Process Engineering Division – Instrumentation and Automation Section – Daily Question – Question dated 2020-08-31

Selection principle: It is primarily based on production safety; the goal is to avoid damaging equipment and injuring operators in the event of a loss of signal pressure. Example: For the control valve of oxygen subjected to pressurized vaporization and fed into the furnace, a normally open valve should be selected. In the event of a regulator failure or a disruption in instrument air, the oxygen control valve should be in the closed position to cut off the oxygen supply to the furnace. To prevent the furnace temperature from rising sharply and damaging the equipment, in case the steam valve is closed while the oxygen valve remains open.

1. Safety principle: When a fault causes the control valve to lose its signal, taking into account factors such as human safety, production safety, and equipment safety, it is determined that in such a situation the control valve should be fully open (air-shut or fail-open) or fully closed (air-open or fail-close)； 2. Product quality principle: When a fault causes the control valve to lose its signal, factors related to product quality are taken into consideration to determine whether the control valve should remain fully open (air-shut or fail-open) or fully closed (air-open or fail-close) in such a situation ； 3. Consumption principle: When a fault causes the control valve to lose its signal, factors related to energy and material consumption are taken into account to determine whether the control valve should be in the fully open position (air-actuated closed or fault-open) or fully closed position (air-actuated open or fault-closed) under such conditions ； 4. Medium conditions: When the control valve loses its signal due to a fault, the medium conditions (crystallization, condensation, evaporation, denaturation, etc.) are taken into consideration to determine whether the control valve should be in the fully open position (air-locked or fault-open) or the fully closed position (air-open or fault-closed) under such conditions.

1. The original rule: The principle for choosing between gas on and gas off is based on safety considerations. 4102 2. Regulation: Turning gas on at 1653 and turning gas off are also referred to as fault shutdown and fault startup functions. That makes it easy to understand. When an accident occurs, that is, when the gas supply is interrupted, from a safety perspective, if it is safer to close the control valve, then an emergency shut-off function is used, which means the valve operates when there is gas flow (opens when gas is present, closes when there is no gas). 3. Pneumatic control valves are divided into air-open type and air-close type. A pneumatic open-type control valve moves in the direction of increasing its opening degree as the air pressure on the diaphragm increases; when the input air pressure reaches its maximum value, the valve is in its fully open state. Conversely, as the air pressure decreases, the valve moves in the direction of closure, and when no air is supplied, the valve closes completely.

1. Safety principle: In the event that a fault causes the control valve to lose its signal, considerations such as human safety, production safety, and equipment safety are taken into account to determine whether the control valve should remain fully open (air-locked or fail-open) or fully closed (air-open or fail-close) in such a situation； 2. Product quality principle: When a fault causes the control valve to lose its signal, considerations must be given to product quality; it is necessary to determine whether the control valve should remain fully open (air-locked or in fail-open mode) or fully closed (air-operated or in fail-closed mode) under such conditions ； 3. Consumption principle: When a fault causes the control valve to lose its signal, factors related to energy and material consumption are taken into account to determine whether the control valve should be in the fully open position (air-actuated closed or fault-open) or the fully closed position (air-actuated open or fault-closed) under such conditions ； 4. Medium condition: When the control valve loses its signal due to a fault, the medium condition (crystallization, condensation, evaporation, denaturation, etc.) must be taken into account to determine whether the control valve should remain fully open (air-actuated or fail-open) or fully closed (air-operated or fail-closed) under such conditions.

Process requirements – Safety considerations

Here we go, daily learning *check-in

1. Safety principle: When a fault causes the control valve to lose its signal, taking into account factors such as human safety, production safety, and equipment safety, it is determined that in such a situation the control valve should be fully open (air-shut or fail-open) or fully closed (air-open or fail-close)； 2. Product quality principle: When a fault causes the control valve to lose its signal, factors related to product quality are taken into consideration to determine whether the control valve should remain fully open (air-shut or fail-open) or fully closed (air-open or fail-close) in such a situation ； 3. Consumption principle: When a fault causes the control valve to lose its signal, factors related to energy and material consumption are taken into account to determine whether the control valve should be in the fully open position (air-actuated closed or fault-open) or fully closed position (air-actuated open or fault-closed) under such conditions ； 4. Medium conditions: When the control valve loses its signal due to a fault, the medium conditions (crystallization, condensation, evaporation, denaturation, etc.) are taken into consideration to determine whether the control valve should be in the fully open position (air-locked or fault-open) or the fully closed position (air-open or fault-closed) under such conditions.

When selecting between air-open and air-close operation modes for control valves, the process unit should be kept in a safe state as much as possible in case of an accident. Pneumatic control valves are divided into two types: air-open and air-close. A pneumatic open-type control valve moves in the direction of increasing its opening degree as the air pressure on the diaphragm increases; when the input air pressure reaches its maximum value, the valve is in its fully open state. Conversely, as the air pressure decreases, the valve moves in the direction of closure, and when no air is supplied, the valve closes completely.