An open path to relief must be maintained on Safety Relief Systems at all times during work exercises. A spare relief capacity, enables continuous production while maintenance procedures take place, and eliminates the need to isolate and shut down the whole process. Typically, twin or multiple safety relief valve systems are fitted with isolation block valves both upstream and downstream of each safety relief valve. Block valves, isolating the spare relief valve must be opened before the block valves are closed. Fitting a valve interlock to the upstream and downstream isolating valves on each safety relief valve (PSV), will ensure safety is maintained at all times.