Backwards Spec Break 3

[zdas04]

I've got a situation where I'm flowing from an ANSI 300 system (PSV set at 600 psig) into an API 3000 wellhead with a potential build-up pressure of 1200 psig if the well is shut in for a couple of days (it is normally less than 40 psig when flowing). I've come across this situation a couple of times before and the solution we used is described in the attachment. No one trusts check valves for isolation, but this approach allows you to verify that the paired checks hold well enough.

The first time I ever used this we did a HazOp on it and the PSM consultant looked at the design and said "that is a legal spec break". We all took him at his word and went on with our lives. We built it and it worked well. We put it on the schedule to be verified when the PSV was tested, and it passed the test every time.

Now I have a client that is saying "why do you say it is a 'legal spec break', is there a document somewhere that defines legal spec breaks and does it include this?". I looked through my library and can't find any requirements for doing anything in particular at a spec break. Reading through all the threads on eng-tips.com most of the discussion is (of course) going from a high pressure rating to a low pressure rating and that requires some sort of positive block (ESD, manual block valve, etc.). The general Interwebz was equally as unhelpful. Going from high MAWP to low MAWP is not the situation I have here, and I'd rather not put an ESD on this line (the ESD valves that this client uses require a person to pump up a hydraulic pressure to open the valve, and it would be tough to pump up the gas and water inlet ESD's and the gas recycle ESD at the same time and if there is a delay the risk of a trip during start up is too high.

Can anyone point me towards something that defines the range of "legal" spec break technologies or is it down to Engineering Judgement which is what my research this morning hints at?

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[BigInch]

If pressure rises very slowly, such that there is virtually no flow, rather flow is so low that it will not swing or completely seal the flapper, or
the pipe settles more on one end, the check is no longer installed precisely level and the swing actually hangs slightly open when there is no/low flow, or
if a rock makes its way into the sealing face of one check, you've only got one check working, and it is plausible that one check can fail...

Since there is nothing you can do, or have to do, to close a check, you can't, by doing nothing at all, actually guarantee that one or both checks are ever actually sealing. You can never say the check is positively sealed when you did nothing. All you can do is cross your fingers.

I hate Windowz 8!!!!

 

[zdas04]

Did you look at the drawing in my OP? You can verify either check very easily. I know that just because a check holds this time does not guarantee that it will work next time, but you could say the same about a PSV or an ESD

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat

 

[BigInch]

You've gotta admit that's comparing apples to oranges.

I hate Windowz 8!!!!

 

[zdas04]

In what way? If I verify that a check valve is not leaking too badly, I have no assurance that it won't get stuck open when I return it to service. If I verify that a PSV lifts at the set pressure and then reseats I have no guarantee that the supply pipe is not plugged with salt. Any verification we do on any safety device has limited scope and limited ability to project into the future, but we live with those limitations and mostly get good enough results.

David Simpson, PE
MuleShoe Engineering

Law is the common force organized to act as an obstacle of injustice Frédéric Bastiat