A dosing pump is selected on the basis of its rated output: a given flow rate at a given pressure. Operators often treat this rated output as a fixed value — the pump delivers X millilitres per stroke, Y strokes per minute, therefore Z litres per hour. In practice, the actual output of a diaphragm dosing pump varies with the back-pressure present on the discharge side of the pump. This variation is not always small, and in applications where dosing accuracy matters — pH control, biocide treatment, chemical neutralisation — it produces real errors in chemical delivery that accumulate silently over time.
Why Back-Pressure Affects Output
A diaphragm dosing pump works by flexing a diaphragm back and forth within a liquid chamber. On the forward stroke, the diaphragm pushes liquid out through the discharge check valve into the discharge line. On the return stroke, it draws liquid in through the suction check valve from the chemical supply.
The volume displaced on each forward stroke is determined by the stroke length — how far the diaphragm travels. In an ideal system with no back-pressure on the discharge, the full stroke length is available to displace liquid. But when there is pressure on the discharge side — from a pressurised process line, a long vertical delivery run, or a high-viscosity chemical that resists flow — the diaphragm must first overcome that pressure before it begins to actually displace liquid forward.
If the diaphragm is not stiff enough, or if the stroke mechanism does not generate sufficient force to fully overcome discharge pressure, the diaphragm compresses slightly rather than displacing the full stroke volume. The pump strokes at the correct rate, but each stroke delivers less than the rated volume. The operator sees the pump running normally. The flow indicator shows normal stroke activity. But less chemical is being delivered than intended.
This effect is most pronounced at high discharge pressures, with high-viscosity chemicals, and at long stroke lengths where the diaphragm is working near its maximum displacement range. It is least pronounced at low discharge pressures, with low-viscosity chemicals, and at reduced stroke lengths.
The Check Valve Contribution
Check valves are a second point of pressure-related output variation. Both the suction check valve and the discharge check valve rely on differential pressure to open and close correctly. If the back-pressure on the discharge is high relative to the spring rating of the discharge check valve, the valve may not open fully on each stroke, restricting flow. If the suction check valve has wear or contamination, it may not seat fully on the forward stroke, allowing liquid to flow back toward the supply rather than forward into the discharge.
In either case, the result is less chemical delivered per stroke than the rated output — and the shortfall increases as discharge pressure increases.
How This Manifests in Practice
The most common scenario is a dosing pump specified for a system at commissioning, where discharge pressure is low. Over time, the system changes: a control valve closes further, a filter downstream becomes partially blocked, a delivery line is extended. The discharge pressure rises. The pump’s effective output falls — but because no one has changed the pump settings, it is assumed to still be delivering its rated dose.
In water treatment applications, this means the active chemical in the system is lower than the control programme assumes. Scale inhibitor residuals fall below effective concentration. Biocide doses fall below minimum inhibitory concentration. The treatment programme appears to be functioning because the pump is running, but the chemistry is actually inadequate.
Pressure-Compensated Pump Designs
Pump manufacturers address this problem in several ways. The most robust approach is a mechanically rigid drive mechanism that maintains full stroke force regardless of discharge pressure variation — so the diaphragm always completes its full stroke and always displaces the rated volume, up to the pump’s rated maximum pressure. This requires a stiffer drive mechanism and higher-quality check valves rated for the expected pressure range.
A secondary approach is a hydraulically actuated diaphragm, where hydraulic oil rather than direct mechanical linkage drives the diaphragm. The hydraulic fluid transmits the drive force uniformly across the diaphragm surface, reducing the localised stress that causes partial stroke compression, and providing more consistent displacement across a wider pressure range.
The Injecta range of motor-driven diaphragm dosing pumps is designed with pressure performance in mind, with models rated to handle the discharge pressures typically encountered in industrial water treatment and chemical dosing applications without the output degradation that affects lower-specification pumps. For applications where back-pressure variation is a known factor, specifying the correct Injecta model at the design stage eliminates a dosing accuracy problem that is otherwise invisible until treatment failure makes it visible.
To discuss dosing pump selection for your specific pressure and chemical conditions, contact us at info@autoflotechnology.com.