Steam Boiler: May 2011

Fire Tube Boiler – Working Principle

Based on use classification, fire tube boiler is categorized as for process or heating boiler. It has range pressure not exceed 450 psi (small scale or medium scale). Design fire tube is regulated in ASME BPV Section I subpart PFT. Fire tube consists of tube and tube sheet.

Figure 1. Fire Tube Boiler

Fire tube design use tubes to flow hot gases and distribute it to the safe point of discharge through stack. Water at the outside of tubes is heated by hot tubes, so there is heat transfer between hot gases inside tubes and water outside tubes. Water is converted to steam for process purpose.

As long as combustion process there are some passes usually used. At the first pass, the hot gases in distributed in fire tube boiler through furnace. In Figure 1, the rear head of lower section had sealed the hot gas from first pass and direct it to the tubes at the second pass to front head. The hot gases from front head is redirected to the third pass then directed again to the fourth pass and the end the hot gas is discharged through stack.

Coal Handling Equipment in Steam Boiler

Steam boiler can be distinguished based on fuel classification. One of fuel types is coal. Steam boilers which use coal as its fuel for combustion certainly need equipment for coal handling. The size and capability of coal handling equipment depend on capacity / rating of boiler, amount of required fuel, storage and location area. The big storage area will be advantage when supply coal is delayed to the power plant area. Generally coal storage capability of boiler is designed for 3 months period or longer.

Some of equipments are generally used for coal handling as the following:

1. Bunker and silo

Bunker is the equipment will be made from steel with protective coating (to eliminate abrasion and corrosion) and used for coal storage in steam boiler. Bunker is connected to coal spreader to remove coal form coal bunker to the furnace. Silo is used for reserve storage and made from steel or concrete.

2. Hopper

Hopper is the equipment which use grate to filter oversized coal form truck or car then distribute coal to conveyor system or feeder.

3. Bucket elevator

Bucket elevator is the equipment for lift coal vertically. This equipment consists of bucket, twin chains and endless chain.

4. Coal weighing

Coal weighing is used to measure of weight of coal when it is received first in the hopper. Coal weighing is used to know the quality and the quantity of coal.

5. Feeder

Coal from hopper to bucket elevator is conveyed and managed by feeder.

Requirement Steam Blow

Before perform steam blow, there are some requirements must be provided. Steam boiler process is used to remove debris and scale in superheater and steam line piping. So considering about safety in operation steam blow shall not be forgotten.

Requirement prior to steam blow are the following; feedwater will be supplied to steam boiler must be treated to result demineralized water and shall meet to other water boiler specification. Feedwater availability should be enough to supply boiler. Install all of required instruments such as level indicator, drum pressure indicator, pressure indicator, superheater outlet temperature and feedwater flow rate indicator.

All pins in permanent pipe hangers should be removed, and hangers should be set to correct cold position before steam blow is excetued. All branch lines that are not steam blown shall be isolated. Prior to the firing of boiler, the supervisor will walk down the entire permanent and temporary piping system to ensure that all preparations have been completed and satisfactory.

Steam Blow before Start up Boiler

Steam blow is performed before starting up boiler. It is necessary to be executed to eliminate foreign material, scale, and debris remaining in the pressure parts especially for superheater, reheater and piping steam line after construction step. The objective of the elimination is to avoid blockages of the steam filter at the steam turbine inlet and damage to the steam turbine. Steam blow requires the use of extensive temporary piping to allow the discharging steam to be blown safely to atmosphere.

Steam blow process should consider about requirement not only for safety, but also consider other requirement, so procedure of steam blow should be designed well. The following below are requirement of steam blow for safety operation:

§ Remove all of combustible materials in steam piping line boiler area such us paper, rags, wood, etc

§ Install silencer at the end of steam piping line to avoid nuisance disturbance.

§ All of personnel include in steam blow process should be equipped with good communication system to make proper coordination each other.

§ Isolate steam supply with closed stop valve when install target plate.

§ Design good temporary piping and support to resist enough the loading during steam blow.

Classification of Boiler Based on Erection

Steam boiler has some classifications. One of them is classification based on erection of steam boiler. The following below are the type of steam boiler regarding to erection methods:

1. Package Boiler

Package boiler is a boiler which all of pressure parts and non pressure parts are fabricated and assembled in workshop. The assembly is performed in workshop completely include equipment of burner, control, mechanical draft and its accessories (see figure 1).

Figure 1: Package Boiler

2. Field Erected Boiler

Field erected boiler is a boiler where some pressure parts and non pressure parts such as steam drum, water drum, are constructed in site area where steam boiler will be placed (see figure 2).

Figure 2: Field Erected Boiler

Requirement ASME for Design Calculation

Design calculation of steam boiler should be performed carefully. Engineers who have responsibility to design steam boiler shall have good understanding about requirement ASME BPV Section I not only for materials, but also for design calculation.

