## What is Relative and Absolute Vibration?

Before we continue the discussion about relative and absolute vibrations, we need to straighten out this matter first. Relative and absolute vibration is not included in one of the vibration classifications (read the following article to find out the classification of vibrations). Relative and absolute vibration is only a method of viewpoint, for measuring vibration.

Relative Vibration

As the name implies, relative is a measuring technique that refers to a certain point that is not silent. Then relative vibration is a measurement of vibration (either displacement, speed, or acceleration) measured relative to the position of the vibration sensor.

We take for example in measuring the engine shaft vibration according to the picture above. The shaft vibration measuring instrument in the form of an eddy current sensor is installed very close to the engine bearing position. So that when a vibration arises on the engine shaft, the sensor will read the vibration whose value is relative to the position of the transducer.

If there is a simultaneous vibration between the shaft and the engine bearing, the vibration sensor will read zero. When this happens, the condition of the installation of the engine foundation against the basic foundation must be considered.

Vice versa, if there is vibration on the engine shaft due to bearing failure, for example, the transducer sensor will read high vibration value relative to the sensor position. In other words, if relative vibration occurs then it is estimated that damage occurs on the engine bearing.

There are two ways to use eddy current sensors, first is using one sensor. If this is used, the sensor is placed just above the center of the bearing. Whereas the second method is to use two sensors that are paired at +45° and -45° from the vertical line of the shaft (according to the image below).

Eddy current sensors require a conversion process so that the reading of the sensor output voltage can be changed to a displacement value. If you want to know the velocity value, you need an integration process from the displacement value, and double integration if you want to know the relative acceleration value of vibration.

Absolute Vibration

Absolute is a measuring technique that refers to a stationary point in free space. Then absolute vibration is a measurement of vibration (either displacement, speed, or acceleration) measured against a stationary point in free space.

Generally absolute vibration measurements only use speed transducers or accelerator. The speed transducer sensor is equipped with a coil as a reference point, while the acceleration transducer uses a mass.

Absolute vibration on a machine shaft cannot be measured directly. A real-time measurement method and calculation is needed.

Measuring the absolute vibration of a machine shaft as shown above, it is necessary to measure the relative displacement of the shaft using the same eddy current sensor as in the measurement of relative vibration. It is also necessary to measure absolute vibration on the bearing using an accelerometer sensor or speed sensor, which is set to have the same axis as the position of the eddy current sensor.

The absolute vibration value of the shaft is obtained by calculating the difference between the relative vibration of the shaft, and the absolute vibration of the bearing. The calculated absolute vibration value of the shaft can be in the form of 0-peak displacement, or also peak-peak displacement as shown in the graph below.

As for calculating vibration speed, an integration process of absolute displacement values ​​is needed. While the value of vibration acceleration is obtained by double integration.

References:

## Classification of Boiler

Boilers are a tool for creating steam. The existence of boilers has been important since its development in the range of the 18th and 19th centuries. Boilers also took an important role in the era of the Industrial Revolution and encouraged various other important discoveries. In subsequent developments many studies have succeeded in bringing out a variety of new boiler designs.

To classify a boiler, we can only do it by looking at it from various points of view. These various perspectives depend on the design of the three constituent components of the boiler, the elements of water, steam, and combustion chamber. For more details, let’s discuss one by one.

Classification of boiler based on the relative position of steam with the combustion chamber

1. Fire-tube boiler

Fire-tube boilers are the simplest type of boiler. This boiler allows it to be applied from low to medium steam requirements. This is possible because the design is not complicated than a water-tube boiler.

As the name implies, fire-tube boilers deliver hot gas from the combustion to the pipes which are covered by water. Hot gas from the combustion of fuel in the combustion chamber (furnace) is passed to these special pipes before being discharged into the atmosphere.

Fire-tube boilers have a very simple design that only requires less space. In fact, many of these boiler designs allow it to be moved to one place to another. However, fire-tube boilers have limited steam production which is only a maximum of 9000 kg/hour with a maximum pressure of 17 bar.

Fire-tube boilers themselves can still be classified into several types:

• Haystack Boiler
• This boiler is the simplest design boiler. Only composed of a giant stove carrying a large pan. This pan-shaped boiler was once inspired by a cooking pot. Whether in the century, how many boilers have started to be developed, but nowadays boilers that are only capable of working at a maximum pressure of 5 psi are rarely encountered. However, the Haystack boiler became the forerunner to the development of a variety of new boiler designs until the discovery of a modern fire-pipe boiler design.

(Credit: Science Museum Group)

• Center-flue Boiler
In the next development, boiler began to be designed more complex. The boiler center-flue is the beginning of the birth of a fire-tube boiler, because the combustion gas is flowed into the water tank through a large pipe before being discharged into the outside air. The flue gas pipe only has one direction away from the furnace.

This boiler is popular after being used as the first locomotive engine. This boiler is quite good at the side of the exhaust gas flow because of the use of the chimney. However, it is not very efficient if it is used to burn too much fuel such as wood or coal.

•

• Return-flue Boiler
Boiler return-flue is a further development of the center-flue type. If the center-flue uses one exhaust gas pipeline, then the exhaust gas pipe in the return-flue boiler is made to have a U-shaped backflow. The purpose of this design is to further improve boiler efficiency. Boilers that developed in the early 19th century were used as locomotive machines to replace boiler center-flue that were not very efficient.

(Credit: Wikipedia: Flued Boiler)

• Huber Boiler
The Huber boiler became the first fire-tube boiler to be more complex than several types of boilers before. This boiler has not used one large pipe as a return channel for exhaust gas, but has used several small pipes or tubes in order to maximize heat transfer from the flue gas to the water in the tank. The shape of the exhaust gas channel after exiting the combustion chamber also has a better design. The design makes gas distribution to be maximized to all pipelines.

• Cornish boiler
Another development of a fire-tube boiler design is the Cornish Boiler. This boiler is a horizontal boiler with a natural draft system, so it requires a high chimney shape to ensure adequate oxygen supply.

This boiler is made from a large water tank with the combustion chamber right in the middle. Flanked by a brick building, such that the flow of combustion gases coming out of the combustion chamber in the middle of the tank will flow back along the outer edge of the tank. Next the brick building will direct the exhaust gas to traverse a passageway under the tank, before finally passing through the chimney and out into the atmosphere.

