There are several methods for controlling the flow of a pump.
One method is to ensure the pump always meets its minimum flow requirements by setting up a recirculation loop from the reservoir, also called a bypass line. When the process demand is low, the pump output will still meet the minimum flow requirements. More information about this type of pump flow control is discussed on this application page about bypass valves.
Another method of controlling pump flow is to use a pump with a variable speed drive which in turn changes the flow of the pump. We do not discuss that method here.
A third method for controlling flow from a pump is to throttle the discharge by opening and closing a valve at the exit of the pump. This method changes the pressure at the pump discharge which results in a specific flow rate based on the characteristic performance curve of the pump. This method is described below.
Simple Pump Flow Control for Non Positive Displacement Pumps
Oftentimes in order to control the flow of a pump, a complicated PID loop is used to electronically control the loop based on the output from a flow meter. The modulating control element is usually either a rising stem flow control valve or 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 such a setup is not desirable. For example, an aggressive media might require a flow meter technology that is prohibitively expensive. In these cases, a simpler pump flow control circuit using an Equilibar Back Pressure Regulator may be preferred (see below).
The circuit above uses a back pressure valve to set the flow rate coming out of the pump to the process. With a non positive displacement style pump, also called a rotary impeller or a centrifugal pump, the output flow 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 pressure and pump output flow is shown on the pump’s flow performance curve, also called a P-V diagram (see graph at right). For every pressure, the pump will only deliver one specific flow rate. Therefore, to control the flow of a centrifugal pump, simply set the output pressure to the point on the P-V diagram that allows the pump to deliver the desired flow rate.
The output pressure of the pump is set using a back pressure regulator. Once the pressure is set, the flow rate to the process is established. The back pressure regulator (BPR) will isolate any changes that occur in the downstream system by making adjustments in order to keep its inlet pressure (the pump’s output pressure) at the target setpoint.
Stainless steel Equilibar BD Series BPV with 2″ NPT ports and an electronic air pressure regulator providing the pilot setpoint signal. Using an electronic pilot signal allows the BPV to accurately control the flow of a pump by setting the pump’s output pressure to the desired point on the pump’s pressure versus flow rate performance curve. Equilibar BPV’s are available in port sizes from 1/8″ to 4″.
Accurate Flow Control with Equilibar Back Pressure Regulators
While any back pressure regulator (BPR) could be used in this control scheme, 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 BPR to respond precisely and with very low hysteresis. The Equilibar is a pilot operated design where the controlled pressure is set by applying a setpoint pressure of compressed gas to the top side of the diaphragm. The advantages of the Equilibar are:
- Can choose to 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.
- Can 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
Video Shows a How a Back Pressure Regulator Can Control the Flow of a Pump
Click on the video link to see an Equilibar back pressure regulator used to control the flow of a pump in Equilibar’s engineering laboratory. The pump is a rotating impeller centrifugal type used to supply cooling water. In this video we demonstrate that the flow rate from the pump is repeatable, allowing you to come back to the same flow rate time and time again. A schematic of the laboratory setup is shown at right.
Schematic of Equilibar Back Pressure Regulator in a pump discharge line to control the flow of a pump as demonstrated in the video. In the demonstration the water is fed back to the storage tank to reduce waste, but it is not used as a bypass line. The manual valve in the recirculation line is simulating process system demand situations.
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.