A test bench is asked to hold 700 bar. The air-driven liquid pump is bolted down, the air regulator is opened, the gauge climbs, then it stops short. The pump keeps cycling but the needle does not move. The operator opens the regulator further, the compressor cuts in, and the gauge crawls up another few bar before sagging again.
Almost every case like this traces back to arithmetic that was never done. The drive air supply cannot deliver the volume the pump needs at pressure. The boost ratio was picked from the target pressure alone, with no allowance for line losses on the air side. Or the outlet is being asked for flow at a pressure where the pump has already run out of it. None of these is a pump fault.
Air-driven liquid pumps and electric plunger pumps: where the boundary sits
Both technologies make pressure, in different ways, and the difference decides which belongs on a given skid.
An air-driven liquid pump is a reciprocating intensifier. Shop air acts on a large piston, a smaller plunger acts on the liquid, and the area difference multiplies the pressure. Nothing electrical touches the wetted end. It stalls safely when the outlet reaches its ratio-limited ceiling and holds that pressure indefinitely without adding heat.
An electric plunger pump drives the plunger from a motor through a crank or a screw. It delivers steady flow, it is far more efficient per unit of output, and it runs quietly. It also costs more to control and puts electrical equipment next to the wet end.
The practical split: intermittent pressure generation, a static hold measured in hours, work inside a hazardous area, or a bench used a few times a week, and the air-driven unit wins on simplicity and installed cost. Continuous circulation at high flow, tight flow control, or a duty cycle measured in shifts rather than cycles, and the electric unit pays for itself. A hydraulic power pack built around an air-driven pump, such as the Maximator hydraulic units range, is a common compromise: the intensifier does the pressure work, an accumulator and manifold do the flow work.
Boost ratio and drive pressure
The boost ratio compares the effective air piston area to the liquid plunger area. More usefully, it is the ratio between output pressure and drive pressure at stall:
output pressure at stall = drive pressure × boost ratio
That single line explains most sizing errors. A pump rated at a given ratio does not produce a fixed output pressure. It produces a multiple of whatever air pressure it is given. Turn the air regulator down and the ceiling comes down with it. Run a long, undersized air line and the pressure at the pump is lower than the pressure at the regulator, so the ceiling is lower than the calculation assumed.
Two rules follow. Size the ratio against the lowest drive pressure you will realistically run, not the highest the compressor can reach, and add margin. A pump whose stall pressure equals the target pressure has no headroom for line losses, seal friction, or a cold morning when the compressor is sluggish. Ratios and limits for each frame are listed in the official data sheet, and that document, not a catalogue summary, is what the sizing should be checked against.
Drive pressure is also the control handle. A regulator on the air inlet sets the output pressure, so the pump can be commanded from outside the hazardous area with nothing more than a pneumatic line.
Flow decay
An air-driven liquid pump is a positive displacement machine, but not a constant-flow one. Flow is highest at free discharge and falls as outlet pressure rises, reaching zero at the stall pressure set by the ratio and the drive pressure. Each stroke has to compress the air and overcome the outlet pressure, so the cycle rate drops and the volume per unit time drops with it.
This is the number that decides whether a pump is right, and it is the number most often left out of the enquiry. A pump that delivers an impressive flow at low pressure may deliver a fraction of it at the working pressure of the bench.
Read the flow curve at the pressure the bench actually runs at. For a hold duty the flow question mostly disappears and the ratio dominates. For circulation the ratio matters less than the flow available at the working point. Values for a specific frame belong in the official data sheet.
Media and seal materials
The wetted path decides the medium, and the seals decide whether the installation survives it.
Water is the easiest case, and stainless wetted parts handle it without drama. Add glycol for freeze protection and the viscosity rises, which alters cycle rate slightly. Hydraulic oil is straightforward for the pump and harder for the seals if the wrong elastomer is used. Aggressive fluids, solvents, and anything with suspended solids need to be discussed before a pump is selected, not after.