For more details, let’s look at the picture on the side, top and front of this Cornish boiler.

• Butterley Boiler

Butterley boilers are the development of the Cornish boiler, which initially aimed to accommodate the needs of boilers in the northern United States which are rich in coal with lower calorific value than the southern mainland. This boiler is similar to the Cornish boiler design but by removing the exhaust gas lines under the water tank.

• Lancashire boiler
The Cornish boiler also has another fire-tube boiler design derivative called the Lancashire Boiler. If the Cornish boiler has only one combustion chamber and at the same time one large fire pipe in the middle of a water tank, then the Lancashire boiler has two combustion chambers that are simultaneously two fire-pipes in the middle of a water tank. The boiler developed by William Fairbairn in 1844 tried to adjust the Cornish boiler design when using coal fuel in the Lancashire area on the English plain, which tends to be difficult to burn in small boilers.

• Locomotive Boiler
The Locomotive Boiler becomes the first complex fire-tube boiler. Even this boiler is still often encountered today. Boilers that are named according to their use as a train driving machine are designed to produce superheater steam. The water vapor will be directly used as a piston drive on a steam engine that is designed to blend into the Locomotive boiler system.

This boiler has also been designed to have a lot of medium sized fire pipes that are smaller than fire pipes in the Center-Flue and Return-Flue Boilers, so that it will increase the transfer of heat energy from the combustion gas to the water.

An important component of the Locomotive Boiler is the presence of a superheater steam valve that is inside a section called the dome. This one-way valve will only open by superheater steam when it reaches a certain pressure. Furthermore, the superheater steam will enter into a steam piston driving medium.

• Scotch Marine Boiler
The Scotch Marine boiler is the most popular fire-tube boiler design used even today. This boiler was originally made to meet the needs of steam in marine engines. Even the legendary Titanic ship uses a total of 29 Scotch Marine boilers.

Scotch Marine boilers have high efficiency. This is obtained because the design of the fire pipe in the water tank is very much. Hot gas from the combustion process comes out of the combustion chamber in the middle of the water tank, towards the fire pipes which are next to the combustion chamber with the opposite direction of flow. Then the exhaust gas flows back to the fire pipes on the upper side with the direction of the direction of the direction of combustion in the combustion chamber. In short, the flow of combustion gases inside the fire pipes seemed to form the letter S.

• Vertical Fire-tube Boiler
Fire-tube boilers that are vertically areanged, known as vertical fire-tube boilers. This type of boiler has design advantages and the manufacturing process is not too complicated. The combustion chamber is under a water tank, with pipes for exhaust gas lines arranged vertically in the water tank.

• Horizontal Tubular Return Boiler

The Horizontal Return Tubular Boiler is similar to other fire-tube boilers that we have discussed. Has a flat arrangement of fire pipes. What is slightly different is the design of the placement of a combustion chamber that is not in a water tank, but is under the tank. The fire pipes in the tank will only be passed by hot exhaust gas from the combustion of fuel in the combustion chamber.

This fire-tube boiler is unpopular and has not been used by many since its appearance in the Ironclad warship era in the mid-19th century. One thing that made it unpopular was the design of the fire pipe that was connected directly to the combustion chamber so that over-heat was often occured on the pipe.

• Immersion Fired Boiler
This last fire-tube boiler has one characteristic that is not haved by other fire-tube boilers. The boiler developed by the Sellers Manufacturing manufacturer is designed so that each fire pipe in the water tank functions as a combustion chamber as well as a hot exhaust gas from the combustion process. So that this boiler has a lot of burners (burners) with an amount equal to the number of existing fire pipes. With an automatic boiler design that is only suitable for using liquid or gas fuels, it is claimed to have a relatively low temperature voltage. This boiler is still marketed to date by the manufacturer Sellers Manufacturing as the owner of the patent design.

2. Water-Tube Boiler
Water-tube boilers have a reverse design with fire-tube boilers. This boiler circulates water through the pipelines with a heat source coming from the furnace. A water tank commonly called a steam drum is one of the characteristics of a water-tube boiler. Steam drum serves as a water tank that is maintained at a certain level to ensure there is always circulating water to the water pipes. Besides that, the steam drum is also to separate steam from water-steam mixture inside the drum. Wet steam coming out of the steam drum will be heated further to produce superheated steam.

The popular water-tube boiler design is using the water pipes as the combustion chamber wall (wall tube). The water from the steam drum drops through a downcomer pipe to a header pipe connected to all the lower ends of the wall-tube pipe. The other wall-tube end that is at the top of the combustion chamber is directly connected to the steam drum. In this part of the wall tube, the phase changes from water to water vapor. This water-tube system produces a closed-loop circulation between the steam drum-downcomer-wall tube-and returns to the steam drum. From the steam drum, only saturated steam will come out.

Even water-tube boilers have slightly more complex designs than fire-tube boilers, but water-tube boilers tend to be able to produce higher quality steam (more superheated). Therefore, water-tube boilers are more suitable to be applied in large industries that are more demanding of high quality steam such as steam power plants.

Based on different designs, water-pipe boilers can be classified as follows:

• John Blakey Boiler (1766)

This boiler designed by John Blakey became the forerunner of the water tube boiler. This boiler is composed of a vertical funace with several connected pipes inside, which are made tilted to form a certain angle. The two ends of the pipe are connected to a smaller pipe.

This boiler was patented by John Blakey in 1766, but not too popular at the time.

• James Rumsey Boiler (1788)

The first functional water-tube boiler was created by a mechanical engineer from the United States, James Rumsey. He is known to patent several water-tube boiler designs, making James Rumsey touted as the inventor of the water-tube boiler. One of the most famous designs is a steam-powered boat.

The boat, which was then made to cross the Potomac River, was equipped with a water-tube boiler. The water tube in the boiler are twisted horizontally, inside a large enough furnace. The steam produced is used to drive a steam piston.

The steam piston, using a single shaft, is connected to another piston underneath it. The second piston serves as a water pump, with water from the river as the media where the boat operates.

The piston shaft, also connected to a large pendulum. The pendulum is connected to an injector pump and an air pump on the condenser.

Steam that enters the steam piston will lift the shaft, so that the water piston also lifted, and sucks the water of the river into the piston cylinder. When the piston reaches the top dead centre, a knob on the shaft will touch a stick mechanism, so the control valve will change its position.

