Dear all,

I have a querry regarding the consideration of pressure thrust in WRC-297 calculations.

Carrying out WRC calculation,
Shall we consider the Pressure thrust also in addition to the radial load.??

according to me,it should not because pressure stresses are not at all loacal stresses.

And if we consider the same,the allowable limit must be increased to 3*Sm.

Please confirm.

Thanks in advance.

Regards,

Harshal Kolhe.

Dear Harshal Kolhe

I have two points:
1. I do not agree that the pressure stresses are not local. The stresses that arise due to gross structural discontinuity. In a junction even peak stresses are possible due to pressure or any load.

2. Regarding considering pressure thrust in addition to radial load, it is a matter of radial load, how it is arrived at and what client wants. if you have taken very conservative nozzle load just based on thum rule or table or some spec, you can compare with pressure thrust and see. If the nozzle loads are actually arrived at from piping analysis say under weight loading or thermal loading (flexibility) then pressure thrust to be necessarily considered; it is an actual loading.
As per UG-22, we shall consider all loadings. Your limit of S, 1.5 S or 3 S do not decide the loading to be considered. The limit is as per stress classification depedning on loading , location and distribution and effect (failure mode)

3. In short pressure thrust shall be considered unless it is already included in the defined loads. Always when we do FEA we include the pressure thrust. (Whetehr we do FEA or WRC we shall consider the same)

Hope I have made my points clear
With regards

_________________
R.SRINIVASAN
Sr. VICE PRESIDENT
Head, Design and Engineering
PED,ISGEC,
YAMUNANAGAR
HARYANA -135001
INDIA

Harshal,

Remember what Sir Isaac Newton said (not the apple bit) but the bit about each and every action has an equal and opposite reaction. So to summise, yes! you should consider the pressure thrust when you do your nozzle local loading analysis for pressure only and/or pressure + external load case.

Now coming to the bit when one may not need to consider Pressure Thrust. One very good excuse for not consider pressure thrust in the analysis is when you have quite a bit of flexibility in the piping system connected to the nozzle, this is typical of high temperature systems, wherein expansion joints are attached into the piping system, and in such a scenario quite a bit of pressure force may get transferred to the bellow and get disipated therein. In this case you would still have some backlash coming onto your nozzle attachment (remember Newton!), but may not be substantial enough to be warranted into the local load analysis. Also note sometimes considering pressure thrust may yield less conservative results, say if you had an axial force acting in a direction opposite to the pressure thrust and the pressure thrust happens to be quite substantial you may end up cancelling your axial force. So a good designer would go slightly beyond the vessel boundary to understand what is causing the external reactions on the nozzle and how. The further course of action may then be determined accordingly.

Does this help? my two bits to your query.
Hope you are keeping well.

Mayur