MAST in On-Off Valve | Explained With a SIMPLE EXAMPLE -Part 1 скачать в хорошем качестве

MAST in On-Off Valve | Explained With a SIMPLE EXAMPLE -Part 1

The MAST value is the maximum allowable stem torque, defined as the greatest torque to which a valve stem can be subjected to during operation without mechanical failure The MAST value is the maximum allowable stem torque, defined as the greatest torque to which a valve stem can be subjected to during operation without mechanical failure MAST (maximum allowable stem torque) is defined as the greatest torque which a valve stem can be subjected to during operation without mechanical failure. so, we need to ensure that the maximum allowable steam torque should be greater than the maximum torque the actuator can deliver. SAFETY FACTOR IN VALVE The next topic that comes during discussion of MAST is Safety margin. Safety margins are determined with the goal that the valve should operate smoothly throughout its service life, irrespective of slight variation in process conditions, valve performance etc. For example, in the oil and gas industry, three different application categories for safety margin are commonly designed for the on/off actuator valves: General Duty like Process On/Off valves – the valve will have an actuator with a torque output of at least 1.3 times the operational torque requirement of the valve (130% or x 1.3 times safety factor) Process Duty like Shut Down and Blow Down valves will have an actuator with a torque output of at least 1.5 times the operational torque requirement of the valve (150% or x 1.5 times safety factor) Safety Duty like Emergency Shut Down valves – the valve will have an actuator with a torque output of at least 2 times the operational torque requirement of the valve (200% or x 2 times safety factor). Although, some end users might go up to 250% ie 2.5 times safety factor higher. Now, the safety factor applied when coupled with the specified design constraints like design air pressure, max pressure etc , can result in selected actuator torques that are exceeding the maximum acceptable stem torque (MAST) specified by the valve manufacturer. The below picture shows a Shutdown valve with hydraulic actuator used to generate large amount of force to actuate the valve Many valve producers can offer higher tensile strength stem materials for large ball and plug valves, which are used in many shut down applications in order to raise the MAST values However, this is not true for smaller ball valves (up to 50mm bore) when it can be challenging to source the stem material to take care of 200% or 250% safety factor requirements because of the limitations in the physical size or valve design. In such cases we might have to look for smarter design like including a pressure regulator in conjunction with a relief valve into the pneumatic circuit control system when faced with valve MAST issues.