Pneumatic Logical Control of the Bypass System for High Pressure Injection Gas Plant

By: Bardo Arce

• Actual Bypass System features an array of 5 Pressure control valves that regulate the pressure to 50 psig about 1200 psig additionally has a KOD and a Bypass flare that handles up 1500 MMcfd gas, pressure transmitters at the plant input and a pressure controller.
• The system has the function of bypass the gas input (1400 MMcfd)  to bypass flare when there is an emergency stop at the plant and needs to stop injecting gas. It also works as a regulator inlet pressure because is necessary that inlet pressure is maintained in 1200psig for gas turbocompressors work in optimal conditions.

• This system is critical and requires fine control to keep the inlet pressure to the injection plant at a fixed value (1200psig). When the pressure increases the value of the pressure control valve for diverting excess modulate and regulate the system.

• Control valves pressure of bypass system has a automatic control loop pressure consists of two pressure transmitters PIT located at the gas line inlet (1200psig), the pressure controller that is located at the PLC process control plant.

• The two transmitters are redundant in 1oo2 votation, either active regulation loop pressure control valve can handle 350 MMcfd each gas, so it only takes 4 valves to divert the total inflow necessary the fifth valve is booking if any valve goes out of service.
• The control loop is completely electronic, but has a disadvantage when the plant is power out. The ups backup that supply the control loop to keep it only has a battery life of 8 hours. So that after this time the valve timing is lost and the valves will be fail safe (open failure) bypassing the entire gas.

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• Solving this problem I had the idea of designing a pneumatic control loop in parallel with the electronic control loop, by Installing a pneumatic pressure and additional solenoid valve. The solenoid valve works like a signal switch between the signal from the I/P electronic and the signal from the pneumatic pressure transmitter (PT).
• The solenoid valve must be energized to operate the electronic control loop and de-energizer for the pneumatic system. If there is a power out, the valve is de-energized and the bypass system works only with the pneumatic pressure loop.

• The system does not work with instrument air, but uses the same process gas from the gasline to power the actuators of the valves, which ensures uninterrupted supply independent pneumatic power plant instrument air.
• These changes are able to improve the efficiency of the bypass gas system, and avoid excessive bypass gas to the atmosphere for failing to effectively the pressure control loop when a contingency happens.

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