Design calculation ideally should follow ASME Code. The following below is summary of requirement ASME for design calculation:

- General Design, see ASME BPV Section I sub chapter PG 16
- Maximum Allowable Working Pressure, see ASME BPV Section I sub chapter PG 21
- Loading on Steam Boiler, see ASME BPV Section I sub chapter PG 22
- Stress Value for Calculation Formula, see ASME BPV Section I sub chapter PG 23 and ASME BPV Section II Part D
- Calculation Formula for Tube Up To 5 in Outside Diameter, see ASME BPV Section I sub chapter PG 27.2.1
- Calculation Formula for Piping, Drums and Headers, see ASME BPV Section I sub chapter PG 27.2.2
- Thickness Greater Than One-Half the Inside Radius of Component, see ASME BPV Section I sub chapter PG 27.2.3
- Welded Access or Inspection Openings under External Pressure, see ASME BPV Section I sub chapter PG 28
- Dished Head Design and Calculation, see ASME BPV Section I sub chapter PG 29
- Design Stayed Dished Head, see ASME BPV Section I sub chapter PG 30
- Design Unstayed Flat Heads and Covers, see ASME BPV Section I sub chapter PG 31
- Design Calculation for Opening in Shells, Headers, and Heads, see ASME BPV Section I sub chapter PG 32.1
- Design Shape of Opening, see ASME BPV Section I sub chapter PG 32.2
- Design Size of Opening, see ASME BPV Section I sub chapter PG 32.3
- Design Compensation Required for Opening in Shells and Formed Heads, see ASME BPV Section I sub chapter PG 33
- Design Flanged in Openings in Formed Heads, see ASME BPV Section I sub chapter PG 34
- Design Compensation Required for Opening in Flat Heads, see ASME BPV Section I sub chapter PG 35
- Design Limit of Metal Available for Compensation, see ASME BPV Section I sub chapter PG 36
- Design Strength of Compensation, see ASME BPV Section I sub chapter PG 37
- Design Compensation for Multiple Openings, see ASME BPV Section I sub chapter PG 38
- Minimum Thickness and Maximum Working pressure for Stayed Flat Plates, see ASME BPV Section I sub chapter PG 46.1
- Design Calculation of Ligaments, see ASME BPV Section I sub chapter PG 52 and PG 53
- Boiler External Piping and Boiler Proper Connection, see ASME BPV Section I sub chapter PG 58
- Nondestructive Examination Requirements, see ASME BPV Section I sub chapter PW 11
- Heat Treatment Process, see ASME BPV Section I sub chapter PW 38 and PW 39

Requirement ASME for Material

ASME BPV stand for American Society of Mechanical Engineer Boiler and Pressure Vessel, it has requirements to set material will be used for steam boiler. ASME BPV Section I and Section II has important role to regulate the using of material. ASME BPV section II has four parts; each part explain about ferrous material specifications, nonferrous material specifications, specification for welding rods electrodes and filler metals, and explain about properties respectively.

The following below is summary sub chapter which can be known about material to be used in steam boiler:

# Material Specification, see ASME BPV Section II Part D
# Carbon Content for Cutting and Construction Process, see ASME BPV Section I sub chapter PW 5.2
# Welding Filler Metal and Electrodes, see ASME BPV Section I sub chapter PW 5.4
# Material Size Limits and Tolerance, see ASME BPV Section I sub chapter PG 5.4
# Plate Specifications, see ASME BPV Section I sub chapter PG 6
# Material Forging, see ASME BPV Section I sub chapter PG 7
# Material Casting, see ASME BPV Section I sub chapter PG 8
# Pipe, Tubes, and Pressure Containing Parts, see ASME BPV Section I sub chapter PG 9
# Material Identified with or Produced to a Specification Not Permitted by This Section, and Material Not Fully Identified, see ASME BPV Section I sub chapter PG 10
# Miscellaneous Pressure Parts, see ASME BPV Section I sub chapter PG 11
# Gage Glass Body and Connector Material, see ASME BPV Section I sub chapter PG 12
# Stays, see ASME BPV Section I sub chapter PG 13
# Material Steam Boiler which is Fabricated by Brazing, see ASME BPV Section I sub chapter PB
# Material for Water Tube Boilers, see ASME BPV Section I sub chapter PWT 5
# Material for Fire Tube Boilers, see ASME BPV Section I sub chapter PFT 5
# Material for Miniature Boilers, see ASME BPV Section I sub chapter PMB 5
# Material for Electric Boilers, see ASME BPV Section I sub chapter PEB 5
# Material for Organic Fluid Vaporizers, see ASME BPV Section I sub chapter PVG 5

Thermodynamics Term

Steam boiler has main task to convert water to high quality steam. When water is heated at atmospheric pressure, water will be converted to steam at temperature 212 F. When water is heated at higher pressure than atmospheric pressure, the boiling temperature will increase.

Enthalpy is the heat energy which is required to heat water from temperature 0 C (3 F) to the boiling temperature. Enthalpy is expressed as Btu/lb (British thermal unit per pound). When temperature from 32 F is increased gradually until 212 F, water will go to the boiling point temperature. If heat temperature is added more than 212 F, the water will converted to steam and the temperature can called as steam temperature.

Saturation temperature is reached when temperature of water is same with temperature of steam. Saturation temperature shows that at the boiling process, temperature is constant. Enthalpy of evaporation which means the heat required to convert water liquid phase to steam phase, will occur when heat temperature is increased from the boiling temperature.

The total enthalpy of saturated steam is the sum of enthalpy of saturated liquid and enthalpy of evaporation. Another means is total heat which is required to convert water and produce steam totally (see figure 1).

From figure 1, can be known that phase 1-2 is water phase, 2-3 is the boiling phase where there are mixture between water and steam at constant temperature. Phase 3-4 is superheated phase where water is converted 100% to steam.

In steam boiler, phases 1-2 occurs in economizer, water drum, downcomer and lower drum, while phase 2-3 occurs on top 1/3 furnace tubes and steam drum. Finally phase 3-4 occurs in superheater and reheater.

