Whether it is refineries, manufacturing plants, or pipeline compressor stations, you will find pneumatic diaphragm actuators working day-in and day-out to help run these facilities. An actuator is a mechanical or electromechanical device that converts energy to provide controlled or limited movements.
In order to operate, pneumatic diaphragm actuators contain a piston enclosed within a cylinder. They are powered by the external air or gas source to move the cylinder so that they can:
● Fully open or close the valve
● Partially regulate the open and close movement
Many industries rely on pneumatic actuators for their application. Hence, while choosing one, there are many factors that need to be taken into consideration. Below listed are some of those factors:
1. Usage Factors
Usage factors play a significant role in the selection of a pneumatic diaphragm actuator. These factors are:
Pneumatic diaphragm actuators need an air supply between the range of 40 to 120 psi to produce the necessary torque. However, higher pressures can be difficult to achieve in certain scenarios. At the same time, if your setup can only provide a low pressure, you will need a diaphragm or piston that has a larger diameter.
Hence, you will need to select an actuator that can deliver the torque you require despite the pressure limitations that you may face.
● Temperature Range
Based on the industry, the working temperatures can vary greatly. Depending on the pneumatic diaphragm actuator and the seals, grease, and bearings used, the operating temperature can range from -40 to 250°F. Depending on your temperature range requirements, you will need to select an actuator that can operate in that range.
● Hazardous Areas
Pneumatic diaphragm actuators are the preferred choice when it comes to hazardous or toxic environments. This is because they are explosion-proof in nature. However, if there is a lack of compressed air, your best bet is an electric actuator.
2. Sizing and Force
For a pneumatic diaphragm actuator, the force refers to the amount of torque required to move the valve from its open to closed position. The force required depends on factors like:
● Type of the actuator
● Valve torque
● Minimum and maximum supply pressure
● Fail mode
What’s more, the size and design of the valve as well as the differential pressure are also taken into consideration. Some of the other factors that the size depends on are:
● Media temperature
● Stem packing friction
Mechanical characteristics of the valve and the valve system.
Since sizing valve actuators is a complex process, it is recommended that trained experts determine this value. If you select an oversized valve actuator, it can damage the stem of the valve. Moreover, these will cost more and amount to wasteful spending.
On the other hand, an undersized actuator won’t be able to operate the valve on demand. This becomes an especially crucial factor in case of emergency shutdown valves.
3. Frequency of Movement
The frequency of movement or operation affects the mechanical drive durability as well as the robustness of the controller. As a result, valves that operate at an infrequent rate (like isolating or regulating duty valves) cause less wear and tear on the mechanical components and controls. Hence, vales that work continuously (like modulating process valves) need more durability in terms of valves used and the pneumatic diaphragm actuator assembly.
The capability of pneumatic diaphragm actuators is defined by the number of starts per hour. So, while selecting a valve as per the duty cycle look for:
● 30 – 60 starts per hour for regulating valves
● 600-1800 starts per hour for modulating valves
Safety is one of the primary concerns that should be taken into account while selecting diaphragm actuators. It is always the best choice to select the safest and the most reliable option considering the application specifications.
For example, if a fire breaks out, the pneumatic diaphragm actuator should be able to maintain the fail-safe position. If the most ideal actuator can’t be used due to other factors, you must use additional components to meet the safety and regulatory specifications.
Cost is obviously one of the most important factors that needs to be considered. As a rule of thumb, the greater your torque requirement is, the more power will be needed. Subsequently, the larger the actuator, the higher the cost.
Furthermore, using additional components to adjust for other shortcomings will also increase the overall upfront cost. However, this will also increase the reliability and durability of the actuator. This may prove more cost-effective in the long term.
Remember, being diligent about quality control and being clear about your technical specifications are the most important practices in procuring the right actuator for your application. By picking the wrong pneumatic diaphragm actuator, you will not only fail to operate the valve but also cause damage to the components. The right actuator will ensure long-term use and fewer unexpected shutdowns.