Why some pumps will burst their own pipes if you close the valve
Close the outlet on a centrifugal pump and it simply churns. Do the same to a positive-displacement pump and it keeps shoving fluid into the blocked line until the pipe ruptures or the pump wrecks itself. That single difference explains why these machines need relief valves, and it starts with how each one moves liquid.
A pump moves liquids, gases or slurries by mechanical action, usually turning electrical energy into fluid energy. They are everywhere: drawing water from wells, aerating ponds, cooling car engines and injecting fuel, moving oil and gas, circulating air conditioning, and even serving as artificial hearts. There are three basic families. In a centrifugal pump the fluid turns through ninety degrees as it passes over a spinning impeller; in an axial-flow pump it keeps going straight; and a positive-displacement pump traps a fixed volume and forces it out.
That trapping is what makes positive-displacement designs behave as constant-flow machines. At a given speed they deliver roughly the same amount whatever the pressure downstream, apart from a little extra internal leakage. Unlike a centrifugal pump, they have no shutoff point, so against a closed valve pressure climbs without limit. The fix is a relief valve on the discharge side, ideally an external one that routes excess fluid back to the supply.
The family is large. Gear pumps carry liquid around the outside of two meshing gears and circulate engine oil in cars. Peristaltic pumps squeeze flexible tubing with rollers, which keeps them clean enough for food, medicine and concrete. Progressing cavity pumps use a long helical rotor turning inside a rubber sleeve to shift sewage sludge full of debris. Reciprocating pumps push with pistons, plungers or diaphragms; they fed steam boilers in the 19th century and survive today in bicycle pumps and hand soap dispensers.
Cheap espresso machines often rely on a vibratory pump, whose only moving part is a spring-loaded piston driven by a solenoid. It is so inefficient that it usually cannot run for more than a minute before overheating. Rotary designs have their own weakness: run them too fast and the fluid can cavitate, eroding the tight clearances they depend on.
Source: Pump
