Nigel, you have some interesting points there. There is no clear-cut procedure for calculating pressure thrust. Here are some aspects to consider:

There are practical issues to modeling the pressure effects in the piping system; for typical piping system the deflection due to internal pressure will be small. It may even be smaller than the geometrical tolerance of a restraint. Therefore, we do not know if that restraint is "active" or not, for the sake of internal pressure. So, that is why pressure thrust has to be (conservatively) estimated and accounted for. The points that Rich outlined in his post are good guidelines to estimate the pressure thrust. Engineers also need to rely on experience when dealing with it.

If one cannot compute the pressure thrust, then let's find a better method to model the thrust load. The typical approach is, applying the thrust load as a radial load on the nozzle, using the WRC 107 module. That approach can produce very high stresses especially the membrane + bending stresses. This can be seen when the nozzle fails with only internal pressure and the thrust loading.

One reason is that, WRC 107 only addresses external loads and not the pressure loading (internal pressure and thrust load). Moreover, if the curves used in a WRC 107 analysis are exceeded, then inaccuracies will be introduced. (In these circumstances, the program uses the last point on the curve.)

An article about pressure thrust is due to appear in our July 2001 newsletter (I will put the link as soon as the newsletter is available). It also contains a comparative study between FEA, WRC 107 and WRC 368 (which models the effect of pressure loading and thrust load) and recommendations on how to account for the thrust load.

Another point, ASME Section VIII area replacement method accounts for the general primary membrane stress in the vicinity of the nozzle, it does not account for the local primary membrane or secondary stresses due to pressure. You should analyze that manway using either WRC 368 or FEA.

I am not sure about your suggestion of modifying the allowable stresses to account for the thrust loading, can you point me to the reference for it.

Feel free share your comments/suggestions.

Regards,
Mandeep