Pumps - An Introduction

What is a pump ? How are pumps classified ?

A Pump is a mechanical device which moves liquid from a lower level to a higher level.  The pump draws the liquid inside pressurizes it and discharges it through the outlet.  A pump is driven by a prime mover which is, generally, an electric motor.  IC engines and turbines can also be used as prime movers to drive the pump.

Pumps are usually classified into two broad categories

Rotodynamic pump and
Positive Displacement Pumps

Rotodynamic Pumps

In these pumps, a rotary device with blades, called the impeller drives the liquid.  The liquid gets kinetic energy in the process.  The kinetic energy is converted into pressure by means of the design of the pump.

The rotodynamic pumps can be divided into

Centrifugal pumps : Here, the impeller with blades drives the liquid radially outwards towards the casing.  The liquid gets pressurized as it exits the pump.

Axial Pumps:  In these pumps, the liquid is driven axially by the impeller.  The flow of the liquid is parallel to the axis of the impeller.

Positive Displacement Pumps

Positive Displacement pumps are another major category of pumps.  In positive displacement pumps, the liquid is drawn into a chamber, pressurized and expelled at the discharge side.

These pumps are in turn classified into two types

Reciprocating Pumps: In these pumps, a piston moves inside a cylinder.  The piston creates low pressure when it moves up.  This sucks the liquid inside.  Once inside, the piston moves down and pressurizes the liquid which is discharged through a port.  The handpump used to pump water is a reciprocating pump. Eg. Plunger Pump

Rotary Pump: In these types of pumps, two rotating gears or screws move inside a casing.  As the screws or the gears move, the liquid is progressively taken into the pump.  The cross section of the casing is reduced as the liquid moves.  This causes pressure at the discharge side.  Examples: Screw Pumps, Gear Pumps

Rotodynamic Pump

A rotodynamic pump is a pump in which the impeller imparts kinetic energy to the fluid. The term Rotodynamic is a broad one encompassing all pumps with rotary impellers.

Centrifugal pumps are a type of rotodynamic pumps.  The impeller of the centrifugal pump draws in water from the suction and pushes the water radially giving kinetic energy to the liquid.

Apart from centrifugal pumps, axial flow pumps in which the water flows radially, parallel to the axis of the shaft, are also called rotor dynamic pump.

Positive displacement pump ?

A positive displacement pump is a pump which draws a fixed amount of the liquid from the inlet and discharges it in the outlet at high pressure.

Positive displacement pumps have an expanding cavity in the inlet and a decreasing cavity near the inlet.  Positive displacement pumps have constant volume.  The pumps deliver a constant flow regardless of the discharge pressure.  The pressure depends on the speed of the pump.

Positive displacement pumps can be further classified into reciprocating pumps, rotary pumps, etc.

Positive displacement pumps should never be operated with the outlet closed. Since the pump works on a fixed volume of liquid,  the pump can get seriously damaged if it is accidentally operated with the outlet closed.

A special pressure relief valve is provided for protection against excess pressure.

Image courtesy: commons.wikimedia.com

A comparison of Centrifugal and Positive Displacement Pumps

Priming
Centrifugal pumps need to be primed separately.  The priming can be manual or through a separate priming arrangments

Positive displacement pumps are self priming as they develop a low pressure which can draw the fluid inside.

Flow Rate
Centrifugal Pumps have a flow rate which is dependent on the discharge pressure.  Positive Displacement pumps have a constant flow rate regardless of the pressure

Viscous Fluids
Centrifugal pumps cannot handle viscous fluids due to increased friction between the impeller and the liquid.  Positive displacement pumps can handle viscous fluids.

Efficiency
Centrifugal pumps have lower efficiency as the viscosity increases. Positive displacement pumps have high efficiency as the viscosity increases

Method of operation
Centrifugal pumps build pressure by imparting velocity to the liquid and then converting it into pressure.  Positive displacement pumps develop pressure by drawing a fixed amount of liquid and pressurizing it.

What are the different parts of the Centrifugal Pump?

The centrifugal pump consists of the following main parts.

The Impeller
The Impeller is the heart of the pump.  The impeller provides kinetic energy to the water entering the pump from the suction pipe.

The Volute
The volute refers to the tubular casing of the pump which increases in size as it approaches the discharge port.  The function of the volute is to convert the velocity of the water from the impeller into pressure.  It achieves this by a gradual increase in volume.

The Suction Pipe
The Suction pipe connects the sump to the pump inlet.

