Updated: Here’s the link to the paper, Choosing the Proper Pressure Control Device for your Application.
Pressure is typically one of the most fundamental control parameters for process manufacturers and producers. Many types of final control elements including control valves and pressure regulators are used to perform this function. How do you decide which to use?I received an advanced copy of a paper being developed by Emerson’s Jake Buford. The paper, Choosing the Proper Pressure Control Device for your Application, does a great job of describing the differences and considerations in your selection process.
Control valves are defined as valves that open and close in response to signals that are monitoring a specific process variable. Examples of these variables include pressure, temperature, flow and many other application criteria. Control valves are typically actuated either pneumatically, hydraulically or electrically. This means a control valve is connected to some sort of plant operating system, for remote operation and for plant operation signaling.
Pressure regulators are self-contained valve and actuator combinations that limit flow through a restricting element to match flow demand downstream. The desired performance of a regulator is to meet any downstream flow demand while maintaining a constant outlet pressure. Pressure regulators do not require any type of external actuation, which is where the self-contained portion of the definition comes into play. Unlike control valves, they operate solely by sensing pressure fluctuations and making corrective adjustments. This means they are usually limited to applications where pressure is the controlled variable.
Jake describes the three main components in a pressure regulator—a restricting element, a measuring element, and a loading element. The restriction part controls the pressure by limiting the amount of flow that can pass through the device. The measuring element:
…senses pressure changes in the system and tells the regulator to open or close to meet system demand.
This measuring element is typically a diaphragm which rises and falls on pressure changes which in turn opens or closes the restricting disk and orifice element. The loading element, typically a spring, controls what the downstream pressure will be by resisting the upward push on the diaphragm.
A control valve consists of multiple components:
…the valve itself, the actuator and the positioner. The valve consists of mostly the same elements as a regulator. Positioners commonly are used to move a control valve to a determined open position to meet the specified process variable being controlled. The actuator is the device that actually performs this opening or closing action.
Jake notes the key difference:
…regulators are self-contained devices. This means that they are powered by system pressure and require no outside inputs. Because of this, regulators are normally limited to only controlling process pressure.
As you consider which may be best suited for your process, here are some key differences. If your application requires more than pressure control, such as flow, temperature, level, etc. than a control valve should be used. If the pipeline size exceeds 10 in/250mm or pressure exceeds ANSI 600, or fluid being controlled is not clean, then a control valve is the best choice. They also offer a broader selection of materials and trims for different types of applications.
Pressure regulators are simpler devices, do not require external power to operate, are typically lower cost, provide a turndown of more than 100:1 versus 20:1 for control valves.