Steam pressure control


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Medium pressure steam header operating at 42 bar.
Presuure loop comprises 2 6inch control valves venting to atmosphere. Valves are presently split ranged.

When an upstream consumer trips, 70tonnes of steam are required to be vented, with minimum disturbance to the header pressure.

What is the most effective control strategy to cater for this scenario. Logic tunable controller parameters? Any suggestions please.
I have done something very similar to this before.

I assume you have some sort of boiler master pressure control which is attempting to maintain the header at 42 bar ? The way I did, was to use a simple PI controller to drive the vent valves, but with a pressure setpoint approx. 0.5 bar above the setpoint used for the boiler master pressure control. The PI controller output was held at zero (valves closed) until the consumer trip was detected. A trip released the controller. The controller was implemented in a single loop controller with inputs for the pressure setpoint and header pressure mv.

On a trip, the steam pressure rises, the vents open to maintain the pressure at 0.5 bar above the setpoint and the boilers start to reduce load. As the boiler load matches the new steam consumption, the vent valves close.

This solution has the advantage that it is simple.