Pump Speed Control in Continuous Manufacturing Processes

Key Takeaways: 

• Pump speed should respond to changing flow requirements and process conditions.
• PLC and control interface requirements should be defined early in system design.
• Motor selection must account for speed, torque, pressure differential, and fluid viscosity.
• The pump, motor, controller, and fluid path should be evaluated as one integrated system.

Article I: How Pump Speed Affects Flow Rate Accuracy in Analytical Instruments, examined how pump speed works with displacement, fluid properties, pressure, and the fluid path to determine actual delivery. Continuous manufacturing builds on that relationship.

Here, the challenge is not only achieving a target flow rate. Fluid delivery may also need to change as production starts, reaches steady operation, changes rate, encounters an interruption, or shuts down. That makes pump speed part of the process-control strategy.

From Flow Control to Process Control

In continuous manufacturing, fluid delivery operates alongside connected process steps. When production conditions change, required fluid-delivery rates may need to change with them.

Engineers therefore need to determine:

• How flow rate changes with production rate
• How multiple fluid streams maintain their required relationship
• How pumps respond during startup, shutdown, interruptions, and rate changes
• How the manufacturing system commands these changes
• Whether the pump, motor, and fluid path support the full operating range

These requirements help determine the appropriate speed-control strategy.

Selecting a Pump Speed Control Strategy

The right approach depends on how frequently flow requirements change and how the pump integrates with the process.

Fixed-speed operation may be appropriate when flow remains constant during normal production. Pump displacement, motor speed, and the fluid path should be selected together for the intended operating point.

Open-loop variable-speed control allows the system to command different motor speeds as flow requirements change. Because actual flow depends on the complete fluidic system, engineers should characterize commanded speed versus delivered flow across the operating range.

Feedback or feedforward control may be appropriate when process measurements or other process information determine pump commands. Depending on the application, variables such as flow, pressure, or mass may become part of the control strategy.

Each approach introduces different requirements for sensors, control logic, response time, integration, and validation.

Coordinating Multiple Fluid Streams

Continuous processes may require more than one metered fluid stream. When production rate changes, the required flows may also change. Maintaining a defined relationship between streams does not necessarily mean commanding every pump to the same speed. Pump displacement, fluid properties, tubing volume, and restrictions can differ between fluid paths.

The control strategy should instead define the required flow for each stream, how those flows change with production rate, and how each flow is translated into a pump-speed command.

The objective is coordinated fluid delivery, not simply coordinated motor rpm.

Managing Process Transitions

Startup, production-rate adjustments, interruptions, shutdown, and restart can move a continuous process away from steady-state operation. Pump-speed control should account for these transitions.

Depending on the application, engineers may need to define:

• The sequence in which fluid streams start and stop
• Pump acceleration and deceleration rates
• Transitions between production rates
• Responses to interruptions or loss of fluid supply
• Conditions required before production resumes

Evaluate these behaviors within the complete process.

Accounting for Differential Pressure and Fluid Properties

A speed command validated at one operating point may not produce the same flow across the production range. Required differential pressure can change with flow and with restrictions from tubing, fittings, filters, and valves. Fluid viscosity and inlet conditions can also affect pump performance.

Evaluate pump speed using the intended fluid and fluid path across the required flow, differential-pressure, and speed ranges.

How Will the Manufacturing System Control the Pump?

Defining pump speed is only part of the control decision. Engineers also need to determine how the manufacturing equipment will communicate that command to the pump.

In an automated process, a programmable logic controller (PLC) or another process controller may command pump operation. That raises several integration questions:

• Will speed be controlled locally or by an external controller?
• What control signal or communication method does the equipment require?
• What are the minimum and maximum speed commands?
• How will start, stop, and speed changes be managed?
• How should the system respond outside its defined operating range?

Industrial equipment may use analog command signals such as 0–5 VDC, 0–10 VDC, or 4–20 mA, as well as other interfaces. The appropriate method depends on the equipment and selected controller, so compatibility should be confirmed for the specific configuration.

Early interface planning helps the pump fit both the fluidic and control architectures.

Motor Technology Is Part of the Decision

The controller can request a speed, but the motor and drive must provide suitable performance under the application's operating conditions.

Motor selection should consider:

• Required speed range
• Available torque across that range
• Expected pressure differential
• Fluid viscosity
• Pump displacement
• Startup and transient conditions

Pressure differential, viscosity, pump configuration, and speed all contribute to the operating conditions the motor and drive must support. Motor technology should therefore be evaluated against the complete operating range rather than maximum rpm alone.

Bringing the Pump and Control Architecture Together

For continuous manufacturing, engineers are not selecting only a pump. They are defining how the pump, motor, controller, control interface, fluid, and fluid path work together.

Fluid Metering's CERAMPUMP® valveless technology uses the synchronized rotation and reciprocation of a ceramic piston within a precisely matched cylinder liner. When paired with an appropriate motor and controller, rotational speed can be used to control fluid delivery.

Before selecting a configuration, engineers should ask:

1. What flow range does the process require?
2. What fluid and viscosity will the pump encounter?
3. What pressure differential is expected?
4. What motor technology and speed range are appropriate?
5. Will a PLC or another controller command the pump?
6. What control signal or interface does the equipment require?

Available motor, controller, and interface capabilities depend on the selected configuration and should be matched to the application's fluidic and control requirements.

Pump Speed as Part of the Continuous Process

In continuous manufacturing, the central question is: How should pump speed change with production, and how will the manufacturing system communicate those commands?  

Answering it requires evaluating flow range, displacement, viscosity, differential pressure, motor capability, control signals, multiple fluid streams, and process transitions as one control architecture. Defining these requirements early provides a clearer basis for selecting the pump, motor, and controller. 

Fluid Metering works with equipment manufacturers and process engineers to evaluate flow requirements, fluid properties, pressure conditions, pump and motor configuration, control requirements, and system integration for reagents, buffers, additives, and other process fluids.

Contact Fluid Metering to discuss your flow range, fluid properties, pressure conditions, motor requirements, and control interface for continuous manufacturing.