When the control valve changes its position, the steam inside the cylinder will be pushed out and enter the condenser. The water piston will also be pushed down, so the water comes out of the cylinder, passing through the nozzle on the back of the ship, thus creating a force for the ship.

When the shaft position down, the injector pump pushes the river water into the boiler, while the air pump pushes the water inside the condenser to exit.

• Julius Griffith Boiler (1821)

This boiler has a fairly simple design, but has a significant impact on the development of the next water-tube boiler design.

This boiler was designed by Julius Griffith in 1821, which is composed of several horizontal pipes in several levels and placed inside the heat source. The horizontal pipes are connected to the twin vertical pipes on either side. At the very end, there is a last horizontal pipe as a gathering place for steam produced, which then comes out of the boiler. This top horizontal pipeline design will next to be the forerunner of the design of the steam drum in modern water-tube boilers.

• Joseph Eve Boiler (1825)

The first sectional water-tube boiler with well-defined circulation was designed by Joseph Eve in 1825. This boiler composed of several vertical pipes with curve variations in the center of the pipe, two larger horizontal pipes as a water reservoir and a steam reservoir, as well as two large external vertical pipes as a circulating pipe between the steam reservoir on the upper side and the water reservoir on the lower side. These two vertical pipes have a function to ensure good natural circulation of water and steam inside the boiler system between the water pipes, reservoirs and external pipes.

• Goldsworthy Gurney Boiler (1826)

The Gurney water-tube boiler design was patented in 1825, first made in 1826, and tested in 1827 by Simon Goodrich.

This boiler is composed of several U shaped pipes laid with one side in the top position. Each end of the U pipe is interconnected with a horizontal pipe with a larger diameter, both top and bottom. Then these two horizontal pipes are connected with vertical pipes to ensure the occurrence of water-steam circulation. There is also a long and large diameter cylindrical tube, vertically standing connected to the upper and lower horizontal pipes. This cylindrical pipe serve as a water reservoir and water vapor.

• Stephen Wilcox Boiler (1856)

This Wilcox design boiler became the first water-tube boiler to use inclined pipe design. These tilted pipes connect the water spaces on the front and back, with the steam room at the top.

This boiler design later developed into Babcock & Wilcox boiler design, and dominated the water-tube boiler market in the late 19th to early 20th centuries.

• Spiral Water-Tube Boiler

If the fire-tube boiler develops along with the train development, the water-tube boiler design was developed in tandem with car technology.

The birth of car technology in 1770 created by Nicolas-Joseph Cugnot, encouraged the development of cars in the 1800s that still use steam engines. Most of the car’s engines use spiral water tube boilers with different designs. Since then, spiral water tube boilers have developed into various uses.

Spiral water tube boiler designs include Climax Boilers, Lune Valley Boilers, Monotube Boilers, The Baker Boiler, Ofeldt Boilers, and many others.

• D-Type Boiler
The first water-tube boiler type that we will discuss is called the D-type because the shape of the boiler is similar to the letter D. This boiler is equipped with two tanks namely the steam drum on the upper side and the mud drum (water tank) on the lower side. These two tanks are connected to many water pipes which are partially arranged vertically, and some are arranged in the shape of the letter D. In the middle of the D-shaped pipes has a functions as a combustion chamber.

• Type-A Boiler
Still because the design is similar to the form of one of the Latin letters, the A-type boiler is named so indeed because its design is similar to the letter A. This boiler has one steam drum but with two water tanks below. The purpose of using these two water tanks is to further extend the life of the boiler because the water pipe will be longer than the D-type design. This boiler has a slimmer design than a D-type boiler, however an A-type boiler cannot produce higher energy-containing steam than the D-type for the same dimensions.

(Credit: Wikipedia: Package Boiler)

• O-Type Boiler
The O-type boiler is the last type of water-tube boiler whose design is similar to one of the letters. This O-shaped boiler has a symmetrical shape with the position of the above steam drum and water tank below. Both are connected with symmetrical water pipes so that in the middle become boiler combustion chambers. This O-type boiler is claimed to be able to produce water vapor faster than the D-type. The low maintenance requirements are also another advantage of this boiler.

• Babcock & Wilcox Boiler
As the name implies, the Babcock & Wilcox boiler was developed by a firm with the same name as the boiler. This boiler design was developed and patented in the mid-nineteenth century. This boiler has only one tank, the steam drum positioned at the top of the boiler. The steam drum is partly filled with water and the other part contains wet water vapor. The typical design of this boiler is water pipes that are designed to be tilted to form a 15 ° angle. This slope serves to ensure the occurrence of natural circulation of water-water vapor in the boiler. On top of the water pipes there is also a further hot steam pipe which functions to further heat water vapor that has been sufficiently hot and escaped from the steam drum to be further heated to achieve superheated quality. For the flow of combustion gases in the boiler is made tortuous so as to maximize absorption of heat from the flue gas to the water fluid.

(Credit: Mech4Study)

• Stirling Boiler
The Stirling boiler is one of the predecessors of the water-tube boiler. These boilers were popularly used in the early 1900s, and are very difficult to find at this time. This boiler has the characteristics of using two kinds of water tanks, steam drum at the top with an amount that is always more than the second tank, the water tank at the bottom of the boiler. Characteristics of the design make the Stirling Boiler can be classified based on the number of water tanks, there are three tanks with two steam drums and one water tank, four tanks with three steam drums and one water tank, and five tanks in the form of three steam drums at the top and two water tanks at the bottom of the boiler. The more number of tanks, the higher ability to produce steam. However, this boiler is old-fashioned and is no longer used because it has relatively lower efficiency values ​​than modern boilers.

Boiler Stirling Tiga Tanki
(Credit: Wikipedia: Stirling Boiler)

Boiler Stirling Empat Tanki

Boiler Stirling Lima Tanki

• Yarrow Boiler
The Yarrow boiler is an important type of high pressure water-tube boiler. They were developed by Yarrow & Co. (London), and is widely used on ships, especially warships.
Yarrow boiler design has the characteristics of a boiler with three water tanks: two tubes of straight water are arranged in a row of triangles with a single furnace between the two. A single steam drum is installed at the top between them, with a smaller water drum at the base of each bank. Circulation, both up and down, occurs in this same bank tube. Yarrow’s specialty is the use of straight tubes and also the circulation in both directions that occur entirely in the tube bank, and does not use external energy or we are familiar with natural circulation.

