Most flow control circuits require a complicated PID loop. The PID is used to electronically close the loop around the output from a flow meter. The modulating element in this type of circuit is usually either a rising stem type flow control valve or is a variable frequency drive on the pump’s electric motor. While these methods are a proven way to control the flow of a pump, there are applications where this degree of sophistication is not desirable. In example the aggressive nature of the media might require a flow meter technology that is prohibitively expensive or the overall budget for the control loop may not allow for the purchase of the required components. In these cases a simpler pump flow control circuit may be substituted. Refer to the pump flow control schematic below.
A Simple Pump Flow Control for Non Positive Displacement Pumps
The circuit above uses the pressure set point of a back pressure regulator to directly set the flow rate coming out of the pump and being directed into the rest of the circuit. The pump draws the liquid media from the reservoir source. The pump is a non positive displacement type pump. These pumps are also called rotary impeller type or centrifugal pumps. In this style of pump the output flow is not proportional to the rotation of the pump. Instead, the flow of the pump is inversely proportional to the output pressure of the pump. When the output pressure of the pump is low the output flow of the pump is high. This relationship between pump output flow and pump output pressure is shown on the pump’s flow curve, also called a P-V diagram. For every pressure the pump will only deliver one specific flow rate. To control the flow of a pump, it is necessary only to set the output pressure to the point on the P-V diagram that allows the pump to produce that flow rate.
The output pressure of the pump is set using a back pressure regulator. Once the pump output pressure is set the flow rate that the pump will deliver to the rest of the circuit is established. The back pressure regulator will effectively isolate any changes that occur in the downstream system. This happens automatically and without intervention as a natural result of how the back pressure regulator (BPR) works. The back pressure regulator will modulate in order to keep its input pressure (the pump’s output pressure) at the target set point. If the downstream system pressure changes the differential pressure across the BPR will also change. However, the BPR will adjust its internal valve so that the output pressure on the pump is accurately maintained. While a flow meter is shown in the schematic above it is not required for the control of the flow rate. The flow meter is shown only to illustrate that if the flow were measured at that point it would be under the control of the back pressure regulator.Request a Quote
Video Shows a How a Back Pressure Regulator Can Control the Flow of a Pump
Click on the video link below to see an Equilibar back pressure regulator being used to control the flow of a pump in Equilibar’s engineering laboratory. The pump used is a rotating impeller centrifugal type that is can used to supply cooling water.
Accurate Flow Control with Equilibar Back Pressure Regulators
While any back pressure regulator could be used to control the flow of a pump, the unique technology in the Equilibar BPR is particularly applicable. The Equilibar uses a flexible diaphragm membrane as the only moving part. This design eliminates friction to allow the Equilibar to respond precisely and with very low hysteresis. The Equilibar is a pilot operated design where the controlled pressure is set by applying a set point pressure of compressed gas to the top side of the diaphragm. The advantages of the Equilibar are:
- Electronically control the pilot pressure or set the pilot using a manually adjustable mechanical regulator. This allows you to control the flow of a pump with an electronic signal or a manual adjustment. The same Equilibar BPR is used in either case.
- Control the flow of a pump with extremely high resolution. The frictionless diaphragm is the only moving part in the Equilibar BPR and responds quickly to subtle changes in either the system pressure or the pilot set point pressure
Rugged Design Handles Extreme Temperatures and Chemistries
One advantage of using a back pressure regulator to control the flow of a pump is that the BPR is a rugged device that is easily manufactured from materials compatible with your process fluids. The Equilibar back pressure regulator is typically manufactured from bar stock stainless steel 316/316L dual cert for maximum durability and chemical compatibility. The flexible diaphragm material is carefully selected to match your application temperature and media. Diaphragm material choices include PTFE, glass reinforced PTFE, Polyimide (Kapton), Viton, Buna-N, PEEK, HDPE, and others as dictated by the application. Since the diaphragm is the only moving part the Equilibar back pressure regulator is extremely easy to rebuild and maintain and the repair parts are relatively inexpensive.
Additional Methods for Controlling Flow Rate Using a Back Pressure Regulator
Click this link to see an application page that shows how a back pressure regulator can be used to control the mass flow of a compressible gas by regulating the differential pressure across an orifice.
Alternate Flow Control System by Controlling Differential Pressure
This video shows how a back pressure regulator can be used to control the differential pressure across an orifice and therefore control the mass flow rate into a system. This is particularly handy when you need to control the mass flow of a compressible gas.
One of our BD series regulators could be a good fit for your pump flow control application.