How internal gear pumps work
As the rotor (outer) gear turns, it drives the idler (inner) gear. When the idler gear comes out of contact with the rotor gear, a low-pressure zone is created. The liquid is drawn into the pump and travels along with the casing to the discharge side of the pump. The gear teeth mesh back together and force the fluid out of the pump.
Proper gear clearance is essential to the long life and efficiency of these pumps. If the clearance is too wide, pump efficiency will suffer. If pump clearance is too tight, as in the case of thermal expansion, the gears will make contact and cause damage or failure.
Unlike a centrifugal pump that cavitates against blocked discharge lines, an internal gear pump will continue to generate flow and build pressure. The pressure will continue to build until something releases it. Oftentimes, this results in a pump failure and a costly component repair or replacement.
Internal gear pumps are oftentimes equipped with an internal or external relief valve. This valve diverts flow when pressure builds in the discharge line, preventing catastrophic failure.