Because of the characteristics of the three drums, Yarrow boilers have greater water capacity. Therefore, this type is commonly used in boiler applications for old warships. Its compact size makes it attractive for use in power generation units that can be transported during World War II. In order to be transported in its time, boilers and auxiliary equipment (fuel oil heaters, pump units, fans etc.), turbines and condensers are installed in their own carriages to be carried through the railroad tracks.

•

• Thornycroft boiler
This boiler was designed by the ship manufacturer John I. Thornycroft & Company. The special design of this boiler is to use just one steam drum on the upper side, with three downcomers so that it is arranged similar to the M formation boiler. However, due to the design of several pipes that have sharp bending, it risks leaking quickly not only because of the possibility of thermal stress, but also because of its own difficulties when needing to be cleaned. Because of these weaknesses, this boiler is not as popular as Yarrow Boiler.

• Tube-Walled Boilers

In the early days of its development, water-tube boilers were not as fast developed as fire-pipe boilers. This is because water-tube boilers require more complex design calculations and manufacturing techniques. But the main advantages of fire-tube boilers that have almost no maximum capacity limit, making the development in the next period, only need to wait for the birth of modern welding and materials technologies.

After electricity was discovered, then the construction of steam power plants began intensively in the early 20th century, the Stirling Boiler type still dominated. To meet this need, numbers of Stirling Boilers are built at once in parallel so they can produce more steam.

Why isn’t the Stirling Boiler made larger, and bigger, so it can produce more steam?

The main reason was the use of fire bricks as the boiler wall. Fire brick walls would certainly be troublesome if you have to be arranged too high, as well as widened, following the boiler design if you want to be enlarged. Beside that, this large-sized wall must be able to isolate the heat energy of the combustion chamber, to ensure maximum heat absorption in the boiler.

Gradually, new innovations were born to replace fire brick walls. The advancement of pipeline material and welding technology are also driving the advancement of boiler wall technology.

Tube and tile boiler walls, became the initial innovation of the boiler wall design revolution. Found in the 1920s, this boiler wall combines a 6 inch diameter pipe with 2.5 inch thick tiles or 4.5 inch thick fire brick. Tube and tiles are arranged alternately, and the outer side of the wall is insulated to maintain boiler efficiency.

The existence of the tube in the boiler wall, has a function to cool the wall so that the thickness of the fire bricks can be reduced from the previous thickness which can reach 22 inches. Since that time, the boiler design continue to grow in both size and capacity.

In the late 1920s and early 1930s, the appearance of the flat studded tube and the loose tube wall constructed boilers. This design was able to increase the absorption of boiler heat. So in those days, boilers that used those two designs were able to receive the highest heat generated from coal combustion.

Large changes in the water-tube boiler wall design occurred in the late of 1950s and early 1960s. Since then, and are still used today, water-tube boiler walls made from long steel tube which arranged in a row and welded each other with certain widths of steel membrane bar between every tube.

This design is much easier to fabricate because making every wall panels can be done at the workshop. Then the wall panels can be much easier to assemble when build the boiler. The process of building boilers has become much more practical, time-saving, and certainly effective in cost.

Not only that, the main advantage of this boiler wall-tube design makes the water-tube boilers could be built even larger and bigger. Known this day, huge water-tube boilers are capable to produce superheated steam of more than 4,000 tons per hour. That’s the same as more than a thousand kilograms of steam produced by this boiler, every second.

Again, in every second!

• Once-Through Boiler

Once-through boiler is a concept of water tube boiler that does not occur a non-vaporizing water circulation. That is, each water molecule only passes through the boiler pipes once. This concept greatly increases boiler efficiency because it is no longer requires a steam drum as a water and steam separator, so there is no need for additional boiler circulation pumps.

This boiler concept is actually not new. The boiler design was once patented in 1824. But the first commercial application of this boiler could only be done in 1923, by a Czechoslovakia inventor, Mark Benson. At that time, Benson only could build 1.3 kg per second boiler capacity. The boilers built to fulfill orders from English Electric Co.. The boiler was originally designed to operate at critical steam pressures. However, due to the frequently damage of the pipeline, boiler operating pressure was then forced to be lowered.

The once-through boiler continues to grow until now. Supercritical and ultra-supercritical boilers have used this concept. So even though this boiler is used in power plants with a capacity of 1000 MW, the efficiency can reach 46%.

Classification of boilers based on water circulation method

In water-tube boilers, the circulation of water in boiler pipes is important to pay attention. In addition to good boiler water circulation, it will increase boiler efficiency, water circulation is also important to maintain boiler durability. This is because the water in the boiler also as a cooling medium, delaying the water circulation, resulting high thermal stress on the pipe. Of course this is very avoided.

Against this background, there are known two types of boilers based on the way of water circulation. Here are both:

1. Natural Circulation Boiler
Boilers with natural water circulation do not use external energy to circulate water in boiler pipes. The water in this boiler is naturally circulated due to the pressure difference between low temperature water and high temperature. Naturally high temperature water will have a relatively lower density. Therefore, the water is getting hotter and the phase changes to steam, the more it will be pushed upwards. Because of this process, the water in the boiler pipes will be circulated.

Boilers with natural circulation include Babcock & Wilcox boilers, Lancashire, Cochran boilers, locomotive boilers, and so on.

Perbedaan Boiler Sirkulasi Natural dan Paksa
(Credit: Wikipedia: Forced Circulation Boiler)

2. Forced Circulation Boiler
Boilers with forced circulation, use additional pumps to help the circulation of water in the boiler. This type of boiler does not need to wait for water phase differentiation to be able to circulate water. With the help of external energy for the process of water circulation, the process of generating steam will not be limited by the size of the boiler. When compared, forced circulation boilers can produce twenty times more steam than natural circulation boilers that have the same volume size.

Examples of forced circulation boilers include Benson boilers, La Mont boilers, Velox boilers, and so on.