Impact of Ash on Steam Boiler

Ash is product of combustion, it is unburned matter. Ash will flow with flue gas through furnace, superheater, reheater, economizer, air heater, multi cyclone / baghouse / electrostatic precipitator and then be discharged out to atmosphere through stack.

Ash can gives bad impact to steam boiler if maintenance program is not performed. Erosion, corrosion, slagging and fouling are the influence of ash. Erosion is reducing of surface metal because of impingement particle in ash with surface metal tubes, so the abrasion occurs. Erosion in steam boiler can be reduced but can not be eliminated, because particle abrasion ash include other unburned particle always presence in flue gas content, can not be eliminated 100%. Some of ways to reduce possibility of erosion are choose highest fuel efficiency and doing routine maintenance such us clean surface tubes with soot blower.

Another impact of ash is corrosion which is caused by either low temperatur corrosion or high temperature corrosion. Low temperature corrosion generally occur in economizer and air heater area because at temperature under 200 C, the water vapor will react with SOx content in flue gas to form sulfuric acid which lead to corrosion. High temperature corrosion generally occur in superheater and reheater area but the causes can not be fully known because amount of reduction regarding to corrosion is very small approximately microns per year.

Another impact of ash are slagging and fouling. Slagging and fouling can be distinguished based on heat transfer. Slagging is the deposits which attached on the surface furnace tubes by radiant heat while fouling attached on superheater, reheater, economizer and air heater by convection heat. Slagging and fouling influence on heat transfer from flue gas to water inside tube can not be optimal.

PH Value in Steam Boiler

PH value of boiler water and feedwater should be maintained. Corrosive qualities of water can be known from PH value because it indicates that quality of water is alkaline, neutral or acidic. But PH value does not show about amount of quantity of alkaline or acidic content in water.

PH value has range between 0 until 14. If PH value is in the middle value (PH 7) so the quality of water is neutral. If PH value fewer than 7 (PH 0-6) so the quality of water is acidic, but on contrary PH value above 7 so the quality of water is alkaline. PH value also states concentration of hydrogen ions and hydroxide ions. Hydrogen ions will dominant in acidic water and hydroxide ions will dominant in alkaline water.

Requirement PH value for boiler water and feedwater is generally above 7 (range between 8.5 and 9.8). It is recommended PH value to protect metal pressure part from corrosion attack. The following figure below is the recommended material construction of steam boiler based on PH velue.

Fig. 1 The Recommended Material Based on PH Value

Source: Book - Practical Guide to Industrial Boiler System by Ralph L. Vandagriff

Failure in Superheater Tube

Superheater tube is one of parts in steam boiler. Superheater is used to heat saturated steam from steam drum to be superheated steam. Failure in superheater tubes is rarely caused by corrosion except the condition of steam boiler is being maintaining or stands by in long term. Failure in superheater tubes is often occurs because of overheating either short term overheating or long term overheating.

Overheating is caused by attached deposit in internal tubes which is carried over by flow of bad quality steam which bring suspended solid or dissolved solid (hardness). The hardness can be carried over with steam because separation process of steam-water mixture can not executed perfectly in steam drum. Deposit can be removed by adding chemical injection (phosphate) in steam drum to make hardness be soluble then the next step is discharge it through blowdown process.

Failure in superheater tubes may also occur during bad start up process. If combustion temperature is increased rapidly without doing water and steam balance flow, the superheater will feel overheating in short term because there is not steam flow which has function as cooler. Steam flow through superheater tubes act as cooler because temperature of steam is lower than temperature radiant combustion. So start up procedure shall be established to perform good combustion.

How to Run Steam Boiler Perfectly

Steam boiler needs requirements condition to perform it perfectly and efficiently. How to run steam boiler perfectly is a question that must be considered. The regulation must be fulfilled as follow:

1. Feedwater will be supplied to steam boiler must be treated in water treatment system. Requirement of contents consist of PH, oxygen, silica, hardness, etc must be met.

2. All personnel involved in work on steam boiler should has good capability such engineer, designer, drafter, fabricator, erector, etc.

3. Circulation of water and steam must be constant.

4. Perform maintenance program, check equipments daily or weekly, clean surface boiler tubes from soot and ash with soot blower.

5. Design good combustion process by choose high quality fuel, enough excess air and well circulation of flue gas.

6. Perfect firing should be performed in the furnace, before flue gas escape from furnace to give highest efficiency.

7. Steam boiler should be equipped with safety gauges such as safety valve, relieve valve, venting valve, water level, and so on.

8. All structures for support steam boiler must have enough strength to resist all of boiler loads.

9. Chemical injection should be supplied to remove hardness.

10. Before running steam boiler, all of equipment should be tested by hydrostatic test, air leakage test, etc.

Deaerator Feedwater Tank

Steam boiler has some systems which running together to produce high quality steam. One of the systems is feedwater system. One of the main parts of feedwater system is deaerator feedwater tank. The mass balance between steam produced by boiler and the feedwater should be maintained in a fluctuating load conditions based on firing rate and steam demand. Furthermore feed water system should be controlled to supply enough water into steam boiler.

The required water must faces water treatment system before supplied into steam boiler, so the water has required parameter contents about silica, scaling, conductivity, PH, dissolved oxygen and so on. The main consideration is how to remove oxygen to prevent corrosion and remove scaling which can isolate heat transfer process in the pressure parts of steam boiler (water wall, header, economizer, etc).

