Control valves come in two sorts: air to open; and air to close. Air to open valves are normally held closed by the spring and require air pressure (a control signal) to open them – they open progressively as the air pressure increases. Air to close valves are valves which are held open by the valve spring and require air pressure to move them towards the closed position. The reason for the two types of valves is to allow failsafe operation. In the event of a plant instrument air failure it is important that all control valves fail in a safe position (e.g. an exothermic reactor’s feed valves (or, perhaps, just one of the valves) should fail closed (air to open) and its coolant system valves fail open (air to close)). The type of valve used obviously impacts on what a controller has to do – changing the type of valve would mean that the controller would need to move the manipulation in the opposite direction. To simplify things in this course we shall assume that we are always using air to open valves – an increase in control action will cause the valve to open and the flow through it to increase.