Classification of boilers based on their working pressure

In accordance with technology advancements, the quality of boiler steam also continues to improve. The boiler designers believe that the higher steam pressure can be achieved, the boiler efficiency will be higher to. So the following are the classification of boilers based on the steam pressure produced:

1. Low-pressure boiler: This boiler produces 15-20 bar of steam.
2. Medium-pressure boiler: This boiler produces steam from 20 to 80 bars.
3. High-pressure boiler: This boiler produces steam pressure above 80 bar.
4. Sub-critical boiler: The critical point of a boiler is a condition where boiler steam reaches a temperature of 560 ° C at a pressure of 221 bar. If a boiler works below these conditions, the boiler is called a subcritical boiler. Typically subcritical boilers are designed to work at 160 bar and steam temperature of 540 ° C.
5. Supercritical boiler: If a boiler works above its critical point, the boiler is called a supercritical boiler. Supercritical boilers have better fuel efficiency than subcritical boilers. Supercritical boilers have a design efficiency value of around 45%. While subcritical boilers can only reach 38%.

This is due to the impossibility of forming bubbles in the supercritical boiler cycle. As a result of the work pressure and temperature above the critical point, the water will not experiencing the nucleate boiling phase (the transition phase from liquid to vapor) and immediately changes the phase immediately to steam. One characteristic of supercritical boilers is that they do not use steam drum components which to separate water from wet steam at sub-critical boiler.

6. Ultra Supercritical boiler:
The working point of the boiler which is high above the critical point, the boiler will be more efficient. To achieve this, more sophisticated and expensive boiler pipe material technology is needed. The last few decades have made possible the manufacture of the material in question, so that at present the boiler design has been able to reach the point of work very far above its critical point. The boiler that we know as the Ultra Supercritical (abbreviated as USC) has an operational point of around 260 bar and a temperature of 700 ° C. This modern boiler has a theoretical efficiency value of up to 50%.

Classification of boilers based on their energy sources

In accordance with technological advances as well, now there are so many sources of energy that can be used as a source of boiler heat. Boilers developed at the beginning of its history only used fossil fuels, now there are several boiler technologies that can use renewable energy.

The following are among them:

1. Coal-Fired Boiler
Coal is the most commonly used fuel in large capacity boilers, including in Indonesia. The price is cheap, abundant (especially in coal-producing countries including Indonesia), high heat values, a number of reasons for the use of coal as boiler fuel to date.

The use of coal as boiler fuel requires special treatment not carried out on other types of boilers. Characteristic of solid coal, the average size of your fist, requires a grinding process before it is burned in the boiler combustion chamber. Of course this is the main objective to facilitate the burning of the coal.

Not only that, the processing of coal boiler exhaust gas is also different from other boilers. This boiler exhaust gas contains ash, carbon dioxide, sulfur, to NOx. Some binding processes for these wastes also need to be considered. Like the use of Electrostatic Precipitator to bind ash, then use Flue-Gas Desulphurization to bind sulfur, to the use of staggered combustion technology to minimize the formation of NOx.

The complexity of coal boiler design, makes the economics of these boilers not as good as oil-fired boilers if used on a small scale. Therefore, coal boilers are more widely used for subcritical to ultra-supercritical production scales.

2. Oil-Fired Boiler
Oil-fired boilers are quite popular for small scale use only. This is due to a much simpler design than a coal-fired boiler. These boilers are generally fire-tube boiler, which only require the main component of the burner and the pipe network for the flow of fire (hot gas) which is made inside the water furnace.

These boilers generally use diesel fuel, or commonly known as High Speed ​​Diesel (HSD). The simple design makes this boiler very suitable for the production of low pressure steam with low steam production capacity.

3. Nuclear Boiler
As the name implies, nuclear boilers use nuclear technology as a source of heat energy. This boiler is very popular for use in nuclear power plants. In nuclear power boilers, the heat energy from the fission reaction inside a nuclear reactor is absorbed by the coolant material which can be gas, liquid, or even liquid metal, depending on the type of reactor. This cooling material then flows into the boiler and is used to heat water so that it changes phase to further steam. The steam that is produced is channeled into turbines to generate electricity in nuclear power plants.

The popular raw material for nuclear reactors is Uranium. Uranium is a type of heavy metal that is not very useful on Earth and is easily found in the oceans and rocks. There are two types of uranium with different isotopes that we know, namely uranium-238 (U-238) and uranium-235 (U-235). These two types of uranium have a major difference in their reactive age. U-238 has a longer reactive life than U-235, which also shows that U-235 is less radioactive than U-238.

One major risk of using a nuclear reactor is of course the radioactive hazard. Therefore, nuclear reactors are always made in a dome that serves to prevent radioactive reactor leakage. Generally the outside of a nuclear reactor is made in a dome shape with strong concrete material which not only serves to prevent radioactive leakage, but also to resist natural disturbances from the outside.

4. Solar Concentrated Powered Boiler
The boiler we will discuss next is very new technology. This boiler uses a very renewable source of energy from sunlight. Although sunlight is only available during the day, this boiler can operate 24 hours a day. Thanks to the special fluid called molten salt, that able to stored heat from the sunlight.

Solar concentrated boilers use main component of large number of mirrors, arranged around a heat-receiver tower. The mirrors are positioned in such a way that the reflection of the sunlight captured by each mirror is reflected centrally to the heat-receiver tower. Each mirror component is equipped with an automatic mechanism, so that it can move following the sun, so that the direction of the sunlight reflection always leads to the heat-receiver tower.

A mechanism is used to circulate the molten salt into a heat-receiver tower. It is estimated that the heat caught in this tower can reach 1500 times hotter than what we normally feel. The heat is absorbed by the molten salt and stored in a special thermal storage tank.

Then, through a heat exchanger, the water absorbs the heat from the molten salt so that the water boils and reaches the superheat temperature. In concentrated solar power plants, this steam is then used to rotate steam turbines and produce electricity.

Many power plants that have used this technology are built in Spain, the United States, South Africa, India, and a little in China.

5. Waste-to-Energy Boiler

Waste power boilers or also known as waste-to-energy boiler are the most environmentally friendly solution to two problems at once: garbage and fossil fuel crisis. Waste production that continues to increase every time becomes one of the energy sources that can be used as boiler fuel.

Waste-to-energy boilers are not much different from other biomass boilers. First, the waste is brought to the facility. Then, the waste is sorted to remove recyclable and hazardous materials. The waste is then stored until it is time for burning. A few plants use gasification process, but most combust the waste directly because it is more efficient. The waste can be added to the boiler continuously or in batches, depending on the design of the plant.

It is known that waste-to-energy boilers have a friendlier emission level than coal-fired boilers. This is due to the absence of sulfur pollutants as contained in coal.

