Centrifugal Pump

WORKING PRINCIPAL

The work takes place at impeller which accelerates the liquid by whirling it through the impeller thus adding centrifugal force and hence acceleration.

When an object is spun around in a circle it is accelerated outward by centrifugal force

When liquid is spun around in a circle, it accelerates outward from the center of the circle due to centrifugal force

Liquid enters the pump suction (nozzle) and then into the eye (center) of the impeller.

When the impeller rotates, it spins the liquid sitting between the vanes outward and provides centrifugal acceleration.

As liquid leaves the eye of the impeller, a low-pressure area is created causing more liquid to flow toward the inlet. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force.

Conversion of Kinetic Energy to Pressure Energy

Centrifugal force creates the kinetic energy.

The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.

CATEGORY: Orientation of Shaft Axis

Vertical

commonly installed in a drilled and cased well

function is to lift liquid (e.g.: fire water pump) from the water level in the well/tank to the surface and provide a specified discharge pressure at the surface.

Horizontal

pump is aligned horizontally

function is to transfer liquid and build pressure

CATEGORY: Number of Stage

Single Stage - has only one impeller

Double Stage - has two impellers

Multi Stage - has more than two impellers

CATEGORY: Casing Split

Radial split / Vertical split

split vertically and the split parts are known as a case and cover
used for high temperature applications due to even thermal expansion of the shaft

Axial Split / Horizontal split

split horizontally into two pieces - the upper case and lower case
permits removal of the complete rotor without moving either piping or motor
limited to temperatures up to 450 ºF. Hotter applications can cause shaft misalignment due to uneven thermal expansion

CATEGORY: Bearing Support

Over-Hung

the rotor is supported in the form of a cantilever

impeller is situated at front

Centre-Hung

the impellers on the rotor are supported with bearings on either side

also called “between-bearing” because impeller is situated in between the bearings

CATEGORY: Support

Foot Mounted

the pump stand on its foot and the foot is bolted to the base plate

Centre-Line

the pump has its own base frame

CATEGORY: Shaft Connection

End suction / Top discharge
the suction is situated at the end while discharge at the top

Top suction / Top discharge
the suction/discharge line is vertical

Side suction / Side discharge
the suction/discharge line is horizontal from the side of the casing

CATEGORY: Direction of Flow

Mixed Flow

specific speed ranges Ns = 3500 to 7000

recognized by having screw like impellers

generally have single inlet

higher flow

Axial Flow

specific speed range Ns = 9000 to 16500

always single entry and are generally single stage

pump cases are concentric with inline inlet and outlet connections

the casing is normally fitted with guide vanes

highest flow low head designs.

Radial Flow

specific speed ranges Ns = 900 to 4500

advantage of providing a smooth flow and is capable of handling fluids with solids contents

low flow high head designs

CATEGORY: Mechanical Construction

Closed

use shrouds that totally enclose the impeller from the suction eye to its edges

prevent liquid slippage from the discharge to suction by using a running joint between the casing and the impeller (wear-ring)

higher pump efficiency

Semi-open

uses a single shroud, usually at the back of the impeller

shroud may or may not have pump out vanes or balance holes to modify the pressure behind the impeller

can pump thin fluids, heavier fluid and slurry

Open

consists of nothing but vanes attached to a central hub

use to pump thick fluid or slurries

less efficient than closed impeller

CATEGORY: Type of Suction

Double Suction

equipped with double suction impellers

develop little axial thrust due to the symmetry of their design

the flow rate is quite high.

This can be managed by having one impeller with two suction eye.

Single Suction
Design to higher axial thrust imbalance flow on one side of the impeller only

COMPONENTS: Casing

kinetic energy in the liquid at the outlet is converted to pressure energy by the pump casing

energy conversion has to be undertaken with a minimal loss to have an insignificant effect on pump efficiency

fluid leaves the casing through the outlet at a higher pressure

Volute;

increases in area from its initial point until it encompasses the full 360° around the impeller and then flares out to the final discharge opening

wall dividing the initial section and the discharge nozzle portions of the casing is called the tongue of the volute or the “cutwater.”

Diffuser;

a spiral-shaped component surrounding the impeller

reduce the velocity of a liquid, decrease slippage, and increase ability to develop flow against resistance

COMPONENTS: Impeller

converts the mechanical rotation to the velocity of the liquid

acts like the spinning wheel in the pump

has an inlet eye through which the liquid suction occurs (impeller eye)

direct the flow of fluid between the vanes

has vanes that enclosed by metal coverings called metal coverings called “shroud”

the flows are categorized by the specific speed, Ns

COMPONENTS: Shaft

prime mover (motor) drives the impeller and displaces the fluid in the impeller and pump casing through the shaft

COMPONENTS: Bearing

rounded pieces of metal (either ball or roller) that support an object and allow it to move smoothly by reducing friction

to keep the shaft or rotor in correct alignment with the stationary parts under the action of radial or transverse loads

COMPONENTS: Wearing Ring

a rotating component and it is housed within the pump casing

to prevent frictional contact, a gap between impeller and pump casing

it is essential to keep this gap or clearance at an optimum value to improve the pump’s efficiency

COMPONENTS: Coupling

a power transmission device that is used to connect the motor (driver) shaft to the power end shaft of the pump

primary purpose of a coupling is to transmit rotary motion and torque from the motor to the pump

COMPONENTS: Stuffing Box

a chamber or a housing that serves to seal the shaft where it passes through the pump casing

consist of packing rings and a gland (end plate) for squeezing and pressing them down the shaft

COMPONENTS: Packing / Mechanical Seal

Packing

Strong flexible ring material that’s control leakage from a stuffing box or cylinder. The present day packing used in pumps are predominantly made of PTFE (Teflon)/Graphite filaments.

Mechanical Seal

A ring-shaped parts that reduce or prevent leaking

A critical refinery pump handling hazardous hydrocarbons would be in all probability built as per standard API 610.

API 610 standard for centrifugal pumps provides guidelines on the minimum recommended wearing ring clearance in line with the pumping temperatures, thermal expansion, and the efficiency of the pump.

E.g.: Horizontal pumps with a centerline support are selected:

API 610 recommends pumping temperature is above 177 ºC (350 ºF).

The seal housing and in some cases even the bearing housing may have cooling water jackets based on this factor.

Dangerous liquids that could be toxic, inflammable, or carcinogenic may demand stringent pump designs. For example, an application in which no leak maybe acceptable under any condition may use sealless pumps.