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This article contains information and details regarding high pressure boilers and their use.
You will learn:
What is a High Pressure Boiler?
Types of High Pressure Boilers
Characteristics of a High Pressure Boiler
The Heat Sources for High Pressure Boilers
And much more …
Chapter One – What is a High Pressure Boiler?
A high pressure boiler is a type of boiler that operates at 80 bars or higher and is used in thermal power plants to generate power. Using water-filled tubes in a metal tank or enclosure, they create power by converting water into steam through thermal energy, which is used to power equipment.
The key to a high pressure boiler is the amount of pressure it creates, which differentiates it from other boiler types. Pressure and steam are created by burning a fuel, such as coal, natural gas, or petroleum. The heated gasses, are circulated by natural convection or forced by a pump. Forced circulation is produced by a centrifugal pump to increase the evaporation rate to 210 k of force per square centimeter. Natural force is incapable of generating the necessary force of a high pressure boiler.
The different forms of high pressure boilers include steam generators, electrical boilers, vertical, horizontal, gas, water tube, and fire tube, to name a few. A key feature of most high pressure boilers is the use of small diameter tubes that increases the heating area between surface area and volume ratio, which improves the rate of heat transfer.
In a fire tube boiler, hot gas is pushed through tubes that are immersed in water, which heat the water to create steam as can be seen in the image below.
Chapter Two – Types of High Pressure Boilers
The majority of boilers work using the same principle of heated water producing steam. The variations between the different types are their geometric orientation, external or internal fire, pressure, water circulation, number of tubes, and the circulation of gasses.
The geometric orientation of a boiler can be horizontal, or inclined with fire on the outside or on the inside, depending on the boiler‘s design. The division of megapascal pressure (MPa) determines whether a boiler is low and high pressure. Boilers that produce pressure above 80 MPa are considered to be high pressure. Low pressure boilers have a MPa below 80 MPa.
Water circulation can be forced or natural. If the water is pumped, it is forced. If it is natural, it is by convection. Gas circulation is determined by the type of tubing, which can be fire or tube. With fire tube, heat moves through tubes in the water. In water tubing, water moves through the tubes.
Tubing for a boiler can be either single or multiple. The traditional Cornish boiler is an example of a single tube boiler since it has only one fire tube. There is a long list of boilers that have multiple tubes, which is modern boiler design.
Types of High Pressure Boilers
La Mont Boiler:
The Lamont boiler is a forced convection water tube boiler that uses a pump to circulate water. It has an economizer to save on energy and increase efficiency, a boiler drum, superheater, evaporator, and air preheater. The combustion chamber for a La Mont Boiler has water pumped through the economizer and preheater into the boiler drum.
The boiler drum serves as a steam separator. The pump pushes the water to a distributor header that has nozzles to send the feed water into the evaporator tubes located in the walls of the boiler to increase surface heating.
The water and steam go into the boiler drum, where the steam is separated and passed to the superheater and on to the turbine. The superheater increases the temperature of the saturated steam to prevent water droplets from entering the turbine.
The Benson boiler has some of the same features as the La Mont boiler without a steam separator and it works under critical pressure. It is a water tube, forced circulation boiler, where the feed water is compressed to supercritical pressure to prevent the formation of bubbles in the water tube. The compressed water feed increases the density of the water such that its density and that of the steam become the same, which reduces the latent heat in the water to zero making it easier to convert to steam.
The function of the air preheater, economizer, superheater, evaporator, and combustion chamber are the same. Unlike the La Mont boiler, the combustion chamber can be mounted horizontally.
The Loeffler boiler was developed to resolve a problem with the La Mont boiler, which was salt and sediment buildup on the surface of the water tube and reduced heat transfer. Like the Benson boiler, water is circulated under high pressure in the water tube.
The Loeffler boiler has radiant and convective superheaters that work together to distribute the steam to the turbine. The radiant superheater heats the saturated steam using radiant energy produced by the burning fuel and is where heat transfer takes place. The steam then passes through the convective superheater that is heated by the flue gases, which further heats the steam before it passes on to the turbine.
The Velox boiler is a forced circulation water tube boiler that uses high velocity to rapidly produce steam. The principle of the Velox boiler is built on the velocity of the gas, which moves faster than the speed of sound and increases the rate of heat transfer.
