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Control valve in split range operation
Addition of control valves in split range operation for wide rangability.
By Ajoy Thomas on 13 April, 2006 - 12:50 am

Can any one tell me selection & sizing of control valves in split range operation to acheive wide range of operation?

Pl explain the type & capacity valve selected to maintain constant loop gain while change over.

Ajoy Thomas

By John Greene on 15 April, 2006 - 10:26 pm

I have a short tutorial on about split-range control. Nothing about the valve sizing, but some practical hints on configuration and tuning.

Basically the loop gain is not only a function of the valve size, but depends also on the process characteristics. So the best way to set it up is as two independent controllers. I have seen countless examples of process instability caused by 'conventional' split-range control and have fixed them by this simple modification.

John Greene
Process Control Consultant
Contek Systems Ltd

By Shailesh C Patel on 19 April, 2006 - 11:32 pm

Dear ajoy

Split range is implemented by two ways:
1) By positioner spring i.e. spring of one positioner for 3 to 9 psi and other positioner for 9 to 15 psi.
2) By two i/p converter (split range is part of DCS).

There is no hard rule to split range of control valve. (it also depends on process & and normally given by process people.) Then you can sizing control valve as normal control valve if you use second scheme. (now days first scheme is no longer used.) Control action and air action again depend on process. (normally if air action is air to open then control action of one valve is direct and other
one is reverse acting.) i hope it is useful to you.

Shailesh C Patel
FDC - Control Systems And Instrumentation
Reliance Engineering Associates(P) Ltd.