The Foot Valve
The foot valve is a non-return valve which is connected on the suction side.  The foot valve prevents the flow of water from the overhead tank which is at a higher level to the sump when the pump is not running.

The Strainer
The Strainer prevents the entry of debris into the pump

The Delivery Pipe
The Delivery Pipe serves to supply water to the tank from the discharge side of the pump.

The Delivery Valve
The delivery valve is a valve at the output of the pump in the delivery line.  The function of this valve is to control the output of the pump.  The delivery valve is closed when the pump is first started during the priming process.  It is then gradually opened.

Common Boiler Concepts

Carbondioxide in Boiler Water

Carbon Dioxide is a constituent of air. As such it get dissolved in water. Carbondioxide mixes with steam to form carbonic acid. Carbonic acid is an unstable compound. It has a tendency to react with steel and can thus corrode piping. Another way carbon is present in the water is in the form of bicarbonates. These carbonates decompose in the boiler to produce carbon dioxide. This carbon dioxide is usually present in the condensate. 

Carbon dioxide reduces the pH of the water. This turns the water acidic which results in further corrosion. Hence, carbon dioxide has to be removed from the water. One simple way of removing carbondioxide is by heating the water. Heating the water reduces the solubility and thus removes the gas. The water should be externally treated to remove the carbonates. Venting at specific locations of condensation can also reduce the carbon dioxide in the system.

      

Design Pressure and Maximum allowable Working Pressure (MAWP) of the Boiler

The Design Pressure of the boiler is the maximum pressure at which the boiler can be operated under normal operating conditions. It is equal to the highest setting of the safety valves in the boiler. 

For instance, if a boiler has two safety valves, the design pressure will be equal to the setting of the valve with the higher setting. The design pressure is calculated based on the stress that the boiler will undergo during operation across its lifetime.

     

Maximum Allowable Working Pressure

This is the maximum pressure that the boiler can withstand. The maximum allowable working pressure is calculated based on the strength of the material, the thickness of the walls, etc. The Design Pressure of the boiler is lesser than or equal to the Maximum Allowable Working Pressure.

     

Super Heater Outlet Pressure

The Super Heater Outlet Pressure is the pressure at which steam is expelled from the super heater. This pressure is depended on the inlet pressure of the turbine. It is generally maintained at 5 percent over the inlet pressure of the turbine. The excess pressure is to offset the drop in pressure between the boiler outlet and the turbine inlet. 

This drop in pressure is due to the piping losses. In fixed pressure boilers, the SH outlet pressure is constant and the turbine inlet pressure is varied with valves in accordance with the load. In variable pressure boilers, the boiler outlet pressure varies with the load.

     

Peak Rating of a Boiler

The Peak Rating of a boiler is the extra evaporation which the boiler can deliver for a specified period such as 2 to 4 hour a day. In some cases, the boiler will be required to operate above the Maximum Continous Rating (MCR) for short period of time. The efficiency during this temporary overloaded operation will be marginally lower. 

The Peak Rating is usually about 110 percent of the normal operating capacity for about 4 hours a day. Any further increase in the Peak Rating will need redesign of the boiler. While the Peak Rating can be used in a contingency, it is best avoided. This is because operating the boiler at peak rating will result in premature aging of the boiler. It will also result in issues such as slagging, fouling, erosion, etc.

     

Maximum Continuous Rating (MCR) and Normal Continuous Rating of a boiler

The Maximum Continuous Rating (MCR) is the maximum output which the boiler can delivery when operated at a specified set of conditions. Alternatively, it can be understood as the minimum assured production of steam in a boiler. The MCR. 

A well designed and maintained boiler will produce an output equal to the MCR value throughout its life. A new boiler can be operated at 8 to 10% above the Maximum Continous Rating. However, the excess capacity is, usually, lost with age.

     

Normal Continuous Rating

The Normal Continuous Rating (NCR) is the rating at which the boiler will be operated normally. The NCR is about 90 percent of the MCR. The NCR is determined based on the rating of the turbine. The boiler is designed to have maximum efficiency at NCR.

       

Boiler Water Treatment

The water in the boiler should be kept within proper chemical paramaters. The treatment of boiler water is intended to facilitate proper heat exchange, protection from corrosion and the generation of steam. Boiler water treatment can be categorized into two main categories. External Treatment in which the water is taken out of the boiler and treated and Internal Treatment in which the water is treated while still in the boiler External Treatment Some of the processes done in external treatment are softening, evaporation, deaeration, etc. 