The Velox boiler has a gas turbine compressor that sends compressed air into the combustion chamber, where more heat is released. The increased rate of heat release enhances the flue gases velocity to the speed of sound. The flue gases pass from the fire tubes at the bottom of the combustion chamber and surround water filled tubes.
Water is forced from the economizer to the circulating pump at high speed causing the heat to transfer from the gases to the water very rapidly. The water and steam move to the water separator and on to the superheater. As with the Loeffler boiler, the flue gasses are used to heat the superheater.
The Cornish boiler is the simplest form of boiler. It is a fire tube boiler that has a long horizontal tube with a single flue that contains the fire. The cylindrical shell of a Cornish boiler is one to two meters in diameter and can be four to seven meters long.
The fuel for a Cornish boil is loaded into a grate through a door, where it burns to produce hot gases that move into the fire tube that is surrounded by water, where the water is heated to produce steam.
Babcock and Wilcox Boiler:
The Babcock and Wilcox boiler is a water tube horizontal boiler with natural draft circulation and multiple tubing. Water enters the tubes by a down take header, is heated by hot flue gases and moves in a zigzag fashion forced by baffle plates. The steam in the tubes moves to an uptake header, where it is collected in a boiler drum. An anti-priming pump filters the steam to remove any water droplets so that only steam is sent to the superheater, which increases the temperature of the steam. The steam from the superheater is collected or sent on to the turbine.
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Chapter Three – Characteristics of High Pressure Boilers
High pressure boilers have characteristics and features that make them the best choice for the creation of energy and power. The main factors that differentiates high pressure boilers from other methods of energy creation are fuel, emissions, and efficiency. Each of these have an influence on the equipment‘s performance and cost. The information below contains some of the important features of a high pressure boiler.
Characteristics of a High Pressure Boiler
Power of Steam:
Water expands 1600 times to form steam, which can carry large amounts of heat. This makes water an economical and ideal raw material for generating heat and power. High pressure boilers heat water to create steam at over 15 psi and have water pressure over 160 psig.
Combustion efficiency is how well the fuel is being used in combustion, which is different from stack loss, the heat carried away by flue gasses and moisture. High pressure boilers are highly efficient and operate with 15 to 20% excess air. All of the fuel is converted into thermal energy.
Thermal efficiency is the heat exchanger's ability to transfer heat from the heating process to water or steam, leaving out radiation and convection loss. In most cases, high pressure boilers have a 95% thermal efficiency.
Fuel-to-steam efficiency is measured by the ASME Power Test Code. It is the ratio of BTU output divided by the BTU input. The input is the amount of fuel used to power the boiler, while the output is the steam produced. The higher this ratio, the more efficient is the boiler. In the majority of cases, high pressure boilers have an 95% thermal efficiency.
Stack Temperature and Losses:
Stack temperature is temperature that is lost from the combustion of the gas in a boiler.
An efficient boiler uses as much of the heat from the combustion as possible. A low stack loss temperature means effective heat transfer and less heat loss. To avoid stack loss, high pressure boilers control the amount of air to avoid excess air, which can cause stack loss. The amount of air needed depends on the type of fuel.
Burners normally require 15 to 20% excess air to burn properly. Keeping the amount of excess air to a minimum results in an efficiently operating high pressure boiler. The evidence of excess air can be found in the rise of the flue gas temperature. An economizer, installed in the flue, can recover some of the lost heat.
Radiation and Convection Losses:
The amount of radiation and convection loss depends on the type, size, and pressure of the boiler. Radiation and convection losses happen because the boiler is far hotter than its surrounding environment. Though it is impossible to totally eliminate radiation and convection losses, they can be controlled by insulating the piping and boiler body.
The American Boiler Manufacturers Association (ABMA) has developed a standard radiation loss chart that can be used to measure radiation loss. The amount of radiation and convection loss is proportional to the surface area of the boiler.
The more heating surface a boiler has per horsepower, the more efficient it is. The normal design of a boiler is 5 to 7.5 lbm/hr of steam per square foot of heating surface. This varies between the different models.
