Beyond the classroom: An expanded view of flow control

In school, engineers are taught that flow control and pressure control are related but distinct concepts — controlling flow focuses on volume while controlling pressure focuses on force. In real-world applications, however, it is often the interaction between the two that gives innovators an opportunity to create powerful new process methods.

Such opportunities have become even more exciting in recent years. Developments in flow chemistry and continuous manufacturing have put a novel twist on longstanding traditions of steady-state flow control for dosing and inline blending. Expanding the traditional model further extends flow control into level control, temperature control, pH control and even coating weight control.

Interaction between pressure and flow control

As innovators push the envelope to adapt processes, it can be helpful to review how flow control and pressure control systems converge to create new process methods.

The flow chart in Figure 1 provides an overview of the interactions where pressure control assists with flow control, and flow control is then used in either a direct application or to assist in the control of other variables.

As illustrated, pressure and flow often work together to enable a specified outcome. In one configuration, flow may be controlled with a constant supply pressure and a variable resistance such as a control valve or solenoid valve. In another configuration, flow control can be achieved by varying the supply pressure to a fixed resistance.

Constant supply pressure with variable resistance

A large manufacturing plant that processes fluids through several steps provides a classic example of flow control in which a fluid with constant supply pressure is sent to a proportional-integral-derivative (PID) control loop with a flowmeter and a control valve. In this situation, engineers could use a globe-style flow control valve acting upon a relatively constant fluid pressure supplied by a pump or header. The flow control valve adjusts in response to a flowmeter and PID controller, providing variable resistance to achieve a desired flow set point (see Figure 2).

Variable supply pressure with constant resistance

A paint spraying system provides an example of using variable pressure and constant resistance (see Figure 3). A pump is needed to keep fluid paint circulating, and the spray nozzle needs various pressures applied to reach the proper flow. A back pressure regulator (BPR) is often used to control pressure in the recirculating loop with the pump. The resistance of the spray nozzle is considered a fixed resistance, which can be approximated by flow coefficient or an orifice equivalent. Considering that virtually all industrial fluid processes are turbulent (even viscous paint spraying), the flow rate can be approximated as the square root of the applied pressure. The BPR provides the variable pressure to control the flow.

Direct flow control applications and modern flow chemistry

The aforementioned steady-state process control strategies have been used in the chemical industry for decades in processes such as dosing and bottle filling.

During the past 20 years, however, innovative developments have resulted in new ways of taking advantage of the interaction between flow and pressure control. Pharmaceutical, fine chemical and agrochemical industries have accelerated the conversion from batch-style reaction kettles to inline blending and continuous reaction flow chemistry using precise pressure and flow control techniques. These developments have been fueled by the availability of extremely reliable and precise Coriolis flowmeters and the desire for simplified and more flexible processes.

In continuous flow reactor processes, precise flow control of reactants ensures proper ratio control for a consistent reaction resulting in a quality final product. Reactor systems are uniquely designed for equilibrium management, residence time and reaction speed. Designs typically meter reactants through precision liquid Coriolis mass flowmeters (MFM) and gas mass flow controllers (MFC), which are MFMs with an integrated solenoid valve. Fluids are metered into a reactor where the conversion takes place inline and continuously.

In the example in Figure 4, a liquid is supplied at constant pressure through a liquid Coriolis MFM paired with a control valve to control flow. In parallel, a gas at constant supply pressure is metered through a gas MFC to control flow. They are mixed together at the reactor inlet and flow continuously through the reactor. Temperature is controlled to optimize the reaction, and an optional BPR may be used to increase reaction rate or control phases of certain byproducts. This design may be used in processes such as hydrogenation.

A similar design would work for inline product blending where ingredients are metered and blended in a modern steady-state continuous manufacturing process to reach a desired ingredient ratio.

Controlling flow to control other variables

Expanding the pressure and flow control interactions even further, flow control can be used to affect other variables such as pH, level, temperature or coating weight.

The case for cascade control

Cascade control uses two feedback loops, with a primary outer control loop sending a set point signal to the secondary inner control loop. Together, they control the final process variable. Cascade control works well when the inner loop responds quickly to set point changes and is controlling a primary outer loop that is slow to respond to changes (level, temperature, pH, etc).

Cascade control loops are commonly designed so that the secondary inner loop controls flow, and the primary outer loop adjusts the flow set point to achieve desired final process variable. The advantage to this arrangement is that the inner loop will respond quickly to process upsets and send a signal to the outer loop, avoiding large excursions in the final process variable.

pH cascade control application

In one case study, a water treatment facility was designing a pH control system where CO2 would be sparged into a body of water to control the pH of the water. (See Figure 5.) The application involved the need to control mass flow over a wide flow range to cater to various volumes of water, while at the same time, reducing time lag between CO2 injection and pH changes of the water.