Seal choice follows the medium rather than the pump. Nitrile is the default for mineral oil. EPDM suits water and glycol and should be kept away from mineral oil. PTFE-based seals cover a wider chemical range at the cost of some mechanical robustness. The seal material named in a data sheet refers to the standard build, and a different medium may need a different build. Maximator valves and fittings sit in the same wetted circuit, so the same compatibility question applies. Compatibility is a property of the pair, not of the pump alone, and a pump that handled water is not automatically suitable for a glycol mix at a different concentration.
Air supply, buffer volume and pulsation control
An air-driven pump is an air consumer, and the air side is where most installations are quietly starved.
The compressor must supply the pump's consumption at the pump's inlet pressure, not at the receiver pressure. A long run of small-bore air line drops pressure and adds restriction, and the pump responds by slowing down. A receiver close to the pump, sized for the surge at the start of a stroke sequence, removes most of this, and a filter-regulator-lubricator at the inlet keeps the air clean.
Pulsation is inherent to reciprocating intensifiers. Every stroke delivers a slug of liquid and then pauses. On a test bench this shows up as needle flutter, and on a fatigue rig it shows up as load ripple. A pulsation dampener or a small buffer accumulator on the outlet smooths the delivery and supplies the first part of any sudden demand, which lets a smaller pump serve a peak load.
For gas work the units in the gas boosters and air amplifiers range are specified on the same three variables: ratio, drive pressure, and flow at the working point.
Five common misselections
Each of these is a real pattern, and each is cheap to avoid at the enquiry stage.
Sizing the ratio to the target pressure with no margin. The bench needs 700 bar, the ratio gives exactly 700 bar on the compressor's best day, and on a cold morning it reaches 640. Add margin for line loss and let the regulator do the trimming.
Choosing the pump before checking the air supply. The compressor is rated for the plant's average demand, not the pump's peak. The pump cycles slowly, the operator blames the pump, and the cause is a 6 mm air line feeding a pump that wants more.
Leaving pulsation to be discovered on site. A gauge that will not settle is read as a pump fault. It is usually an unsmoothed reciprocating output, and a dampener fixes it for a fraction of the cost of a different pump.
Matching seals to the pump instead of the medium. A standard build is ordered, the medium is a glycol mix or a solvent, and the seals fail in weeks. The seal question belongs on the enquiry next to the medium.
Expecting constant flow at rising pressure. An electric plunger pump behaves closer to constant flow, and a specification carried over from an electric installation assumes the same. The air-driven unit does not work that way.
If the duty is high-pressure liquid work, the high pressure pumps range and the S-Series pump are the natural starting points. For a complete circuit rather than a bare pump, the Hydroline EasyX modular line assembles pump, accumulator and manifold as one package. Both are supplied through our Maximator brand page.
FAQ
Can an air-driven liquid pump hold pressure without running?
Yes. Once the outlet reaches the pressure set by the drive pressure and the ratio, the pump stalls and stops cycling. It holds that pressure through its check valves with no further air consumption beyond seal leakage. That is why these pumps are used for long static holds on test benches.
Why does flow drop so much as pressure rises?
Each stroke must compress the drive air against a higher outlet pressure, so the cycle rate falls. Volume per stroke stays roughly constant, so volume per minute falls with it. The relationship is not linear and should be read from the flow curve in the official data sheet rather than extrapolated.
What air pressure do I need to reach a given output pressure?
Divide the target output pressure by the boost ratio to get the minimum drive pressure, then add margin for line losses. The required figure is usually higher than that division suggests, and the compressor and air line must be sized for it at the pump inlet.
Which seal material should be used for water, oil and glycol?
Nitrile is the usual choice for mineral oil. EPDM suits water and water-glycol mixtures and should not be used with mineral oil. PTFE-based seals cover a wider chemical range. The final choice belongs with the medium specification and the official data sheet.
How do I know which pump frame to start from?
Start from three numbers: the working pressure, the flow required at that pressure, and the drive pressure available at the pump inlet. Those three narrow the range quickly. Model-specific ratings are in the official data sheet.
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