Separate your shutdown system from control systems

Today,for cutting costs,many plant owners are trying to incorporate shutdown actions using the control system itself. For example, if there is a control valve that has to close when a predetermined shutdown point is reached, the instrument air to this valve is cut off using a solenoid valve and the valve is designed to "fail close".You must always keep your shutdown system independent from the control system. This is important from an emergency safe shutdown point of view. There is an interesting case study presented in this link where the shutdown system was designed to be independent from the control system. The control system valve did not close during an emergency due to a failure of a solenoid valve but the separate shutdown system acted safely.
I have investigated many process incidents where the shutdown system was connected through the control system and it failed to operate. In fact in one of the Ammonia plants in Europe a friend told me that every critical shutdown valve is provided with redundant solenoids for greater reliability. Have a relook at all your critical shutdown systems. Its better to be safe than to be sorry!