Last revised: October 31, 2024

Reviewed by: Scott Orlosky, consulting engineer

Flow controllers are electric devices that monitor and provide feedback to control flow-rate variables as part of a process control application. They can be used in conjunction with pumps, valves, blowers, and counters in fluid flow systems. Flow controllers can also be found in manufacturing to count items on a conveyor belt or used to monitor air speed or quantify particulate flow through air or liquid. Flow controllers read the output of a sensor and use that information to open or close gates or valves in response to the signal input.

Flow controllers work by measuring process variables, comparing them to the desired value (compute an error value between actual and the desired value) and then adjusting the flow rate to compensate for the difference. The goal is to drive the error to a value of zero.

In classic control theory this error is used as a forcing function to make adjustments to the flow rate or density of particulates, depending on which is most effective. There are several common ways to use feedback from the error signal and the value of process variables to drive the process back on track.

Limit control — Limit control establishes two set points that are determined by the ideal flow rate + or – an offset value. The controller ensures that the flow rate never goes above the high value, or below the lower value. This is called on-off or "bang-bang" control and on average it produces the right quantity output. It is simple to operate, but not ideal since each on-off cycle is an abrupt change and this type of control can cause oscillations.

Linear control — Flow controllers with linear control use one element of a classic PID (Proportional-Integral-Derivative) type of control. Linear control is just the Proportional part of the control feedback loop. This works by applying a multiplier to the value of the error signal. The bigger the multiplier, the more quickly the controller will drive the output toward the target value. As the actual output moves closer to the target value, the smaller the adjustment becomes (it diminishes linearly with the error – hence linear control). In theory, once the difference between the desired value and the actual goes to zero, then the controller stops adjusting.  The drawbacks in real life are that it rarely goes all the way to zero error because there is always some noise in the system and once the forcing function gets down to the noise level the signal gets lost in the noise.  Also, system response delays can cause the output to overshoot the desired value which causes oscillations. The solution is a full PID control.

Proportional, integral, and derivative control — Proportional, integral, and derivative (PID) control requires real-time system feedback. PID is a common control technique that monitors the error between a desired variable value and the actual value, and adjusts the control accordingly. PID uses an intelligent I/O module or program instruction, which provides automatic closed-loop operation of process control loops. The action starts with a linear control which can move fairly quickly toward the target value. This response is dampened by the derivative portion, which acts on the rate of change of response — controlling overshoot and undershoot; integral corrects any long-term offset by looking at the accumulated error and drives that to zero.

• Proportional control alone — The control signal is proportional to the error between the reference and feedback signals.
• Proportional plus derivative control — The error signal is differentiated to get the rate of change. This type of control is used to increase the controller’s speed of response, and control oscillations in the output.
• Proportional plus integral control — The error signal is integrated and is for eliminating steady state or offset errors. This may also be called automatic reset/bias/offset control.

Feedforward — Feedforward control is a direct control or compensation from the reference signal. It may be open loop or in conjunction with PID control and does not require a sensor since it is driven by the existing measured system variables.

Fuzzy logic — Fuzzy logic is a type of logic in which variables can have imprecise values (as in partial truth) rather than a binary status (completely true or completely false). Information and rules can be inexact, and calculations can be made on these imprecise values.

Advanced or nonlinear control — Advanced or nonlinear control uses process control strategies beyond PID loop control, such as dead-time compensation, hysteresis, lead/lag, adaptive gain, neural networks, and fuzzy logic.

Specifications

Specification for flow controllers include controller functions, number of channels, type of output and user interfaces.

Controller Functions

Common functionalities for flow controllers are:

• Rate indication and control — This functionality monitors, indicates, and/or controls the rate of the material or media throughout the system.
• Batch or totalizer indication and control — This functionality totalizes and indicates the amount of the material, media, or state of a process variable.

Input and Control Channels

To choose a flow controller, there are several important piece of information: the number of inputs, control outputs, and number of zones.

• Number of inputs — The number of inputs is the number of signals sent to the controller. This number includes process variable signals such as temperature, pressure, level, speed, etc., and status input such as on/off.
• Control outputs — The number of control outputs is the total number of outputs used to control, compensate or correct the process. This number includes analog control outputs as well as relays and switches which activate or deactivate control devices.
• Number of zones — The number of zones or areas monitored and maintained by the controller.

