It is maximum allowable torque a stem of a quarter turn valve can be subjected to without mechanical failure. Engineering unit Nm OR lb-in
During valve operation, torque delivered by actuator (Pneumatic or electric) at any stage, should not exceed MAST value. If it happens stem may be subjected to mechanical failure.
In some projects it has happened that MAST value calculated by valve vendor was higher than actual, even than it resulted in failure of stem.
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Source: Maximum Allowable Stem Torque (MAST)
Source: MAST Calculation