Flue Gas Passes:
The number of passes flue gas makes before leaving the boiler is a good method for examining boilers. As the gas moves around the boiler, it cools, which changes its volume. The more passes it makes increases the efficiency of the boiler and increases the gasses velocity.
The performance of a boiler depends a great deal on the burner, controller, and boiler working as an integrated system. For a boiler to be efficient, the burner must make optimum use of the air supply and eliminate excess air. The productivity of the burner is critical to the cost of the boiler's operation. Air levels specifically are set to ensure proper combustion from the burner.
Chapter Four – Heat Sources for High Pressure Boilers
The different models of high pressure boilers are divided by their type of fuel, which is a major consideration since it substantially affects high pressure boiler operating costs. Depending on the regulations of the area where the boiler is used, there can be challenges in regard to the environmental impact of the choice of fuel. Having an understanding of the various fuels can be helpful in selecting the right high pressure boiler for its location and use.
A recent trend in high pressure boiler selection is choosing a boiler that can operate on more than one fuel, such as natural gas, propane, or #2 fuel oil. A major benefit of being able to use different fuels is the ability to adjust fuel consumption as fuel costs change.
Types of Heat Sources for High Pressure Boilers
Coal has been a fuel source for boilers for over a hundred years. For many modern boilers, it is pulverized into a powder. Anthracite coal is the hottest burning coal and is considered to be clean burning since it produces more heat and less smoke.
Biomass, organic fuel, is gaining popularity because it is eco friendly and produces low emissions. There are several forms of biomass fuel, escribed below. For ease of handling, feeding, combustion, transportation cost, and storage, biomass is densified into pellets or briquettes.
Rice husks have become a biomass fuel because they are easily available and are produced in large quantities as the result of rice production. Using rice husks eliminates the problem of their disposal. Another benefit of using rice husks is the ash produced, which is high in silica. Rice husks are an unproblematic fuel that does not cause corrosion, fouling, or sintering.
Wood chips are produced by putting wood through a wood chipper that converts them to the proper size to be used as fuel. Boilers that run on wood chips are referred to as multifuel boilers since they can run on pellets made from a variety of waste from crop production. Wood chip boilers have a feeding mechanism to continuously feed the boiler.
Gas boilers can use propane or natural gas. The gas is ignited in a combustion chamber and heats the heat exchanger. Gas type boilers are the most common form of boiler and inexpensive to install and operate.
Oil boilers have the same operating principle as a gas boiler. The fuel is ignited in a combustion chamber and heats the heat exchanger that heats the water. Oil boilers are not eco-friendly since oil burning produces CO2. Although there are problems with oil boilers, they can be more efficient, depending on their model and design. They are also more expensive to operate because of the changing prices of crude oil.
Electric boilers use electric heating elements to heat water directly. They are cleaner burning but not as efficient and far more expensive than any of the other boiler types. They are simple to operate and a good choice where there are strict emission standards.
Chapter Five – High Pressure Boiler Construction
Boiler construction requires an understanding of the purpose of a boiler, which is to generate steam from water. Though there are several purposes for the production of steam, the main purpose of an industrial high pressure boiler is to generate and produce energy and power to drive equipment and machinery.
Each type of boiler has a different design and geometric orientation. Despite their differences, there are common elements for most boilers, which are the burner, combustion chamber, and heat exchanger.
Accessories and mountings are necessary for the boiler to produce energy efficiently. Accessories include economizers, superheaters, feed pumps, controllers, and air preheaters. Mountings include feed check valves, steam stop valves, fusible plugs, blow off cocks, critical safety valves, and water level indicators.
Basic Boiler Elements
The burner is the heat source for the boiler. It requires fuel to burn and is activated by the thermostat that tells it to produce heat. When liquid fuel is pumped into the boiler, it is changed to a vapor by a nozzle and is ignited. Solid fuels, like wood, coal, and biomass, are fed into a grate in the combustion chamber before being ignited.
The combustion chamber, sometimes called the firebox, is where the fuel is burned. It is made of cast iron or steel and able to withstand temperatures into the thousands of degrees Fahrenheit. Combustion chambers are sealed to hold in the heat between firings to increase efficiency. The heat from the combustion chamber is transferred to the heat exchanger.
The grate is a platform located inside the combustion chamber where solid fuels like wood and coal are placed to burn. It is very similar to any form of grate with bars spaced sufficiently to allow air flow to feed the burning fuel.
