Understanding the Pressure Stress Calculations in the DesignCalcs WRC-107 Implementation

Owned by Former user (Deleted)
Dec 11, 2015
2 min read

General

  • The WRC-107 method only considers external loads and does not consider pressure.
  • The pressure stress equations used in the DesignCalcs implementation of the WRC-107 method are for primary membrane stress caused from internal pressure.
  • The stresses caused by the external loads are typically classified as primary local stress and secondary bending stress (although this is not always the case).
  • If pressure is considered for these calculations, the primary membrane stress from pressure will be added stresses determined from the external loads.

Pressure Stress Calculation: Elliptical Host

The pressure stress for an elliptical host in the WRC-107 method assumes that the host can act like a sphere at the location where the attachment/nozzle is placed. The diameter entered must represent an effective spherical diameter at this location. The formula is derived from the UG-27(d) formula from ASME SC VIII-I (setting E=1).

Elliptical Host Pressure Stress vs. Elliptical Head Actual Stress

The Actual Stress formula for an elliptical head is derived from ASME SC VIII-I Appendix 1-4(c).

As you can see, these equations are not the same and thus will produce different results.

In addition, the elliptical head form has a location to enter thin out; the WRC-107 form lacks this field, so if the math needs to consider local thinning at the attachment/nozzle, the thickness must be reduced manually to reflect that. The Actual Stress in the elliptical head report also considers adjustment for the joint efficiency as joint efficiency is typically applied as a penalty to the allowable primary stress.