Internal Treatment Internal treatment involves conditioning the water inside the boiler through chemicals. Internal treatment is generally done in low or moderate pressure boilers. Internal treatments is intended to prevent water hardness and the formation of scales. to prevent sludge from settling in the boiler walls. To prevent foam carryover by providing anti foam protection. To remove oxygen from the water to maintain water alkalinity to prevent corrosion.

     

Overheating in Boilers

Overheating in boilers occurs usually in the boiler tubes. This problem is seen when the boiler is first commissioned and a short while later. It usually does not appear after the plant has been stabilized. Scale formation in the tubes can be a reason for overheating. Scale formation prevents heat transfer and can cause localized overheating. Overheating can also occur if there are changes in the boiler operation such as a change in fuel or any change in any other significant parameter.

      

Silica in Boiler Water

Ordinary Silica is insoluble in water. But when silica combines with other materials such as lime and soda, it can form scales which are very difficult to remove. Soda and lime are used in softening units. Use of silica based lubricants in the thermal plant as well can also result in silica entering the boiler water. Another source is the presence of unreacted silicon in the feed water. If silica is not removed in time, it forms deposits in the turbine nozzles and change the direction of the steam. 

The velocities and pressure drops are changed inside the turbine resulting in reduced efficiency. Uneven nozzle flow can result in torsional vibration due to uneven loading of the blades. This can result in vibrations. Silica deposits in the boiler are difficult to remove. They equipment has to be dismantled and physically cleaned. Blasting aluminium oxide on the surface is also a method used in the removal of silica deposits.

      

British Thermal Units and Boilers

Boiler Capacities are often denoted in British Thermal Units. One British Thermal Unit is defined as the amount of heat required to raise one pound of water by one degree Fahrenheit. While the BTU has generally been replaced with the more popular unit, the Joule, Boilers and the Heating industry still use the British Thermal Unit. 

One BTU is equal to 1.06 Joule BTUs are also used for indicating the energy in fuels. Oil has a BTU of 138000 per gallon. Natural Gas has a BTU of 1075 per cubic foot. A bigger unit is the MMBTU which stands for one million BTU. The M is the Roman number for thousand. MM stands for a thousand thousand which is one million.

Draught in Boilers

Draught or Draft refers to the pressure difference between the burner and the atmosphere.  This pressure difference or draught causes the air to flow from the burner to the atmosphere.  The residue of combustion such as waste gases, soot, etc are carried away by the flow of air.

Draught also has a great role to play in combustion.  The flow of fresh air into the burners is necessary for proper combustion.  Hence, the draught system should be designed such that the combustion can take place properly. 

The draught of a combustion system can be measured using a manometer when the furnace is in operation.  One end of the manometer is connected to the furnace while the other end is left open to the atmosphere.  The pressure difference indicates the draught of the system. 

Types of Draught in Boilers

• Natural Draught where the draught occurs naturally due to the pressure difference between the furnace and the atmosphere. 
• Induced Draught where the draught occurs by means of fans which create a negative pressure in the furnace causing fresh air to enter
• Forced Draught where the draught occurs due to fans which provide combustion air and create a positive draught in the furnace.  This  drives the air through the chimney

Steam Jet Draught

Steam Jet Draught refers to the Draught created by using a jet of steam.  The steam generated by the boiler can be used for this purpose.  The jet of steam is used to create an airflow which will cause the flue gases to exit through the chimney.

If the steam jet is applied near the stack of the chimney, the negative pressure it creates draws the flue gases from the furnace into the Chimney.  This is known as induced draught.

If the jet is applied below the grate, the steam pushes the flue gases in the direction of the chimney.  This is a forced draught.

The Steam Jet draught is a simple mechanism.  No external equipment such as compressors or blowers are required.  The steam when used below the grate cools the firebars and prevents the clinkers from sticking to the bars.

Boiler Classification based on Pressure

Pressure is a very important parameter in boilers.  The boiler and all the connected equipment are designed to withstand the pressure developed by the steam.  Pressure, is an important criterion to classify boilers.

Classification on the basis of pressure

Low to medium pressure boilers

These boilers have an operating pressure of less than 10 bar.  Natural circulation is sufficient for these boilers.  Typical application are in industries.

High pressure boilers

High pressure boilers have an operating pressure of 10 to 14 bar.  They have forced circulation.

Super high pressure boilers

Super high pressure boiler are also used for utility applications.  The operating pressure is above high pressure boilers but generally lesser than 17 bar. 