The purpose of a heat exchanger is to transfer heat without making direct contact with the water. With a boiler, heat transfers from the heated gasses from the combustion chamber through piping, as can be seen in the image below. It is a central part of a boiler for the creation of steam.
The steam drum acts as a separator for the steam and water mixture. Steam leaves the steam drum at the top and goes to the superheater, while water is removed at the bottom. Separation takes place in two stages – primary and secondary. The primary stage removes most of the steam from the water. The secondary stage is a separator scrubber to capture the final droplets.
Boiler accessories are not mounted on the boiler but are necessary for the boiler to operate efficiently.
The economizer is where the water enters the boiler, and flue gas increases its temperature. Economizers are energy improving devices that lower costs by saving fuel and making the system more efficient.
The steam produced in a boiler is wet or saturated water vapor. The water droplets it creates are harmful. The purpose of a superheater is to remove or evaporate the water droplets by further heating the steam.
Boiler feed pumps feed a boiler with feed water that is equal to the amount of steam being produced. In modern boilers, feed pumps are centrifugal.
Boiler controllers monitor the operation of a boiler and check for abnormalities or dysfunctions. The aquastat of the controller controls water temperature to prevent the boiler from firing too often. Controllers monitor water levels and the feed rate to prevent low water levels, which can cause a boiler to overheat and fire too often.
The air preheater (APH) heats the air using recovered flue gases. The heated air enters the boiler and increases its thermal efficiency. The recirculated flue gas cools after heating air, and before it enters the gas stack. Air preheaters are either recuperative or regenerative.
Boiler mountings are necessary for the efficient operation and safety of a boiler. As their name implies, they are mounted on the boiler.
Water level indicator:
The water level indicator provides a reading of the level of the water in the boiler and is a safety device. Most boilers have two water level indicators. A water level indicator has three cocks and a glass tube. The first cock keeps the glass tube connected to the steam, while the second cock connects the glass tube to the water. The third cock indicates whether cocks one and two are clear.
The pressure gauge gives a reading of the pressure inside the boiler.
All boilers are required to have a pressure gauge, which can be mounted on the front or side. A Bourdon pressure gauge is typically used on a high pressure boiler.
The safety valve protects the boiler against extreme high pressure inside the drum. It allows for blowoff when the working pressure gets too high. As an extra precaution, and as the steam is released, it makes a loud, uncomfortable noise to warn people in the area. The varieties of safety valves are lever, dead weight, high steam low water, and spring loaded (as seen in the image below).
A stop valve controls the flow of steam and can completely stop it. It manages the steam supply to use and can be operated manually or be part of the control system.
A fusible plug is found in a firetube boiler as a protection against the boiler overheating. During normal operation, the fusible plug is covered in water to keep it stable. When the water level falls, the plug is uncovered and its fusible metal melts, allowing water to enter and put out the fire in the boiler to protect the fire tubes.
Blow Off Cock:
The blow off cock is used to remove unwanted deposits in the bottom of the boiler or drain off water. When the cock is open, the deposited material leaves under pressure.
Feed Check Valve:
The feed check valve monitors the water supply as it enters the boiler and prevents water from escaping if the pump fails or has low pressure. It is also known as a no return valve and is placed just below the water level in the boiler.
Chapter Six – High Pressure Boiler Safety
Though high pressure boilers are essential to the production of energy and provide excellent power, they are a potentially dangerous piece of equipment and have to be monitored closely. Regardless of the many safety features on a high pressure boiler, if the pressure is not contained, the steam energy will cause the boiler to explode and release metal and superheated steam.
There are certain conditions that can lead to dangerous failures of a boiler.
Melt down is when the metal of the boiler melts. It is usually the result of a low water supply. This condition will not result in an explosion but can severely damage the boiler. Though there may not be an immediate explosion, if the situation remains unresolved, an explosion will occur.
Thermal shock is when cold water is applied to an overheated surface. The cold water expands dramatically and becomes steam, which causes an explosion.
Combustion explosions are the result of ignited gas and can happen inside or outside the boiler.
The purpose of a boiler is to create steam pressure, which has to be closely monitored. All boilers are required to have safety measures in place to prevent this from happening.
