Industrial valves, in their principal form, consist of a valve body and a closure element which is an internal component that can be moved. This either stops or restricts fluid passing through the body of the valve. For the automation of the valves, actuators are incorporated in the valves. The most efficient method of understanding the valve actuation process is by starting with the exploration of the valves that the actuators are going to automate.
Various types and designs of valves are utilized in different industries. These valves are classified by size, type, industry application, pressure class, and some other considerations. The actuator required for the automation of the valves heavily depend on the four fundamental parameters which will be discussed in the succeeding sections.
#1 Motion of the Valve Operation
Classifying valves according to motion means taking a look at some of the distinguishable differences between the valve designs.
Valves designed with closure elements that require a linear motion can use:
- A linear cylinder powered with fluid, or
- A diaphragm with a smooth stem
Alternatively, to control threaded valve stems, multi-turn actuators with a motor powered with electricity or fluid can be used.
Valves that have a closure element that is operated through a linear movement require a mechanism that converts rotary motion to a linear one. This allows a multi-turn actuator to automate the valve. Although the movement of the valve closure element is linear, the actuator’s movement is rotary. So the rotary movement is converted to a linear one by the valve stem nut.
To attain the desired movement, an actuator that outputs a linear motion are used for valves that do not have this conversion mechanism. A popular example of these valves is pneumatic linear actuators. These actuators are used in the operation of process control valves.
For the majority of part-turn valves, the valve stem needs to be rotated in a quarter turn to shift the valve from an open to the closed position and vice versa. The standard movement of these types of valves is a 90° angle. This allows actuators with pre-known travel distance to be used in the valves. The element of closure on quarter-turn valves can be:
- A disc such as butterfly valves
- A sphere such as ball valves
- The cone frustum such as in tapered plugs, or
- A combination of the before-mentioned types
The most popular type of quarter-turn valves is ball valves. You can check out the XHVAL ball valves for this.
#2 The Force Required to Move the Valve
The required force needed to operate a valve in varying ranges of motion is crucial in the actuator selection process. For valves with sliding stems, the force required would be either a push or a pull in one direction. While for quarter-turn or multi-turn valves, a torque or rotary force is required.
The required torque for the valve operation generally depends on:
- The sizing of the valve
- The design of the valve, and
- The pressure differential through the valve
Generally, the pressure differential when the valve is closed creates the most significant force required by the valve to shift the valve into an open position.
When the valve’s closure element is large, then the force applied to the seats is also large. For specific types of valves like globe valves and gate valves, the calculation of this force is possible. A reasonable estimation of the force demanded by the valve can be acquired for the sizing of the actuator.
These are the other factors you should also be considering:
- The friction of the stem packing
- The temperature of the fluid or gas, and
- The valve’s mechanical features
However, the accurate calculation of the torque demanded by the valve is impossible to calculate independently in quarter-turn valves. You need to do this by measuring the size of the valve in various conditions physically.
Various actuator designs and classifications are required to cover the whole range of output forces. These output forces are needed to cover the wide spectrum of various sizes of valve and classes of pressure. When a greater torque is demanded by a valve, the required actuator is larger and is more costly.
#3 The Speed of Valve Operation
The amount of power needed from an actuator depends on the operational speed required by the application of the valve.
The quantity of work done throughout a specific allotment of time defines what power is. To compute the power of a valve actuator, the following are multiplied with each other:
- the force required by a valve, and
- the distance on which that specific force will be applied (called valve travel)
For example: If the work is needed to be completed for about a minute, then the required power would be twice the power demanded in the valve operation for two minutes.
This is a crucial parameter for electric actuators because the quantity of work done controls the power of the actuator’s motor. While for a fluid power actuator, this parameter affects the exhaust and supply lines size including the control valve size. Whatever the case, the cost is crucially impacted along with the actuator’s physical size.
#4 Frequency of Movement
The frequency required for the operation of the valve affects the durability and sturdiness of the mechanical drive and the controller. Actuators and valves that are used for the isolation and regulation of fluid flow are only needed to be operated infrequently. This results in less wear and tear on the mechanical controls and components of both the actuator and the valve.
Modulating control valves, on the other hand, need to be operated constantly. For these types of valves, a required resistance degree to the possibility of wearing on the assembly is needed. Having mentioned this, it is vital to include modulating requirements to the factors in actuator selection. Not only this, but the controls of the actuator must also be capable of persistent changes without failing of overheating.
The factors discussed in this article are just some of the principal characteristics of a valve that you need to consider in choosing a valve actuator. In the real world, numerous options are needed to be given into account in the selection of the right valve actuator appropriate for your application. Although you can only find here the key factors you should be considering, these will help you decide on your ideal actuator