MAST means Maximum Allowable Stem Torque. It is the maximum torque the valve stem and associated drive train are permitted to experience. Actuator selection must satisfy two conditions at the same time: provide enough torque to operate the valve under the specified conditions, and avoid applying torque above the valve’s MAST.

Collect the valve torque data

Obtain manufacturer torque values for the actual valve size, seat, pressure differential, temperature, media, and operating direction. Rotary-valve data may include break-to-open, run-to-open, end-to-open, break-to-close, run-to-close, and end-to-close torque.

Apply the project sizing basis

Use the factor of safety or application factor required by the valve manufacturer and project specification. Factors may account for service, frequency, ageing, seat condition, and uncertainty. Do not invent a blanket factor without checking the manufacturer’s guidance.

Required actuator torque = valve torque requirement × approved sizing factor

Use minimum available supply

For pneumatic or hydraulic actuators, size at the minimum credible supply pressure after regulator, line, and temperature losses. For spring-return units, verify both the powered stroke and spring stroke across the entire travel.

Compare torque curves, not just one number

Scotch-yoke, rack-and-pinion, diaphragm, and electric actuators produce different torque profiles. Compare available actuator torque against required valve torque at the relevant positions throughout the stroke.

Check MAST at every condition

The selected actuator’s maximum output, including stall torque, spring end torque, hydraulic pressure, or electric torque setting, must not exceed the allowable valve stem torque unless the assembly has an approved mechanical or control limit. This check protects the stem, keys, couplings, and internal valve parts.

Complete the rest of the selection

  • Fail-open, fail-closed, or fail-in-place action
  • Stroke time and emergency response
  • Ambient temperature and hazardous-area rating
  • Duty cycle and modulating frequency
  • Manual override and lockout
  • Mounting kit, coupling, and alignment
  • Limit switches, positioner, solenoids, and air set
The selection window: actuator output must remain above the required valve torque and below MAST, for every relevant stroke position and operating condition.

Technical references

This article is general educational information. Apply project specifications, current manufacturer data, applicable codes, and qualified engineering judgement.