Pressure Relief Instrument Air Failure
The consequences of instrument air failure should be evaluated in conjunction with the failure mode of the control valve actuator. It should not be assumed that the correct air failure response will occur on these control valves (fail open, closed or in position). Some valves may stick in their last operating position and the choice of which valves are assumed to fail in their last operating position would be governed by maximizing the relieving load. If the failure position of the valve helped minimize the relieving rate, it should be assumed that the valve would not move. The relief valve size should be based on the failure of the most critical component of the air supply system. Following determination of the consequences of air failure, the designer should size relief devices based upon specific knowledge of the system including control valve flow characteristics and piping layout.