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.
Maximum Allowable Working Pressure