Explain the difference between 'direct acting' and 'reverse acting' control valves.

The difference between direct acting and reverse acting control valves lies in the relationship between the control signal and the valve's stem position, which in turn affects the flow through the valve. In a direct acting control valve, an increasing control signal causes the valve stem to move further into the valve body, increasing the flow through the valve. So, a higher control signal results in a higher flow rate. For example, in a pneumatic direct acting valve, increasing the air pressure applied to the valve actuator causes the valve stem to open further. Conversely, in a reverse acting control valve, an increasing control signal causes the valve stem to retract from the valve body, decreasing the flow through the valve. Thus, a higher control signal results in a lower flow rate. In a pneumatic reverse acting valve, increasing the air pressure applied to the valve actuator causes the valve stem to close further. The choice between direct acting and reverse acting valves depends on the specific application and the desired fail-safe behavior of the system. For example, if it is desired that the valve fails open in the event of a loss of control signal, a direct acting valve would be used. If it is desired that the valve fails closed, a reverse acting valve would be used.