The valve industry is made up of many types of valves that are used in countless types of applications, locations and service conditions. Most fall into one of three operation types: multi-turn, quarter-turn or linear.
Along with this variety are a number of means to operate the valves either manually or via an actuator. These actuators fall into three types: electric, pneumatic or hydraulic. This column focuses on how to properly size an electric actuator for use on a linear valve.
THE WORK OF LINEAR VALVES
Typical linear valves include gate, globe, pinch, diaphragm and needle valves. Depending on the application requirements, these valves may be used in on/off (open/close), inching, jogging or modulating service. A linear valve uses a simple design principal whereby an element is typically pushed and pulled to open or close the valve. This pushing and pulling operation is accomplished by an external force, which is defined as thrust. Thrust is typically measured in pounds (lbs) or newtons (N), and it can be generated by an electric actuator.
When sizing an electric actuator for use on a linear valve, several key inputs must be considered and provided to the actuator manufacturer. At a minimum, the manufacturer must know the thrust required to operate the valve, the required operating speed (typically defined in inches or feet per minute), stroke length of the valve, the available power supply for the actuator, plant control interface or communication scheme, type of valve operation (open/close, inching, modulating, etc.), and environmental and operational conditions of the valve and actuator. We’ll take a closer look at each of these to further understand how they play a role in the selection process.
The required thrust to operate the valve will help the actuator manufacturer initially decide what type of product may be best suited for the application. Depending on the thrust requirements, linear electric actuators can be configured in two basic ways: a direct-mount linear actuator (Figure 1) or a multi-turn actuator combined with a linear output drive (Figure 2). Typically, a direct-mount linear actuator is used for smaller thrust applications (up to around 3,000 lbs) while the multi-turn actuator with a linear output drive is used for larger thrust applications.
Additional inputs that must be considered are required operating speed and stroke length. Some linear valves must operate very fast—these speeds are typically defined in feet per minute—while others may require a much slower operation—typically defined in inches per minute.
Along with the speed of the actuator, the overall required valve stroke length must be considered. It is possible for an actuator manufacturer to create a product that operates at the desired speed; but if the stroke length is not considered, the actuator could reach its end of travel before the valve is full open or full closed. On the other hand, if the stroke length is considerably long and the operation is slow, the actuator may need to be sized and configured for an extended operation period or duty cycle.
As far as configuration, available power supply and plant control interfaces are critical for actuators. Typically, the power supply for linear electric actuators will be DC, 1-phase power (1-ph) AC, or 3-phase power (3-ph) AC. The specific voltage for the actuator motor (i.e. 120 volts AC [VAC] 1-ph, 460VAC 3-ph, 24 volts DC [VDC], etc.) must be provided to properly configure the actuator.
Along with the power supply, understanding the plant control interface or interfaces is critical. This control interface could be as simple as a 24VDC, contact closure signal sent to the actuator to tell it to run open or close. It also could be a signal sent via an analog signal (4-20mA) or a fieldbus protocol (i.e. Profibus, HART, Modbus, etc.) instructing the actuator what to do. The actuator control interface may also be required to send a feedback signal back to the control room or programmable logic controller indicating various parameters such as valve position, fault indications or actuator diagnostic information.
THE VALVE’S OPERATING CONDITIONS
Understanding how—and how often—the valve will be operated factors into the selection and sizing of a linear actuator as well. Will the service be an open/close operation where the actuator will only be required to operate the valve from full open to full close? Will the application require various starts and stops to inch, jog or modulate the valve to control the process? Another criterion that may need to be considered and discussed with the valve manufacturer is the differential pressure across the valve. This could impact the thrust requirements and operation of the actuator.
If the valve will be inched, jogged or modulated by the actuator, understanding how many starts within a specific time period is critical. A higher number of starts within that period (typically defined by the number of starts per hour) may require the actuator to be configured differently to support these operational requirements. If these numbers are unknown, the actuator manufacturer often can define starts per hour for which the selected actuator is rated.
Finally, the expected environmental and operational conditions of the application must be factored into the sizing and selection equation. Will the valve and actuator be installed indoors or outdoors? Does the product need to be submersible? In what ambient temperatures will the product be operated? Does the actuator need to be explosion proof? If so, to what standards and classifications? Does the product need any special corrosion protection? Will the available power remain constant or is there concern the available voltage may experience under- or over-voltage conditions. These may seem like small details, but they often play a very important role in the overall selection of a suitable linear electric actuator.
Sizing and selecting an electric actuator to operate a linear valve can be a very simple process provided the necessary information is communicated between the customer and actuator manufacturer. When this needed data is not provided, the selection process becomes much more difficult, which often will lead to field-related issues. When it comes to avoiding such problems, it’s better to provide the actuator manufacturer with too much information instead of not enough.