Last month we discussed the internal operation of reciprocating pumps, one of two types of positive displacement pumps. This month we will analyze how the other type of PD pump – rotary pumps.
Rotary pumps have rotating gears instead of the backward and forwards motion of cylinders like reciprocating pumps. In some cases, they can operate sealless with the speed of the rotating component creating a liquid ring. This liquid ring traps gas between the liquid ring and pump chamber to create a superficial seal while running. This motion also creates suction at the inlet.
There are multiple variations of rotary pumps. Here are the types of rotary pumps and how they operate:
Gear Pumps are a common type of rotary pump that consists of two interlocking gears. They can be either internal or external with distinction depending on the size and rotation motion of the gears.
An external gear pump has gears of the same size that rotate in opposite directions. Sometimes motion is driven by the other. These pumps create suction by trapping fluid between the gear teeth and outside casing by both gears. Fluid pressure and flow are ensured by tight tolerances between these components preventing fluid from leaking towards the discharge side and interlocking gears that keep liquid from flowing in the center.
Tight tolerances between these components ensure the fluid does not leak towards the discharge side and as the gears interlock as they rotate together to prevent fluid flowing through the center, which would disrupt the pressure. There are three types of external gear pumps: spur, helical, and herringbone.
An internal gear pump operates by using different sized gears where a smaller one rotates around larger stationary gear. This functions similarly to external gear pumps, creating flow by trapping fluid between the cavities of the two gears. Both internal and external pumps need to be well lubricated by the fluid they pump and should never run dry because of the friction mechanics inside. They are ideal for high viscosity fluids such as oils and paints.
Two other common rotary types are lobe pumps and vane pumps. Lobe pumps operate almost identically to external gear pumps except the lobe shape means there is no direct contact. This reduces the certainty of wear that comes with gear pumps. Vane pumps mount aspherical components on a shaft that is off-center from the inner chamber. The spherical component contains moveable vanes within that extend when the space increases. These vanes, when extended, trap fluid between them as they rotate toward the discharge port. This flexibility of the vanes comes from spring-loaded designs, or they are put under hydraulic pressure.
Lobe pumps are commonly found in chemical and food industries. They provide low shear and are easy to clean and sanitize making them ideal for sanitary, pharmaceutical, and biotechnology applications. Vane pumps feature durable designs as vanes are resistant to flowing solids. They are predominantly used in applications with low viscosity fluids common in the automotive industry with transmission systems, fuel loading, and transmission oil.
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