Please check the allowable stress values for Primary +Secondary stresses (Pm+Pl+Q) in WRC calculation.

PV elite computes the allowable stress for Induced primary+Secondary stresses as "3 x average of stresses allowed at minimum(38°C) and at maximum (Input design temperature)". Please clarify following in this regrard :-

1) WRC calculation is being done to evaluate the effect of mechanical loads (pressure and external Nozzle loads). NO temperature loading included. Then please clarify why average stress is being calculated for comparision. Instead it should be caluclated as "3xS at input design temperature).

2)If average stresses are to be considered then it shall be as per ASME Div 2 part 5 cl no 5.5.6.1(d) where it allows you to consider the maximum of "3 x S avg" of "2 x Yield avg" where Sav and Yavg is caluclated at lowest and highest temperature during operation. This is valid only when you know the min and maximum operating temperature. metal temperature (under the effect of ambient temp) during start up can not be considered as lowest operating temperature. Any way this is apllied when equipment is under cyclic/or undergoing temperature change during operation. The limitation of use of 2xYield avg is when ratio of min yield strength and ultimate strength at room tmperature is less than 0.7, else allowed stress has to be "3xSavg only"

If we are following div 2 criterial as explained above and if Equipment is not in cyclic/undergoing change in temp then
The allowed stress should be max of "3xS (at design temp)" or "2x yield (at design temp)". And this shall be used by PV Elite as allowed stresses for induced Primary+Secondary stress evaluation.

Please advise on above.

Hello Avedhesh

In the case of primary + secondary stresses, ASME Section VIII, Division 2 states that the stresses must not exceed Sps. In section 5.6.1, Sps is defined as 3*S or 2*Yield Stress. This is the definition (as you rightly point out):

d) The allowable limit on the primary plus secondary stress range, PS S , is computed as the larger of the
quantities shown below.
1) Three times the average of the S values for the material from Annex 3.A at the highest and lowest
temperatures during the operational cycle.
2) Two times the average of the y S values for the material from Annex 3.D at the highest and lowest
temperatures during the operational cycle, except that the value from paragraph 5.5.6.1.d.1 shall be
used when the ratio of the minimum specified yield strength to ultimate tensile strength exceeds
0.70 or the value of S is governed by time-dependent properties as indicated in Annex 3.A.

As stated above from Division 2, Sps is either 3S or 2Yield, taken at the average of the hot and cold temperatures.

I agree that primary + secondary stress can occur only at the design temperature (you have a point). However, I am of the opinion that the secondary stresses are derived principally from the attached pipe work as the piping thermally expands. The thermal expansion naturally occurs between two temperatures. Thermal stresses are defined as (among other considerations) as secondary stresses. This is defined in the ‘Hopper’ diagram in Figure 5.1 in Division 2.

Therefore, there is some justification in the way PV Elite handles WRC 107.

Dear All

WRC Calculation to be considered as a elastic-stress analysis method. In Elastic stress-analysis method secondary stress is not evaluated, pl. refer. ASME SEC VIII Div-2, 5.2.2.2.(a),(b),c).

Now, as you have mentioned 5.5.6 – Ratcheting Assessment and definition of SPS. Pl. note Ratcheting assessment is only applicable for Fatigue loading condition.
Let suppose we consider most sever case of ratcheting, i.e. vessel under shutdown & starting with piping load & comparing Pl+Pb+Q with SPS. Therefore, Average allowable stress or average yield stress is adequate consideration & CODECALC & PVELITE doing correctly. Otherwise Result of Pl+Pb+Q may be ignored, if required.