Biggest Waste-to-Energy Power Plant will be built in Shenzhen, Cina

Waste-to-energy power plants have been used for more than two decades in Sweden. And now many have been built in China, the United States, and many other countries.

## Classification of Vibration

Classification of Vibration – In general, vibrations can be classified into several ways:

1. Free and Forced Vibration
Free Vibration. If a system is initialised with interference, so it vibrates by itself, then the vibration is called free vibration. No external force works on the system. The motion of back and forth of a pendulum is an example of free vibration.

Forced Vibration. If a system is subjected to an external force (more precisely the repetitive force), then the vibrations that arise on the system is known as forced vibrations. The vibrations that arise on a working diesel engine is one example of forced vibration.

If the frequency of an external force is exactly same as the vibration frequency of the system, a condition known as resonance occurs. Resonance is very dangerous. Damage from the structures of buildings, bridges, turbines, and airplane wings is often associated with the resonance of the vibrations.

2. Undamped and Dumped Vibration
If there is no energy lost or dissipated due to friction or other resistance during vibration, then the vibration is known as Undamped Vibration. Whereas, if a vibration experiences a gradual reduction of energy, it is called Damped Vibration. In various systems, the value of the damping is so small that it is often disregarded for most engineering purposes. But also vice versa, there are other systems that put damping system into important components, shock absorber in vehicles for example. Consideration of damping becomes extremely important in analyzing vibratory systems near resonance.
3. Linear and Nonlinear Vibration
If all the basic components of a vibration system the spring, mass, and damper behave linearly, the resulting vibration is known as Linear Vibration. However, if one or more of these basic components behaves nonlinearly, then the vibration is called Nonlinear Vibration. Differential equations are made to describe the behavior of linear and nonlinear vibration systems. If the vibrations are linear, the superposition principle applies, and the mathematical analysis technique is well developed. For nonlinear vibrations, the superposition principle becomes invalid, and the analytics technique becomes more difficult. Since all vibration systems tend to behave nonlinearly as oscillation amplitude increases, knowledge of nonlinear vibrations is more developed in handling practical vibration systems.
4. Deterministic and Random Vibration
If the value or magnitude of the excitation (force or movement) acting on the vibration system is known at any given time, the excitation is called deterministic, and the resulting vibration is known as Deterministic Vibration.

In some cases, excitation is nondeterministic or random; excitation values ​​at certain times can not be predicted. In this case, extensive excitation data may indicate some statistical regularity. Under these conditions it is possible to estimate averages such as the mean and mean square values of excitation. Examples of random excitation are wind speed, roughness of the road, and ground movement during an earthquake. If the excitation is random, the resulting vibration is called Random Vibration. In this case the vibration response of the system is also random; and that condition can only be explained through statistical calculations.

## What is Water-tube Boiler?

What is Water-tube Boiler? – Water-tube boiler is a type of boiler with pipes containing circulated water, which is heated by a fire on the outer side of the pipe. The water-tube boiler has the opposite design with the fire-tube boiler. This boiler circulates water through the pipelines with the heat source coming from the furnace. These pipes, which become the water-vapor circulation pipes, are inside the combustion chamber blanket or the combustion hot gas duct. In modern water-tube boilers with large production loads, there are several parts of water pipes designed to be the walls of the boiler’s combustion chamber. The pipes are usually known by the term wall-tube.

A water tank, commonly called a steam drum, is one of the water-tube boiler character. Steam drums serve as a water tank to ensure there is always enough water circulated to the water pipes. Otherwise, steam drum also serves to separate the saturated steam with water. Saturated steam coming out of the steam drum will be heated further to become superheated steam.

Modern water-tube boiler, equipped with water pipes designed as the boiler walls (wall-tube). Water from the steam drum goes down through a pipe called downcomer to the header pipe (lower ring header) connected to all the lower ends of the wall-tube. The other end of the wall-tube at the top of the combustion chamber is directly connected to the steam drum. In the wall-tube, there is a phase change from water to steam. This water-pipe system produces a closed water circulation between the steam drum – downcomer – wall-tube and back to the steam drum. From steam drum only saturated steam will come out. In superheater boilers, the saturated steam will be heated further into superheated steam.

Even though the water-tube boiler has a slightly more complex design than a fire-tube boiler, this boilers tend to be more capable of producing higher steam quality, as well as a much larger capacity. Hence the water-tube boiler is more suitable to be applied to large industries, such as steam power plants, that demand more quantity, as well as high steam qualities.

## Working Principle of Hydraulic System

Working Principle of Hydraulic System – We are certainly familiar with the above heavy equipment. A lot of heavy work using this equipment. This equipment is designed to conquer heavy loads raised, or for the purpose of digging. The system used as a “fork” driver is a hydraulic system. Hydraulic system is a system that uses liquid fluid power to do a simple job. The hydraulic system is an application of the use of Pascal’s Law.

Hydraulic machine, supply pressurized hydraulic fluid to a hydraulic motor or hydraulic cylinder to perform certain work. Hydraulic motors produce a rotating motion that can be used to rotate heavy loads such as pulleys, chains, etc. Hydraulic cylinders produce back and forth motions that are widely applied to heavy equipment, water gates (at dams for example), or also for large valves. Hydraulic fluid is controlled by flow control valve and passed through hydraulic tube.

The hydraulic system can be simply explained through the picture above. The first image shows that by using a hydraulic system, it takes a smaller (F) force to be able to lift a larger load.

F2 = F1 • (A2/A1)

While the second picture explains the principle of using a hydraulic motor on a pulley. And it takes a smaller torque to be able to rotate pulleys with a larger load (large torque).

Tmotor = Tmotor • (Vmotor/Vpompa)

Hydraulic Circuit

A hydraulic system consists of hydraulic pumps, pipelines, control valves, hydraulic fluid tanks, filters, actuators (cylinders or hydraulic motors), and other devices as a complement.

The picture above illustrates a hydraulic system that works to drive hydraulic cylinder piston. The working fluid collected in the tank is pumped by a hydraulic pump so that it has specific pressures. Fluid flows to the solenoid valve, this valve regulates the movement of hydraulic cylinders. If the cylinder position lengthens (advance) then the solenoid valve will go to the left, so the fluid can push the piston forward. When the solenoid valve is directed to the right, the hydraulic cylinder will retract. At the time of movement in the cylinder, then there is some hydraulic fluid is wasted. This fluid returns to the tank through a special pipeline.