Super critical boilers

Supercritical boilers have an operation pressure higher than 22.5 bar

Miniature Boilers

These are boilers with very small capacity with a pressure less than 6.8 atmospheres and a gross volume less than 0.1415 cubic metres.

Supercritical Boilers

The Critical pressure in boiling a liquid is that pressure above which there is no clear change of state between the liquid and the vapour phases.  Simply put, water turns into vapour without boiling.  Above a pressure of 22.1 MPa, water reaches this state.  

In supercritical boilers, water is boiled at a very high pressure.  At that high pressure, there is no clear distinction between the water and vapour phases .  The fluid can  no longer be called liquid or vapour.  It becomes what is known as a super-critical fluid.

Supercritical Boilers are generally used in Turbine systems.  When the supercritical fluid drives the turbine, it loses pressure.  As the pressure drops below the critical point, the supercritical fluid becomes a mixture of water and steam which then passes through the condenser.

Supercritical Boilers are boilers in which the working fluid is above the critical pressure.  At this pressure, water changes into steam without boiling.  This intermediate state is known as a super critical liquid.  

Supercritical boilers are used in Turbine systems.  

Advantages of Supercritical Boilers

The advantages of supercritical boilers over sub critical boilers are

Efficiency

Supercritical boilers are more efficient that sub critical boilers.  They consume less fuel.  The efficiency rating of supercritical boilers is in the range of 32 - 38 % while that of ordinary boilers is in the range of 32% - 38%.

Reduced Operating Costs

As the efficiency increases, there is a natural reduction in fuel costs which translates into reduced operating costs

Lower Emissions

Due to less fuel being burnt, there are lower emissions.

Higher Initial Costs

The downside is that super-critical boilers have higher initial costs as the boiler and the systems have to be designed to withstand higher pressures.  

Advanced Water Chemistry

Supercritical boilers require very pure water.  Even small levels of impurities can cause deposits on the turbine blades. 

Deaeration in Boilers

Deaeration refers to the process of removing dissolved gases such as oxygen and carbondioxide from the water in a boiler. Dissolved Oxygen in Water causes corrosion by the formation of rust on the

surfaces of the boiler and the piping (rust). Carbondioxide which is dissolved in the water forms carbonic acid which also causes corrosion.

Classification of Deaerators in Boilers

Hence, it is essential that these two gases are removed from water. 

Deaerators can be classified into 

Mechanical Deaerators

 

These Deaerators separate the gases by a mix of high temperature and mechanical action Chemical Deaerators 

Chemical Deaerators

Chemical deaerators work by passing the water through chemicals which absorb the oxygen and the carbondioxide. 

Vacuum Deaerators

Vacuum Deaerators or Membrane Contractors work by passing the water through hollow fibres. The water is made to pass on the outside of the hollow fibre. A vacuum is created on the inside. The gases pass through the membrane on to the inside and drawn into the vacuum pump. 

Vacuum Deaeration is a method of removing dissolved gases from water. Removing dissolved gases from water is necessary as they can cause corrosion.

   

Working of Vacuum Deaerators

The principle on which Vacuum Deaerators are based is called Henry's Law. 

Henry's Law states that the gas solubility in a solution reduces as the partial pressure of the gas above the solution decreases. The Deaerator consists of a tower with baffles. The tower is made of galvanized or reinforced steel. Water is drawn to the top of the tower and made to fall through the baffles. The falling water is in the form of thin films. 

This creates a large contact area between the water and the air. A vacuum pump creates a vacuum in the inside of the tower. This lowers the air pressure on the inside. The dissolved gases in the water are drawn to the vacuum and are removed. In some vacuum deaerators the water is increased in lower the solubility. The amount of vacuum to be created depends on the temperature of the water. Vacuum deaerators are highly efficient and can deliver water with dissolved oxygen less than 5 ppb (parts of billion). 

Chemical deaeration is the use of Chemicals to remove the dissolved gases, usually oxygen. Chemical deaeration is usually used after Mechanical deaeration. Even after mechanical deaeration, all the oxygen will not be removed. 

 A chemical known as an oxygen scavenger is used. This ensures that all the oxygen has been removed. A common oxygen scavenger is Sodium Sulphite. Sodium sulphite reacts with the trace amounts of oxygen. Sodium sulphite, however, cannot be used at high pressure as it can decompose to acidic gases which can increase corrosion. 

Another oxygen scavenger is hydrazine. Hydrazine reacts with oxygen and produces volatile compounds which do not dissolve. Hydrazine also does not cause corrosion. However, the downside is that it is a carcinogen (cancer causing substance) and thus has to be used very carefully. It may be banned in the future. 

