A valve is a pressure vessel with internal moving parts; and when they move, they encounter resistance from other parts and from the flow. The recording of the pressures and forces associated with overcoming these resistances can be referred to as “valve signature”. For actuator sizing, it is critical to obtain accurate valve thrust/torque data. This is especially true for spring-return actuators as conservative or exaggerated valve thrust/torque data can lead to extremely oversized and expensive actuators.
The thrust or torque required to move the valve, in either direction, will vary throughout its stroke. In industry, the following terms are used to describe the various data points required for actuator sizing:
End to Close
Force/Torque required to seat the valve
End to Open
Force/Torque required to fully open the valve
Break to Close
Force/Torque required to start closing the valve
Break to Open
Force/Torque required to start opening the valve
Run to Open
Force/Torque to keep the part movement at a constant velocity during the open stroke
Run to Close
Force/Torque to keep the part movement at a constant velocity during the close stroke
(fig.1 – see table for details)
The thrust or torque values will vary depending on the design and brand of the valve. For example, a pinch valve will have a very small BTO value compared to its ETC value. Conversely, a gate valve will have a BTO value greater than the ETC. This results in dramatically different actuator sizes for fail-open applications for the same size valve.
If you have more questions about the valve signature and understanding thrust and torque values, Cowan’s valve automation experts can help you size the optimal actuator for your application.