The system above is not much different from the hydraulic system that has a piston actuator. It’s just that here the actuator is a hydraulic motor for its use of torque (torque). The solenoid valve adjusts the direction of rotation of the hydraulic motor. Unlike the more complicated electric motors required to be able to rotate in both directions, hydraulic motors are easier to apply when needed to rotate in both directions.

## What is Fire-Tube Boiler?

What is Fire-Tube Boiler? – Do you remember what boiler mean? Boiler is a vessel that serves to heat water. In principle, a pan is also a boiler, but it’s not a boiler of this kind that we will discuss. Long ago, mechanical engineers created boilers by simply increasing the size of the ‘pan’. Then gradually they design the boiler with more complex again. One important principle is that larger surface contact between the heat source and the heated water, the more steam produced by the boiler in equal size. Then came the idea of ​​making pipelines in the gigantic ‘pan’, so the pipes flowed the fire – or at least the hot gases – through the amount of water housed by the giant ‘pan’. This is the forerunner of the fire-tube boiler. So the definition of a fire-tube boiler is one type of boiler that flows the heat of the combustion process into one or more pipes, which were inside a sealed container filled with water.

Fire-tube boiler is the simplest type of boiler. This boiler can be applied from low to medium water steam requirements. This is possible because the design is not complicated as the water-tube boiler. The size of the fire-pipe boiler is also relatively smaller, and allows it to be moved very easily. That advantages make this boiler is very popular when developed in conjunction with the steam engine. In the 19th century until the beginning of 20th century, fire-tube boilers were developed massively to meet the transportation needs of the time. Steam locomotive, naval ships, and early models of cars, became the most sophisticated models of fire-tube boiler transportation in that period.

As the name, the fire-tube boiler flows the hot gas to the water-coated tubes. Different pipeline designs from the various fire-tube boilers, are to maximize the heat absorption of the combustion gases. The water level inside the boiler tank, must be maintained to avoid overheating. On the other hand, this boiler is also equipped with a safety relief valve avoid the explosion from over pressure.

Many types of fire-tube boilers are also equipped with advanced steam heating systems to produce superheated steam. Nevertheless, the fire-tube boiler has limited steam production of only 2500 kg/h with a maximum pressure of 10 bars only.

## Types of Superheater Boiler

Types of Superheater Boiler – Superheater is a subcritical boiler component that serves to heat the saturated steam, at constant working pressure, so it becomes superheated steam. In its development since the beginning of the 20th century, along with various boiler design races, some engineering experts patented the design of different superheater. Here are the types of superheater boiler, according to the patents:

Radiant superheater is a superheater positioned in the boiler’s combustion chamber, so the superheater pipes instantly absorb the radiant heat from the combustion inside the furnace. In modern water-tube boilers, these superheater radiant pipes are placed hanging over the top of the boiler furnace. These pipes will absorb the second greatest heat energy, after the wall tubes (raiser/evaporator tube).

At radiant superheater, the more steam flows in the superheater pipes of radians, the steam temperature output are decreasing.

2. Convective Superheater

As the name implies, the convective superheater is the superheater pipes of the boiler which placed in the flow of the flue gases that still contain heat. These convective superheater pipes will absorb heat from combustion exhaust gases convectionally. This concept aims primarily to maximize heat absorption from combustion.

In contrast to the radian superheater, the characteristic of the convection superheater is that the more steam flows in the convection superheater pipes, the superheater steam temperature output increased.

3. Separately Fired Superheater

Separately fired superheater is a superheater that is placed separately from the main boiler, which has own separate combustion system with the main boiler. This superheater design is not like the radians or convection types that still use the combustion heat inside the furnace, but instead put additional burners in the area of ​​superheater pipes. This type of superheater is not popularly used, and even tends to be extinct due to efficiency of combustion ratio with steam quality that is not better than other superheater types.

4. Combination Radiant and Convection Superheater

The last superheater type is the most popular, and still applied today. This superheater simultaneously combines two opposite characteristics between radiant and convection superheater, resulting in more homogeneous superheated steam temperature output in various steam flow. The graph below will explain these characteristics.

In modern subcritical boilers, the superheater component of radians is subdivided into several stages. As in the subcritical boiler diagram below for example, after passing the Primary Superheater which is a convection superheater, steam is streamed sequentially to the Platent Secondary Superheater, Intermediate Secondary Superheater, then the last is the Final Secondary Superheater. This design aims to maximize the absorption of radiant heat from combustion inside the furnace.

## Superheater Working Principle

Superheater Working Principle – Superheater is a subcritical boiler’s component that heat the saturated vapor, at constant pressure, so it becomes superheated steam. Superheater technology has been used since the use of steam engines early 20th century. The main purpose is to increase the heat energy contained by the steam, so that increasing the thermal efficiency of the engine. Until now the use of superheater is still very popular, especially in large water-tube boiler steam power plant.

Picture above is a simplified of a subcritical water-tube boiler. This water-tube boiler is composed by two water tanks on the bottom and top. Both tanks are connected with pipes that we know as the raiser tube. The heat from the combustion will first pass through the raiser tube, heat the water inside the pipe. Water than reaches its saturation point and turns the phase into saturated steam.

Saturated steam is still mixed with liquid water so it needs a mechanism to separate the saturated steam with water. This is the function of the top side tank. This tank is commonly known as steam drum. The liquid water will remain in the steam drum and will be recirculated by the raiser tube. While the saturated steam will exit the steam drum and go to the superheater pipes. The superheater pendant will absorb heat by convection and radiation from the flue gas of combustion, until saturated steam dried and become superheated steam. Superheated steam have a greater heat energy content than saturated vapor.

Above is a much more complex and modern subcritical boiler scheme. This boiler is very popular used in steam power plants. The concept is not much different from the previous subcritical boiler principle. The superheater components in modern subcritical boilers made into several levels to fulfill the needs of the quality and quantity of superheated steam produced. In the diagram the superheater is shown by red pipes.

The subcritical boiler’s combustion chamber is composed of vertical raiser tubes that will circulate the water from and to the steam drum. In modern subcritical boilers, only one water tank is used as a steam drum on the upper side of the boiler.

