ASME B31.3 302.2.4 - Pressure / Temperature Variations in Piping 3

[mechengr12]

I have a question involving ASME B31.3 302.2.4 Allowances for Pressure and Temperature Variations. Does this portion of the code allow for pressure variations in components such as valves? I have a piping system in which in the worst case the pressure may temporarily spike above design and allowable pressure-temperature rating of the valve set forth by ASME B16.34. There is no relieve valve or rupture disc in the system so I can not take into account the allowable 10% given by B16.34. Can I account for this variance in a valve or am I limited to B16.34?

In my case I have a Class 900 Valve at 300F which is rated for 1680psig. Pressure could spike to 1748 psig. Is this allowed by B16.34 without any type of Relieve device in place (2.5.1 Safety Valves, Relief Valves, or Rupture Disc)?

 

[LittleInch]

B16.4 2.5.2 does allow variances in excess of the rating which are the responsibility of the user.

For this level of variance, what I think you need to do is compile a report giving all the relevant details, codes, wording etc and show that in piping terms you remain inside the requirements (note 302.2.4.d) - max 1000 variances over 25 - 30 years?? ) explaining how and why the pressure spike cannot exceed your stated value and noting all the normal 10% / 20% clauses.

Then it is up to you to conclude whether this is satisfactory and submit it to the owner or someone further up the line for review / approval.

My concern would primarily be how you can calculate such a precise overpressure (and not one say 100 psig higher) and why you can't put in some sort of relief valve. This would need to be explained very well in your report / technical note.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mechengr12]

The overpressure is calculated on a worst case scenario which would be very atypical for operation to reach and a few things would have to line up for it to happen. Is it ok to follow the rules of B31.3 302.2.4 for the piping including the valves? A relief valve in this system would be extremely difficult, and costly. Our pressure excursions that we have tracked in the past have never exceeded the maximum allowable rating of the flanges. This is one of those cases where someone identified an extrememly unlucky possibility of overpressure and we want to make sure we can justify it through code.

Thanks again for your help.

 

[LittleInch]

As I said, yes it is "OK", but IMHO you should document this to avoid any issue in the future if this valve just happened to fail for some totally spurious reason then the "overpressure" cannot be blamed for it. Lawyers tend not to see things as engineers do so the answer to the question "did you exceed the stated pressure" would have to be "yes". This way it can be said "Yes, but this was approved by the client / owner / competent engineer".

It sounds like you have all the data available, just gather it together and write it down.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[MJCronin]

Excellent advice LittleInch...... BRAVO !

MJCronin
Sr. Process Engineer