Closed impellers have shrouds on both sides of the vanes, while open (or semi-open) impellers have only one shroud opposite the impeller entrance (see Figure C 6). Closed or open impeller designs are usually available for mixed flow pumps.

The balancing device on centrifugal pumps is designed to fully or partially compensate axial thrust generated by the pump rotor. If the balancing device consists of a balance disc, the entire axial thrust of the pump rotor is usually balanced. In this case, an additional thrust bearing is not required.

Balance line is used to balance the centrifugal pump shaft from axial thrust. Due to the axial thrust, pump bearings and internals will get damaged. To nullify this effect, a tapping from discharge end is connected to a balancing drum.

1 Axial thrust: Axial forces in a single-stage centrifugal pump. Axial forces acting on the rotor in the case of a single-stage centrifugal pump. The axial impeller force (F1) is the difference between the axial forces on the discharge-side (Fd) and suction-side (Fs) impeller shroud.

Also known as End Float. Movement of one shaft along its centreline due to the freedom of movement permitted by a journal bearing or a sleeve bearing. This adjustment should be set before performing vertical or horizontal moves.

To balance radial thrust on volute casing pumps, a double volute is installed instead of a single volute. This volute type consists of two partial volutes which have the same effect and are arranged at an offset of 180° ending in the same discharge nozzle.

Classification of Pumps

• Dynamic. Centrifugal pumps. Vertical centrifugal pumps. Horizontal centrifugal pumps. Submersible pumps. Fire hydrant systems.
• Positive Displacement. Diaphragm pumps. Gear pumps. Peristaltic Pumps. Lobe pumps. Piston Pumps.

The volute casing is designed to guide the flow out of the impeller in order to convert the fluid flow’s kinetic energy into static pressure; it serves to collect the fluid discharged from the impeller and route it to the discharge nozzle (also see Pump casing).

A volute is a curved funnel that increases in area as it approaches the discharge port. The volute of a centrifugal pump is the casing that receives the fluid being pumped by the impeller, maintaining the velocity of the fluid through to the diffuser.

An impeller, typically made of brass, bronze or iron, may either be open or enclosed. An enclosed impeller has a front and back protective shroud, whereas an open impeller does not. Open impellers are often used in low-power applications and the majority of the impeller weight is supported by the hub.

When the flow from the pump is restricted, forcing the pump to operate at a reduced capacity, the flow from the impeller is reduced, and the volute does act as a diffuser, creating an increasing pressure from the cutwater all the way around to the casing throat. The maximum pressure rise occurs at shut-off (zero flow).

In the case of the assembly drawing shown the diffuser contains 10 vanes as compared the volute casing which only has one. Also, while an impeller is placed in the center of a volute, an impeller generally sits directly adjacent to a diffuser and pushes water into the diffuser vanes.

The purpose of the volute is to convert the liquid’s velocity to pressure as the impeller discharges the liquid through the casing nozzle. This is achieved via a gradually increasing cross-sectional area in the volute that increases the fluid’s flow-through area, thus increasing the static pressure of the fluid.

Cavitation occurs when the liquid in a pump turns to a vapor at low pressure. It occurs because there is not enough pressure at the suction end of the pump, or insufficient Net Positive Suction Head available (NPSHa). When cavitation takes place, air bubbles are created at low pressure.

What are the effects of cavitation? The surface of conduit damaged and produce cavities on metallic surfaces. Considerable vibration and noise due to sudden collapsing of bubble. Cavitation effect efficiency of hydraulic machines.

Perhaps the most obvious effect of cavitation on pumps is the material damage that cavitation bubbles can cause when they collapse in the vicinity of a solid surface. Eventually, this will lead to fatigue failure and the sub- sequent detachment or flaking off of pieces of material.

intransitive verb. : to form cavities or bubbles.

The obvious symptoms of cavitation are noise and vibration. When bubbles of vapour implode they can make a series of bubbling, crackling, sounds as if gravel is rattling around the pump housing or pipework.

Cavitation results from tissue necrosis and abscess formation and may be observed on plain radiographs in 38 to 62% of cases (106, 273). Computed tomography frequently reveals multiple nodules and pleural involvement, with cavitation in up to 80% of cases (407).