I'm sorry if this question have been answered before, but just bear with me, I got a 30" plug valve, and this system need to be upgraded with an actuator, I may need to remove the plug and put a new ball valve, but of course (budget). My question is finding a rough figure of torque needed for the plug valve - to choose the actuator, or how much is the torque  relatively to a same size ball valve, p is 300 psi.

i suggest you to look for the tech. brochure of the existing plug valve which most probably shows the output torque of your valve.
Regarding the ball valves , you should first decied what kind of ball valve you might use ; conventional , floating trunionion etc. which all depends the application anyway.

In general a plug valve will have a higher torque than an equivalent size ball valve. This is due to the increased seating area of a standard rotary plug valve.    

Being that you will require a 30" ball valve it will most likely be trunnion mounted and have a substantially lower torque requirement than your current plug valve. Due to the port geometry of most plug valves, you will probably find that a Reduced Port ball valve will have the same Cv as the full port plug valve it is replacing. Further reducing the oprating torque.

As cemo said, the easiest way to get an accurate torque figure is by contacting the valve manufacturer and inquiring. This is the easiest way of determining the torque required to actuate the valve.

If truly necessary you could determine the torque of your valve by measuring the force required to operate the valve. Use a long handle and measure the force required to operate the valve. Measure the distance from the stem centre line to the applied force. Multiply the distance(inches) by the force (lbs) and you will have your torque requirement in in/lbs.

This figure will be very accurate, but a suitable factor of safety(I use 1.25) should be added before purchasing a suitable actuator.

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