Centrifugal pump working

Centrifugal pump working

Centrifugal pumps are a sub-class of dynamic axisymmetric work-absorbing 

Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volutechamber (casing), from where it exits.

Common uses include water, sewage, petroleum and petrochemical pumping; a  centrifugal fanis commonly used to implement a vaccum cleaner The reverse function of the centrifugal pump is a water turbine converting potential energy of water pressure into mechanical rotational energy.

History

According to Reti, the first machine that could be characterized as a centrifugal pump was a mud lifting machine which appeared as early as 1475 in a treatise by the Italian 

How it works

Like most pumps, a centrifugal pump converts rotational energy, often from a motor, to energy in a moving fluid. A portion of the energy goes into kinetic energy of the fluid. Fluid enters axially through eye of the casing, is caught up in the impeller blades, and is whirled tangentially and radially outward until it leaves through all circumferential parts of the impeller into the diffuser part of the casing. The fluid gains both velocity and pressure while passing through the impeller. The doughnut-shaped diffuser, or scroll, section of the casing decelerates the flow and further increases the pressure.

Description by Euler

A consequence of Newton’s second law of mechanics is the conservation of the angular momentum (or the “moment of momentum”) which is of fundamental significance to all turbomachines. Accordingly, the change of the angular momentum is equal to the sum of the external moments. Angular momentums ρ×Q×r×cu at inlet and outlet, an external torque M and friction moments due to shear stresses Mτ are acting on an impeller or a diffuser.

Since no pressure forces are created on cylindrical surfaces in the circumferential direction, it is possible to write Eq. (1.10) as:[3]

(1.13)

Euler's pump equation

Based on Eq.(1.13) Euler developed the head pressure equation created by the impeller see Fig.2.2

(1)

(2)

In Eq. (2) the sum of 4 front element number call static pressure,the sum of last 2 element number call velocity pressure look carefully on the Fig 2.2 and the detail equation.

u2=r2.ω the peripheral circumferential velocity vector

u1=r1.ω the inlet circumferential velocity vector

ω=2π.n angular velocity

w1 inlet relative velocity vector

w2 outlet relative velocity vector

c1 inlet absolute velocity vector

c2 outlet absolute velocity vector

Velocity Triangle

The color triangle formed by velocity vector u,c,w called "velocity triangle". This rule was helpful to detail Eq.(1) become Eq.(2) and wide explained how the pump works.

Fig 2.3 (a) shows triangle velocity of forward curved vanes impeller ; Fig 2.3 (b) shows triangle velocity of radial straight vanes impeller. It illustrates rather clearly energy added to the flow (shown in vector c) inversely change upon flow rate Q (shown in vector cm).

Efficiency factor

,

where:

is the mechanics input power required (W)

is the fluid density (kg/m3)

is the standard acceleration of gravity (9.80665 m/s2)

is the energy Head added to the flow (m)

is the flow rate (m3/s)

is the efficiency of the pump plant as a decimal

The head added by the pump () is a sum of the static lift, the head loss due to friction and any losses due to valves or pipe bends all expressed in metres of fluid. Power is more commonly expressed as kilowatts (103 W, kW) or horsepower (hp*0.746 = kW). The value for the pump efficiency, , may be stated for the pump itself or as a combined efficiency of the pump and motor system.

Vertical centrifugal pumps

Vertical centrifugal pumps are also referred to as cantilever pumps. They utilize a unique shaft and bearing support configuration that allows the volute to hang in the sump while the bearings are outside the sump. This styleof pump uses no stuffing boxto seal the shaft but instead utilizes a "throttle bushing". A common application for this style of pump is in a part washer

Froth pumps

In the mineral industry, or in the extraction of oilsand, fourth  is generated to separate the rich minerals or bitumen from the sand and clays. Froth contains air that tends to block conventional pumps and cause loss of prime. Over history, industry has developed different ways to deal with this problem. In the pulp and paper industry holes are drilled in the impeller. Air escapes to the back of the impeller and a special expeller discharges the air back to the suction tank. The impeller may also feature special small vanes between the primary vanes called split vanes or secondary vanes. Some pumps may feature a large eye, an inducer or recirculation of pressurized froth from the pump discharge back to the suction to break the bubbles