1.3.1 Centrifugal pumps

Centrifugal pumps are fluid-kinetic machines designed for power increase within a rotating impeller. Therefore, it is also called the hydrodynamic pumping principle. According to I this principle, the fluid is accelerated through the impeller. In the outlet connection of the' centrifugal pump, the resulting increase in speed is converted into delivery head.

As referred to previously, centrifugal pumps have prevalent application in liquid pipe-1 line transmission systems because they are capable of handling variable heads and flowl rates [8]. These pumps can handle multi-products and other liquids over a wide range of I viscosities and other properties [9].

Centrifugal pumps are further classified as radial, mixed, and axial flow.

Radial Flow. Pumps in which pressure is developed principally by the action of a centrifugal force are classified as radial flow pumps. Generally, such pumps with a single inlet impeller have a specific speed below 4200, and pumps with double suction impellers have a specific speed below 600. Liquid enters the impeller at the hub and flows radially to the periphery.

Mixed Flow. Pumps in which head is developed partly by centrifugal force and partly by the lift of the vanes on the liquid are classified as mixed-flow pumps. This type of pump has a single inlet impeller with the flow entering axially and discharging in an axial and radial direction. Specific speeds generally range from 4200 to 9000.

Figure 1-3 Family of kinetic pumps

Axial Flow. Axial-flow pumps are sometimes called propeller pumps and develop most of their head by the propelling or lifting action of vanes on the liquid. These pumps have single inlet impellers with flow entering axially and discharging nearly axially. Specific speed of this type of pump is usually above 9000.

Other Classifications.

• Single-stage pump: pumps in which total head is developed by one impeller

• Multi-stage pump: pumps having two or more impellers acting in series in one casing

• Volute pump: pumps having a casing made in the form of a spiral or volute

• circular casing pump: pumps having a casing of constant cross-section concentric with the impeller

• Diffuser pump: pumps with diffusion vanes

• Horizontal pump: pumps with the shaft normally in a horizontal position.

• Vertical pump (dry pit-type): vertical shaft-type pumps located in dry walls

• Single-suction pump: pumps equipped with one or more single suction impellers.

• Double-suction pump: pumps equipped with one or more double suction impeller

Centrifugal pumps are designated as rotating either clockwise or counterclockwise. To de­termine the type of rotation for horizontal pumps, stand at the driving end facing the pump and note the type of rotation. For vertical pumps, determine the rotation by looking at the top of the pump.

1.3.2 Reciprocating pumps

Reciprocating pumps are generally classified into three major divisions: direct-acting, crank and flywheel, and power pumps.

Direct-Acting. A direct acting pump is a reciprocating, steam-powered pump, in which the steam piston is directly connected to the liquid piston or plunger through a piston rod. The length of the stroke is determined by the action of the steam in the steam cylinder. Pumps of this kind can be either simplex or duplex.

Crank and Flywheel. The crank and flywheel pump is also generally steam-powered, with a crankshaft on which a flywheel is mounted for storing energy during the early part of the stroke. The flywheel then imparts the stored energy to the liquid piston or plunger during the latter part of the stroke, after the steam is cut off in the steam cylinder. The stroke length is determined by the throw of the main crank.

Power. Power pumps are driven by power from an outside source applied to the crank­shaft of the pump. Three types are as follows:

• simplex pump: a reciprocating pump having one liquid piston or its equivalent single- or double-acting plungers

• duplex pump: a reciprocating pump having two liquid pistons or their equivalent single- or double-acting plungers

• triplex pump: a reciprocating pump having three pistons or their equivalent single or double acting plungers.