A Pressure Regulator is a Flow Control Valve
Gerald Bloom, Plast-O-Matic Valves, Inc.
As pointed out in many engineering texts, a pressure regulator is actually a control valve. The CE article discusses the ordinary regulator, whether spring-loaded or pilot-operated, as an economical substitute for a control valve with pneumatic or electric actuator and a separate controller.
For really precise control of pressure, flow rate or temperature (the three most common process variable), a piloted-operated pressure regulator with some electronics added can fit into many process applications. This is shown schematically in the attached sketch, (forgive me for neglecting them! Viscosity and PH can also be controlled with this setup. All that is required is a sensor with an electrical output signal and a process that can be affected by flow rate).
The control system shown is called a “closed-loop” system. An ordinary spring-loaded regulator is a good example of a simple closed-loop system:
1) The spring is adjusted to the desired force (say 100lbs.).
2) Process fluid (for example, sulphuric acid) is allowed to flow through. Initially, the regulator is wide open.
3) As flow builds up, downstream pressure rises.
4) Assuming the regulator diaphragm area is 5 square inches, the regulator will begin to close off as downstream pressure rises.
5) At the regulator valve opening where downstream pressure equals 20 psig, the system reaches equilibrium. That’s because 20 psig x 5 in ² = 100 lbs force to oppose the spring force.
In the case of the simple regulator, the diaphragm is both the transducer (pressure sensor) and actuator. However, a simple pressure regulator, including the pilot-operated (air-Loaded) type, has several shortcomings when compared to the more elaborate system shown in the attached sketch. For example:
1) It is limited to pressure sensing only.
2) It offers proportional control only (thus “droop”).
3) Sensing is very local (at the outlet of the regulator), rather than at the point of action.
If we think of the regulator as a controller rather than as “just a pressure regulator”, we may see much broader application and implication of this product line.
Control can be applied to a variety of functions:
- Pressure – To obtain very precise control of pressure at any desired (downstream) point in the system.
- Flow – Because pressure influences flow rate, a flowmeter signal can be used to vary upstream pressure in response to system changes in order to maintain a steady rate.
- Temperature – Output from a thermocouple or RTD (or thermistor) can maintain a constant temperature, for one example by controlling flow through a heat exchanger.