Oxygen Attack

Oxygen Attack refers to the corrosive action of dissolved action on the boiler.  Dissolved oxygen causes pitting on the boiler surface.  Oxygen enters the boiler through the feed water.  Though, the deaerators remove a large amount of oxygen, the oxygen that remains can cause corrosion. 
When the feed water is heated, the oxygen becomes even more aggressive resulting in severe corrosion.

If the water contains ammonia, this results in corrosion of components containing copper and copper alloys such as bearings. 

Corrosion also results in deposits on the heat transfer surfaces which affect efficiency. 

Corrosion caused by oxygen is usually localized.  Oxygen Corrosion can also be extensive. 
Oxygen Attack is not monitored and prevented can result in failure of the boiler components. 
Heating the feed water reduces its solubility and reduces the dissolved oxygen.  Mechanical deaerators can further reduce the dissolved oxygen level.  Finally, chemical deaerators such as sodium sulphite can scavenge the remaining oxygen ions. 

Blowdown in Boilers

Significance of Blowdowns

Blow down in Boiler is a very important procedure.  The Blow down helps flush the boiler of impurities which may accumulate as the water evaporates.  If the blow down is not carried out, the impurities can reach dangerous levels which can result in the formation of scales in the pipelines and the formation of sediments due to precipitation.  Scaling and deposit formation reduces the heat transfer between the boiler and the water and affects the efficiency. 

Impurities can also cause foaming which results in the loss of water which gets carried away in the steam. 

There are two ways in which blowdowns can be carried out in Boilers.  The Bottom blowdown and the Surface blowdown. 

The Bottom blowdown is done by opening a drain at the bottom of the boiler.  The boiler pressure pushes the impurities and deposits out. 

Surface blowdown is done to remove the impurities which have formed a foam on the surface of water.  The foam needs to be removed for optimum heat transfer.  A pipe placed at the water level in the steam drum is used for this purpose.  Opening the pipe causes the water on the surface to be vented. 

The duration and frequency of blowdown depends on the boiler design and the conditions of operation.  It also depends on the levels of the contaminants in the feedwater and thus on overall water quality.  The boiler blowdown rate should be determined uniquely for each boiler installation. 

Surface Blowdown

Surface Blowdown in boiler is carried out to remove dissolved substances at the surface of the water.  That is, it is used to remove impurities which are in the liquid or dissolved phase.  Impurities which are in solid phase precipitate to the bottom where they are removed by the bottom blowdown.

The impurities and dissolved substances tend to form a layer of foam on the surface of the water.  This layer of foam needs to be removed to reduce the level of the dissolved substances. 

Since the blowdown involves removing water from the surface, it is called surface blowdown.
Surface blowdown is done by a pipe which is made to float a few inches below the water surface.  The pipe is connected to the outlet by means of a swivel joint.  The pipe can thus freely float in the water.  The pipe is held afloat by means of a float.

The pipe has a needle valve at its end.  The size of the valve opening can be adjusted based on the frequency and amount of blowdown required during each session.  Today, Automatic blowdown controllers which can control the rate and volume of the blowdown are also available. 

Bottom Blowdown

Bottom blowdown in boilers is used to remove impurities which have fallen to the bottom as precipitates.  These impurities are in the solid phase.  Bottom blowdown is done by means of a valve connected to the bottom of the valve.  When the valve is opened, the impurities are flushed out by the boiler pressure.

The steam collected during the blowdown can be removed into a steam flasher and a heat exchanger.  The heat can be recovered and the steam can be recirculated after passing through the flash tank and the deaerator.

The duration and frequency of the blowdown is determined on factors such as size of the boiler, water quality and the location and the operating load.

A proper blowdown programme improves efficiency and reduces maintenance costs.

 

Boiler Blowdown Rate

The Boiler Blowdown rate refers to the rate at which the blowdown should occur in an operating boiler.  It describes the blowdown in kilograms per hour.

The Boiler blowdown rate depends on the quantity of the impurities and the limits of tolerance for the employees.  The Blowdown rate is a product of the steam consumption and the ratio of the level of the TDS to the difference between the maximum allowable TDS and the actual TDS. 

q_BD = q_S f_c / (b_c - f_c) 

where

q_BD    the blowdown rate in kg/hour

q_S is the rate of steam consumption in kg/hour

f_c is the total dissolved substances in ppm

b_c is the limit of the total dissolved substances in ppm

Boiler Blowdown Percentage

The Boiler Blowdown percentage refers to the amount of boiler water drained during a blowdown to the total quantity of the boiler feed water.  This is a very useful value

The formula is 

This value is a very important parameter.  The boiler blowdown percentage can range from 1% for high quality feed water to 20 % for low quality feed water. 

