Clarification on interpretation of pressure/temperature excursions (para 302.2.4)

[NovaStark]

Good day all,

I have been looking through some of the threads related to this paragraph as stated in ASME B31.3 for pressure & temperature variations in process piping:

Subject to the owner’s approval, it is permissible to exceed the pressure rating or the allowable stress for pressure design at the temperature of the increased condition by not more than (a) 33% for no more than 10 h at any one time and no more than 100 h/y, or (b) 20% for no more than 50 h at any one time and no more than 500 h/y

Thus say I have a piping system of design conditions 1,000 psig @ 500 degF (A106 Gr.B).

For pressure rating, I assume this means that I can go up to 1.33*1000 psig or 1.2*1000 psig (for the durations stated).

But for the allowable stress, does that mean that I am to find the allowable stress for 106 Gr. B @ 500 degF, then find 1.33* allowable stress then corresponding temperature from the stress table ? OR is it (1-0.33)* allowable stress and then find that temperature ? Or is this interpreted differently ?

 

[mk3223]

It's correct for the exceeding design pressure part, i.e. 1.33*1000 or 1.2*1000.
For the allowable stress at the temperature, it can be 1.33*S or 1.2*S.

 

[LittleInch]

"Pressure rating" tends to mean the max pressure rating for a particular flange rating.

This assumes that this is the limiting factor and not the pipe wall thickness. Otherwise at your 1000 psig, the flanges would be class 600 and are already good for 1,205 psig at 500F for A105 material.

Basically it allows for the fact you need to test to 1.5 times the design pressure and flanges are good for 1.5 times their max working pressure, so 1.33 for a few hours isn't that big a risk.



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

 

[NovaStark]

Thanks for the clarification.

So now in the case of temperature variation where I have 1000 psig @ 600 degF. How would I use the 1.33*S with this?

It should not be the same as taking S@600 degF and then finding P based on the piping thickness and comparing against the design MAWP (assuming here that MAWP > design pressure).

Otherwise using a higher allowable stress would give me a false indication that my piping will always be okay.

 

[mk3223]

IMO, one may "think" it this way as the piping system is allowed to be overpressurized for the specified time duration due to the process excursions.

 

[NovaStark]

That seems to be the understanding from the code.

But I still don't understand if my interpretation of the temperature case is correct or not.

 

[davefitz]

For components working in the creep range ( above 800F) the allowable stress decreases exponentially with temperature, so the allowable overpressure at a 100F overtemperature may actually imply a de-rating of the design pressure. Of coourse creep damage is a slow process, but there can also be piping expansion issues with overtemperature operation that can lead to piping hitting bumpers or other interferences, which leads to greatly increased stresses at the elbows. There are cases of pipiong failures due to long term operation above design temperature.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[NovaStark]

I agree that creep is an issue for certain materials and temperatures. However I was just referring to short duration excursions.

But as I said, my interpretation on the temperature issue is still a bit unclear.