The main functions of deaerator feedwater tank are:

1. Remove oxygen and non condensable gases.

2. As storage tank to maintain and supply enough feedwater into steam boiler

3. Increase temperature feedwater until saturated temperature.

Water which is treated from demineralization process and from condensate is sprayed into through nozzle deaerator. Low pressure steam is injected to deaerator to remove gases content and dissolved oxygen and then discharge them through deaerator venting. In addition to remove oxygen content, low pressure steam is also used to increase temperature up to 130 140 C. The water which gas content and dissolved oxygen had been removed enters to feedwater tank. Moreover chemical contents such as sodium sulfite (Na2SO3) or hydrazine (N2H4) are injected to reduce oxygen content in feedwater.

Feedwater Temperature

One of specifications of steam boiler should be stated is feedwater temperature. Every steam boiler type has different feedwater temperature. For process boiler whose low capacity and does not need heating for its treatment, the feedwater temperature required is low approximately 85 C. But for power boiler which has high design pressure, the feedwater temperature required up to 270 C.

There are some differences of temperature feedwater will be supplied to steam boiler and it has some levels temperature. The differences depend on system and type of boiler such as for Heat Recovery Steam Generator (HRSG) which does not has heating cycling process for condensate, the feedwater temperature flow through HRSG about 30-40 C. For process boiler, the condensate output from condenser has temperature 85 C and the feedwater should be injected with sodium sulfite before distributed into steam boiler to prevent possibility pitting corrosion occur in the metal of pressure parts. The feedwater can be distributed to economizer first if engineer / designer want to increase efficiency boiler.

For higher temperature needed approximately 105 C, the feedwater is heated and the oxygen is removed in one equipment which named as deaerator. Deaerator can also help to increase feedwater temperature until 130-140 C. For the higher temperature again, a power plant system usually use High Pressure Heater (HP Heater) located after deaerator to increase temperature up to 160 C - 270 C.

Properties of Steam

Boiler output which is required to generate turbine generator is steam. Engineer or designer of steam boiler must have good understand about properties of steam. The properties of steam should be known are specific volume, specific entropy and enthalpy.

Specific volume of steam is volume (expressed as m3) divided by unit mass (expressed as kg) of steam at certain pressure. Specific volume will different if steam is produced at different pressure. Specific entropy of steam is the amount of energy (expressed as Joule) which be divided by unit mass (expressed by kg) at certain temperature (expressed as C). Enthalpy is measure energy (expressed as Joule) which is divided unit mass (expressed by kg).

The properties of steam can be seen in the steam table of ASME International Steam Tables for Industrial Use and the calculation properties of steam can be performed based on code International Association for the Properties of Water and Steam (IAPWS). At saturation condition by means that steam and water are mixed, steam will has different properties based on temperature and pressure. If the mixture heated exceeds saturation temperature, the steam will be superheated, but on contrary if the mixture cooled under saturation temperature, the water will be sub cooled.

Gaseous Fuel in Steam Boiler

Steam boiler can be classified based on fuel; one of the fuels is gaseous. Gas can be generally divided as three categories; natural gas, butane, and propane. Natural gas is gas which be found in offshore area, while butane and propane are gases which be found in crude oil and had been separated from it. Natural gas is usually used for fuel combustion in steam boiler or for industry such as manufacturing of steel. Butane and propane are generated as LPG (Liquefied Petroleum gases)

Gaseous fuel generally has unit to express calorific value at pressure and temperature in normal condition. The unit is Mega Joules per Cubic Meter or can be symbolized with MJ/m3. Butane and propane are kept in designed place which has pressure and the unit can be expressed as Giga Joules per metric Ton (GJ/tonne). The details explanation of natural gas, butane and propane as follow:

Natural Gas:

Specific gravity compared to air : 0.59
Specific gravity liquid compared to water : -
Calorific value (MJ/m3) : 38 – 39
Calorific value (GJ/tonne) : -
Flammability limits air/gas ratio % by volume : 5.3 – 14
Volume gas m3 perkg of liquid at normal pressure & temperature : -

Butane:

Specific gravity compared to air : 1.49 – 2.1
Specific gravity liquid compared to water : 0.57 – 0.58
Calorific value (MJ/m3) : 121.8
Calorific value (GJ/tonne) : 49.3
Flammability limits air/gas ratio % by volume : 1.8 - 9
Volume gas m3 perkg of liquid at normal pressure & temperature : 0.42

Propane:

Specific gravity compared to air : 1.4 – 1.55
Specific gravity liquid compared to water : 0.5 – 0.51
Calorific value (MJ/m3) : 93.1
Calorific value (GJ/tonne) : 50
Flammability limits air/gas ratio % by volume : 2.2 - 10
Volume gas m3 perkg of liquid at normal pressure & temperature : 0.54

The Role of Valve in Steam Boiler

Valve has important role in steam boiler. Valve is one of required equipments in steam boiler to control flow of water or steam. Control velocity, debit, volume of fluids, and to make flow start or stop, are some of roles of valve.

Valve can be classified by internal configuration. Based on internal configuration valve can be classified as globe valve, ball valve, check valve, non return valve, gate valve, and butterfly valve.

Gate valve is valve that is used to make flow fully stop or fully open, there is minimum restriction in this type and usually used for straight line flow. Globe valve is valve which is used to regulate flow, and the fluids will has more resistance if flow through globe valve than gate valve.

Check valve is valve which is used for flow one way direction or non return direction. Non return valve is used to eliminate steam flow back into a steam boiler if the pressure of steam line piping is under the pressure of header in steam boiler; this type is automatic stop check valve.

Butterfly valve is valve which is used for regulate and isolate flow, it allow for quick stop flow. Ball valve has similar function with butterfly valve, the differences is ball valve use spherical disc to regulate flow.

