Managing pressure is a fundamental control challenge for most process manufacturers and producers. A frequent question is whether a pressure regulator or control valve is best for a particular application. At last fall’s Emerson Exchange, I live-blogged a great presentation by Keith Erskine with Emerson local business partner, Puffer-Sweiven and Fluor‘s Vince Mezzano on this topic, Regulators versus Control Valves: What’s the Best Fit?
Keith and Vince teamed up on a great article in Valve magazine also titled Regulators versus Control Valves: What’s the Best Fit? Their article’s executive summary:
For today’s up and coming engineers and plant designers, understanding the differences between what regulators can do and what control valves can do will be critical to making the best choices.
This educational article highlights how they differ, what each does, and their use in certain applications.
While both regulators and control valves are used in pressure control applications, they have unique differences:
The design of a typical control loop allows control valves to manipulate a range of process variables depending on which variable is measured for control. Examples of this include valves with capabilities for control of flow, level, temperature and pressure. The process control variable is measured by a sensor/transmitter and then communicated to a host control system, which is typically a distributed control system (DCS).
The main operational difference between a control valve and a regulator is that, contrary to the control loop design mentioned above, regulators are process-powered valves without the need for an external power or instrument air source to operate. A regulator typically applies the pressure of the controlled process fluid against a diaphragm. This diaphragm then opposes a compressed spring to achieve force balance with the diaphragm at a given set pressure.
Because pressure from the process is applied directly to a regulator’s diaphragm, inlet and outlet pressures must be considered in the design. Control valves can handle larger pipe sizes and higher pressures. They are also better suited for corrosive fluids since regulators will almost always have some elastomer material in contact with the process fluid. Regulators are much faster acting since they don’t have the latency of a control loop.
Here is a table of the relative advantages of the two technologies:
The article describes the operation of several types of regulators including direct-operated and pilot-operated. Pilot-operated regulators are designed to reduce pressure droop, defined as:
Pilot-operated regulators have 1-3% droop instead of 10-20% droop for direct-operated regulators. This improved droop performance is achieved by:
…deviance from setpoint to achieve full capacity.
…adding a small, direct-operated regulator (the “pilot”) to the main regulator, which introduces gain to the system and increases sensitivity to changes in the controlled pressure. Pilot-operated regulators have much larger orifices as well, which allows higher capacities as well as heightened accuracy compared to self-operated regulators.
Keith and Vince share five application examples to help illuminate best fits for regulator, control valve, or a combination of both. These applications include: blanket gas control, plant feed gas supply, severe service, differential pressure control, and heater or boiler fuel gas control. You’ll want to read the article to see the rationale for the selection of the best technology.
Pressure control valve selection will always be a critical aspect of facility design and maintenance. By understanding the capabilities and functionalities of both control valves and regulators, engineers tasked with selecting control valves or regulators can start with an optimal solution in mind or reduce rework for their pressure control applications.