the kicker and main line valves is the same standard, I can't see why the same system can't be standardised across the system and added to at least the main door?

However if there are currently other mobile systems without interlocks then not sure what the issue is?

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

 

[zdas04]

The idea of a key interlock system scares the heck out of me. The procedure I recommend for launching pigs is:
[ul]
[li]With the side valve open, the barrel isolation valve shut, kicker valve shut, open barrel vent[/li]
[li]Open closure and load pig into the reduced-diameter section of the launcher barrel, shut closure[/li]
[li]Slowly open kicker and purge air from barrel [time for purge set by launcher configuration on a case by case basis], shut vent and open kicker fully[/li]
[li]Open barrel isolation valve (which has zero dP with the kicker open, so the pig won't move)[/li]
[li]Slowly move side valve towards shut, at some intermediate position the pig will launch[/li]
[li]When the pig sig indicates that the pig has exited the barrel, fully open the side valve, shut the barrel isolation valve, shut the kicker, and vent the barrel[/li]
[/ul]

That procedure always works without damaging pigs or sticking them in the barrel, I don't see how a key interlock would work with that procedure.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[jonjose123]

Hi LittleInch and zdas04,

Thanks for the response.

As client is insisting to provide these interlock sequence, we have have informed them accordingly that sequencing operation will be 100% safe only if you interlock all the associated valves rather than providing only for Kicker line and main line.

Second case : The kicker line has two valves and its not specified which valve of these two to be interlocked.
As normal case it shall be the first valve near to the trap to be provided interlock.

IS there any other reason why we need to provide the interlock on to the first valve of Pig trap.

Regards,
J

 

[LittleInch]

jonjose,

You need to provide a little diagram so we know which valve you're talking about.

I can understand key lock systems, especially if the operation is infrequent and / or the operators are not well trained.

The problem is that some actions require gently opening or closure and others (venting / draining) take an indeterminate period of time and there is no time delay feature in any of the key lock operations.

So two things
I don't understand which valve you're referring to and most importantly
"sequencing operation will be 100% safe " is an impossibility. What you can say is that a key driven sequencing operation can reduce the probability of mal operation to as low as possible, but 100% safe? not with a pig trap - there are too many things to go wrong (blocked drains / vents, lack of valve sealing, jammed valves / doors etc etc)

If you approach a pig trap and see strange bars and poles lying next to it, be very careful. The operators have found new and exiting ways to over stress your door closure / isolation valves....

If you follow the rules / procedures, use well maintained equipment and have no real dirt or debris then all can go very smoothly and safe. Get it wrong and it can have lethal consequences.

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

 

[jonjose123]

Thanks Littleinch,

true that 100% safe operation of pig is impossible.

The sketch of PIG Trap is attached for reference

 

[zdas04]

I'm not a big fan of P&ID for field equipment. You have way too many valves, vents, instrumentation, etc. for practical use. I think that interlocks are the least of your problems. For the purposes of communication here is a very functional launcher/receiver pair (I've built several hundred of this style) with the bits and pieces labeled (notice that I have a vent in the throat near the barrel-isolation valve? The purpose of that vent is if/when the barrel isolation valve leaks I can open the vent while I'm opening the kicker valve to keep the leakage from pushing the pig out of the throat, no need for the "pressure balance line" which provides an excellent opportunity to not launch pigs if it is left open)

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[dik]

Is BRT/536 the pig trap?

Dik

 

[LittleInch]

Personally I really like the pressure balance line.

Like any other system you need to use it properly, but it prevents any differential pressure building up behind the pig in a receiver if the pig has become stuck for some reason as it comes into the trap. This prevents it shooting out at any point.

In a launcher it allows the space in front of the pig to be pressurised the same as behind it and not pushed up against the barrel isolation valve.

If you leave it open when launching then just close it and away the pig goes.

BRT 536 is the Barred Tee.

The pig trap is mobile and connects in at the points marked A and B. Unfortunately the screen grab has ended just a little bit to the left hand side.

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

 

[zdas04]

I inherited several launchers/receivers that had balance lines and people kept leaving them open for launching and the lines were big enough that the pigs wouldn't move until the side valve was all the way shut and the upstream pressure had build 50-100 psi (and the dP through the balance line was more than was needed to launch the pig). I found that they also would often not allow a pig to seat so about 20% of the time the pigs never launched. With the drawings I attached, if you have a leaking barrel isolation valve, you open the vent in the throat, open the kicker until pressure equalizes, shut the vent and open the barrel isolation valve. I want the pig fully engaged in the throat for launching and "pushed into the barrel isolation valve" is a success.

In a receiver, the balance line prevents a slow moving pig from exiting the throat and they can be really difficult to pull out (the stupid rope handles seldom survive the first removal from the throat so the operators tend to open the closure and then "crack" the barrel isolation valve, launching the pig into the weeds).

In the system I operated we ran 20-30 pigs per week, and pretty much every stupid move an operator could make happened sometime in the 13 years I was accountable (most of them repeatedly). What I kept finding was that the more complex the equipment or procedure, the more creative ways the operators could find to screw it up.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[dik]

LittleInch and zdas04... thanks, that clears things up a bit...

Dik

 

[LittleInch]

so the operators tend to open the closure and then "crack" the barrel isolation valve, launching the pig into the weeds

Holy smoke! That's why I think the OP's client is looking to physically prevent something like that happening. Get in the way of that and you're history.

Operators do though find new and inventive ways to "operate" in a way never envisaged by the original designer alright.



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

 

[jonjose123]

Thanks Littleinch and Zdas04 for the clarifications.

As the drawing depicts...the kicker line nozzle N8A which will be connected to Point B... Our main question arises , in any case if we have to provide an interlock for kicker line which valve should be provided the interlock.. I believe it should be the first valve near the nozzle N8A rather than the other valve near point B as it provides trap isolation. Is there any other view onto this.. and Interlocking was asked only for kicker line valve and main line valve...

And I believe providing interlock for only these two valves (Kicker line valve and main line valve) doesn't provide any operational safety...

Regards,
J

 

[LittleInch]

It should be the furtest valve from the trap on the permanent pipework. This allows you a second valve to close if the first one is passing.

Agree though that it is pointless unless the result of closing both valves is required before this action then permits you to do something else like open the pig tap door. If there is no further restriction then why bother closing the valves??? Makes no sense. A key driven system is one which needs a key released from the previous action to permit a follow up action. If there's no follow up action which needs the key then the system is meaningless.

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

 

[jonjose123]

Thanks Littleinch for the clarification..

Yea, as u said... it is meaningless to provide interlock of there is no followup action.

We are trying to get the client convinced regarding the smatter.

Regards,
J