Types of Blowdowns

Two types of blowdown can be carried out in boilers. They are,

Intermittent blowdown

Intermittent blowdown, as the name suggests, is the blowdown performed at frequent intervals.  The general rule is to do the blowdown for 2 minutes in 8 hours.

This method requires increases in the feedwater input to the boiler.  Feedpumps of large size may be required for this method.

With each blowdown, a significant amount of energy is lost. 

Continuous blowdown

Continuous blowdown involves a steady discharge of concentrated boiler water and its replacement by a constant input of feed water.  TDS and steam purity are maintained at a given load.

Once the discharge rate of the blowdown and the feed rate are set, it requires no operator intervention.

The heat lost during continuous blowdown can be recovered by blowing it into a flash tank and generating flash steam.

The blowdown which leaves the flash tank will still have heat which can be recovered.  This is done by using a heat exchanger to heat the make-up water.

Package blowdown heat recovery systems which can be customized are available.

Benefits of blowdown control

The benefits of blowdown control are

• Reduced cost of pretreatment.
• The quantity of makeup water required is less.
• The maintenance downtime is less.
• The boiler life is increased.
• The amount of chemicals to treat the water is less.

Scaling and Slag Formation in Boilers

Fouling in Boilers

Fouling is a phenomenon where the hot flue gases and the ash precipitate and settle down in the places where the flue gases exit the boiler.

This layer which is formed reduces the gas flow into the Selective Catalytic Reduction tubes.  This can result in poor effluent treatment of the gases.
Fouling is generally removed by soot blowers. 

Slag Formation in Boilers

Slag formation occurs when the temperature of the gases exiting the furnace is above the fusion temperature of the fly ash.  At these temperatures, the fly ash melts and gets deposits on the sides of the furnace.

Slag Formation can lead to problems such as

• reduced heat transfer from the combustion gases inside the furnace to the water
• It can lead to further overheating of the boiler gases which, in turn, leads to further deposition.
• Leads to unpredictable behaviour in the boiler

It is necessary to maintain furnace temperature below the fusion temperature of the ash.  The fusion temperature of the ash can be obtained by testing ash samples at laboratories. 
Slagging can also be prevented or minimized by a Slag Screen Arrangement in Boilers.

Thermal Spray in Boilers

Thermal Spray is a protective coating made on the tubes of the boiler.  The Thermal spray prevents corrosion, damage to the tubes and unscheduled breakdowns.   The material used for coating is usually an alloy.

Alloys based on Iron with added Chromium are used.  Low carbon steel can also be used as a thermal spray as it resembles the weld overlay.  Aluminium based thermal sprays are also used. 

There are different methods of applying the Thermal spray.  The metal is melted by using an electric arc or a gas flame and sprayed on to the tubes.

Scaling in Boilers

Evaporation of water causes the impurities and minerals in the water to concentrate.  Scale Formation in boilers when impurities precipitate from water.  Scaling also occurs when matter which is suspended settles down to the bottom.

Scaling forms usually on heat transfer surfaces.  Scaling affects the heat transfer and thus the overall efficiency of the boiler.  Severe scaling can cause blockages which can be very expensive to remove. 
Some of the contaminants which can form scales are calcium, magnesium, silica.  Calcium and Magnesium form their carbonate and sulphate salts.  These salts get deposited on the tubes and other internal surfaces of the boiler and other equipment. 

Scaling can be prevented by using good demineralized water as feed water.  Effective water treatment and maintaining good water chemistry can prevent scaling to a large extent. 

Silica in Boiler Water

Ordinary Silica is insoluble in water.  But when silica combines with other materials such as lime and soda, it can form scales which are very difficult to remove.  Soda and lime are used in softening units.

Use of silica based lubricants in the thermal plant as well can also result in silica entering the boiler water.  Another source is the presence of unreacted silicon in the feed water. 

If silica is not removed in time, it forms deposits in the turbine nozzles and change the direction of the steam.  The velocities and pressure drops are changed inside the turbine resulting in reduced efficiency.  Uneven nozzle flow can result in torsional vibration due to uneven loading of the blades.  This can result in vibrations. 

Silica deposits in the boiler are difficult to remove.  They equipment has to be dismantled and physically cleaned.  Blasting aluminium oxide on the surface is also a method used in the removal of silica deposits.