Newton's Law of Viscosity gives the relationship between the shear stress and the shear rate or the velocity gradient of a liquid that is subject to a mechanical force.

The law is not universal in its application.  It is applicable to some liquids while other liquids do not obey it.

Newton's Law of Viscosity can be mathematically described as

t =  m dv/dy

where t = shear stress
           dv/dy is the velocity gradient

Liquids which obey Newton's Law of Viscosity are called Newtonian liquids.  Liquids whose response cannot be described by this law are called Non-Newtonian Liquids.


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A Newtonian liquid is one in which the viscosity of the liquid is constant regardless of the stress applied.  That is, the viscosity of the liquid is the same if it is left alone or agitated vigorously.

In other words, Newtonian liquids obey Newton's law of viscosity which states that the shear stress is proportional to the rate of strain.

Water is a common example of a Newtonian liquid.  The viscosity of water is same whether it is still or in an agitated state.  In contrast, a solution of water and corn starch will be liquid when still but will become highly viscous when agitated.

Liquids in which the viscosity changes in response to the application of stress are called non-Newtonian liquids.


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The centrifugal pump is not self-priming.  The pump needs to be primed before it can function.  Priming involves filling the pump casing with water and removing the air inside.


When the centrifugal pump is started for the first time, the casing should be filled with water until it is filled completely, the delivery valve should be closed and the pump should be started.  When sufficient pressure is built, the delivery valve can be gradually opened.

Priming can be done manually for the first time by means of special openings provided in the pump to fill water.  For subsequent start ups, special arrangements can be made such as having a footer valve in the suction pipe.  This ensures that the pump remains flooded even when it is not running. Another method is to have a separate smaller pump to remove the air in the main pump.

In some installations, the pump is placed at lower level than the sump so that the pump is always flooded.




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The Volumetric Efficiency of a Pump refers to the ratio of the total ouput of the pump to the total output of the pump without losses.


It can also be described as the ratio of the theoretical flow of the pump to the actual flow.  The theoretical flow of the pump can be calculated from the pump design data.  The theoretical flow of the pump is the product of the displacement per revolution and the number of revolutions.

The actual flow is measured by a flow meter which is connected to the pump output.

For instance if the displacement per revolution of a pump is 120cc/ rev and the pump has a speed of 1000 rpm, the theoretical flow rate would be 120x1000 = 120000cc or 120 litres per minute.

If the actual flow measured is around 115 litres per minute, the volumetric efficiency of the pump can be calculated as (115/120) x 100 = 95%

The volumetric efficiency can be reduced by reducing the leakages from the pump.


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The volute refers to the part of the casing which expands along its curvature around the pump.  The primary function of the volute is to convert the kinetic velocity of the water developed by the impeller into pressure.  

The volute achieves this by gradually increasing in area along the pump as it approaches the discharge port.  This causes a gradual drop in velocity and an increase in pressure.  

The volute also serves to minimize circulation losses and balances the forces around the impeller. 


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Waterjet cutting is a method of cutting materials using a pressurized jet of water.  The water at high pressure up to 90000 psi.

The primary advantage of waterjet cutting is that not heat is generated in the process.  This the material which is cut does not undergo any transformation in the surface due to the process.  This is different from other mechanical cutting processes where heat is generated and a coolant is used.

The water jet can be incorporated into a CNC system which can cut the material according to a preset pattern.

Sometimes, an abrasive is added to the water in order to cut hard substances such as granite.

Video of a WaterJet






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Galling refers to a very severe type of wear in which microscopic particles of the material of one component is transferred to the other component.  This occurs due to severe mechanical wear which can cause the materials to fuse due to heating. 

This type of wear, generally,  occurs in components which have high load and which operate at low speed.  Poor lubrication is also a reason for galling.  Galling if normally found in components such as bearings, pistons of IC engines and in shafts. 

Hardened materials have a lower tendency to tall.  Soft materials are more vulnerable to galling.

Galling can be prevented by ensuring adequate lubricating and by choosing the materials of the mating surfaces carefully.  The materials can be from materials with different hardness levels such as different grades of steel. 

Bolts are particularly vulnerable to galling action.  Due to improper tightening techniques heat can be generated between the bolt and the thread.  Ensuring proper tightening techniques which prevent heat generation can prevent galling. 


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Wear is the loss of material due to friction with another surface caused by motion.  Wear causes weakening of components and ultimately breakdown of materials. 

Wear resistance, therefore, is a highly desirable property in materials.  Wear can be minimized by selecting proper materials which can withstand the expected wear during the design stage itself.

Nevertheless, no matter how good the material selection is, there will always be some wear during operation.

Wear resistant coatings increase the wear resistance by forming a protective layer over the surface experiencing friction.  These coating can be applied as a spray or by means of a special forming process.

There are different types of wear resistant coatings. Ceramic Coatings, Phenolic coatings, Polymer and Epoxy coatings are some of the types of coatings available. 

The type of coating is chosen based on the working environment and the degree of protection desired.


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In engineering, wear resistance refers to the property of materials to resist wear and tear during normal operation.  Wear resistance is a desirable property in moving parts such as bearings, wheels and tyres etc.

There are different mechanisms of wear.  Some of them are adhesive, erosive, cavitation and fretting.  Wear generally occurs as a combination of different mechanisms. 

All materials have wear resistance as a standard specification of materials.

Wear resistance is evaluated by specialized tests which simulate normal operation. 

By understanding the level of wear expected during normal operation, materials can be made wear resistant.  Wear resistance can be achieved by special manufacturing process and appropriate choice of materials. 

There are also other types of wear resistance methods such as special coatings and sprays.





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The losses in the pump can be categorized into three categories

Mechanical Losses
The first type of losses is the mechanical loss.  This loss is due to friction between the pump components such as the impeller and the bearings.

Hydraulic Losses
These losses occur due to the energy expended in overcoming the friction between the fluid and the impeller surface

Volumetric Losses
Volumetric Losses occur due to re-circulation of the fluid within the pump.  This does not contribute towards the development of pressure.

Leakage Losses
Leakage Loss is caused due to leakage of the liquid from the pump due to reasons such as malfunctioning of seals, etc.


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