P91 Heat Treatment

[DSB123]

Dear Members,
I would like your thoughts on the following:

We have a new piping system which is in A335 P91 material. The post weld heat treatment has been carried out at 750 Deg C +/- 10 Deg C. Some of the heat traces show the temperature to be 741 Deg C. Bearing in mind the accuracy of the measuring devices (+/- 0.75%) the temperature could be as low as 734 Deg C. Is this acceptable and will the resultant metal structure be acceptable?

 

[metengr]

davefitz;
Interesting information. I am not sure what this means in terms of creep deformation for design purposes. Are these weld reduction factors to assure that creep deformation remains at or below some percentage over 10,000 or 100,000 operating hours in calculating allowable stress?

Also do the weld reduction factors apply to seam welds and girth welds from a generic viewpoint?

 

[davefitz]

The factor W only applies to sustained loads to welds at elevated temperatures. Normally girth welds are exposed to stresses perpendicular to the seld plane of only 50% of the governing membrane stress ( ie 50% of circumferential stress), so one can ignore the W factor for most girth welds. That changes if there is a sustained bending moment or shear stress also applied to the girth weld, such that the resolved max stress ( using Moore's circle or from FEM analysis) exceeds W * allowable stress at operating temp.

For longitudinal welds , the factor W would always apply since the governing membrane stress is perpendicular to the plane fo the weld. Again, one would add to this membrane stress any sustained loades such as torsional loads or shear stresses.

Becht claims the approach matches that of ASME sect III subsection NH, adjusted for the different safety factors used in B31.3.The factors used are for 100,000 hr durations and prorated as per case N-253.