Valve can be also classified by function in steam boiler, Based on its function valve can be categorized as vent valve, safety valve, control valve, safety shut off valve, and so on. Vent valve is used for release air or steam when steam boiler is being filled with water; it is usually located on the highest level. Safety valve is used for protect steam boiler if operating pressure exceed maximum allowable working pressure, the valve will automatically pop up to release excess pressure. Control valve is automatic control to regulate how many percent debit flow of fluid will be allowed. Safety shut off valve is used to close and stop line flow of fluid.

Outlet Temperature in Superheater

Superheater is one of pressure parts in steam boiler. Superheater has some classifications. Outlet temperature in steam boiler is temperature of steam when leaves superheater that will be distributed to steam turbine. Outlet temperature in superheater is usually higher than outlet temperature in another pressure parts.

Based on use classification, process heating boiler has outlet temperature in superheater not exceed 350 C and the steam temperature is generally not adjusted. This condition is difference with power boiler, steam outlet temperature leaves superheater is usually above 400 C and need to adjust it.

Adjust steam outlet temperature in power boiler can be performed by using desuperheater or attemperator. The function is decrease outlet temperature if the temperature is higher than expected. One of methods to decrease steam temperature is using spray desuperheater. It spray water flow from feed water piping, so steam flow receive low temperature from water then decrease temperature it self. Location of spray desuperheater is usually between primary superheater and secondary superheater. Another way to adjust steam outlet temperature is maintain steam temperature control constant until load generally 70%. – 100% maximum continuous rating.

Steam temperature control can be executed by control excess air, burner management system, and flue gas circulation to gain maximum efficiency. Flue gas circulation should be controlled to give certain temperature when pass through both primary superheater and secondary superheater also ensure they had received enough heat transfer to generate expected steam outlet temperature.

Erosion, Corrosion Water Hammer and Leaks in Economizer

Economizer is one of heat exchanger equipment in steam boiler that absorbs heat from flue gas then transfers it to feedwater. Economizer can help steam boiler to increase efficiency. Some of failure regarding to operation of economizer can be occur because of erosion, corrosion, water hammer and leaks. So, the preventive maintenance should be performed.

Possibility of corrosion can be occur if there is concentration of oxygen either high or low concentration. Corrosion can be prevented by removing oxygen content in feedwater. Deaerator has important role to make feedwater free from oxygen.

Steam boiler which uses coal as fuel combustion will has a lot of fly ash particle. It will impact to metal tube erosion even steam boiler is operated with high excess air and on maximum continuous rating; possibility of erosion is highly occur. If certain amount of fly ash is localized in certain area then forming plugging, this condition will reduce flow area and increase the velocity of flue gas, so potential erosion on economizer tubes will mostly occur and the result economizer tubes will leak and rupture. Preventive maintenance to clean plug and fly ash attached on surface tube must be executed routinely. Rotary soot blower type is suitable applied to this preventive maintenance in economizer.

Another warning regarding to economizer operation is water hammer. At certain condition, level water gauge can not read water level in steam drum correctly, the result is boiler control allow feedwater is not distributed into steam drum for a while. So steam will flow enter and become trapped in economizer. When feedwater is distributed again through economizer although with low pressure, it will impact to water hammer because of steam is encountered immediately with feedwater on certain pressure. To overcome this condition, venting should be installed to make the steam out from economizer or supply feedwater constantly.

Water Softener in Steam Boiler

Water softener is one of water treatment efforts to make feedwater is suitable for requirement of steam boiler. Water can be called hard if it is tested with soap will be difficult to make foam and water can be called soft if it can porduce foam easily. There are some types of water softener process, can be operated manual or automatic and can be multiple tank or single tank. The most popular type is the zeolite softening process. In this process, magnesium and calcium ion contain in water will be removed and elminate it until there is not hardness.

Water softener process use ion exchange to make hardness is removed and change it into soluble sodium ions easily. This matter should be performed because hardness (magnesium and calcium ions) will be scale while sodium is soluble. So when water is supplied into steam boiler pressure part, scale will not occur and do not disturb heat transfer process. Furthermore, zero hardness will reduce amount of chemical injection needed.

Sodium ions can be taken from salt or sodium chloride (NaCl). Clhorine anion (Cl-) and sodium cation (Na+) will be formed when salt is mixed with water because salt will be dissolved in water, very cheap effort.

Hydrostatic Test in Steam Boiler

Hydrostatic test in steam boiler is one of non destructive examinations which should be performed for new steam boiler or steam boiler which has been repaired such as replace water wall, superheater, reheater, economizer and so on to. Hydrostatic test must be done to make sure that steam boiler is free from leakage before start up is performed. Before hydrostatic test is executed, inspection for all welds and installation of pressure parts should be done. Moreover inside and outside of steam boiler and pressure part shall be free from all dirt, scale, paint, debris, dirt, weld slag, rust, oil, spatter and all other foreign matter.

Before perform hydrostatic test (hydrotest) in steam boiler, the following matter should be prepared as follow:

Non-destructive examination shall be verified to confirm acceptance as per Code and specification.
Dimensional inspection.
Full visual inspection for all welds.
Boiler pressure part installation is completed.
No painting work on pressure part before hydrostatic test.
Inside and outside of boiler and pressure part shall be free from all dirt, debris, weld slag and spatter, oil, rust, scale, paint and all other foreign matter.
Prepare and check valve line-up.
All gags shall be installed on safety valve.
Vent and drain are to be provided for the system before commence of hydrostatic test.
All temporary fitting, blind flange shall be suitable for the test procedure and should not have defect that may cause failure during hydrostatic test.
Fill boiler with treated water and free from mineral or other suspended matter (Demineralized water).

