Functional Description of Liquid Pumps

HASKEL air driven liquid pumps are "ratio" devices using low pressure air against a large piston area to generate force (force= pressure * area). This force is transmitted through a smaller area plunger or piston in the pumping section to magnify the pressure of a liquid by a factor equal to the nominal area ratio. Using this principle, a large, low pressure air flow can be used to generate a smaller higher pressure liquid flow.

AIR DRIVE FUNCTION: For all but the "M" series pumps, the cycling system, built into the air drive end caps, provides the continuous reciprocating action of the pump. It is composed of an air piston assembly, two pilot valves, and an air cycling valve. The action of the valve depends on an "unbalance" of areas exposed to the drive pressure and the pilot pressure. With the pilot pressure vented, the air drive pressure works against an area of the valve spool to move it to a position routing the air through a flow tube to the pilot vent end cap. This drives the air piston towards the valve end cap. When the air piston contacts and opens the pilot fill valve in the valve end cap, the pilot chamber is pressurized. This pressure, acting on a larger area of the spool than the drive air, moves the spool to its other position. In this position, the pressure on the air piston is shifted to drive the piston towards the pilot vent end cap. When it reaches the cap, it opens the pilot vent valve, permitting the pilot chamber to vent (through the pilot tube) and shift the spool to its original position. The cycle of pressurizing and venting the pilot chamber continues to repeat automatically as long as air pressure is available and the outlet pressure is below stall pressure.

For the "M" pumps, the action is the same, except that the pilot vent function is built into the valve end cap and plunger/piston assembly and the return force is provided by a mechanical spring. The drive air is only used for the "pumping" portion of the cycle, and no "flow" and "pilot" tubes are required.

PUMPING FUNCTION: The pumping section consists of a hydraulic body to contain the pressure, a plunger or piston to move the pumped fluid, and check valves to control the flow direction. The size and shape of this section will vary with the size of the air drive, the nominal pump ratio, and the check valve configuration. Its operational mode is basically the same for all liquid pumps.

The plunger/piston is mechanically connected to the air drive piston and extends through a high pressure seal package into the pumping chamber. The reciprocating (back and forth) movement of the plunger/piston alternately increases and decreases the liquid volume in the hydraulic body (the difference between the two volumes is the displacement per cycle). When the volume is being increased (suction stroke), the inlet check valve opens and fluid enters the pumping chamber. When the plunger/piston reverses direction, the inlet check valve closes to trap the fluid, and the outlet check valve opens to direct the flow into the downstream system (pumping stroke). When the stroke is complete and another suction stroke is started, the outlet check valve closes. This prevents system fluid from returning (back flowing) into the hydraulic body, and permits new fluid to enter the body through the inlet check valve. The process is repeated automatically each cycle until a force/balance condition is achieved between the drive force of the air drive and the downstream pressure on the plunger/piston (stall).

The plunger displacement causes output flow. The pressure is generated by system resistance (back pressure), not the total force available. The "excess" force available over that required to generate pressure is used for "cycle" rate, and determines the output flow. With no back pressure, all of the energy is used for cycle rate, and it will cycle rapidly. As the system builds in pressure, the excess energy decreases, and the pump will cycle progressively slower until it reaches the "force balance" point and then it will stop (stall) because all of the energy is required to generate the desired pressure.

Stopping the pump traps and holds the downstream pressure. The maximum outlet pressure produced by these pumps is a function of the "nominal ratio" and the air drive pressure. For example, an AW-35 (Nominal 35:1 ratio; Actual 40:1 ratio) pump with 100 psi air drive pressure will produce between 3500-4000 psi at the stall condition (because the internal friction of the pump absorbs some of the energy).

When the resistance downstream of the liquid section is high enough to reach the "maximum" pressure noted, the force of the air drive is balanced by the force of the internal friction and downstream pressure against the end of the plunger. The pump stops and holds pressure with no flow. If the downstream pressure is reduced due to leakage, the use of fluid downstream, or if the air drive pressure is increased, the pumping action will re-start automatically because the forces are no longer "balanced". This is an advantage in any application requiring extended periods of constant pressure, because there is no power used at stall as would be required by an electric or hydraulic motor drive that must run continuously.

SYSTEM DESIGN CONSIDERATIONS: The most important factors in maintenance of any mechanical equipment are proper selection and proper system installation. Pumps should be selected to provide the best match for the application (flow rate, pressure rating, type of duty, etc.).

The system should provide an inlet screen on both air and fluid supply lines to prevent particulate contamination from damaging seals and sealing surface finishes.

Permitting pumps to operate "unloaded" for extended periods of time can result in excessive seal wear and higher maintenance cost, as well as shortening the useful life of the pump. Running "unloaded" is equivalent to taking your car out of gear at full throttle...not very good for service life.

Provide an adequate supply of fluid to compensate for system leakage. Fluid depletion can cause the pumps to run unloaded with the possibility of damage, as noted above.

For applications involving "high purity" fluids (i.e. - electronic 'chip' washing), downstream filtration is recommended since no dynamic seal (one that is moving with respect to the mating part) can be "particle free".

Supply piping for the air drive and fluid pumping sections should be at least the size of the connections to obtain the rated catalog performance of the pump. Too small a line size on the air drive supply will result in slower operation and less output. Too small a fluid supply line can cause "cavitation", loss of prime, and failure to pump the expected flow rates. Too small a downstream line will cause excessive resistance, making the pump think it pumping against a higher system pressure, and it will slow down accordingly and put out less flow.

Where the pump pressure capability is greater than the system design pressure, a relief valve should be installed with adequate flow capacity to prevent over-pressurization.