The boiler log is a record of boiler operations and is updated regularly. It includes information regarding safety checks and any malfunctions.
Water Level Checks:
This is the most important part of boiler maintenance, since low or high water levels can lead to major problems. There are two methods for checking the water level. The first is the gauge glass, which is the easiest. The second method is checking the cocks when the gauge glass is not working.
During blowdown, the water in the gauge glass should fill quickly when the blowdown valve is closed. If it happens slowly, there may be a blockage in the water flow.
The capacity of a safety valve is measured in the pounds per hour that it can handle. Its capacity has to be the same or more than the boiler‘s pounds per hour rate. Safety valves are tested by activating the try lever. When the try lever is released, the disk of the safety valve should move to the closed position.
Modern boilers have a flame safeguard as part of their control systems. It is designed to monitor the burner and shut off fuel if fuel is not being supplied. A flame scanner measures the flicker of the flame and its intensity. It scans the visible, infrared, and ultraviolet light to determine the strength of the burning fuel.
One of the problems of working with water is rust and corrosion. For boilers, corrosion weakens the metal and causes pitting in the tubes, piping, and various components. Water treatment ensures the boiler‘s efficiency and lengthens its life span.
Carryover is when droplets of water get into the steam line, which is caused by a high water level, impurities in the water, or high surface tension in the water.
When doing a blowdown, the boiler should have a light load and be at a normal operation water level. Opening of the valve should happen slowly and with caution to avoid injury.
Handling Steam Valves:
When opening a steam valve, safety glasses or shields should be worn as well as gloves. The valve should be opened very slowly to equalize the pressure and warm the downstream equipment. Traps should be drained as well.
Pumps are normally maintenance-free devices. The temperature of their bearings should be checked regularly. Also, they should be examined for any vibrations. Regular maintenance should include lubrication of the pump bearings and a check of the flexible couplings and pump seals.
Steam traps can cause a feedwater pump to be steambound, where the feedwater becomes too hot and turns to steam. A feedwater pump can pump water but not steam. To correct the problem, the pump needs to be cooled.
The above listed measures are a few of the actions that need to be taken when dealing with a high pressure boiler. Each organization and manufacturer has their own safety standards that should be strictly followed.
The Occupational Safety and Health Administration (OSHA) and the National Fire Protection Association (NFPA) have strict guidelines and regulations for high pressure boiler safety.
Chapter Seven – High Pressure Boiler Regulations
The oversight groups for boiler standards and regulations include OSHA, NFPA, and the American Society of Mechanical Engineers (ASME). OSHA and NFPA have developed safety protocols and regulations regarding proper handling of high pressure boilers in specific situations.
OSHA‘s boiler regulations are designed to prevent serious injuries. The standards that have been established are designed to meet specific conditions for individual industries.
OSHA Standard 1926:
Pressure devices need to be inspected for safe installation, have regular inspection, and testing.
Boilers are in compliance if they have a current and valid certification provided by a qualified inspector.
Specific requirements for various types of pressure devices and equipment, which are outlined in Subparts F and O.
NFPA 85: Boiler and Combustion Systems Hazards Code, 2019 Edition
NFPA 85 specifies the operating safety and preventive measures for uncontrolled fires, explosions, and implosions as part of Section 1.1. It establishes the requirements for design, installation, operation, training, and maintenance of boilers and pressure devices.
ASME Boiler and Pressure Vessel Code # 138:
The ASME Boiler Code is the national standard for public safety, product reliability, and industrial efficiency and has increased research efforts in industrial groups as well as provided stability to contractual relations and certification. It has become the international standard for accreditation programs.
Boilers are a form of shell and tube heat exchanger that use boiled water to create steam or high pressure hot water.
A key component of high-performance high pressure boilers is their ability to produce superheated steam, which is not possible with all high pressure boilers.
The variations between the different types of boilers are their geometric orientation, external or internal fire, pressure, water circulation, number of tubes, and the circulation of gasses.
Boiler construction requires an understanding of the purpose of a boiler, which is to generate steam from water.
There are several oversight groups that have specific standards and regulations regarding high pressure boilers. The spectrum of control groups include international, national, state, and local organizations that provide regulations and stipulations.
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