Requirement of gauge and record during hydrostatic test in steam boiler (hydro test):

A minimum of 2 (two) pressure gauge shall be used for pressure tests. One shall be a pressure temperature recorder with a range suitable for recording of the test pressure. The full scale of the other pressure gauge shall preferably 1½ to 4 times of test pressure.
The test pump and the line to be tested shall be provided with the separate indicating test gauges. In case of testing between a high and low point of the module, gauges shall be placed one at the top and one at the bottom to measure the pressured drop.
The pressure gauge shall be calibrated and still valid before used. Any gauges showing incorrect zero error more than + 1% of full-scale range shall not be used without repair and recalibrate.

Hydrostatic test must follow the determined procedure which is generally as follows:

Install suitable temporary fitting such as blind flange or end plate to close opening and ensure all of openings are closed except venting valve.
Fill the steam boiler with treated water, free from mineral or other suspended matter at temperature not under 70 F or ambient temperature.
Inspect leakage when steam boiler is being filled, remove air through venting valve, close it if there is not leakage and air is totally removed.
Increase pressure gradually with maximum pressure rate 5 kg/cm2 until 1.5 times of maximum allowable working pressure for new boiler and 1.2 times for repaired boiler.
Record pressure, time and temperature during test and inspect all of pressure parts of steam boiler at holding time of hydrostatic test approximately 10 minutes. 6. After hydrostatic test, the pressure should be release in a manner and at rate so as not to endanger personnel or damage equipment.

Steam Separator in Boiler

Steam separator has main function to separate water and steam and this equipment is usually located in steam drum. Water surface in steam drum is turbulent, so make it easy to mix between steam and water. The principle of steam separator is make steam flow is changed in any direction. Because of density of steam is lighter than water make steam can be distributed easier than water. The water droplet which has higher density will be separated and dropped from steam. Moisture will be removed by steam separator to eliminate damage and erosion if water or wet steam is distributed to the steam line.

There are some types of steam separator. For small scale steam boiler, steam separator consists of dry pipe which has a lot of holes at the top and two holes at the bottom half. The mixture steam-water is directed through the top half holes dry pipe, turbulent moving force the mixture to separate between water and steam. Steam will flow to steam line and water will drop through bottom holes (see figure 1).

Fig. 1: Dry Pipe Steam Separator
Source: Book - Boiler Operator's Exam Preparation Guide - Theodore B. Sauselein

For big scale boiler which has complexity equipment, steam separator method use centrifugal force for better result. The mixture steam-water is forced to move around the cyclone and make the rotation (see figure 2). The more turbulent moving force the mixture separate easily.

Fig. 2: Centrifugal Steam Separator
Source: Book - Boiler Operator's Exam Preparation Guide - Theodore B. Sauselein

Optimizing Heat Transfer in Steam Boiler

Heat transfer within steam boiler can be performed by three ways and by their combination; radiation, conduction, and convection. Optimizing heat transfer should be done to maximize efficiency of steam boiler. The followings are the ways to optimize heat transfer:

1. Control excess air

Composition of excess air should be controlled, if excess air is too much will effect temperature of flue gas low because of cooling from excess air, but on contrary if amount of excess air is too low will cause flame temperature is high.

2. Keep clean boiler tubes

Tubes receive heat from combustion furnace by radiation and heat is distributed to all of surface metal tubes by conduction. The presence of soot, ash and scale will obstruct heat transfer because they are poor conductor. Any deposit on outside surface tubes can be cleaned by soot blower, while any scale in inner side tubes can be prevented if water treatment is undertaken well.

3. Keep optimum flue gas and feedwater

Feedwater should be treated free from sludge and scale to ensure water can receive good heat transfer and can flow with a certain velocity. Optimum flue gas can be taken by doing routine maintenance and choose optimum fuel.

Combustion of Coal in Steam Boiler

Coal as one of fuels for combustion process contains fix carbon, ash, volatile matter and moisture. Coal can be classified based on them. In the combustion system of boiler, coal needs oxygen and heat (certain temperature) to perform firing. Coal can be fired by two methods; underfeed stoker firing and overfeed stoker firing, for more details see here.

When we discuss overfeed stoker firing in which approximately 8 inch coal bed is arranged above stoker grate, air should be distributed from bottom the grate and a layer of ash are passed first. Oxidizing zone occur when oxygen content in air pass through the first layer of burning coal which contain fix carbon. When burning process, heat temperature will increase and drying raw coal from moisture and volatile matter.

Over fire air as excess air is needed to perform complete combustion because distributed air from bottom grate can not burn volatile matter and carbon monoxide perfectly. Composition of air combustion should be controlled, does not be over and less regarding to efficiency of combustion. At firing which has low rate, level of over fire air should be minimized to eliminate possibility of cooling on volatile matter, so the combustion will be incomplete. At firing which has high rate, amount excess air is determined by the ability to totally mix over fire air, carbon monoxide and volatile matter to perform complete combustion.

Classification of Coal as Fuel in Steam Boiler

Steam boiler can be classified by fuel. One of them is solid fuel which contains coal as fuel in steam boiler. Coal can be classified based on percentage of volatile matter, ash, fixed carbon and moisture (see table 1). Based on these matter coal can be named as peat coal, bituminous coal (1), bituminous coal (2) and anthracite.