The water in the raiser tube will absorb the heat directly from the combustion process. The water from the raiser tube goes back to the steam drum, and will be separated between the saturated steam phase and the liquid water. Liquid water will be re-circulated through the raiser tube, while the saturated vapor go out to the first stage superheater pipe (primary superheater). Primary superheater is also commonly known as Low Temperature Superheater (LTSH). LTSH pipes absorb heat conventionally from combustion exhaust gases.

From LTSH, the steam will pass consecutively the Secondary Superheater Platform, Intermediate Secondary Superheater, and the Final Secondary Superheater. This steam produced by Final Secondary Superheater is called superheated steam or dry vapor. One phase of water that actually gas phase. It contains no moisture at all, and stores very high heat energy, much higher than the saturated vapor.

## What is Saturated Steam?

What is Saturated Steam? – Saturated steam is a condition where water vapor is at the equilibrium of pressure and temperature equal to liquid phase water. Saturated steam becomes a phase transition between liquid phase of water to its pure gas phase, or commonly known as superheated steam. When the water is in this phase of transition, there is mixing between the liquid phase of water with gas phase of water (saturated steam) in proportion to the amount of latent heat absorbed by the fluid.

Saturated steam begins to form just as the water reaches its boiling point, until all the energy from latent heat is absorbed by water. While all latent heat has been absorbed by water, and the amount of vapor phase has reached almost 100% compared to its liquid phase, that is the end of the phase of saturated steam. The process of reaching almost 100% of the vapor phase occurs at a constant pressure and temperature. Furthermore, if thermal energy continues to be fed to saturated steam, there will be an increase in fluid temperature and encourage steam to turn the phase into superheated steam.

According to the water phase diagram above, the phase of saturated steam can only form along the saturated curve. The lower limit of the saturated curve is the triple point, while the upper limit of the curve is the critical point. Water in more than triple-point conditions will not experience a phase of saturated steam. Water that has a pressure above 22.1 kPa, if it continues to be heated will immediately turn the phase into supercritical steam.

The mixture between water vapor and liquid water in the saturated steam can be determined in amount by using a saturated steam diagram. This diagram uses pressure as the Y axis and the enthalpy as the X axis. This saturated steam diagram is made of a curve. Half of the curve from the lowest point to the top is called the saturated water curve. This part curve becomes the boundary between liquid water with the saturated steam phase. For the right curve from the top of the curve to the lowest point is called the saturated steam curve. This curve becomes the boundary between the phase of saturated steam and superheated steam phase. Right at the vertex of the curve is a critical point, the same point as the critical point in the phase diagram of water.

Since the saturated steam is in constant pressure, a certain amount of saturated steam is represented by a horizontal straight line connecting a point on the saturated water curve to another point on the saturated steam curve. The point on the saturated water curve (hf) shows the enthalpy value of saturated water, ie how much heat energy required for water at pressure P per one unit of mass can reach saturated water. While the point on the saturated steam curve (hg) is the total enthalpy value required so that the water reaches 100% of steam.

The simple relationship is:
hg – hf = hfg

Where:
hf = enthalpy saturated water
hg = enthalpy saturated steam
hfg = difference of enthalpy required saturated water to achieve saturated steam

In other cases, the enthalpy value given to water is not as large as hg, ie only by hmix. The hmix point is anywhere along the horizontal line. In this case the saturation vapor is a mixture of vapor with water whose ratio can be easily determined using the following equation:
$x=\dfrac{h_{mix}}{h_{fg}}$

So:
hmix = hf + x . hfg

Where:
x = comparison of the amount of water in the overall vapor mixture of saturation
hmix = enthalpy mixture

## Steam Turbine Components – All Components in Detail

Here I will explain the steam turbine components.

1. Stop Valve
The stop valve of the steam turbine serves to isolate the turbine from the steam stream and also to quickly stop the supply of steam to the turbine under certain conditions. For example in case of loss of electrical load at a steam power plant from the the grid – we know it as a load rejection – the stop valve will quickly close in a split second. This is useful for avoiding overspeed on the turbine due to the presence of pressurized steam entering the turbine but no electrical load on the generator. The stop valve opens by the working of the hydraulic actuator and closes by the spring.

2. Control Valve
Control Valve is to control the flow of steam into the turbine in accordance with the existing load. In the steam power plant, the control valve opening depends on the amount of electrical load in the generator.

3. Electrohydraulic actuators on Stop and Control Valve
The actuators for stop and control valve on steam turbine power plant use the “fail-safe” principle. That is, the valves are opened by the hydraulic actuator and closed by force from the spring. The differences between the stop valve and control valve actuators is on the stop valve does not need to use the valve position sensor as in the control valve. The stop valve uses only a limit switch sensor.

4. Extraction Steam Line and its Check Valve
Extraction steam is a steam taken from certain stages in a steam turbine that is used for many things, such as preheating water (feedwater before boiler entry), turbine sealing system, sootblower system, etc.

In the pipeline extraction steam shall be installed check valve to prevent backflow from the steam. For example in the case of load rejection above, the flow of water back into the turbine, especially on the side of the superheater turbine will cause a termal stress on the steam turbine components.

The check valve is commonly using Swing Check Valve or Power Assisted Swing Check Valve type. Swing Check Valve opens up by the large differences in steam pressure. And in the event of steam flow interuption (as in the case of load rejection) this check valve will close as the result of the weight of the valve itself. While the Power Assisted Swing Check Valve uses additional actuators to close the valve. Otherwise, to open the valve does not need to use the actuator. It will open by the pressure difference of the steam flow in the pipe.

5. Bearing
The steam turbine use bearing as a part to reduce friction between the shaft (the rotating part) with the casing (stator). Bearings are equipped with circulating and pressurized lubricating oil. To compensate the gravity of the turbine the journal bearing is used, whereas to compensate for the axial forces arising from the steam flow inside the turbine, thrust bearing is used.

6. Hydraulic Turning Gear
It is a mechanism to rotate the turbine rotor at initial start or at the time after shut down to prevent distortion/bending resulting from uncoordinated heating or cooling process on the rotor. This system uses a hydromatic motor whose rotating power comes from a high pressure hydraulic system.

7. Balance Piston
Balance Piston on steam turbine is to compensate the emergence of axial forces due to the flow of the steam insidenthe turbine. This component mitigates the work of thrust bearing.