Flow controllers can have multiple control modes or functions, which may or may not use different inputs and outputs. Also, multiple control loops may be linked to improve controller performance and/or stability.

Controller Output

• Switch or relay — Some controllers are able to handle high power switching such as relays and optoisolators.
• Update rate — This is the frequency with which devices take readings and adjust their output.
• Flow controllers can have PLC and discrete control and can be compatible with TTL type I/O.

User Interface

Displays for flow controllers can be simple analog indicators, numeric or alphanumeric digital readouts, or video terminal displays. User interfaces are similar.

• Analog interfaces can have switches, dials and potentiometers.
• Digital user controls are typically keypads, menus and other digital interfaces such as PLCs.
• A remote computer can also program these controllers. Common computer interfaces are serial and parallel, but other options such as SCSI or network connections may be specified.

Flow Controllers FAQs

How do different types of flow controllers impact the efficiency of a system?

Proportional control adjusts the flow rate in proportion to changes in the input signal. It offers moderate accuracy and is suitable for most systems. However, it is usually more expensive to implement compared to simpler systems like on/off control 

With on/off control the flow control valve is either fully open or fully closed based on the input signal. It is simpler and less expensive but less precise. Suitable for applications where precision is not critical.

PID Control is a type of proportional control that also considers integral and derivative components. It balances precision and simplicity, reducing overshoot and undershoot, which can improve system stability and efficiency.

Flow limiting devices are passive devices like orifices and flow nozzles that limit the flow rate. They are effective for applications requiring a fixed flow rate without active control. They can be simpler and more cost-effective but lack flexibility.

Higher flow capacity valves can improve system efficiency by reducing the cost of fluid transfer.

Systems with higher throughput valves are more efficient and have lower operational costs. Conversely, restrictions in valve capacity can significantly increase energy usage and operational costs.

Mass flow control measures the particulates in the flow, while volumetric flow control measures the volume.

Mass flow control is generally more accurate and reliable than flow control.  This is important in research applications involving biological or chemical reactions. It improves the quality of research data by providing more consistent results. Other applications are injection molding applications, where precise amounts of raw material must be carefully measured.

Digital controllers can reduce energy costs and noise compared to analog controllers, which require a constant bleed.

Digital controllers are more energy-efficient and quieter, making them suitable for applications where energy consumption and noise are concerns.

Variable frequency drive (VFD) pumps are favored for their increased efficiencies and energy savings. They offer better control and lower energy consumption compared to standard centrifugal pumps. However, any increase in flow needed downstream can significantly increase energy usage.

Progressive cavity pumps (PCP) provide better control for applications requiring low flows and high pressures. They offer more precise dosing and mixing, leading to more consistent results.

When should flow control be used?

When it is necessary to regulate the flow rate or volume of a fluid, gas, or steam to achieve a desired output. 

In applications where the flow rate or volume of a fluid, gas, or steam needs to be regulated, such as in oil and gas, power generation, water and wastewater treatment, and food and beverage production.

Flow control helps avoid Issues such as overpressure, overheating, cavitation, erosion, and water hammer. These all can cause damage to piping systems, valves, and other components. 

What are the benefits of flow control?

Improved efficiency, accuracy, and reliability in controlling the flow rate or volume of a fluid, gas, or steam. It can also reduce operating costs, improve product quality, and enhance safety.

How do different types of flow controllers impact system efficiency?

• Proportional Control: Moderate accuracy, suitable for many control systems.
• On/Off Control: Simpler and less expensive, but less precise.
• PID Control: Balances precision and simplicity, reducing overshoot and undershoot.
• Flow Limiting Devices: Effective for fixed flow rates without active control.

What is the difference between mass flow control and volumetric flow control?

Mass flow control measures the number of particulates in the flow, while volumetric flow control measures the volume. Mass flow control is generally more accurate and reliable, especially in research applications or other situations requiring accuracy.

What are the advantages of digital controllers over analog controllers?

Digital controllers can reduce energy costs and noise compared to analog controllers, which require a constant bleed.

Flow Controllers Media Gallery

References

GlobalSpec—Valveless flow control: The paradigm shift from centrifugals and control valves to progressive cavity pump