Table 1: Classification Coal in Steam Boiler
Source: The Boiler Operators Handbook - graham & Trotman

Percentage of volatile matter can be defined as percentage of coal component exclude moisture, which is heated until temperature 905 C, then measure the weight loss. Percentage of moisture is percentage of wetness content or condensed liquid in coal which can be dried on temperature 105 C. Percentage of fixed carbon is percentage of carbon content when volatile matter is driven off. Percentage of ash is a residue from firing coal which can be burned.

High percentage of volatile matter will make coal can be burned quickly and more generate smoke and low percentage of volatile matter coal can be burned slower and produce less smoke.

Water Level Indication in Steam Boiler

Steam boiler use water level indication to detect the water level in steam drum. Steam drum usually uses gauge glass as its indicator. A lot of rules determine to use and arrange two gauge glasses especially for steam boiler which has maximum allowable working pressure above 400 psi or above 28 bar to reduce possibility of breakage which can cause the operator in danger. Gauge glass is a tool consisting prismatic glass which is design to resist high pressure (see figure 1) and allow the operator to see water level in the normal condition, under normal and above normal water level.

Fig. 1: Water Level Gauge Glass in Steam Boiler
Source: Book - The Boiler House - The steam and condensate lopp - spirax sarco

Water level can be designed for remote or direct viewing. Sometimes water level is difficult to be seen clearly because there are steam and many bubbles at the water surface and high pressure / temperature make more water fluctuation.

Water level indicator has two types for direct viewing and remote viewing. Each type has kind of product type:

1. Water level indicator for direct viewing

Tubular gauge glass
Reflex type gauge glass
Transparent gauge
Bicolor gauge
Port type bicolor gauge
Multiple gauge

2. Water level indicator remote viewing

Manometric gauge
Electronic gauge

Design Water Level in Steam Boiler

Steam boiler has a important function to produce steam with the best quality and efficient. Combustion process give heat transfer to water inside water wall tubes then water will be evaporated continuously to generate steam. In the process of steam release, steam boiler needs a certain amount of water surface area. Tolerance of height water level slightly exceeding normal water level should be considered if at any time the increase in load which allow the rise elevation of water level but still within safety limit.

Engineer who design steam boiler should make sure that steam can be produced in calculated area of normal water level and steam can be distributed with the correct velocity. Specific the lowest height of steam which can be generated above normal water level should be determined. The normal water level should be maintained, so supply of feedwater must be maintained, do not be shortage.

If steam boiler is performed with insufficient feedwater or under normal even low water level then steam boiler will has high possibility to explosion and damage. Therefore some actions should be taken such as maintain water level and give good actions if low water level is detected. The actions are turn on the alarm and turn off the operation of steam boiler.

Short Term Overheating in Steam Boiler

Steam boiler failure is not only caused by long term overheating but also short term overheating. Short term overheating can occur if temperature of pressure part (tube, drum, pipe and header) increases rapidly in short period exceeding limitation of design temperature. It can occur at the pressure part which has high operating temperature such as superheater, reheater, water wall, roof wall, boiler bank, screen tubes and downcomers and rarely occurs in low operating temperature of pressure part such as air heater and economizer.

When the tube has insufficient cooling from water flow inside and receives excessive heat input from combustion process will lead to short term overheating. To identifying that steam boiler is occurred this condition; operator may identify it with microscopic examination on the failure section tube (see figure 1).

Effort to reduce possibility of short tem overheating should be performed such as ensure feed water before supplied into steam boiler is fulfill the requirement, inspect on area which occur debris accumulation such as U bend-tube, drum and header. Circulation of water, blowdown, corrosion, and all of potential matter which disturb heat transfer should be monitored.

Air Temperature for Combustion Process

There some things which need to be considered to carry out combustion system of steam boiler. They are control of air supply, mixing of air and fuel, also temperature and time required for combustion. At this time will discuss the importance of air temperature for the combustion process.

Air temperature should be set to perform complete combustion. If any changes of air temperature will influence the amount of required air temperature to be supplied into steam boiler and amount of excess air. Practically, there is not fix rules to set air temperature. If amount of excess air is high will lead to heat loss, high temperature in stack and will decrease efficiency of overall boiler. But on contrary, if amount of excess air is too low will impact to incomplete combustion, wasted fuel, and too much soot attached on the surface of pressure part.

Practically, to perform efficiently burner is adjusted initially at amount of excess air 15%. The lower temperature of air will make the greater amount of excess air, and vice versa. In every increase of air temperature 40 F will influence the required excess air decrease approximately 5%. The outside temperature will always changes in day and night, this condition will influence to the changes of air temperature in the furnace. So, equipment analyzer for combustion is required to help operator maintain efficiency of steam boiler on top level.

Air Leakage in Steam Boiler

Combustion should be performed completely to enhance efficiency of steam boiler. Operator need to check all of apparatus of combustion. One of them is air as one of source for combustion process. Combustion need excess air to supply air beyond required air to perform combustion completely. Excess air can be distributed through the burner or stoker and through wall furnace tubes. When maintenance or start up new steam boiler is performed, air leakage test must be done.

There are some ways to check air leakage. One of the ways is by burn small fuel such as wooden or fireplace butane lighter in area of potential leakage. If there is leakage, the fire will come out of the furnace. Another way is to give pressure with bomb which contain smoke, if there is leakage smoke will appear in the outside of furnace. Potential leakage can occur at seal area although furnace was welded.

