Stroke length vs. Speed Control: How Each Affects Turndown

In chemical dosing and process fluid handling, selecting the proper control method, stroke-length adjustment vs. speed control, directly influences the achievable turndown ratio and dosing accuracy. Turndown ratio, defined as the ratio of maximum to minimum flow while maintaining specified accuracy, is vital for applications that require flexible, precise dosing

We’ll compare stroke-length control and speed control, then explore how combining both approaches can dramatically expand turndown capability.

Understanding Turndown Ratio

Turndown ratio refers to the operational range of a metering or dosing pump, expressed as the maximum flow divided by the minimum controllable flow at an acceptable accuracy level. For example, a pump rated at 10 GPH that can accurately deliver as low as 2 GPH has a turndown ratio of 5:1. Higher turndown ratios enable accurate dosing over broader ranges, which is essential in applications such as boiler chemical feed, wastewater treatment, or food-grade chemical blending.

Metering pump manufacturers often guarantee turndown performance based on controlled volume dosing, with typical standards designed to maintain ±1 % or better steady-state accuracy across the entire range.

Why Turndown Ratio Matters for Users:

• Accurate low-flow dosing: Especially when chemical usage varies by season or plant batch size.
• Avoiding over- or under-injection: Helps reduce costs and maintain process control.
• Flexibility in system design: High turndown allows oversizing pumps safely for expansion or variable loads.

Stroke-Length Control

How Adjusting Stroke-Length Affects Pump Turndown

Stroke-length control adjusts the distance the pump’s diaphragm or piston travels during each stroke. By mechanically shortening the stroke, the pump delivers a smaller volume of fluid per cycle while maintaining stroke frequency. This method provides consistent dosing intervals and maintains shear profile, making it ideal for applications that require regular pulsing and minimal flow variation.

In most mechanically actuated diaphragm pumps, stroke length adjustment is achieved via a micrometer dial or an electric positioner. This control approach typically delivers a turndown ratio of 10:1, meaning the pump can reliably reduce flow to 10% of its maximum rating while maintaining dosing accuracy/

Benefits of Stroke-Length Control:

• Maintains dose frequency for steady chemical reaction rates
• Ensures uniform stroke profile for each dosing cycle
• Reduces mechanical stress compared to extreme speed reductions
• Provides reliable low-flow operation without risking motor stall

Considerations:

• Mechanical stroke adjustment may not support real-time, dynamic process control
• May limit responsiveness in highly variable flow conditions
• Resolution is limited by the adjustment mechanism, typically in fixed increments

Stroke-length control is often preferred in systems where maintaining consistent flow characteristics is more important than extreme range variability. When paired with speed control, it becomes even more powerful—something we’ll explore in Section 5.

Speed Control

Using Stroke Frequency to Adjust Flow and Expand Turndown

Speed control varies the rate at which a metering pump cycles, without changing the stroke length. This is typically achieved with a variable frequency drive (VFD) or pulse control system, which adjusts the motor speed or solenoid actuation rate. By lowering stroke frequency, the pump delivers less fluid per minute while maintaining full stroke travel and dosing precision.

Depending on the motor and control system, speed adjustment can provide a turndown ratio of 10:1 to 100:1, offering significantly greater flexibility than stroke-length adjustment alone. Solenoid-driven diaphragm pumps, for example, can operate at a rate as low as 1 stroke per minute, allowing for extremely low flow rates while maintaining dosing accuracy.

Benefits of Speed Control:

• Enables wide turndown for variable dosing applications
• Ideal for automated systems with PLC or flow signal integration
• Simplifies flow rate changes without manual adjustment
• Reduces mechanical complexity compared to adjustable stroke designs

Considerations:

• At very low speeds, dose frequency drops, which may cause inconsistent chemical reactions in some processes
• Low-speed operation may reduce drive torque, increasing the risk of stalling in high-viscosity or high-pressure applications.
• Pulsation may increase at lower frequencies, depending on pump design

Speed control excels in applications with highly variable flow requirements, frequent changes in dosing rates, or remote monitoring needs. For many facilities, this method simplifies automation and enhances dosing accuracy, particularly when combined with digital flow measurement or batching controls.

Impact on Turndown & Application Selection

Comparing Control Methods and Best-Fit Use Cases

Each flow control strategy, stroke-length adjustment, speed control, or both, offers different advantages depending on the process, accuracy requirements, and system design. Here’s a comparison to help clarify:

For facilities requiring precise low-flow capabilities, solenoid-driven diaphragm pumps offer a significant advantage. These designs can operate reliably at speeds below 1 stroke per minute, achieving accurate micro-dosing without compromising stroke consistency or drive performance (Murphy & Dickey, Modern Pumping Today, PSG Dover).

When to Use Stroke Length vs. Speed Control

Choosing the Right Control Strategy for Your Application

To determine the best metering control method, consider the following:

• Do you need pulse uniformity and consistent dose frequency?
  Opt for stroke-length adjustment. This ensures that each dose remains the same size, which is essential in timed or cyclic feed applications, such as boiler treatment.
• Do you need extremely low-flow rates or highly variable dosing?
  Choose speed control, or better yet, combine stroke and speed adjustment for the widest turndown range and responsive flow tuning.
• System constraints, such as motor torque or startup lag?
  Be aware that low-speed operation may require oversizing, pulsation dampening, or enhanced motor control.

For help configuring the right system, Illinois Process Equipment provides expert pump selection and application support. From diaphragm dosing systems to digital solenoid metering pumps, IPE works with leading brands to match your flow control needs with proven technology.

Selecting between stroke length and speed control comes down to understanding your dosing requirements, system limitations, and performance goals. While each method offers advantages on its own, combining both can unlock exceptional turndown ratios and precision for demanding applications. Illinois Process Equipment assists facilities in implementing efficient and flexible metering systems, providing expert support and access to top-performing pump technologies.

Illinois Process Equipment (IPE) specializes in engineered pump solutions for chemical dosing, metering, and fluid handling applications. We offer a wide range of metering pumps with stroke-length, speed, and combined control capabilities to meet your system’s unique requirements. Contact us to explore the best stroke-length vs. speed control options for your turndown needs.