Common Maintenance Issues for Economizer
A boiler is a pressure vessel in which heat from combustion of fuel is used to convert water into steam. The pressurized steam can be utilized for process operations or to power steam engines or steam turbines. The feed water system supplies water for steam generation. Feed water is a mixture of condensate return (steam from the process requirements) and makeup water. The fuel system is made up of the fuel feeding system and the combustion system. The combustion system generates the heat required for combustion. An economizer is a part of the feed water system; it is a heat exchanger used to recover heat from the flue gas. It increases the temperature of water as it enters the steam drum or furnace walls. The economizer is the last water heat transfer unit upstream of the air heater. Economizer is located in the second pass of the boiler.
Types of economizer
Economizer can be classified on the basis of economizer surface, and boiler efficiency.
On the basis of economizer surface:-
- Bare tube coiled economizer are more common. The economizers can have an inline or cross-flow configuration. These economizers are efficient and occupy very little space.
- Finned economizer has welded fins on the outside surface of the economizer tubes. Finned economizers are more sensitive to flue gas environment. So, finned economizers are not recommended when the flue gas is coarse. Also, cleaning the surface of the finned economizer is a concern.
On the basis of boiler efficiency-
- Non condensing economizers are the most common variety. They are placed in the flue gas duct near the exit of the boiler.
- Condensing economizers are used in natural gas fired boilers. Condensing economizers reduce the temperature of the flue gas below its dew point. The flue gas condenses moisture and allows the use of latent heat and sensible heat for heating the water. Condensing economizer can help improve efficiency of the boiler by 10-15%. It can be used only when the flue gas does not have sulphurous, nitrous, or particulate corrosive matter.
Working principle of the economizer
An economizer is a tubular heat exchanger used to preheat the boiler feed water before it enters the steam drum. The flue gas exiting the boiler carries a lot of heat. The economizer utilizes this heat to heat the water entering the boiler. The water is preheated in the economizer in the liquid phase at elevated pressure.
Function of the economizer
- The economizer helps to improve the efficiency of the boiler. It helps to lower the temperature of the flue gas. Adding an economizer can improve boiler efficiency by 3-5%.
- Increasing the boiler efficiency can help reduce operating costs of the boiler by reducing fuel costs. The economizer helps you to get a faster return on investment.
- It reduces the chances of thermal shock due to fluctuating water temperature in the steam drum or furnace walls.
Common problems while operating economizer
Tube failure is one of the major problems occurring in the boiler. 30% of tube failures occur in the economizer. Economizer tubes face fireside (external) and waterside (internal) corrosion.
The fireside corrosion is a function of flue gas components, tube metal temperature, and the method of firing. Fireside corrosion is caused by soot, sulfur corrosion, poor maintenance, and deviating from boiler operating procedures.
Waterside corrosion happens due to contamination of oil, scale and sludge, and excess oxygen.
The different types of failures in the economizer are:-
- Oxygen pitting can cause problems in the economizer. Oxygen pitting is the most common type of waterside corrosion. The phenomenon is due to excessive quantities of oxygen in the economizer. It can also occur due to leakages in the air in the feed pump or poor pre-water treatment.
- The caustic attack is a localized loss in the ID of the tubes. It is caused by deposits in the tubes which prevents proper heat transfer.
- Caustic embrittlement is a specific type of stress corrosion. It is caused by the presence of free NaOH in the boiler water.
- The acid attack occurs due to a corrosive attack on the internal tube surface which results in irregular pitting. It is caused due to poor water treatment or insufficient post-cleaning of residual acids.
- Dew point corrosion or sulfur corrosion is more common in high-pressure boilers. It occurs when sulfur and oxygen in the flue gas and water temperature is below 130 C.
- Hydrogen Attack causes inter-granular micro-cracking. It results in loss of ductility or embrittlement of the tubes. Hydrogen attack is associated with heavy deposits inside the tube along with water with low pH.
Preventing economizer tube failure
- Careful analysis of the boiler tube is necessary to ascertain the cause of cracking. To avoid waterside corrosion, the water should be treated properly as per standards mentioned in the operating manual.
- Using a deaerator can also prevent oxygen pitting.
- Frequent blowdown during boiler operations helps to reduce sediment before it forms scale or sludge.
- Operators need to keep the external surface of the economizer clean.
- Soot blower in the economizer region helps to remove soot.
- Instrumentation to measure flow rates and pressure drops in the gas sides helps give warnings to prevent tube failure
- During the annual maintenance, the deposits in the waterside and blockages in the tube need to be examined.
- Gas side maintenance should be done when flue gas temperature at higher.
Why Mago Thermal?
Mago Thermal Private Limited (MTPL) has been designing, manufacturing, and commissioning boilers for over 20 years. We offer engineering services including engineering consultancy, operation, and maintenance. Our experienced boiler operators will take complete responsibility for the efficient running of your power plant. We can provide manpower backup and provide spares, if required.
MTPL has a well-documented process that allows for quick problem resolutions. Outsourcing boiler operation can result in cost savings.
Mago Thermal provides engineering, consultancy, procurement and commissioning of multi-fuel industrial steam boilers for benefit of businesses. For more information on Steam Boilers and Industrial Power Plants, contact us.