Centrifugal pumps

Introduction

Pumps are machines which adds to the energy of a liquid or gas causing an increase in its pressure and thus movement of the fluid.

One of its type is centrifugal pumps,these are Roto-dynamic pumps and not self primed and used mostly for low and moderate pressure range.These are basically radial flow or axial flow type(here we are discussing radial type).

Lets start with its working principle

Water is led to the suction eye of the rotating impeller. The water gains energy by the centrifugal action of the pump and is discharged to the volute outlet casing. The volute is created by increasing the area of the outlet port and is greatest at outlet from the pump. By this design the kinetic energy of the water is converted to pressure energy. 

Construction

Impeller

Impeller is the rotating part inside the pump casing.It consists of series of backward curved vanes and mounted on a shaft which is connected to the shaft of a prime mover.

Casing 

Air tight passage surrounding the impeller,designed in such a way that the kinetic energy of the water discharged at the outlet of the impeller is converted into pressure energy the water leaves the casing and enters the delivery pipe.

Following are the types of casing:

Volute casing

Vortex casing

Casing with guide blades 

Mechanical seal

Sealing is provided by a mechanical seal or by packed gland. For the former cooling water is supplied from the discharge side of the pump. For the latter cooling is provided by the allowance of slight leakage, lubrication is by a grease filled manual lubricator. 

The pump unit shown above relies on the driving motor bearings for alignment. For larger pumps a leaded bronze or brass bush may be fitted positioned just below the seal. For the largest pumps, especially those fitted with an inducer the shaft may be extended below the impeller fixing and a second bearing fitted.

Wear rings

wear ring act as a guide for rotating part and maintains alignment of the shaft.For efficient operation it is important to ensure that leakage from the high to low pressure side is kept to a minimum. This is achieved by the use of wearing rings. Traditionally these are fitted to the casing, to increase the longevity of the impeller wear ring tyres may be fitted.

NPSH

(Net Positive Suction Head)

Keeping it straight n short the NPSH is the total head required to make the liquid flow through the suction pipe to the pump impeller.

Available NPSH = Hes + Hs - Hfs - Hvs - Hvap 

Here,

Hes = pressure head acting on the liquid surface at the suction inlet

Hs = the vertical distance of the pump centre from the surface of the liquid

Hfs = friction head loss in the piping system

Hvs = head loss due to high velocity

Hvap = vapour pressure of the liquid

(Vapour pressure is the the pressure at which the liquid is vapourized at atmospheric temperature, if the combination of temperature and pressure within the suction pipe such that vaporization occurs, the efficiency of the pump decreases)

In application,the available NPSH must always be greater than required NPSH.

Available can be calculated knowing the details of the suction piping while the required NPSH is given by the manufacturer and varies with design,speed and capacity of the pump.

Factors affecting available NPSH and how to improve 

To make it easier for you we will just discuss in brief points

Frictional losses : Eliminate the obstruction such as valves,strainers and other fittings.

Avoid loops in the pipe lines and also sharp bends.

Replace the corroded pipes.

Heat the liquid in case of highly viscous liquids.

Temperature of the fluid : In order to avoid head loss due to vapour pressure make sure that the temperature of the fluid is within the limits.(It is provided by the manufacturer )

Position of the pump : well of course if the liquid can flow with the help of gravity it would be easier life for pump.

(Priming is not required if centre of pump is below the liquid level,we will discuss this soon relax chum)

Lenght and diameter of the pipeline should be decided where cautiously.

Cavitation

This is very a serious problem incase of centrifugal pumps.

U might know that water boils below its actual boiling temperature under vaccum conditions, same phenomenon causes cavitation and its a serious problem occurring mostly close to the impeller. 

      This is because disturbances in the water flow cause rapid localized pressure variations. This can lead to instantaneous vaporization and bubble formation. 

      When these bubbles collapse there is a rapid in rush of water. When this occurs near to a surface this slug of water can strike at very high speed and lead to destructive erosion and removal of protective oxides thereby increasing rates of corrosion and causing severe damage.

How to prevent cavitation 

The pressure of the liquid flowing in the system should be kept above its vapour pressure, for water the absolute pressure head should above 2.5 m

We can use special material or coating such as Aluminum bronze and stainless steel which are cavitation resistant.

Cavitation may result in making the system less efficient which makes it inevitable for healthy work environmental.

Priming

As mentioned above the centrifugal pumps are not self primed but what does this means?

Well whenever a pump is started it creates vaccum inside the casing of the pump resulting into the movement of the fluid from the suction side to the casing (fluid flows from high pressure side to low pressure side) but sometimes the pump is not able to create enough vaccum to suck the fluid which in general language we say that it is taking air,this phenomenon decrease the efficiency of the operation since there is actually no transfer of the fluid is taking place but the pump is still running.

To avoid this we have to prime the pump through external means that is the suction pipe,casing of the pump and a portion of the delivery pipe upto the delivery valve is filled with the liquid to be pumped before starting the pump.

(This can be easily done by fixing the pump below the level of the liquid surface)

There are many ways to prime the pump which would be discussed in our coming post.

Characteristics curves

In order to predict the behavior and performance of the pump when the pump is working under different flowrate,head and speed it is very important to know the characteristics of the pump or any machinery.

Main characteristics

The main characteristic is obtained by measuring the discharge by varying the speed during the test at constant head (discharge incase of centrifugal pumps is directly proportional to the speed).

Discharge characteristics

The input power curve for pumps shall not pass through the origin.It will be slightly away from the origin towards the y-axis as even to start the motor some power is consumed in order to overcome the mechanical loss.

From the input power curve it is observed that minimum power is consumed when there is no flow and when the discharge head is at its highest.

It can be seen that efficiency curve of the pump is convex which means that pump is most efficient somewhere between max and minimum discharge head and discharge conditions.

Because of the shock and eddy losses caused by the impeller blade thickness and other mechanical considerations there will be some rise in head loss with flowrate or discharge (both are same).

Eddy losses 

While studying about pumps this question alway crossed my mind that what is actually eddy losses if it isn't the case for you,great you are smarter and if it is don't worry. 

Basically eddy losses means losses in the surroundings.Its obvious that at high speed its hard for fluid to have a streamline motion and there will be some part ( although very little) which does not follows the path and the power is wasted this is known as eddy losses which you will also come across while studying about electromagnetic flux.

Multistage pumps

When centrifugal pump has two or more impellers then they are called multistage centrifugal pumps.

The impellers could be in series (on the same shaft) or they are in parallel (having separate shafts).

When in serious they increase they head generated proportional to the number of impellers in series.

In parallel they increase the discharge proportional to the number of Impellers.