AODD pumps and diaphragm dosing pumps deliver flow in discrete slugs — one slug per stroke, twice per revolution. The flow is not continuous; it is a series of pressure pulses separated by brief pauses. In a short, low-pressure system with few sensitive components, this is inconsequential. In most industrial dosing installations, it is not. Pulsation causes real mechanical damage and process errors that accumulate quietly over months until a pressure gauge fails, a pipe joint weeps, or a pH reading drifts without explanation.
What Pulsation Is and How to Quantify It
Pulsation in a dosing system is the periodic variation in line pressure caused by intermittent delivery. An AODD pump operating at 60 strokes per minute (30 cycles per minute) generates 60 pressure peaks per minute in the discharge line. The amplitude of these peaks — the difference between peak pressure and average pressure — depends on the pump’s stroke volume, the operating pressure, and critically, the compliance (elasticity) of the downstream piping system.
In a rigid piping system with no dampening, each discharge stroke sends a pressure wave down the pipe at the speed of sound in the fluid — approximately 1,400 m/s for water-based solutions. This wave reflects off closed valves, pipe elbows, and dead ends, and the reflected waves interact with subsequent pulses. The result is not a simple sine wave but a complex pressure profile that can include peaks significantly above the average operating pressure. Measured pulsation amplitude in an undamped AODD installation is commonly 30–80% of average operating pressure.
Bourdon Tube Gauge Failure
Mechanical pressure gauges using Bourdon tubes are the most visibly affected instrument in a pulsating system. A Bourdon tube is a curved metallic tube that straightens under internal pressure and returns when pressure is released. Each pressure cycle flexes the tube. In a continuous-flow system with stable pressure, the tube is flexed once and held. In a pulsating system, the tube flexes and relaxes 60–120 times per minute.
Metal fatigue from cyclic loading accumulates with each flex. Bourdon tubes in pulsating AODD installations typically fail by cracking at the highest-stress section — usually near the tip or at the point of maximum curvature change — after 10–50 million cycles. At 60 pulses per minute continuous operation, 10 million cycles represents approximately 115 days. Gauges that should last years fail within months. The failure mode is often a slow weep at the crack, resulting in process fluid contamination of the gauge internals and a sticky, inaccurate reading before final failure.
The standard mitigation — before considering a pulsation dampener — is to fit a snubber or needle valve on the gauge port. A snubber allows slow pressure equalisation while blocking the full pulse amplitude from reaching the gauge mechanism. This extends gauge life significantly but does not address pulsation in the rest of the system.
Electronic Pressure Transducer Drift
Electronic pressure transducers used in control loops and data logging are subject to a different failure mode. Most industrial transducers use a sensing diaphragm that deflects under pressure, with the deflection converted to an electrical signal by a strain gauge or capacitive element. Cyclic pulsation stresses the sensing diaphragm in the same way it stresses a Bourdon tube — fatigue crack propagation in the diaphragm or bond layer, or plastic deformation of the sensing element that shifts the calibration baseline.
The insidious aspect of transducer pulsation damage is that it manifests as calibration drift rather than catastrophic failure. The transducer continues to output a signal, but the signal no longer accurately represents average line pressure. In a closed-loop pH or concentration control system, a transducer reading 0.2 bar high translates directly into an under-dosed system — the control logic sees adequate pressure and does not increase pump output. The process error may not be detectable without independent verification.
Pipe Joint Fretting and Joint Failure
Every pressure pulse creates a small mechanical impulse on pipe joints, fittings, and supports. In a plastic piping system — PVC, CPVC, PP, PVDF — solvent-welded or threaded joints experience cyclic tensile and compressive loading as the pipe wall expands and contracts with each pulse. Over time, this fatigue loading causes micro-cracking at the joint root, progressing to weeping leaks and eventually joint separation.
The failure pattern is characteristic: joints that held pressure successfully during hydrostatic testing fail after months of pulsating service. The pulse frequency and amplitude determine how quickly fatigue accumulates. A joint that would last decades under stable pressure may fail within two years under continuous AODD pulsation without dampening.
Threaded connections in rigid materials are particularly vulnerable. Thread engagement concentrates stress at the thread roots, and repeated cycling opens micro-gaps in the thread contact zone that allow chemical ingress, accelerating corrosion or chemical attack on the thread itself. PTFE thread sealant does not prevent this — it seals the gap but cannot resist the fatigue loading.
Check Valve Chatter and Seat Wear
Dosing systems typically have check valves at the pump outlet and at the injection point. In a pulsating system, these check valves open and close with each pressure pulse, not just at the start and end of a dosing event. At 60 pulses per minute, a check valve in an undamped AODD discharge line opens and closes 60 times per minute continuously. The ball or disc impacts the seat on each closure.
Seat wear from this impact loading is progressive. Elastomeric seats deform plastically over time; harder seats develop impact pitting. A worn check valve seat allows backflow between pulses, reducing net dosing accuracy and allowing downstream process fluid to contaminate the dosing line. In pH control applications, backflow of process water into a sodium hydroxide dosing line causes localised precipitation and blockage. In biocide dosing, backflow contaminates the chemical supply drum.
pH Probe Overshoot and Process Errors
In pH control loops, the pulsating delivery of reagent causes concentration fluctuations at the injection point. Even if the average dose rate is correct, instantaneous concentration spikes with each pulse create local pH excursions that the pH probe detects. The control system sees a higher pH reading during the pulse peak than is representative of the bulk process fluid. In a tightly tuned control loop, this triggers the control system to reduce dosing, resulting in under-treatment of the actual bulk fluid between pulses.
The solution to probe overshoot is not to retune the control loop — it is to smooth the delivery upstream. A pulsation dampener on the dosing pump discharge converts the intermittent slug flow into near-continuous flow before the chemical reaches the injection point. The probe then sees a representative, steady reagent concentration rather than a series of peaks and troughs.
What Pulsation Dampening Actually Does
A pulsation dampener — such as the Fluimac Damper range used with AODD installations — is a gas-charged accumulator installed on the pump discharge. The gas pre-charge absorbs the energy of each pressure pulse by compressing slightly, then releases that energy during the stroke pause to maintain flow. The result is that instantaneous flow variation, which might be 0–100% of average flow without dampening, is reduced to 5–15% of average flow with a correctly sized and pre-charged dampener.
The pre-charge pressure, not the dampener volume alone, determines performance. A dampener pre-charged at the wrong pressure — most commonly at atmospheric pressure because no one adjusted it at commissioning — provides minimal benefit. Correct pre-charge is typically 60% of operating pressure. The mechanical damage and process errors described in this article are preventable, but only when the dampener is both correctly sized and correctly pre-charged.
To assess pulsation dampening requirements for your dosing system, contact Autoflo at info@autoflotechnology.com.