Automated Process Control
Using Dome-loaded Pressure Regulators
Electronic pressure regulators (EPR's) can easily control inert gases in low and medium flow rates. However, in order to control process liquid and gas pressures, a dome-loaded pressure regulator is often used in conjunction with the EPR.
Dome-loaded means that the set-point to the regulator is provided by a fluid (typically air) pressure on the top dome port. Dome-loaded regulators can have a ratio of 1:1 (process to set-point pressure), or can have magnifying ratios such as 10:1.
Dome-loaded pressure regulators, coupled with electronic pressure regulators can provide a comprehensive method of providing computer control to a process liquid or gas stream. This approach has certain advantages over traditional pressure control valves, which typically have a delayed response time, limited flow ratios, and depend on PID loop tuning. Large chemical plants, however, typically use pressure control valves instead of pressure regulators due to economic scale factors.
Dome-loaded pressure regulators come in both "back pressure" or "pressure reducing" varieties. Back pressure regulators control the inlet pressure, and open to reduce upstream pressure. Pressure reducing control their outlet port and open to increase downstream pressure (see more about the differences).
Several example applications are shown below:
Dome-load Back Pressure Regulators
Liquid Pump Application
The schematic above shows how a dome-loaded back pressure regulator can be used to control the flow rate to a dispensing application. Instead of using a complex variable speed pump, the output pressure of any type of pump can be controlled by the BPR, which allows only enough fluid to return to the supply vessel as required to limit the dispensing pressure. The PID flow control loop controls the signal to the electronic pressure regulator, which thereby controls the set-point of the dome-loaded BPR with an air pressure signal.
While this example does use a PID flow controller, the reaction time of this system is infinitely faster than that of a PID controlled control valve. That is because the control valve action has to swing suddenly with every change in dispensing flow rate. However, the BPR will hold the header pressure constant without any change in controller output. The PID is therefore only used for small adjustments to the header pressure, and can be tuned slower while also enjoying instantaneous reaction from the regulator.
Dome-loaded Back Pressure Regulators
Gas Reactor Application
The schematic above shows an electronic pressure regulator and a dome-loaded back pressure regulator controlling the pressure of a laboratory reactor, such as might be used in catalyst research. Two separate Mass Flow Controllers (MFCs) are used to inject reactants in the vessel. The vessel pressure is monitored by the process computer. The pressure in the reactor is commanded by the computer through an electronic pressure regulator, which produces an air or nitrogen pressure signal to the dome-loaded BPR. The BPR allows only enough gas to escape from the reactor to maintain the precise desired pressure setting.
Dome-loaded Pressure Reducing Regulator
The schematic above shows how a dome-loaded pressure reducing regulator with 10:1 ratio can be used to maintain high pressure in a vessel. While electronic pressure regulator is ranged 0-100 psig, the 10:1 ratio on the PRR controls the process outlet pressure at 0-1000 psig. Ratio regulators are very popular for higher pressure ranges because of the ready availability of instrument air and electronic pressure regulators in ranges up to 100 psig.
Ratio regulators are very convenient for high pressure applications. Electronic pressure regulators are commonly available in ranges up to 0-100 psig, but can be purchased from Proportion-Air up to 1000 psig. More expensive pressure controllers can also be purchased up to about 4500 psig.
Learn about electronic pressure regulators from Equilibar.
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