Question

how can fluctuation in control valves response effect the process (other than deviation in controlled variable) ?

in terms of safety.

Answer 1

The fluctuation in control valves after a period of operation, does significantly affect the process in terms of safety. The more change in the controlled variable is to be brought about (for more amount of time), more the valve is used.

After being used for a long time, the valve may undergo physical inaccuracies in terms of manufacturing defects. These are brought about by rust, friction produced in each movement or defects caused due to pressure it has to bear in the process line.

When this happens, the valve may give lead to giving a delayed response.

For example, take a valve that controls the fuel intake to an engine or burner. If the valve is to be closed, and it does not do so immediately due to the wear and tear, it may lead to an accident as too much fuel may be combusted. Or if the valve fluctuates and does not stay open at a particular prescribed level, enough fuel may not reach the engine and the process may shut down.