Split Range Control on two valves - HC900-C70R


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I need to control two valves with a split range-control. One valve is fully opened and the other is 30-40% open end. Does anyone know how to do this?

This is the work description i received:

Gas from V-5 flows into the first stage scrubber V-1811, where the entrained liquids are separated from gas. Compressor suction pressure is controlled via two pressure control valves PCV-120A and PCV-120B. The control mode for these valves shall be split range-control. The PCS algorithm shall be such that the open and closed analogue values can be changed by PCS engineer (not operator) as and when required.
This change may be necessary for optimizing the pressure control if the compressor suction pressure will deviate from original design parameters. These valves are pneumatic operated are fail to close

The Valve PCV-120B is equipped with open and close positions indicators and are connected to PCS. The PIC-120 shall control the operation of these valves so that valve PCV-120B will open first fully opened and if still need more demand the other PCV-120A will start open up to 30 to 40 %.

The pressure transmitter is the input to an analog input function block. Its analog value is the process input to a PID function block.

The output of the PID block connects to two AO function blocks.

One AO function block is ranged for 4-20mA over 0-100% of the PID output, which drives the positioner for PCV-120B.

The other AO function block (FB) is ranged for 4-20mA over 70-100% of the PID output which drives the positioner for PVC-120A. The AO block has an 'Input Range' setting, which in this example would be
Range Lo = 70
Range Hi = 100

So it's easy to configure the turn-on point for the staged control.

In order to comply with the requirement for an engineering change to the 'split range' range, there are two approaches.

One approach is to require the use of the HCD development software, connect to the controller, make a change, download it, hot start. Whenever a change is made to the controller configuration, a concurrent change has to be made to the Control Station HMI because any controller change changes the controller database).

The other approach is to use a WCON (write constant) function block to write a value from the Control Station (or whatever HMI is used) to the Analog Output FB's 'Range Lo' index value, thus changing the index value (Range Lo value) on a demand basis. The Write Constant function block is in the auxiliary FB group. This method is subject to whatever level and depth of security is implemented on the HMI.

The analog output's help file or documentation in the Function Block Reference Manual shows the index # for Range Lo (as well as all the other internal block values). The analog variable that you connect to the X pin on the WCON FB has an assigned Modbus address that can be used to write a value to the WCON FB. Enable logic is needed as well to force a one-time write.