Ball Valve Torque Calculation Formula

[Sid89]

Is anyone aware of any empirical formula to calculate the torque of a ball valve? There are formulae like Roark's to find MAST but how can we mathematically compute the Break to Open, End to Open torque values for a valve??

 

[3DDave]

It depends on how much you know about the construction of the valve. Presumably if one knew every dimension of every part and what the characteristics of the fluid it was controlling one could make a good estimate. If not everything is known then it seems unlikely. MAST is simple at only one material and cross-section is required and that measurement is possibly known from a user made measurement.

 

[gerhardl]

Usually torque is a measured value under known and optimal/ideal conditions.

For a single ball valve experienced torque can vary from an observed ideal torque up to about double or triple value over time.

This is the reason that reliable producers set a limits for operational values when giving out a torque value.

Non ideal conditions, abrasion, stickiness and residues, maintenance and operational metodes, pressure, flow etc. can give a multitude of higher values. Five or ten times higher torque than ideal is not unheard of, but indicates that another type of valve should be selected, or operational data and maintenance should be improved.

When selecting electrical actuators it is recommended both to give as long operational time as possible to give a smaller actuator. On the other hand it is generally recommended (especially for larger valves) to leave enough time to eventually add an extra 1:2 gear to double the time and output force, if it should be proven to be necessary.

Conclusion: Calculation of torque is done, but based on practical observed data and conditions for some valves in a range, given known data for fluid and flow.

 

[Mconlon]

Entirely depends on the design of the valve itself. There is too much variety from one manufacturer to another to be able to calculate this. The seat type/material and packing style/torque add a lot of friction before you even consider factoring in the process. One extra turn of the packing nuts could drastically increase the required torque.

For actuator sizing I always use the full differential pressure for the given ANSI class of the valve plus a safety factor.

 

[Sid89]

So there is no generic formula to calculate the torque of a ball valve mathematically?

 

[Mconlon]

You can calculate the required torque to shut off against full differential pressure, but there’s no standard for the valve itself. Having a metal seat vs soft seat alone will make a radical difference in break torque.

 

[bcd]

There is no simple formula you can copy. Is it a floating ball or trunnion. Is it soft seated or metal seated. Does it have graphite packing? What are the characteristics of the media flowing through the valve, lubricating oil or sticky plastic? How fast does the valve cycle? What temperature ranges can the valve be used in?

As you can see, there are many variables that affect the required torque. So even if you do develop and equation for your situation, you still should verify by testing. Your estimates for coefficient of friction can easily be off by 100% in a calculation.