Occasional loads

Pressure and weight act for as long as the plant is running. Other loads occur only part of the time: wind, snow and ice, earthquake, the reaction force of a discharging safety valve, a pressure surge. Piping codes group them as occasional loads and assess them separately.

Like sustained loads, they are external forces that do not diminish when the pipe deforms. They therefore count as primary loads and not as thermal expansion.

The allowable stress for sustained loads includes a margin that also covers long-term effects. For short, rare events part of it may be used. The codes therefore raise the allowable stress for the sum of sustained and occasional loads.

ASME B31.3 permits 1.33 times the allowable stress at operating temperature. EN 13480-3 uses an increase factor that depends on the type and duration of the load. In every case the increase applies to the pipe stress – not automatically to supports, structure and equipment nozzles.

Occasional loads are each superimposed on the sustained loads, but not all with one another. The strongest storm and the design earthquake occurring together is so unlikely that the codes do not require this combination.

In return, each load is applied in all relevant directions: wind from two horizontal directions with both signs, earthquake horizontally and vertically. Which combinations are to be formed in detail is laid down in the project specification.

Wind is applied as a static distributed load on the line. Earthquake is calculated as an equivalent static load or with a response spectrum, depending on the requirements. Impulsive loads – safety valve, pressure surge – are dynamic by nature; they are either calculated as a time history or approximated by a static force with a dynamic load factor.

Occasional loads rarely govern the stress, but often govern the supports.