on-off valve interlock

[hswang2]

Dear professional engineers
I have a simple and mundane question need to bother you.

I have a storage tank and the product there need to be send to drumming area and isotank area.(Please see the attached drawing)
The on-off valve A will be closed through the interlock trip if the tank level is high high. Beside, the on-valve C and E can't be open at the
same time. If on-off valve C open, the interlock will trip to close he conrol valve D.Also, there is a local hand-switch push buttom needed
to be actived when product is selected to transfer to isotank.

My question is
Any valve can be open/close to protect the system if operater open the control valve D by accident or
the control valve C can cause any on-off valve failuer?
Thank you very much.

 

[JLSeagull]

My initial thought would be to eliminate the control valve from the interlock logic. Consider each of the on-off isolation valves for the interlock logic.

I would consider a low level interlock to trip the pump; or open the B isolation / recycle valve. The logic description suggests that you open valve B during the transition when switching between drum and Isotank.

If I were to include logic for the control valve I might command it to begin closing if valve C is open (belt and suspenders). I would not consider the control valve D position regarding the operational commands for valves B, C or E.

That is my thought at this instant.