A piston pump traps fluid in a sealed chamber and pushes it out with a moving plunger. Follow the crank, the piston and the two check valves from suction to discharge.
A piston pump is a positive-displacement pump: instead of throwing fluid outward like a centrifugal pump, it traps a fixed volume of fluid and pushes that volume along with a moving plunger. Because the fluid is gripped and shoved rather than spun, the pump can build very high pressure and will keep pushing against almost any resistance.
From the outside it is just a body with a pipe in and a pipe out. The working parts — the crank, the connecting rod, the piston and the pair of check valves — are all hidden inside, and they are what make the pump deliver fluid one way, stroke after stroke.
A motor turns the crank. Because the crank pin is offset from its centre, the connecting rod is swung and slid back and forth as the crank goes round — round-and-round rotation becomes a straight-line stroke.
The connecting rod pulls the piston back. As the chamber grows, the pressure inside falls below the pressure in the inlet, and fluid is pushed up the inlet pipe. The inlet check valve lifts off its seat and lets the fluid in.
The crank keeps turning and now drives the piston forward. The chamber shrinks, the inlet check valve seats and seals, and the fluid has nowhere to go but out through the outlet check valve and up the outlet pipe.
The two ball valves face opposite ways. One opens only for fluid coming in, the other only for fluid going out, and each snaps shut the instant the flow tries to reverse. Together they turn the piston's back-and-forth motion into a one-way flow.
Because the piston only pushes on its forward stroke, the output arrives in bursts instead of the steady stream you get from a centrifugal pump. In practice an accumulator, a damping chamber or several pistons out of phase are used to smooth the flow.
It is a pump that moves fluid by trapping a fixed volume and physically pushing it along, rather than by spinning it. A piston pump is one type: the piston sweeps a volume of fluid out on every stroke.
One lets fluid in and the other lets it out, and each seals when the flow tries to reverse. Together they turn the piston's back-and-forth motion into a one-way flow through the pump.
The piston only pushes fluid on the forward stroke, so the output arrives in bursts. An accumulator or a multi-piston design is often added to smooth the flow.
It can move some air, but running without liquid wears the seals and piston quickly and removes the lubrication the fluid provides. It is best kept primed with liquid.
A centrifugal pump throws fluid outward with a spinning impeller for a smooth, continuous flow. A piston pump grips a fixed volume and pushes it out, giving high pressure and a pulsing flow regardless of the system's resistance.