When plants struggle with inconsistent chemical addition, foaming, corrosion, or missed pH targets, the culprit is often poor control of the dosing rate. This means that even a reliable metering pump can deliver unstable results if the stroke length is not set correctly. In other words, the stroke setting is the heart of repeatable metering. A clear grasp of stroke in a pump, how it interacts with stroke frequency and backpressure, and how to verify delivery will turn a touchy system into a predictable one.
Why Stroke Length Matters in Metering and Dosing
What makes stroke length so important for a stroke dosing pump?
- Direct impact on flow rate: A longer stroke displaces more volume per cycle, so flow increases at the same speed.
- Turndown without losing accuracy: Adjusting stroke length can maintain check valve seating quality at low rates, improving repeatability compared to speed-only control in some ranges.
- Process stability: The right combination of stroke length and speed reduces overshoot in pH or ORP control and limits slug dosing.
- Mechanical health: Excessively short strokes may lead to poor valve dynamics and siphoning. Excessively long strokes at high speed may increase pulsation and wear.
If stroke control is ignored, you may see erratic setpoint control, frequent over-dosing, pump cavitation, air binding, and failed compliance targets.
How Stroke Works in a Dosing Pump
What is “Stroke” in a Pump?
Stroke is the linear travel of the diaphragm or plunger during one cycle. Stroke length is the adjustable proportion of that travel, usually set as a percentage on the pump head or via an actuator.
How Stroke Length Produces Flow
Delivered flow per minute ≈ displacement per stroke × strokes per minute.
This means that stroke length modifies displacement, while stroke frequency modifies how often the displacement occurs.
Common Architectures
- Mechanical diaphragm metering pumps: Manual micrometer adjusts crank eccentricity to vary stroke length.
- Hydraulic diaphragm pumps: Oil-backed diaphragm with stroke adjustment on the hydraulic side for high-pressure duties.
- Plunger metering pumps: Plunger travel defines displacement and is adjusted via crank mechanism.
- Solenoid pumps: Often vary effective stroke by pulse width or frequency. Some models simulate stroke length through electronics.
Variants of Control: Stroke Length vs Speed vs Hybrid
| Control Method | How It Works | Best Use Case | Pros | Watch-outs |
|---|---|---|---|---|
| Stroke length control | Adjusts diaphragm or plunger travel | Low flow with high accuracy at constant speed | Maintains valve dynamics at small flows | Too-short strokes can cause valve chatter |
| Speed control | Varies strokes per minute via VFD or pulses | Wide flow changes with quick response | Good dynamic control and lower pulsation at modest rates | At very low speed, check valves may not seat reliably |
| Hybrid control | Set stroke near optimal, trim flow with speed | Critical applications needing wide turndown | Combines stability and range | Requires commissioning to find sweet spot |
Benefits and Advantages of Getting Stroke Length Right
- Performance: Tighter control bands for pH, ORP, chlorine, coagulants, antiscalants, and polymer feed.
- Operational ease: Reduced need for manual “tweaks” and fewer alarms.
- Compliance and quality: Repeatable dosing supports discharge permits and batch specifications.
- Asset protection: Less corrosion and scaling due to steady chemistry.
- Sustainability: Lower chemical consumption and fewer off-spec batches.
How to Choose the Right Stroke Strategy for Your Dosing Pump
Core Selection Factors
- Required flow and pressure: Define maximum, minimum, and normal operating points with backpressure.
- Turndown ratio: Decide whether you need 10:1, 20:1, or more. Hybrid strategies often give 100:1 with better repeatability.
- Chemical properties: Viscosity, off-gassing tendency, crystallization risk, and compatibility influence stroke settings and materials.
- Valve dynamics: For gassy fluids like hypochlorite, longer strokes at lower speed often seat valves better.
- Controls integration: Do you need a stroke control of dosing pump with 4–20 mA, pulse, or Modbus to work with PLC or PID pH controllers?
