Valve/Actuator Characteristics

Resolution is the smallest change in the position of a control valve stem. This limits the ability of the control system to position the valve precisely to the desired position. If the adjustment increments are so large that the flow alternates between too high and too low, a lower resolution valve/actuator may oscillate continuously around the desired setting. Although resolution is an important factor in designing a stable control system, some users do not fully understand the concept of resolution.

Sticky action is a combination of "sticking" and "friction". In valves, especially in or near the closed position, control signals for incremental motion are by forces from the actuator that need to be overcome before the valve can respond.

Backlash This is a characteristic feature of valves and other mechanical devices and arises as a result of clearance between mating parts. When a reversal of direction occurs, no movement occurs until all the clearance is taken up.

"Whenever we change direction, we need to pass through dead space. It tends to be highest near the closed position, especially for rotations. Suppliers will not provide you with deadband and resolution data near the closed position unless you specify otherwise."

Rotary valves have piston actuators that convert linear motion into rotary motion to drive the valve. Inherent in the design are many mechanical connections with backlash (pin joints, rack and pinion, or grooves) that create more backlash than designs such as diaphragm actuators with fewer connections.

Deadband is sometimes confused with hysteresis, which is the phenomenon of different valve positions caused by the same signal, depending on whether the valve is open or closed. In control valves, this effect is much smaller than deadband. In some places hysteresis is an important consideration, but in valves it is usually less than 0.1 per cent.

Selecting the best valve and actuator for the job

Up-and-down travelling (shut-off) valves typically have minimal clearance and sticking and offer a wider throttling range than rotary valves. However, they may not be suitable for viscous fluids that increase friction or for handling slurries. In addition, they can be expensive if sized large.

Diaphragm actuators offer many advantages. They typically have a smaller backlash compared to the 1% of piston actuators, often with a resolution of 0.1%. For greater thrust, high-pressure diaphragm actuators can now operate at pressures up to 90 psig (621 kpa); for conventional diaphragm actuators, the maximum atmospheric pressure is 30 psig (207 kpa).