what decide valve torque? 3

[7431]

I am curious about how to decide the valve torque? just pick up from handbook or national standard data?
and what about stem diameter, stem thrust? if need caculate, how?

 

[JLSeagull]

I generally select the valve to meet my process requirements. I then use the manufacturers' torque tables for selecting pneumatic actuators for automated valves.

The torque is a function of the particular valve design - and the working pressure. A ball valve with a Teflon seat ring requires less torque than the same valve body with a graphite seat ring suitable for fire rating. The manufacturer either publishes the torque or can furnish that data.

I recently obtained data on the 87/88 series Apollo valves. There are four sets of tables, one set for the 87/88A100 series and one for the 87/88 200 series all Class 150 and another pair of tables for Class 300. The first column is port size for Class 150 with rows for each size from small such as NPS 1/2 to large, such as NPS 12. The pressure tables range from 0 increasing by 100 psig to the maximum for the flange class. A torque value exists in each block corresponding to the size and pressure. The next to last column shows the LSST with torque and the last column shows an adder for option 24. Option 24 is the graphite packing and seal.

The actuator torque at minimum air pressure must exceed the valve torque. Actuator data includes the torque from the piston and torque from the spring. Further detail applies to the curve between the start and end of travel.

 

[7431]

thank you! make sense of it!

 

[JimCasey]

Everything JLSeagull said is accurate. Ball valves torque is determined by the seat material, the ball material, the process fluid lubricating (or fouling) the seats, the pressure differential loading the ball against the seat, the design of the seats (some seats are preloaded for application reasons but this causes higher torque). As he suggested, all these factors are considered in the manufacturers' torque tables. Some manufacturers also publish a liat of safety factors for actuator sizing....if the valve sits two weeks between actuation, it will take a set and need a mighty heave to make it start moving.

You did not specify ball valves in your question, though. Butterfly valves also have torque consideretions:
Type of valve, seat material, process fluid velocity and density, packing material all affect the torque required. Again, there is no national standard and there is much variation between similar valves from different nmanufacturers. Consult the manufacturer's literature.

 

[7431]

thanks, JimCasey!

 

[johnsmith2]

All above mentioned factors account for the torque. But, I would like to say sometimes, manuafacturers just copy and modify the torque with similar design and functions of other competitors. Of course, they include the safety factor to over-estimate the torque value, and in turn, larger size of actuators are used instead. So, they can earn more money. For some good one, they will measure the torque or send them to laboratory during the prototype testing.

 

[7431]

Thanks, Johnsmith2. get the shadow of the fact.