Best practice for flange break management (use of finger tight joints) 1

[dean427]

Hello

I apologise in advance for any ignorance here as I am not a mechanical engineer.

I am wondering if there are any industry standards or recognised best practice documents available for flange break management? I will try and articulate the specifics of what I am looking for below...

We had a safety incident on our facility during nitrogen reinstatement testing of a unit. The unit to be tested was isolated from the downstream plant by the use of actuated ball valves. Unfortunately, the ball valves were passing and this allowed nitrogen to flow into the downstream unit which contained a joint which was only finger/hand tight. The joint then failed releasing high pressure nitrogen close to where a team of operators were working.

In my view - all joints which are assembled should be tightened/torqued straight away to prevent the possibility that someone may make the system live and the joint could fail - this is how the infamous Piper Alpha incident started. In our circumstances we did not bring the downstream system under pressure intentionally but the risk of the isolation valves passing was not recognised which led to this incident.

However, I am keen to understand if it is industry best practice to mandate that all joints are immediately torqued after assembly to minimise this risk - it seems like common sense to me following Piper Alpha, but our company procedures do not seem to cover this issue.

Any help or advice is greatly appreciated!

 

[mk3223]

IMO, the N2 leaking is a near miss incident and could be a serious issue in safety. It shows that a single block valve is insufficient for isolating the system safely. As a minimum, consider to use a double block and bleeder or a mechanical blind for the positive isolation.

Suggest to involve the Management, O&M and contractor for a safety root cause investigation, and create a workable process and procedure for the piping testing, which may include the following items as a minimum:

- required mechanical method of the positive isolation for the system testing
- required O&M responsibilities in the testing procedure

 

[racookpe1978]

No.
You cannot mandate that every flange be made up and fully tightened every time. During a long replacement/repair process, the pipe spools may need to be fitted up (for cleanliness, for example), or for future adjustment, or while the next spool (or its mating equipment) is loose to permit the two to be be lifted up into position. Twisting, sliding the final spool into place, cold spring, poor original flanges - any number of dozes of reasons can cause the the joint to be made up temporary. Including also, fitup of the second, third or fourth spool for continued welding and construction and measurement.

Failure was of the leak-by in the valves for the pressure test. Which should generally NOT be pressurized gas, but water specifically because of such problems with contained energy. Now, why was the system at high pressure without initial leak checks of the boundaries?

 

[LittleInch]

Yes there are a number of guides such as the one in the link below.

Most companies will have their own isolation philosophies and permit to work systems for such events, some more robust than others

Also how you could overlook an isolation valve passing is beyond me. That's your issue not the downstream pipework which was not inspected or passed fit for service. Sone sort of double isolation or positive isolation should have been used.

https://www.google.com/url?sa=t&sou...FjAAegQIARAB&usg=AOvVaw3REgnXwk1IK0gZHu9wGtcm

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

 

[dean427]

Thanks very much for the responses, much appreciated.

@mk3223 - Operations did not check the flange break tags on the downstream joints - they assumed they were all torqued and that a single valve isolation was adequate just to get the upstream system up to test pressure. They did not envisage that there could be un-torqued joints downstream.

@racookpe1978 - thank you for your insights. I take on board your point. The reason for the blanks in question was due to an isolation valve being removed for overhaul which was several days in duration. I feel that if any joint is to be left assembled but not torqued that this timescale should be limited. I would initially suggest a maximum of one shift or 12 hours but perhaps I am being too conservative with that? Thoughts? Also, the reason we use nitrogen is that the facility is a natural gas processing facility - we like to avoid water in the system as can cause havoc due to gas hydrates if not thoroughly dried.

@LittleInch - Agreed - passing valves should have been thought about. However, the main issue for me here is that the potential for a finger tight joint was overlooked rather than the fact the isolation may have been inadequate. Thank you for the link. I also had a read of reference 16 of the document - I don't think anything formally covers the interim period between assembly and tightening.

I think the crux of the matter is that the flange break register needs to be thoroughly checked before the introduction of any pressure.

I wonder if there is a convenient way to highlight assembled but not tightened joints more effectively?