I was asked by the engineers at a chemical manufacturing site here in the United Kingdom if I could have a quick look at a level control problem in a reactor coolant water drum. They have 2 very large reactors that generate a lot of waste heat that is removed from the reactors by generating steam in cooling coils.
The two reactors share a single water drum that provides water to the steam generating coils. If one reactor comes off line because of a shutdown system trip, the level in the drum rapidly falls to the point where the second reactor may trip.
This behaviour is very strange because the water is not leaving the system, yet the drum level rapidly falls. We think that because the source of heat is removed from the reactor steam coils, any steam in the reactor coils quickly condenses and the volume is replaced with liquid water from the coolant drum, causing a fall in level.
The normal level control works very well at normal conditions, adding water to the system to replace the water leaving as steam. However the control loop cannot react fast enough when a reactor trips.
The process operators have developed a method of managing this situation by adding water directly into the system, overriding the normal control loop. However this is not convenient for them since when a reactor trips the process operators have many important actions to take and looking after the water drum level is job they don’t need at that time.
We have come up with a simple piece of logic that detects a reactor trip and then performs the actions that an operator would normally do (puts the level control into MAN, and opens the water valves to 100%). Once the liquid level stops falling, then the control loop is put back into AUTO and is allowed to close the water valves as the level recovers.
This is a great example of working with our clients to help them get added value from their DeltaV system.
Added by Jim: The DeltaV track in the Emerson Exchange 365 community is a great place where DeltaV system users and subject matter experts come together to ask questions and solve problems in a peer-to-peer manner.