Every stroke of an AODD pump delivers a discrete pulse of fluid. This is not a defect — it is the fundamental operating principle of any positive displacement pump. But pulsation is also one of the most overlooked sources of process problems downstream, and the stroke rate you set has a direct and predictable effect on what that pulsation looks like at your instrumentation, your piping joints, and your process end.
Getting the relationship between stroke rate and pulsation right matters more than most operators realise — especially once you understand that reducing pulsation is not simply a matter of running faster.
What Pulsation Actually Is
When the AODD pump’s diaphragm completes a discharge stroke, it pushes a volume of fluid into the discharge line over a short time period — typically a fraction of a second. This creates a pressure wave that travels down the discharge line at the speed of sound in the fluid. The fluid velocity during the stroke is high; the velocity between strokes drops to near zero as the diaphragm resets. The result is a repeating cycle of pressure rise and fall — pulsation.
Pulsation amplitude is the magnitude of these pressure swings. Pulsation frequency is how often they occur per unit time. Both are determined by the pump stroke rate and the stroke volume, and both affect downstream equipment differently.
How Stroke Rate Changes Pulsation Character
A higher stroke rate does not eliminate pulsation — it changes its character. At high stroke rates, the pulses are more frequent but each one carries a smaller volume, because the pump is completing shorter, faster strokes. The pulsation frequency increases, the peak-to-peak pressure amplitude of each pulse decreases, and the flow appears smoother on average.
At low stroke rates, the pulses are less frequent but each one carries more fluid in a single, larger discharge event. The pulsation frequency is lower, but the peak pressure on each pulse is higher and the inter-stroke drop in line pressure is more pronounced. Low stroke rates produce a more aggressive pulsation signature despite the pump running slowly.
This is counterintuitive to many operators. “Run it slower” is a common first response to pulsation complaints — but it usually makes the pressure swings worse, not better.
What Pulsation Does to Downstream Equipment
The consequences of pulsation depend on the amplitude, frequency, and what sits downstream of the pump.
Flow meters. Turbine meters, paddlewheel meters, and some magnetic flow meters depend on steady flow to produce accurate readings. High-amplitude pulsation causes the flow meter to read inconsistently — oscillating between falsely high and low values on each pulse cycle. The meter average may be close to correct, but the instantaneous readings are not. For dosing applications where instantaneous flow is used to control a process, this introduces real dosing error.
Pressure gauges and transmitters. Repeated pressure cycling fatigues Bourdon tube gauges and damages the sensing elements of pressure transmitters over time. A gauge that shows significant needle oscillation at the pump discharge is accumulating fatigue damage. Snubbers — small orifice restrictors installed in the gauge connection — damp the pressure peaks reaching the sensing element, but they do not eliminate pulsation in the main line.
Piping and fittings. High-frequency, high-amplitude pulsation imposes repeated stress cycles on pipe joints, threaded fittings, and valve bodies. This accelerates fatigue at threaded connections and screwed fittings — particularly where the thread form creates a stress concentration. In chemical service, pulsation fatigue in screwed fittings is a common source of drip leaks that are incorrectly attributed to seal failure or corrosion.
Process control valves. A control valve positioned downstream of a pulsating AODD pump sees the flow demand change many times per minute. If the valve is sized and calibrated for steady flow, it will hunt and oscillate trying to maintain setpoint against a pulsating input. This affects process quality and accelerates valve seat wear.
The Pulsation Dampener’s Role
A pulsation dampener installed at the pump discharge is the primary tool for reducing pulsation amplitude before the discharge line. The dampener works by absorbing each pressure pulse into a gas-charged bladder or diaphragm, which then releases the stored energy smoothly between pulses, creating a more even flow profile in the line.
The effectiveness of a pulsation dampener depends on its gas pre-charge pressure relative to the system operating pressure, and on the stroke volume it needs to absorb. Sizing a pulsation dampener requires knowing both the pump stroke volume and the line pressure — not just the pump flow rate. A dampener correctly sized for a pump at 40 strokes per minute will be undersized if the same pump is later run at 80 strokes per minute with a different air pressure, because the stroke volume and discharge pulse profile have both changed.
The Fluimac Damper range — in D20 through D50 sizing codes — is designed for use with AODD pumps in chemical service. Pre-charge pressure should be set to approximately 60% of the operating line pressure at the dampener location. If the pump operating pressure changes significantly, the pre-charge must be re-adjusted to maintain dampening effectiveness.
Finding the Right Stroke Rate for Your System
The optimum stroke rate for pulsation management is usually in the middle of the pump’s operating range — high enough that the inter-stroke drops in pressure are small, but not so high that the pump is running near its maximum air consumption rate. For most AODD installations, this means operating at 40–80 strokes per minute with the air supply set to deliver the required flow at that rate.
A needle valve on the exhaust line is the simplest tool for controlling stroke rate independently of the discharge pressure. Restricting the exhaust slows the rate at which the spent air chamber empties, which slows the diaphragm reset and reduces stroke rate. This is one of the few AODD operating adjustments that improves pulsation, diaphragm life, and air consumption simultaneously — provided the restriction is not taken so far that exhaust back-pressure builds to a level the air valve was not designed to handle.
If you are experiencing pulsation-related problems downstream of an AODD pump — flow meter inaccuracy, pressure gauge oscillation, or fatigue at pipe connections — contact Autoflo at info@autoflotechnology.com. We can review your stroke rate, pre-charge setting, and dampener sizing to identify where the issue originates.