FREQUENTLY ASKED QUESTIONS
The boiler, also called the Steam Generator, is an apparatus designed to convert a liquid into vapor.
The process of combustion is a high speed, high temperature chemical reaction. It is the rapid union of an element or a compound with oxygen that results in the production of a heat-essentially; it is a controlled explosion.
Fuel fed to a burner is converted to heat. The water in the boiler absorbs the heat but not all the heat released from the fuel is used to heat the water. Some of the heat is wasted in the process. The heat balance of a boiler consists of accounting for all the heat units in the fuel used or wasted. It is a balance because it is the sum of all the heat consumed. Heat balance of a boiler is found by using the following equation:
A = B + C
A = heat energy available in the fuel
B = heat energy absorbed any the water in the boiler
C = heat losses
Efficiency of the unit = B/A
The following are the energy losses in a boiler:
• Gases of combustion to atmosphere
• Incomplete combustion
• Moisture in fuel
• Moisture in air used for combustion
• Water vapor produced from the burning of hydrogen
• Unburned combustibles
Boilers are called ”one-pass,” ”two-pass,” ”three-pass,” or ”four-pass,” which is determined by the number of times the heat released in combustion is conducted through the boiler before the gases exit the vent. The furnace is which the combustion takes place is counted as one ”pass.” If the tubes are arranged so that they are actually an extension of the combustion chamber, as in a vertical tube unit, then they are not considered as a separate pass and the unit is then called a ”one-pass” boiler. If the tubes are arranged so that the combustion gases must make a 180° turn to enter them, then each group of flue passages requiring a reversal of gas direction is referred to as a separate ”pass.” For example: a three-pass boiler consists of a furnace (first pass) from which the gases exit with 180° turn in to a course of flue tube tubes (second pass) from which they exit with a 180° turn into another course of flue tubes (third pass) before exiting into the vent.
Commercial boiler ratings may be stated in several ways, but are always directly related to the amount of heat that will be produced when the boiler is fired or a specified fuel input under specified conditions. Boiler can be rated according to:
• Thousand of Joules per hour (KJ)
• Boiler Horsepower (BHP)
• Kilograms of steam per hour
Horsepower is a unit of measurement of the ability of a boiler to evaporate water, usually given as the ability to evaporate 34 lb. (15.6 kg) of water an hour, into dry saturated steam from and at 212° F (100°C).
The common boiler designs are as follows:
• Fire Tube – the combustion products from the burning fuel are conducted to the boiler flue outlet through flue passages (tubes) that are surrounded by water. The tubes may be arranged horizontally or vertically above the furnace.
• Water Tube – the hot combustion gases are directly over the tunes in which the boiler water circulates. Metal or refractory baffles direct the gas flow to improve heat transfer.
Boiler scales are insoluble scale formation on the internal surfaces of the heat-absorbing components of the boiler. The scale coats the heat transfer surfaces, acting as an insulator to impede heat transfer. Most scale formations in the boilers can be traced from the presence of silica deposits, hardness and other impurities such as iron, silica, copper oil, etc. found in the boiler water.
Boiler scale increases the metal temperature, which could result in eventual failure of the pressure parts due to overheating. Scale formations in the boiler are responsible for lost efficiency, increased maintenance and operating cost, not to mention lost revenue due to outages and downtime.
Chemical treatment will give you the following results:
• Increase your boiler efficiency.
• Reduce your fuel, operating and maintenance costs.
• Minimize your maintenance and operational downtime.
• Protect your equipment from corrosion and extend your equipment lifetime.
Chemical additives can be used to correct problems caused by impurities. It is used to treat boilers to improve feedwater quality and steam purity; these chemicals can be injected directly into the feedwater or steam.
The factor of safety (5) by PSME code makes your boiler thicker hence the life-cycle will be longer compared to other brand which uses a factor of safety of 3 and lower.
Majority of the scale formation problems can be avoided if you conduct a thorough water analysis prior to boiler operation. Understanding the make-up of your boiler water will give you an informed decision on the potential problems you may encounter and identify appropriate water treatment program, maintenance scheduling and solution. Normally, pre-softening the water before feeding it to the boiler is the first step in eliminating scale formations. Even when the make-up is soft, there is still a need for chemical scale inhibitors to form inside the boiler. Hence, a complete program on proper treatment must be determined. Such program requires the right balance of chemical treatment and control, including sludge build-up, PH levels, oxygen removal, condensate treatment and alkalinity levels.
Corrosion is the reversion of a metal to its ore form. Iron, for example reverts to iron oxide, commonly knows as rust, as a result of corrosion. The process of corrosion is a complex electro chemical reaction and it takes many forms.
