Actuators & Controls
- Published on Friday, 14 December 2012 12:01
- Written by Rich Oaks
The key to successful sizing or selection of the right valve actuator for a job is communication. That communication occurs between the specifier of the automated valve, the valve supplier and the actuator supplier. The selection process itself is straightforward—actuator manufacturers publish selection or technical data charts. The task of the person who is sizing or selecting consists of comparing the project’s valve actuation requirements against the actuator manufacturer’s published data.
The information required to properly select an actuator comes in two parts. The first part consists of the application requirements such as the ambient conditions, enclosure requirements, operating time or speed, and control interface. These criteria are defined in the actuator specification and related documents. That specification describes the detailed features the designer requires of the actuator.
Sometimes the specification includes critical actuator sizing requirements such as a factor of safety to be applied to the unfactored valve operating torque. Because this is a crucial part of communication, it’s critical that the valve supplier pass along that actuator specification to the actuator supplier.
The specification usually consists of a written document with control schematics, and in the case of electric actuators, a wiring diagram. Another critical piece of information is the actuator power medium—the available air or hydraulic pressure for a fluid power actuator or electrical mains power (voltage, phase and frequency) for an electric actuator.
The second part of the information required consists of valve operating requirements. Typically, valve operating criteria consist of: valve type and size, type of actuation (multi turn, part turn or linear), and torque and thrust required over the full cycle of valve operation. The torque/thrust requirements include the seating and unseating positions and the run torque required during mid-travel, valve stem characteristics including diameter, key dimensions, and thread pitch and lead as appropriate.
Other necessary information might include required valve and actuator mounting interface and dimensions, space considerations, and gearboxes provided by the valve supplier that can turn a part-turn requirement into a multi-turn requirement. These criteria vary by valve type and vary from manufacturer to manufacturer. The valve manufacturer is the best and most reliable source for that information.
Sometimes the information given to the actuator supplier is not complete or the actuator supplier has additional questions. Unfortunately, many written specifications are not as clear as they should be or have conflicting requirements. The actuator supplier is trained to identify those discrepancies. That’s when additional two-way or three-way communication among the specifier, valve supplier and actuator supplier is necessary.
When the actuator supplier has all the required information, the selection process is usually clear-cut. The actuator supplier combines the specification needs with the valve operating criteria and determines the overall valve actuation requirements. Those requirements are then compared with the appropriate actuator manufacturer’s published technical data or selection charts.
Actuator manufacturers have specific products and resulting charts for multi-turn, part-turn and linear actuators. Those charts list specific actuator models and the performance criteria of those models. Criteria include torque rating, thrust rating as appropriate, stem diameter acceptance, available operating speed or closing time, mounting flanges and weight. Any of those criteria can be critical or determining factors in selecting the particular model.
For example, a specific actuator model may meet the valve operating requirements for torque and thrust, but the valve stem diameter is too large for that model. That causes the actuator selection to be increased to accommodate the stem diameter. In this case, the valve stem diameter is the critical factor in selecting the actuator.
Again, communication comes into play—dialog between the valve supplier and actuator supplier on the subject of stem diameter could lead to the valve supplier decreasing the stem diameter, allowing the smaller, more optimal actuator to be selected.
When an electric actuator is selected, the stall torque of the selected actuator should receive attention. Position limit sensors, torque sensors and phase correction devices furnished in state-of-the-art electric actuators protect the valve from motor stall, provided the actuator was properly commissioned on the valve. There are isolated cases, however, where the valve may encounter the stall torque of the actuator, which is up to 50% above the published torque rating, particularly if the actuator motor controls are not provided by the actuator supplier. For this reason, assuring the valve and valve actuator mounting interface can withstand that stall torque is critical.
Open communication of all specification and valve operating data will result in proper actuator selection and achievement of the ultimate goal: a correctly applied automated valve.