PASS/NOZZLE-FEM 3.5. Program Manual

5.5. Calculation of allowable loads

For assigning of allowable loads acting on the nozzle, calculations are made of maximum stresses caused by action of single forces, moments and pressure in turn; i.e. in the course of calculation based on the assigned single force or moment, all other loads (including pressure) are put to zero (0.0). For each type of stresses (membrane and total ones) a minimum safety factor is defined subject to a mesh factor.

At determining of the allowable load its unit quantity is multiplied by the obtained minimum safety factor. The obtained results form a table of individual allowable loads (table 5.3).

Note:

1. When exceeding any loading component, a reinforcing of nozzle connection is required. Or this loading component must be reduced.

Then allowable loads on the nozzle without pressure are calculated. In that case allowable loads are defined upon condition that within their simultaneous impact on the nozzle at zero pressure the maximum stresses won't exceed the permissible values. Calculation practice shows that such condition is met if load rate is equal to $1/3$ of allowable individual loads.

Under influence of design pressure allowable loads are reduced by multiplication to the $\left(1-\alpha_p\frac{p}{[p]}\right)$ value, where $p$ - design pressure; $[p]$ - allowable pressure value, calculated by bending stresses; $\alpha_p$ - factor, depending from normative document. When you choose normative document for strength estimation by PNAE G-7-002-86 [17] factor $\alpha_p$ is 0.87, which takes into account, that under influence of pressure with loads the local membrane stresses shall not exceed $1.5[\sigma]$, while under influence of the pressure only - $1.3[\sigma]$. In other cases, $\alpha_p$ is assumed to be 1.0.

The table 5.4 shows the results of calculating the allowable loads with their joint action.

Note:

2. When exceeding one or several loading components, a supplementary calculation of strength is required

This table can be recommended for assignment of allowable loads influencing on the nozzle at strength calculations of vessel pipe manifold.

It should be noted that allowable loads at the design pressure are conservative. As a rule, at their simultaneous application, with pressure considering, maximum stresses amount to 50-80% of allowable values. For this reason a reference to Table 5.4 points that if the loads exceed the allowable values, for drawing up a conclusion on operability of branch connection it is necessary to make a supplementary calculation with the loads obtained. At that the loads shall not exceed the defined values in any case (table 5.3).