Corrosion in the boiler proper generally occurs when the boiler water alkalinity is low or when the metal is exposed to oxygen bearing water either during operation or idle periods. The oxygen causes very localized corrosion to occur in the form of pitting. These pits are small and deep pinpoint holes, which eventually penetrate tube walls. While basic corrosion in boilers may be primarily due to the reaction of metal with oxygen, other factors such as stresses, acid conditions and other contaminants infiltrating the boiler system can cause low pH levels to develop. High temperature and stresses in the boiler metal tend to accelerate the corrosive mechanism. In the steam and condensate system corrosion is generally the result of contamination with carbon dioxide and oxygen. Specific contaminants such as ammonia or sulphur bearing gases may increase attack on copper alloys in the system. Corrosion is caused by combination of oxide layer fluxing and continuous oxidation by transported oxygen.
Corrosion control techniques vary according to the type of corrosion encountered. Major methods include maintenance of the proper pH, control of oxygen, control of deposits, and reduction of stresses through proper operations practices. Deaeration and recently the use of membrane contractors are the best and most diffused ways to avoid corrosion removing the dissolved gasses, which are mainly oxygen and carbon dioxide. Corrosion can be chemically treated with oxygen scavengers such as sodium sulfite.
A deaerator is a piece of equipment or device that is widely used for the removal of oxygen and other dissolved gases from the feedwater to steam-generating boilers. In particular, dissolved oxygen in boiler feedwaters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides. Water also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion. Most deaerators are designed to remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or less.
When considering a deaerator with your boiler system, you have a realize that the deaerator is a preventive maintenance tool. Hence, it really should be used in every boiler application to extend the life cycle and the operational efficiency of your equipment (with the exception of a hot water heating system that uses absolutely no make-up water). However, industry standard suggest that a deaerator be used when:
a. Your boiler plant operates over 75 psig.
b. Any boiler plan with limited capacity.
c. Boiler plants that use 25% or more cold make-up water.
d. Any boiler plant that relies on continuous boiler operation.
The following are Boiler Classifications according to ASME:
1. Power Boiler
a. Process Boilers
b. Power Boilers
c. High Pressure Boilers
2. Heating Boilers
a. Commercial Boilers
b. Industrial Boilers
c. Heating Boilers
d. Low Pressure Boilers
Flame safeguard system is a set of controls used on a boiler to ensure safe burner operation. Primary functions include:
• A safe way of starting and shutting down the burner. This can be accomplished either automatically or manually.
• A flame safeguard system also starts the burner in the proper sequence. For example it will purge the combustion chamber of gas, light the pilot and then open the main gas valve.
• The flame safeguard system will also continually monitor burner operation when the boiler is on-line.
• The system will protect the boiler from excessive pressure or temperature conditions.
• It will also regulate the firing rate according to the demand for heat or steam.
• Finally, it will standby during down time, waiting for the signal to start the burner once again.
The most common measure to prevent foaming is to maintain the concentration of solids in the boiler water at reasonably low levels. Avoiding high water levels, excessive boiler loads, and sudden load changes also helps. Very often, contaminated condensate returned to the boiler system carry-over problems. In these cases the condensate should be temporarily wasted until the source of contamination is found and eliminated. The use of chemical anti-foaming agents, mixtures of surface-active agents that modify the surface tension of a liquid, remove foam and prevent the carry-over of fine water particles in the stream, can be very effective in preventing carry-over due to high concentrations of impurities in the boiler-water.
Foaming is a condition that occurs in boilers when a steam contamination is formed due to high concentration of soluble salts, suspended solids or organic matters in the boiler water. Bubbles or froth actually build up on the surface of the boiler water and pass out with the steam. It is generally believed that specific substances such as alkalis, oils, fats, greases, certain types of organic matter and suspended solids are particularly conducive to foaming. In theory the suspended solids collect in the surface film surrounding a steam bubble and make it tougher. The steam bubble therefore resists breaking and builds up foam. It is believed that the finer the suspended particles, the great their collection in the bubble.
Turndown ration is the ratio of maximum fuel input rate to minimum fuel rate of a variable input burner. Traditionally burners on firetube boilers operate in the 5:1 turndown ration range depending on fuel and size. High turndown burners are considered those with ratios of 10:1 or greater.
The flue gas outlet temperature at a known steam flow rate is a good indication of the boiler performance. An increase in flue gas outlet temperature at the same steam flow rate recorded in the previous data log is an indication log either scale formation or ash build up.
A very clear(transparent) flue gas is an indication of too much air, a blackish flue gas is an indication of poor combustion and a whitish flue gas is an indication of the right mixture of fuel and air ratio, i,e, efficient combustion.