Baker Hughes Masoneilan SVIII AP, Installation And Maintenance Manual, Page: 173 / 204

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Masoneilan SVI II AP Installation and Maintenance Manual =
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13. Air Supply Requirements
Air Supply Requirements
A high quality air supply greatly improves the quality of control and reduce maintenance costs of
pneumatic equipment. See ANI/ISA-7.0.01-1996 - Quality Standard for Instrument Air. Air supply
failure requires special attention to minimize process effects. Design and apply all process
equipment to fail to a safe condition. This includes failure of the air supply. SVI II AP is designed to
fail to a condition of low or no air pressure. Choose control valve actuators to move the valve to
a safe condition when air pressure is low or absent. For example, a valve supplying fuel to a
combustion process is normally equipped with an Air to Open valve. In other words, the fuel flow
is shut off if air fails.
Additional process precautions can be taken. When the air supply recovers, the setpoint to the
valve must be at a value that continues to hold the valve in its safe condition, or to move it to a
known safe condition. To do so, put the control system sending the control valve position
setpoint into manual mode and set to 0%. After the air supply has stabilized at its correct
pressure, the setpoint can be moved to its operation point in accordance with the plant’s safe
start-up procedures. An additional precaution required on critical processes with an ATO control
valve is to install a shut-off valve that supplements the control valve by moving to a safe
condition on air failure, and remains in that condition until all necessary requirements for safe
start-up have been met.