How To Specify a Control Valve?

Control valves play an integral role in establishing and maintaining an efficient process for a number of industrial and commercial applications. More than that, they are tasked to handle all kinds of fluids at varying temperature extremes. When selecting a control valve, you must consider not only your application, but the body style, material, and trim construction design you need.

Moreover, it is important to evaluate how the control valve’s features work in tandem with the application’s fluid requirements, fixed flow control, ability to handle a variety of flows, the valve’s delivery time, and connectivity for remote monitoring. While control valve manufacturers and distributors will help you choose the appropriate valve for your application, there is information that you need to provide them that will narrow down your choices.

Valve Specification

Valve specification begins with identifying four essential components that are a part of the control loop: the transmitter, the actuator, the positioner, and the valve. The transmitter is the portion of the control valve that forwards signals to a computer for operators to interpret. The control valve anatomy consists of the actuator that opens and closes the valves, the positioner which converts signals into pneumatic signals, and the valve that blocks flow.

The positioner is typically mounted onto the valve and connects to the instrument air supply which allows the positioner to provide the actuator with air to control valve position and offer diagnostic information for the valve. At the same time, the actuator receives a pneumatic signal from the positioner. The signal becomes amplified to overcome process pressure and provides indication in the case that a signal is lost with failure directions such as “open, closed, last.” There are two main types of actuators: diaphragm actuators and piston actuators.

Diaphragm Actuators

Diaphragm actuators have compressed air applied to a diaphragm with a high surface area, and at the other end of the actuator, a spring dictates fail direction. Moreover, these actuators are low-friction, which means that they provide a quick response to small changes and a longer response time for larger changes in a process.

Piston Actuators

While diaphragm actuators are typically the industry standard, piston actuators are used in their place when the stroke of a diaphragm is too short or the thrust may be too small. Additionally, piston actuators can be single-acting or double-acting, can withstand higher input pressure, and can offer smaller cylindrical volumes which can act at a high speed. Though a double-acting actuator provides better thrust and more precision, it also requires a volume tank that needs maintenance, as well as a locking mechanism to fail in any position other than the last.

Beyond the kind of actuator that fits best for your application, there are several options available for valves. Each valve has distinctive internal mechanisms and transforms the overall process.

Linear Globe Valves

Linear globe valves are typically used for their versatility to regulate flow over a wide range, their ability to mitigate noise, and the ease with which you can access the valve internals during maintenance. Moreover, the valve consists of a disk or cylindrical plug element placed atop a stationary ring seat in a spherical axis.

When it comes to linear globe valves, there are three varieties: stem-guided, cage-guided, and top- and bottom-guided globe valves. For general and less-aggressive applications, stem-guided and top- and bottom-guided valves are better suited for the job. In contrast, cage-guided valves are less cost-effective, but perform in a broader scope of applications with greater flexibility in mitigating noise or cavitation. Nonetheless, each variety can be controlled by electrical, pneumatic, manual, or hydraulic means of actuation.

Linear Angle Valves

Linear angle valves are similar to linear globe valves, but are better equipped to handle more severe applications, for example: applications with larger flow rates, and greater visual and auditory pollution. Stem-guided and cage-guided options are available in this variety, and the actuators normally paired with these valves are pneumatic or hydraulic due to the high internal forces and rapid response time needed for these applications.

Linear Three-Way Valves

For industrial applications, linear three-way control valves are the most common selection. This kind of valve has one inlet and two outlets, or vice versa, which facilitates the mixing of liquids from two pipes into one, or separating water from one pipe into two. Typically, linear three-way valves are used for air-handling units, water chillers, boilers, and fan coils. This valve is not ideal for severe applications, but is useful for applications that require pressure or flow control in one specific section of the piping system.

Rotary Valves

Rotary valves utilize the rotation of a passage or passages in a transverse plug to control the flow of liquid or gas in a system. While globe and angle valves have a more tortuous flow path, rotary valves offer a more inline, less tortuous path, while still providing comparable flow control, range, and resilience. This is ideal for an application where fluids contain solid elements and cleanliness cannot be maintained.

When communicating this data to a manufacturer or representative, it can be quite difficult to differentiate each control valve and its intended application. Luckily for you, ASAP Purchasing represents the evolution of purchasing through our parts purchasing platforms and unmatched supply-chain network. We can match you with a representative that will assist you with valve selection for your specific application. Utilize our search engine to access manufacturer insights, detailed PMA and Technical Standards catalogs, and more. For a quick and competitive quote, email sales@asapsemi.com; we are available 24/7x365.

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