Evaluating Valves in Cyclic Service 1

[Xianglu]

Pressure components , pipes, welds, fittings, in a piping system, subject to cyclic fatigue condition could be assessed using Sec VIII, Div 2. How can I evaluate the suitability of valves in terms of different classes?
Thanks in Advance.

 

[davefitz]

Unless you are a valve mfr, the only valve component you can reliably model and estimate fatigue life for is the pressure boundary. As a first approximation, you can estimate the fatigue life based on cyclic applications of pressure and therma l stresses using either the german boiler code trd 301 annex 1 or the newer european norm EU 13445 appendix B.

Alternately you can make a finite element model of the pressure boundary and use rules for analysis provided in ASME sect VIII div 2 or you can use the european norm methods, which may be better suited for the output from finite element models.

Of course, there is a lot more to proving the valve is suitable for cyclic service than just the pressure boundary. Threaded bolts will loosen under cyclic alternating stresses, so a cyclic service valve will miimize the amount of threaded components. Another issue is the different rate of heating up of the valve body vs the cage vs the trim/plug; under fast heat up cycles some slip clearances will bind and the valve will be bound up in a fixed position- not good if a fail open or fail closed action is relied on for safety.

The best method is to find 5 dozen exact same applications with the exact same valve that has laste 30 yrs in cyclic service- good luck.