How does a peristaltic pump work?

A rotor turns inside a curved track. Mounted on it are shoes, or on smaller pumps rollers, and each one presses the hose completely flat against the track. That full closure is called the occlusion, and it is what separates the suction side from the discharge side.

As the rotor turns, the occlusion travels. The volume of medium trapped between two occlusions is carried round to the outlet and pushed out. Nothing meters it and nothing throttles it. One revolution moves the volume held in the occluded length of hose, multiplied by however many shoes are in contact.

Because the closure is complete, there is no path back. That is why a peristaltic holds its flow against rising discharge pressure where a centrifugal would simply slip.

The return stroke is the half of the cycle people forget, and it is the more interesting one.

Once a shoe passes, the reinforced hose springs back to its circular bore. That recovery increases the volume behind the shoe with the suction line still connected, so it pulls a partial vacuum and draws medium in. The pump is not waiting for atmospheric pressure to push liquid into a flooded suction. It makes its own suction on every revolution.

Two things follow. It is genuinely self-priming, and it will lift dry: Crane rates the ELRO range at 9.5 m of dry suction lift with no foot valve and no additional accessories. And it clears a line of air without vapour-locking, which is why these turn up on tanker offloading and sump emptying.

The hose's ability to recover is doing real work here. A hose that has taken a set, or gone hard with age or temperature, primes worse before it fails outright.

Flow is set by the bore of the hose and the speed of the shaft. Halve the speed and you halve the flow. Raise the discharge pressure and the flow barely moves.

That makes the pump repeatable enough to meter with, and it makes speed the control you size against. It also means the pump has no natural pressure ceiling. Close a valve on the discharge and it keeps displacing until something gives: the hose, a fitting, or the drive. A relief path is part of the installation, not an optional extra. This is the most common way a peristaltic installation is got wrong.

The medium touches the bore of the hose and nothing else. There is no mechanical seal, no gland, no valve, no impeller and no close-running clearance in the wetted path.

That is the whole engineering case for the type. A mechanical seal needs liquid to lubricate it, so on a lobe or centrifugal pump a dry run usually ends in a failed seal. A peristaltic has no seal to lose. Grit does not wear a clearance open, because there is no clearance. Acid does not attack a seal face, because there is no seal face.

What the medium does reach is rubber, so the compound has to suit it. That decision determines how long the pump runs between changes, and it is covered on the peristaltic pump hose page.

Every advantage above is bought with the same component, and the bill arrives in three ways.

The hose is a consumable. It is flexed flat and released on every revolution, so it fails by fatigue in the reinforcement. Replacement is scheduled maintenance, not a fault, and the pump has to be sited where you can get at it.

The flow pulses. Occlusion is discrete, so the discharge arrives in slugs rather than a smooth stream. Where downstream instrumentation or the process is sensitive to that, a pulsation damper goes on the discharge.

The hose sets the pressure ceiling, not the drive. Fitting a bigger motor does not raise it. On the Crane ELRO range the mobile Series T and Series M stop at 1.5 to 2 bar, while the stationary IP and XP pumps are coded in bands reaching 13 bar. If your duty needs more head than the hose will hold at the flow you want, a peristaltic is the wrong type however well it suits the medium.