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Variation in temperature (B31.3, 302.2.4 (f) (1)

Variation in temperature (B31.3, 302.2.4 (f) (1)

Variation in temperature (B31.3, 302.2.4 (f) (1)

(OP)
thread378-243938: Effect of internal pressure.
We had a variation in operating temperature for approx. 72 hours in a steam line. The system design is 540 psig @ 665 F. Material is A106 grade B. The piping affected is downstream of the desuperheater on a pressure let-down station (1400 psig @ 880F to 475 psig @ 630 F). The line ran at 724 F for the 72 hours. I understand the 33% for no more than 10 hour and the 20% for no more than 50 hours when it comes to pressure variations but I am not clear how this clause applies when the variation is temperature only. Can anyone clarify?

RE: Variation in temperature (B31.3, 302.2.4 (f) (1)

You only need to find the allowable pressure at the higher temperature.  The higher pressures allowed under transient cases assume that the criteria is the allowable stress at the DESIGN temperature (I here assume that your NORMAL OPERATING condition is 540 psig at 665 F and that the "DESIGN" envelop is from 1400 psig at 880F to 475 psig at 630 F.  

So, start with the allowable stresses and equivalent pressures for the A106 material used at the given DESIGN temperatures, as found in the material tables.

If your system material allows
1400 psig at 880 F, and
 475 psig at 630 F

and we linearly interopolate those values as given in the table, (those are table values right?)

At 724 F, the allowed pressure would be,
1400 - (1400-475)/(880-630) * (880-724) = 822 psig

So, we find that your allowed pressure at 724 F is 822 psig.
What you didn't say is, what the pressure was when you were operating at 724 F, but if it was NOT higher than 822 psig, you would not have reached the normal design allowable for the 724 F temperature, and would not have to classify under the extended transient allowables.  You may have gone above normal opeating conditions, but that's not important for the pipe.  Design conditions rule.  Any pressure less than or equal to 822 psig would be considered to be within the allowed normal design limit envelop at 724 F, but you may still want to consider tightening up on that process control.

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