How a Liquid Pump Works

How a Liquid Pump Works

Major Features


number1 Air Inlet Port

number2 Air Drive Tube

number3 Drive Piston

number4 Upper Tappet Valve

number5 Pilot Air Tube

number6 Spool Valve

number7Lower Tappet Valve

number8 Pilot Vent

number9 Outlet Muffler

number10 Inlet Check Valve

number11 Outlet Check Valve

 Drive Air Flow

 Exhaust Flow


Air Drive Section

This section comes standard with a lightweight piston consisting of a seal inside a hard-coated aluminum barrel. The size of the air piston remains the same for all air driven pumps in a given series. The drive air forces the piston down on the compression or pressure stroke. The air then forces the piston back up on the suction stroke. Unlike other liquid pumps, air drive line lubricators are not necessary due to the low friction forces of the design and lubrication during assembly.

Hydraulic Section

In this section, the hydraulic piston/plunger is attached to the air piston and its bottom section is housed inside the hydraulic pump head. Its size determines the pressure ratio of the pump, which in turn designates output flow and maximum pressure capability. Its purpose is to pull liquid into the hydraulic body through the inlet check valve and push it out through the outlet check valve at an elevated pressure.

These check valves are spring loaded and direct the passage of liquid through the pump. During the suction stroke of the hydraulic piston/plunger, the inlet check valve opens to its maximum. The liquid is pulled into the pump while the outlet check valve is held shut by a spring and differential pressure. On the pressure stroke, the inlet check valve is closed as the hydraulic piston/plunger moves the liquid out through the outlet check valve.

A seal is positioned around the hydraulic piston/plunger and is one of a few parts that may wear. The seal’s purpose is to hold the liquid under pressure during cycling and to prevent both external leakage and leakage into the air drive. Various seal materials and designs are utilized based on the liquid to be pumped, operating temperature and pressure rating.

NOTE: With most pumps, a separation or distance piece may be utilized between the air drive section and the hydraulic section. This allows for total separation and contaminant-free operation.

Spool Valve

This section is comprised of an unbalanced, pilot operated, lightweight spool which moves the compressed air to either side of the air piston, depending on position. The air piston moves pilot valves at the end of each stroke, alternately pressurizing and venting the large area of the spool, allowing it to control the air flow to the air piston, providing automatic cycling. The main drive air is vented through an exhaust muffler. On the larger pumps, an unregulated pilot air port is used to overcome friction and differential pressures which enables excellent pressure control. This is also an ideal place to use any pump control devices.

Air driven liquid pumps work on a standard reciprocating differential area principle utilizing a large air drive piston connected to a smaller hydraulic piston/plunger to convert compressed air power into hydraulic power. The nominal ratio between the area of the hydraulic plunger and the air drive piston is shown by the dash number in the model description and estimates the maximum pressure the pump is able to generate. The actual ratio can be higher than the nominal so the pump will still cycle when the ratio of the output hydraulic pressure to the air drive pressure equals the nominal ratio. Consult technical information chart in the catalog for actual pressure ratios. Example: an S35 has an actual ratio of 1:39.

This means that the actual ratio of the area of the air piston is 39 times the area of the plunger.


P.R  = Pressure Ratio          = 1:49
PA   = Air Drive Pressure    = 80 psi
PO   = Maximum Outlet      = 39 x 80 = 3,120 psi

If the air drive pressure is raised to 100 psi then the outlet pressure will be near 3,900 psi at stall. The maximum air drive pressure rating on all pumps is 160 psi.

When drive air is initially introduced to the pump, the pump will cycle at maximum speed, providing maximum flow and also functioning as a transfer pump by filling the test piece or actuator with liquid. The pump will then begin to cycle at a slower rate as the outlet pressure rises and offers more resistance to the reciprocating differential piston assembly. The piston assembly then stalls when the forces balance, i.e. when air drive pressure x air drive piston area = outlet (stall) pressure x driven hydraulic plunger area.

The hydraulic pressure drop (hysteresis) needed to cause the air driven pump to restart is very low due to little frictional resistance from the large diameter air drive piston seal and hydraulic seal. This can be as low as two times the pump’s ratio under certain conditions.

The minimum air drive pressure to operate a pump is 15 psi and the maximum is 145 psi depending on pump used.

Double Air Head Pumps

The pressure capabilities of the pumps can be increased without affecting the hydraulic plunger size, by stacking two air pistons, which doubles the pressure ratio. The double air head pumps use less air than other pumps with a single air piston of similar area due to only one of the two heads being pressurized on the return stroke.

Double air head pumps are identified by the suffix -2 in the pump model number. Example: a nominal 1:100 ratio pump (L100) with two air heads is described as an L100-2, 1:200 ratio.


Maximator pumps can be installed in any position, but vertical is best for longest seal life. All connections to the pump, both liquid (inlet and outlet) and air drive lines, must be run with equal or greater size than the connections in the pump.

Drive air should be filtered between 5µ and 40µ and have a maximum dew point of approximately 50°F. Wet air can cause icing and will wash out seal lubricant. For very dry air (dew points below 0°F) a lubricator may be required.

The maximum recommended height of a pump above the fluid level is 10 ft. for LO and L pumps, 7 ft. for S pumps and 3 ft. for PPO and PP pumps.

Special seals for various services are available. Contact your local distributor or High Pressure Technologies directly.