Generally, air leakage test should be performed when there are signals as follow; flue gas exhausts temperature at stack decrease, volume of flue gas increase and O2 level increase. Furthermore, air leakage can affect to reduce function of O2 analyzer because infiltrated air disturbs O2 analyzer to read O2 content in flue gas.

Classification of Boiler Based on Use

Boiler can be classified based on several types. One of classification of boiler is based on use, they are:

1. Power Boiler

The use of boiler in power plants and big industrial boiler can be called power boiler.

2. Process Boiler

The use of boiler in industrial process can be called process boiler.

3. Steam Heating Boiler

Steam heating boiler is boiler which use firing directly from fuels (oil, electricity, gas, and solid fuel) and has operating pressure less than 15 psi.

4. Hot Water Heating Boiler

Hot water heating boiler is used in industrial process to produce hot water and heating building. Boiler has temperature boiler under 250 F and operating pressure boiler less than 160 psi.

5. Hot Water Supply Boiler

Hot water supply boiler is used in industrial process and building to supply hot water. Operating temperature and operating design are same with hot water heating boiler.

Design Process Steam Boiler

Steam boiler should be designed systematically. There are some engineers who work for design steam boiler. They are performance engineer, mechanical engineer, project engineer and drafting engineer. Each engineer has task respectively. First task is usually started by performance engineer such as determine classification and type of steam boiler will be designed; calculate design pressure and temperature each part of steam boiler; issued Piping and Instrument Diagram (P & ID); design size of steam boiler part like steam drum, water drum, water wall tubes, superheater, economizer, boiler bank, riser, downcomer, etc; design capacity of steam, fuel consumption and so on.

After performance engineers did their task, the next step will be continued by mechanical engineers to perform calculation requirement minimum thickness each pressure part, design structure of steam boiler, piping stress analysis, and all of requirement calculation will be needed based on ASME BPV Section I (for design steam boiler), ASCE (for design structure), AWS (for design welding) and so on. If all of calculation had been performed, drafting engineers has task to draw both in 2 dimension and 3 dimension.

Project engineers should arrange well schedule, so steam boiler can be designed, fabricated, and constructed on time. Prepare all of documents (drawing and specification for welding process, fabrication and construction. Project manager should monitor and ensure that fabrication and construction are performed as per design.

Effect of Scale in Steam Boiler

Water treatment should be performed to feedwater before to be supplied into steam boiler to remove scale content in feedwater. Bad treatment will cause a lot of scale content in feedwater. It can give bad effect in the steam boiler process as follow:

1. Influence amount of heat loss in overall boiler.

Scale which be attached in inner surface of pressure part (tube and header) will block heat transfer by conduction from metal tube or header to water because scale in the middle position between metal and water has role as isolator. So, the blocking heat transfer will impact to amount of heat loss. Practically, 2 mm thickness of scale attached in the surface will make heat loss approximately 2% until 3%.

2. Reduce efficiency in the steam boiler

Higher heat loss will influence lower efficiency of steam boiler because of less heat absorb in the furnace will make excessive fuel consumption and temperature of stack outlet

3. Overheating in boiler tube metal

Water flow in inner tube also has function as cooler because temperature of water is very lower than temperature of firing. If scale occurs in inner surface of tube, cooling function will be reduced, so metal temperature will be higher than temperature design and will impact to overheat. This condition will lead to failure and damage in pressure part.

4. Influence to corrosion forming

The deposit of scale will cause corrosion forming because scale composition can compound with metal tube or header to form corrosion.

The Role of Blowdown in Water Treatment

The role of blowdown is necessary to treat water in steam boiler. Blowdown is used to eliminate concentration of impurities and steam boiler should be free from scale. Amount of blowdown should be balance, do not too high and do not too low. If too high, the cost for chemical treatment and energy loss will high too, but if too low will make high concentration of impurities. There is not standard to regulate the amount of blowdown but practically it range between 1% until 25% regarding to the flow of feed water.

The flow of blowdown can be intermittent or continous flow depending on its use. In the classification of steam boiler based on tube type, there are two types of boiler, water tube steam boiler and fire tube boiler. In the water tube boiler, concentration of water in steam drum is controlled with continuous blowdown but in the mud/water drum or bottom header is controlled by intermittent to reduce possibility disruption of circulation in the steam boiler. In fire tube boiler, the flow of blowdown can be done with either intermittent or continuous. The amount, duration and frequency of blowdown can be controlled depend on requirement of water treatment, operating condition and design of steam boiler.

Flue Gas Treatment in Steam Boiler

Some of treatments of flue gas in steam boiler should be performed to reduce bad effect of emission (NOx and Sox) before distributed to atmosphere. The followings are treatment to reduce content of NOx and SOx

1. Treatment to reduce content of NOx

a) Select low content NOx of fuel such as for coal must less than 1.5%

b) Increase sizing of furnace to reduce temperature and NOx production.

c) Restrict the use of oxygen by low excess air when react with nitrogen to form NOx.

d) Use over fire air to add combustion perfectly.

e) Install flue gas filter such as multi cyclone, baghouse or electric precipitator to reduce NOx content in flue gas.

f) Use mechanical ash collection like hopper to collect unburned fuel.

2. Treatment to reduce content of SOx

a) Select low content of SOx fuel.

b) Use scrubber wet non regenerative, use additional chemical like limestone to react with SOx to be combined compound. The compound can be disposed or be sold for gypsum.

c) Use scrubber wet generative to make faster the process of collection by use sodium carbonate.

d) Use scrubber dry, collecting solid product result from heating process of flue gas, so dries up the moisture. The moisture can be taken from adding chemical in water based spray to flue gas.