What is a Pump?

Pumps are machines designed to add energy to fluids. They typically do this by using a rotating element to push the fluid in one direction.

Types of Pumps

While there are several types of pumps in existence, there are two types of pumps discussed here in detail, and two more are mentioned in passing. One type is called a centrifugal pump, the other is called an axial flow pump. The functional difference between the two is in the direction that flow goes through the pump.

In an axial pump, the flow into (called "suction") and the flow out from (called "discharge") both go in the same direction as the axis of rotation of the blades. Similar to a fan¹ , axial pumps are usually used to make fluid go faster without increasing its pressure. Submarine propellers are used in this way, though their purpose is more to move the boat than the water.

In a centrifugal pump, the direction of the discharge is at a right angle to the direction of the suction. This is achieved in a different way from an axial flow pump. When liquid flows into a centrifugal pump, it moves onto an "impeller," which is similar to a merry-go-round. The impeller spins, causing the liquid to get flung out away from the impeller similar to one getting flung off from a fast-spinning merry-go-round. The liquid then is channeled to the discharge port by a circular casing around the impeller. Since the fluid is flung equally in all directions, the net increase in velocity is zero. However, as the fluid does now have more energy than it did when entering, it gets discharged with more pressure than it entered² .

Peristaltic pumps work in a similar way to intestines, in that liquid is run through a tube which is squeezed in such a way as to push the liquid along. The advantage of these pumps is that nothing touches the fluid except the tubing, so this sort of system is often used to move bodily fluids, as during open-heart surgery.

Piston pumps use a rotating bar with a notch cut out of one area. As the piston rotates in a chamber, the notch passes the inlet hole and "catches" a precise amount of liquid. As the piston rotates, the notch passes the discharge hole and "releases" the liquid. The advantage here is that very precise amounts of liquid can be released.

There are many other types of pumps available, though they are not mentioned here for the sake of brevity.

Uses of Pumps

In fluid systems, pumps are used to keep the fluid moving in a useful way. When fluids move, they are frequently required to move upwards through pipes. The pumps provide enough push to keep the fluid going uphill. Also, fluids going through pipes encounter friction, like anything else. Pumps help overcome this friction loss to keep everything moving.

Cavitation and Waterhammer

One of the least desired conditions an axial or centrifugal pump can encounter is cavitation. All fluids have a "vapor pressure," corresponding to a certain temperature. At this temperature, the liquid will boil if the pressure is the vapor pressure or below. As temperature increases, the vapor pressure increases. In pumps, this concept happens in a reversed way. If pressure decreases too much³ , then the fluid will vaporize no matter what temperature it is. When that happens, gas bubbles pass through the pump. This is called cavitation.

Cavitation is bad because gases don't push on things as well as fluids do. So when there is gas on one side of an impeller blade and liquid on the other, there is a force applied on the blade due to the pressure imbalance. This force may be enough to damage the pump.

Furthermore, when the vapor bubble collapses back into liquid, it causes an immense pressure pulse. This pressure hits the nearest solid object (usually the pump impeller) and generally will carve little pieces out of it. Ship propellers generally have little pits in them because of these bubbles. This effect is called "water hammer."

Where to Find Pumps

It is safe to say that wherever one finds liquids in pipes⁴ , a pump is probably nearby. Pumps can be found in cars, refrigerators, air conditioners, power plants, ships, and airplanes, to name a few places. Without pumps, human civilization as it is known today would not exist.