Allow. forces & moments for high pressure/temp valves

Hi,

First of all a little question: ¿You are responsible for the valves only or for the complete piping system?

In the first case, your client can ask you to include in the calculation the nozzle loads, but he must give you the corresponding values.

In any case, normally in all piping systems all items (but straight pipe) are usually designed to be more resistant than the straight component. We consider the transition valve end to pipe O.K if the quotient between allowable stress (for the specific material and according to the design code) and the section modulus is greater in the valve. In your case you should consider the design temperature allowable stresses of course.

Even you don't mention, I would recommend to include in the loads the maximum torque/thrust produced by the motor actuator in the case of motor stall. It would affect usually to the stem which can fail due to buckling or the bolting between actuator and valve yoke.

I woud recommend you too, if it's your responsibility to use motor operators with compensating belleville spring in the stem nut to compensate thermal expansion loads if valve is closed in cold condition and warmed later on.

Good luck

Thanks for ur reply Mr.kwetz,

My answer to your question is that I am responsible for the whole piping system including the connecting equipment nozzles, valves, stress distribution and usage of proper supports etc.,

Yes i have received the allowable forces and moments for the connecting nozzle of the start & end point of the piping system.

The stress analysis was completed to satisfy the above allowable forces & moments, stress distribution and proper usage of supports provided in the piping system.

While modelling a valve in the stress analysis software, we will be giving the total weight of the valve given by the valve supplier (including acutator - motor /pneumatic weight) and the thickness if the valve will be considered as three times the normal thickness of the straight pipe. Also the valve material is equivalent to pipe material only.

I am not clear about your recommendation of usage of belleville spring in the stem. Please explain in detail.

Thanks in advance

regards

arbaskaran

Hello,

I think that you have made your task if you've included the nozzle loads provided by the client.

Regarding my other comments i will try to be more clear (excuse me but my mother language isn't english and perhaps I don't explain very well):

First: For the calculation of stem and bolting you should consider the maximum electric operator deliverable torque and thrust (motor stall condition) that usually can be obtained from the motor operator manufacturer. If this is not considered, a failure of torque switches, or a simple change in the phase sequence could result in failure of those items.

Second: In some Combined Cicle Power Plants recently I was consulted about several failures in main steam valves, from broken stem to upper bearing part of the motor operator completely destroyed. When I analized those cases, I found in one case that the closing torque switch adjustement was unadequate and resut in the broken stem, and in the other case, there were two parallel units, and one of the interconnecting valves was torque seated during cold shutdown and remain closed when one of the units started up. The thermal expansion of the stem (It was a 32" gate valve) generated stresses enough to break the upper bearing part of the motor operator.

So I recommended to change the operator by other model incorporating a belleville spring which allows, trough stem nut displacement, compensate the thermal expansion.

That's all