Back Pressure Regulators in Flow Control
Traditional flow control schemes use variable orifice control valves in conjunction with flow transmitters and a closed-loop PID controller.
In an alternative method of flow control a dome-loaded back pressure regulator (DLBPR) can also be used to control flow by controlling the pressure drop across an orifice or an adjustable metering valve.
Reasons to consider a Back Pressure Regulators for Flow Control
Wide Flow Range
Typical control valves operate in a flow range of 10:1 to 15:1. It is common to use multiple flow control valves in parallel when an application exceeds these flow rate ratios. The Equilibar back pressure regulator can easily be configured to control flow rate through a 100:1 flow rate range.
Isolation from downstream pressure changes
By using a BPR, your flow control scheme is automatically buffered against changes in downstream pressure. A back pressure regulator will automatically adjust to keep its input pressure at setpoint regardless of changes in its output (vent) port. When using a traditional flow control valve, any change in downstream pressure or restriction will require a PID control adjustment. This takes time and may be disruptive to your critical process. By using the BPR, the pressure drop across the metering valve is held steady and quickly responds to disruptions downstream.
Equilibar dome-loaded BPRs have ultra high resolution, zero hysteresis, and zero dead-band. This can be useful where traditional control valve deadband (or “stiction”) do not have adequate precision. Small adjustments may be made to the differential pressure resulting in high resolution pressure control. See details for high resolution flow control application.
Simple flow control without a flow meter
In some applications, the purchase of a flow meter may not be economical or practical, such as with severe service conditions. By controlling the pressure upstream and downstream of an orifice, simple flow control can be achieved with a flow meter. The flow rate of most turbulent fluid systems is highly proportional to the flow rate, raised to the second power. (See orifice calculator)
Demanding service conditions
Equilibar BPRs have only one moving part, and are therefore easily configurable for the following severe and demanding service conditions:
Open Loop Flow Control
The schematic below shows open loop flow control. This works for controlling the flow of liquids and gases. The upstream pressure reducing regulator (PRR) sets the P1 pressure to the orifice. The back pressure regulator sets the P2 pressure to the orifice. By adjusting P2 with the back pressure regulator the differential pressure is controlled. If the orifice is of a known Cv or has had its flow characterized, the differential pressure across the orifice effectively controls the flow rate. Open loop flow control is demonstrated in the video.
Improved Valve Turn Down Ratio
By replacing the fixed orifice in the previous schematic with a modulating variable orifice valve, a much wider range of flow rates may be achieved. Traditionally flow control valves may have a turn down ratio of only 10:1. Turn down is the ratio between the highest flow that can be controlled and the lowest flow that can be controlled. A 10:1 turn down means that is 10scfm is the highest flow when the control valve is wide open then 1 scfm is the lowest flow the control valve will likely be able to set with reasonable accuracy.
Traditionally if a higher turn down ratio was required, say 15:1, a second smaller flow control valve would need to be mounted in parallel. This is quite costly and requires a sophisticated control to allow the two valves to smoothly transition. An alternative approach is to retain the original larger 10:1 flow control valve. When low flow rates are required the differential pressure may be reduced allowing the flow control valve to produce lower flow rates.
Closed Loop Flow Control
In the schematic below, a pressure reducing regulator is used to provide a stable gas pressure to the upstream of a Metering Valve. The Back Pressure Regulator has its set-point adjusted by an electronic pressure regulator (EPR) to control the downstream pressure on the Metering Valve. The flow transmitter is monitored by the PID controller to keep the process at the desired flow set-point. One real advantage of this is the reduced workload on the PID circuit and the increased speed of response in the system. Changes in the downstream system pressure are immediately and automatically compensated for when the back pressure regulator modulates to keep its input pressure (P2) at setpoint.
Flow Control Against Pump/Compressor Flow Curve
In the schematic below, the process works in the same manner. However, a pump or compressor can be used to supply a consistent pressure on the upstream of the Metering Valve. This schematic is valid for both process gases and liquids.
Interestingly, it is not necessary that the pump or compressor provide a constant pressure; as the flow rate varies, it is normal for the pump or compressor output pressure to vary. So long as the pressure at any given flow rate is relatively stable, the PID controller is able to provide good control. In fact, for some pumps or compressors, the Metering Valve could be eliminated altogether, with providing flow control by simply adjusting the point on the “pump curve” that the process is operating.
Learn more about Equilibar back pressure regulators.
Learn how Equilibar back pressure regulators can provide high resolution flow control for applications such as temperature control.