Small vs Large Installations
- Small skids and OEM panels: Fixed stroke near midrange and pulse-speed control for simplicity.
- Large plants and variable loads: Set stroke around 70 to 80 percent for valve stability and modulate speed with PID for dynamic response.
Quick Decision Checklist
- Is minimum accurate flow achievable without dropping stroke below 20 to 30 percent?
- Can the pump maintain check valve seating at the lowest rate?
- Do we have a backpressure valve and calibration column installed?
- Is the suction line NPSH adequate to avoid cavitation at long strokes and high speed?
Considerations: Efficiency, Cost, and Sustainability
- Lifecycle cost: Proper stroke settings reduce diaphragm and valve wear, cutting spares.
- Energy efficiency: Speed control saves energy at lower flow. A hybrid approach avoids extremes that cause rework.
- Chemical stewardship: Stable stroke settings minimize over-dosing and neutralization by-products.
- Safety and design: Add backpressure and anti-siphon valves, pulsation dampeners, and secondary containment around tanks.
Practical Setup: From Bench to Plant
- Start with hydraulics right: Keep suction short and flooded, add a foot valve with strainer, and eliminate air pockets.
- Set stroke midrange: Begin around 50 to 70 percent. This keeps valve dynamics healthy.
- Trim with speed: Use VFD or pulse rate to meet the target flow under closed-loop control.
- Verify with a calibration column: Measure actual delivery at several points and note the linearity.
- Tune your PID: If working with pH or ORP, start with proportional control, add integral slowly, and avoid aggressive derivatives.
How Stroke Length Interacts With Process Dynamics
- Backpressure: A stable backpressure using a valve reduces flow ripple and keeps the stroke-volume relationship linear.
- Viscosity and temperature: Cold, viscous fluids need slower strokes for full chamber fill.
- Gassing fluids: Use longer strokes and lower speed to allow bubbles to exit and valves to seat.
- Pulsation and mixing: Where slug dosing is a concern, add a dampener or split chemical addition at two points.
Quick Buyer’s and Commissioning Checklist
- Target flow range, pressure, and required turndown
- Minimum accurate flow without dropping stroke too low
- Backpressure valve, anti-siphon, and pulsation dampener
- Materials compatibility for fluid and temperature
- Calibration column installed and used during startup
- PLC or controller integration with clear scaling
- Spare wet-end kits and a documented maintenance interval
Frequently Asked Questions
What is stroke length on a dosing pump?
It is the adjustable travel of the diaphragm or plunger that sets the displacement per stroke, expressed as a percentage of the maximum stroke.
Is it better to control flow by stroke or speed?
Both work. If you need wide turndown and responsive control, use speed. If you need valve stability at very low flows, increase stroke and reduce speed. Many experts prefer a hybrid approach.
How does stroke affect accuracy?
Very short strokes can cause poor check valve performance and inconsistent delivery. Keeping stroke above 30 to 40 percent often improves repeatability.
Does stroke length change discharge pressure?
Stroke length primarily affects displaced volume. Discharge pressure depends on system backpressure and pump capability, not the stroke setting alone.
How do I know my stroke setting is correct?
Use a calibration column to measure delivered volume at several setpoints. The plot of setpoint versus delivered flow should be near linear with minimal hysteresis.
What about pulsation at long strokes?
Longer strokes at low speed can increase per-stroke volume ripple. A pulsation dampener and backpressure valve reduce this effect.
If you are looking for stable, low-waste chemical feed, start by mastering stroke length. Set stroke for healthy valve dynamics, trim with speed for range, and verify with a calibration column. Depending on your chemistry and load swings, a hybrid strategy will usually deliver the best mix of accuracy, responsiveness, and diaphragm life. Evaluate your minimum accurate flow, install basic hydraulic accessories, and standardize your commissioning steps to turn dosing into a predictable, compliant process. The Verito Engineering Team can help guide you with the right system. Contact us for detailed information.