Gentlemen

I have been asked to investigate whether the wall thickness specified in our Material Spec for the Flare Header (at Design Pressure) is sufficient to prevent the pipe from rupturing under the influence of the short term elevated temperature of a jet fire. The pipe is 16" Sch 30 and the material is A 106-B. The expected temperature of the jet fire is 600°C and the design pressure is 10 BarG.

Clearly this temperatue is beyond the recommended temperature limit of 427°C according to Appendix A of B31.3 which cautions that this material is "not recommended for use with temperatures in excess of 427°C (800°F) since there is the possibility of the carbides in these steels being converted to graphite."

With these parameters in mind I have made two attempts in justifying a considered response.

Firstly I applied the variables to equation (3a) B31.3 Para 304.1.2 and used the miniscule allowable stress of approximately 1ksi (6.895 MPa) at 600°C (1112°F). The result showed the pipe to be of insufficient thickness to contain the pressure under these conditions (not to mention the carburization effect on the metal).

My second attempt was to make reference to B31.3 para 302.3.6 which states "At temperatures greater than 427°C (800°F), as an alternative to the use of 1.33 times the basic allowable stress provided in Table A-1, the allowable stress for occasional loads of short duration, such as surge, extreme wind or earthquake, may be taken as the strength reduction factor times 90% of the yield strength at temperature". Using an allowable stress based on this paragraph showed that the wall thickness is sufficient to withstand the conditions.

My dilemma is self evident in that I now have two opposing answers.

Several questions can be raised here, such as; how long will the pipe be exposed to this scenario? How fast will the crystalline nature of the material be changed such that it's mechanical properties can no longer be relied upon? Will personel have the time to evacuate the vessel before the pipe fails as it surely must if subject to prolonged heat of this nature? How long is a piece of string? However, I would like to pose a question which is more specific than these. I would like to know your opinion on the veracity of the second method I used in the scenario I have outlined. In other words, can the jet fire be regarded as one of the "occasional loads of short duration" such that the application of B31.3 para 302.3.6 can be applied in Eq.(3a)?

Best regards
Alex

my opinion is as follows.

It depends upon whether the jet fire is a condition expected during service, of if the design check is a "just in case" type of check. See 301.2.1. If it is the latter, you should set the criteria as part of the engineering design.

Thanks for taking the time to answer me Chuck

Indeed, from the reference 301.2.1 " The most severe condition is that which results in the greatest required component thickness and the highest component rating" and it would appear that the jet fire may be construed as an event that could take place during normal service. However fires of any kind on board vessels such as these tend to be sufficient reason for a total shut-down or evacuation and from that aspect the jet fire shouldn't be considered part of 'normal' service. It is certainly "severe" and therefore the component thickness should be designed accordingly.

But, further reference is made to para. 302.2.4 in regard to design pressure considerations and the pipe in question certainly seems to qualify for “Allowances for Pressure and Temperature Variations.” According to this paragraph “The most severe coincident pressure and temperature shall determine the design conditions unless all of the following criteria are met” and there follows a list of these criteria. So, having met all the criteria, am I to presume that this jet-fire condition can safely be described as something else which does not come under the heading and details of “the most severe condition” and therefore does not qualify as being part of the design equations for determining the greatest component thickness under these conditions? If that is so, then would the second method I described in my enquiry yesterday be an acceptable calculation to perform in this instance?

I take your point that a “just in case” calculation should encompass any expected fluctuation expected in normal or severe service, but this is an atypical one-off situation and I am therefore still in two minds about concluding this topic simply by doubling the wall thickness of a 16” flare header for an incident that may never happen.

Your further consideration of these points would be most appreciated.

Regards
Alex

Alex, a couple of thoughts

1. I know our local reglatory body takes a dim view of the 302.2.4 section unless there is a tracking system in place... and if you go over the limits in 302.2.4 then do you shut the plant down...may or may not be an issue

2. Just out of curiousity, if you think about all the time spent proving your point (either way) and getting approvals (owner approval above), is it cheaper to do the engineering or to use the thicker wall pipe...

Of course, I don't know what the answer is for your situation, let us know what you end up doing.

-Shannon

The decision has to be made, and ultimately it is the owner's responsibility, as to what you are trying to accomplish. I would suspect that a jet fire in your circumstance is like other fire cases, and not part of any design conditions.

If the objective is to make sure that it does not fail under a short term jet fire (and possibly require repair or replacement), then you will need to establish criteria that assure integrity relative to short term high temperature conditions. This can include consideration of yielding, buckling, tensile overload, etc. The occasional load stress limits which you cite should be a conservative criteria, since they are code rules for loads of reasonably short duration. But if you are simply trying to prevent collapse, other criteria, possibly less conservative, may be considered.

Regarding the allowances for variations paragraph, all that is saying is that you may use that paragraph which permits short term variations above the design conditions, if the conditions cited are met.

Chuck, Shannon

I certainly appreciate your comments. Thank you.

The thing of it is that I am representing the owner and it is up to me one way or the other to technically approve/disapprove the current design. There will doubtless be further discussions on this topic in-house and I would tentatively say that time is not particularly an issue at this stage (and what a luxury it is to be able to say that for once Shannon!). The focus is not as much on asset protection as it is on personnel safety in the event of a jet fire. But there are indeed many considerations to be taken into account and I therefore wanted to have the benefit of the wider view of experience that this forum offers in reviewing this topic with me and in providing a certain consensus of considered opinions and interpretations based on the Code. As you say Chuck, ultimately a decision has to be made and with that in mind I definitely intend to make an informed decision, so again thank you for your contributions gentlemen.

Best regards

Alex