Design load case

The design load case is the combination of pressure, temperature and other actions for which a piping system is dimensioned. Wall thickness, the pressure rating of flanges and valves and the choice of material all follow from it. It does not describe normal operation but the most severe condition the system has to withstand for sustained periods.

Pressure and temperature belong together here: what governs is the combination of coincident values that requires the greatest wall thickness or the highest rating.

A plant rarely has only one condition. Start-up, shutdown, cleaning, steam-out or an upset can produce other pairs of values than normal operation. Each pair is checked; the most severe governs.

The key word is “coincident”. The highest pressure and the highest temperature of a line do not necessarily occur together. Combining both maxima into a case that never happens in operation means designing for a condition that does not exist.

In the wall thickness calculation it supplies the calculation pressure and temperature. In the piping class it determines the pressure-temperature rating of the flanges. In the pipe stress analysis it is one of several load cases: sustained loads are verified with the design pressure, whereas thermal expansion uses the temperatures of the operating load cases – or, if the project specification requires it, the design temperature as well.

Margins on design pressure and design temperature look cautious but have consequences. More pressure means more wall thickness, and a thick-walled pipe is stiffer. More temperature means more calculated thermal expansion. Together they raise the forces on anchors, supports and equipment nozzles.

An exaggerated design thus shifts the problem from the pipe to the equipment. The values should therefore be justified – by the process design, the overpressure protection and the modes of operation that can actually occur.

The errors almost always lie in the definition, not in the calculation.