Very simple valve interlock system

Did you know it's possible to achieve high integrity process control using valve interlocks? The simplest system involves two valves, each fitted with a key interlock device. In the diagram below standard symbols are used to represent the valves. The circles with the letter 'A' inside represent the interlock. The designation 'A' refers to the key.

So, in this diagram the requirement is to ensure that both valves are closed during change over.

Illustrated Status:

• Valve V1 is open, but unlocked. (We know this because the interlock symbol (circle) has a line through it indicating that the key is present and trapped).
• Valve V2 is closed and locked.

The only way to release the key (A) is to close the Valve V1. The key will only release when the valve is closed. Therefore, the changeover sequence is as follows:

1. Close Valve V1.
2. Release Key A (Both valves are now locked closed).
3. Insert Key A into Valve V2.
4. Unlocked and open Valve V2.

The important feature of interlocks of this type is that when correctly fitted to the valve, it's impossible to make a mistake. The 'human element' is removed and the operator cannot deviate from the correct changeover sequence.

Click on the link to see valve interlock hardware.