Boiler Water sampling involves taking a sample of boiler water to analyse for dissolved substances. This is necessary to determine the level of TDS (Total Dissolved Substances) in the water and, consequently, the quantity of the blowdown.

Samples taken from points such as the level gauge glass, inlet for the feed water or the level control mechanisms are usually inaccurate.

It is dangerous to take a sample from the boiler shell as the water will be at pressure and may flash into steam causing injuries to the operator.

The safe method of taking boiler water samples is by use of a small heat exchanger.  In this method, cold water is used to cool the sample being taken.  This eliminates any risk of flashing.  The sample is also more acccurate.

Another method is to use a TDS sensor.  The sensor reaches into the shell of the boiler and can continually monitor the TDS in the water boiler.

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.

Piston valves function by a piston which moves in a cylinder.  The movement of the piston serves to open or close the valve.  The Piston has a hole through it.  As the hole moves up and down, the valve is opened and shut.  Piston valves can be used in media such a steam, water, compressed air, etc.

The valve can be mounted in the vertical or horizontal direction.  The valve has a handle which is to be rotated.  The rotation causes the piston to move in a linear direction.

Piston valves are also to be seen in musical instruments.

Methane Fermentation is a process of eliminating organic matter in the sludge.  In the process, methane is generated along with carbondioxide.  Methane can be used as a fuel.

Methane Fermentation involves digestion of organic matter using bacteria.  The bacteria are called methanogens.  

The digestion happens in the absence of oxygen (anaerobic digestion).  Anaerobic digestion will reduce the organic matter by about 50 percent.  It is posible to extract about 1000 litres of gas from one kg of organic matter.

The residual matter can be used as fertilizer.

Sludge Conditioners are chemicals which are added to sludges before they undergo the de-watering process in water treatment plants.  The dewatering process removes the water and the solid particles.

A sludge is a colloidal solution with solid particles suspended in water.  For dewatering to be effective, the smaller sludge particles should join together and form larger particles.

This is done by adding sludge conditioners.  Sludge conditioners work by altering the pH value of the sludge.  This results in coagulation.  The water and the particulate matter separate.  Different sludges have different pH value for coagulation.

Once the sludge has coagulated, it can be easily dewatered using centrifuges or vacuum filters.

Adiabatic Flame Temperature is the temperature which will be achieved if a combustion process is complete.  It assumes that there is no change in work, no heat transfer and no change in kinetic and potential energy.

The hypothetical temperature will be achieved in the event of a stoichiometric combusion ( a combustion without any residues)

The actual temperature which will be obtained is usually lower than the adiabatic flame temperature. This is due to factors such as heat transfer, incomplete combustion, etc.

Every fuel will have an adiabatic flame temperature.  It is the theoretical maximum temperature which can be achieved using the fuel

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.

Boiler Feed Water Pumps are used to supply water into the boiler.  The water may be fresh water or the water from the condenser.    The size of the Boiler Feed Water Pumps depends on the capacity of the boiler.

The operation of the Boiler Feed Water Pumps is dependent on the water level.  A level switch is used to switch the pumps on or off.

There are generally two pumps with one running and one as a standby.

Boiler Feed Water Pumps are of the centrifugal types.  They are usually multistage pumps.    The pumps are driven electrically or by a turbine.  Turbine driven pumps are preferred as the cycle efficiency of the boiler increases.  

Co-firing or Co-combustion refers to the combustion of two fuels simultaneously.  For example, biomass fuel can be burnt with coal.  In paper plants, the pith, which is a byproduct of the manufacturing process, can be burnt along with coal.

The fuel which is added to the main fuel is called additional or auxiliary fuel.

Co-firing has many advantages. Co-firing is cheap as it uses fuel which would otherwise have gone waste.  This reduces the cost of steam generation.

Co-firing also has environmental benefits.  Co-firing results in more efficient combustion.  The green house gases emitted are lesser than those emitted when burning a single fuel.

In certain countries, the governments incentivize cofiring as it is beneficial to the environment.

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.

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.

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.

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.