How to Use DC block to configure permiisive for both open and close commands

Is it possible to configure permissive to close and permissive to open using DC block, if so kindly let me know how to do so....

Thanks for your time Slight smile

Senthilkrishnan

6 Replies

  • Use a DCC plus EDC, and Force Setpoint?
  • If using just a DC block the permissive input only applies to active states and you only have one input that you really cannot configure from within the block. I imagine that you want the operator only to allow the valve/device to open when certain conditions are met and then only allow the operator to close when different conditions are met.

    I think one option to achieve this is to use a CALC block that controls operator commands to the DC blocks and also checks that your required premissions are in place before executing a state change. You might have to check also what you want the DC block to do when the device needs to interlock.

    The operator faceplate will have to write the operator open/close request to a parameter within the module and not directly to the DC block in order to this solution to work. I do not know your DCS structure and layout, so this is just a guess.

    One more question is what your device does when the DCS output for some reason loses power (does it maintain its last state)?

  • I have done this with an EDC and used a transfer block to switch a connection to the Passive State between 2 parameters from the same named set as the EDC setpoints. This lets you change which state the permissive condition in the DCC block applies to. For example, in the DCC, condition 1 prevents opening, condition 2 prevents closing. When condition 1 is true, Passive_State = Close, when condition 2 is true, Passive_State = Open. You can use the DCC parameters P_OUT_D_x to read the state of each permissive condition.

    With a DC block in one direction you use the normal permissive functionality. For the other way you can use its DELAY_TIME and a conditon + action looking at the SP_D to push the setpoint back before OUT_D has changed.

  • In reply to RobPerry:

    Thanks for the detailed explanation, but Rob the problem is we are using and have to use only the old DC block and individual condition block for permissive as EDC / DCC update is rejected by customer :(.

    Yes, I am planning to add logic before sending the active command to the DC block, but we aren't planning to send the command if the permissive is not met and the operator has to reinitiate the command again after making the permissive ok. 

  • In reply to Martin v. Bergeijk:

    We are not going to upgrade to DCC / EDC blocks. We have to use the DC block only
  • In reply to fkitzmann:

    Thanks for the reply....
    Yes, I have been thinking along this line..to bring the commands out to logic before sending them to DC block, also in our case we have field commands which will be clubbed with operator command, but it would mean update in class & FP

    Also if we show permissive symbol in FP in both open & close cases, it will be confusing. One of the methods I thought of is to use PERMISSIVE symbol near each command and write logic to trigger that symbol instead of showing the original permissive from DC .. Not sure if this is acceptable ??


    About power failure, I have to check