For decades, piston pumps have been used to meet a wide variety of transfer, extrusion, coating and dispensing requirements across many industrial and commercial applications. The versatility of piston pumps makes them a workhorse in everything from pressure washing, abrasive blasting, bonding, chemical dosing, potting, paint spraying, lubrication and pavement marking to sanitary dispensing, to name a few. With this versatility in mind, the fluid type and viscosity of the material being transferred—and the associated flow rate and output pressure for those materials—must be considered to ensure that the correct pump is selected for the application.
A piston pump is classified as a reciprocating positive displacement pump that uses a piston or plunger to draw and move fluid through a sealed cylinder.
The fluid inlet of the piston pump is often immersed directly into fluid, which is then drawn into the pump via negative pressure as the plunger reciprocates. The combination of check valve timing and plunger motion allows fluid to be dispensed on both the up and down stroke, delivering output pressures up to 15,000 per square inch (psi) and flow rates up to 60 gallons per minute (gpm).
Image 1 shows fluid flow examples for the three types of piston pump lower-end configurations.
As fluid viscosity increases and it becomes more difficult to draw fluid into the pump, a follower plate can be attached as part of a piston pump package and is either powered pneumatically or by gravity that forces fluid into the pump, as seen in Image 2.
Piston pumps are powered by a variety of motor types including pneumatic, hydraulic and electric.
Pneumatic motors are the most common choice due to their reliability, ease of maintenance, good power density, stall capability without
motor damage and inherent safety in volatile atmospheres.
Hydraulically powered pumps offer even better power density than pneumatics as well as quieter operation. Hydraulic piston pumps tend to be the preference for mobile applications where the power take-off (PTO) of a vehicle can be used, or when working in cold conditions where motor icing could cause challenges. Piston pumps powered by an electric motor are energy efficient, but because of low power density, an electric pump cannot deliver the pressures that many industrial applications demand, limiting use to lower viscosity fluids and applications mostly involving transferring or spraying.
Application Pump Types
Multiple pump ends are available for piston pumps to successfully handle a range of fluid properties. These are called lower-ends and they are available in two-ball, four-ball and chop-check types. Two-ball and four-ball style piston pumps are designed for low- to medium- viscosity fluids typically found in transferring, washing and coating applications. For extrusion applications involving high-viscosity materials up to and beyond 1 million centipoises (CPS), both heavy-duty chop-check and two-ball pump packages can be used.
Demanding extrusion tasks often require additional accessories such as a pneumatic ram assembly and a follower plate to provide additional force to move the fluid into the pump, thermal blankets to reduce material viscosity or fluid regulators to ensure accurate dispense control.
Although there is a measure of capability crossover, each piston pump type has limits. It is advisable to consult the manufacturer or dealer when sizing or selecting a pump for a specific application.
Handling Sticky Situations
When using a piston pump to transfer challenging media such as shear sensitive ultraviolet (UV) inks or adhesives with suspended solids, special tubes and plunger coatings and packing seal materials are used. These minimize shear and thermal energy, as well as provide resistance to any abrasion that can occur. A stainless steel pump combined with ceramic-coated cylinders and piston rods offer waterborne benefits and abrasion resistance for more endurance and reduced wear when working with harsh materials. All of these features combined help prevent premature pump failure.
Understanding the fluid and the ultimate goal of the application is essential to maintaining product integrity. Some fluids require shear control or thermal assistance to get them to a viscosity at which they can be transferred easily while minimizing the risk of irreversible fluid damage.
Shear-sensitive materials are diverse and cover everything from lotions and soaps to pastes, cornstarch, inks, some adhesives and certain paints.
Such materials require low internal velocities and gentle pumping action to limit damage and maintain their properties during transfer applications. In the microelectronics industry, many sealing compounds include the addition of fillers for enhanced performance.
This results in a mix of solid and viscous properties that can present challenges in effectively managing the application without damaging the pump or material.