CH-13 Boiler Safety

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INDIABOILER DOT COMTUTORIAL FOR SECOND CLASS BOILER ENGINEERS PROFICIENCY EXAMINATION

DLP/BOE-II/ 1- 01092001

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CHAPTER - 13

Boiler Safety

A. IntroductionBoiler and pressure vessels have many potential hazards in common, as well as hazards

unique to their specific operations. These vessels hold gases, vapors, liquids, and solidsat various temperatures and pressures, ranging from almost a full vacuum to pressures of

thousands of pounds per square inch. In some applications, extreme pressure and

temperature changes may occur in a system in rapid succession, imposing special strains.

Major boilers and pressure vessels explosions occur, because of their spectacular nature.

The problem is to ensure the integrity reliability throughout the working lives of all

boilers and pressure vessels and, in the event of any defect occurring, to limit itsdamaging effects.

In the interest of safety, rules for boiler operation and maintenance have been set forth

by manufacturers, engineering societies, and most importantly, governmentaljurisdictions (IBR). The engineer and boiler operator should be familiar with those rules

specifically applying to the local situation.

Causes of accidents:

Accident causes fall of broadly into two groups.

1. Personal - involving the human element of care:-

Persons falling short distances Lifting and handling

Stepping on objects

Hand tools

Objects falling

Falls from a height

2. Technical involving the collective element of planning and control, and theimpersonal aspect of physical protection:-

Machinery

Hot substances Electricity

Transport

Poisonous Substances Explosions

The Technical accidents are usually serious and although as a percentage of the total number ofaccidents they are of low incidence, their prevention requires considerable foresight in planning


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and control and in provision of adequate physical protection. The potentially serious accident is

covered by the requirements of the Factories Act.

What now follows is primarily concerned with the prevention of accident producing situations

resulting from technical causes. The emphasis is on safety in plant operation and it should beremembered that with the larger units ad advanced operating conditions there is greaterpossibility of technical trouble

Design and control systems:All plant is designed for a certain conditions. Although operators have no control over design,

they must be aware of any limiting features in the design. For example, the limiting design

features of a boiler include:-

1. Maximum permissible steam pressure.

2. Maximum superheater metal temperature.

3. Minimum air heater temperature.

Plant must be operated to ensure that it is kept within the design working limits. To this end all

items of plant should have operating instructions and these should be explained, if necessary, tothe personnel who are responsible for their operation. Some items require very simple

instructions. Others may require very complex instructions.

Many items of plant have automatic controls. There is nothing new about this; float switches ontanks have existed for many years, and in fact boiler control systems have been in existence for

over thirty-five years. The only difference is that todays control systems are more complex.

Modern units have whole sequences of operation that can be performed automatically.

However, it must be remembered that although a correctly working automatic system has nohuman weaknesses, neither has it human intelligence. A computer will only do what it has been

programmed to do.

Automation takes care of many routine operations such as steam/load output variations and

logging, and this reduces the tedium of the work. On the other hand, the operator has greaterresponsibility in abnormal conditions for ensuring that the plant is kept within its design limits

and it is essential the emergency drills for abnormal conditions are included in the operating

instructions. These emergency drills require practice to ensure that the operator not only knowswhat to do, but has practical experience.

The majority of control systems have built-in safety features known as fail-safe systems. for

example, if the control system of oil fired burners is actuated by compressed air then thee fuel

valve is pneumatically operated, and if the air fails the fuel valve will shut.It also ensures that the fuel valve cannot open unless the control air is available. Many examples

of the fail-safe system are found in power stations and operators should be able to list some

examples in their own particular plant.

Although equipment may be designed to fail-safe an operator is subject to human errors, such as

operating the wrong valve or pushing the wrong control button. This is rarely the result of lack

of knowledge, but is more often of forgetfulness, so that the problem becomes one of remindingthe operator where he is and the function of the control he is about to operate. In short, adequate


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identification and labeling of plant, equipment and controls is the first essential for correct

operation, and it is of utmost importance during the commissioning period of new plant.

It is proposed to highlight certain aspects of plant operation where hazards can arise.

Boilers:

Over Pressure:

Each boiler has a maximum permissible working pressure and to prevent the pressure risingabove this level, safety valves are fitted. The superheater safety valves are set lower then the

steam drum safety valves in order to ensure a flow of steam through the superheater.

To lift these safety valves unnecessarily, involves a loss of water and heat, but they have to befloated after each statutory inspection. (Floating is the term used for controlled lifting of safety

valves to ensure lift at the correct over-pressure).

Loss of water:The water level indicator is one of the essential fittings that have to be fitted on all boilers.

Another is the pressure gauge. Although various indirect indicators are fitted, these must be

considered as aids to the operator and in the event of trouble with the water level in the boilerdrum the local gauge is the absolute gauge to be relied on. The Safety Rules require the water

level gauge to be tested daily and the mode of testing and blowing down should be clearly laid

down in the operating instructions.The loss of water on a boiler is a most serious condition. It can arise from any of the following:-

1. The failure of the feed pumps

2. Incorrect operation of feed check valves and/or feed regulators

3. Sudden demand of steam lifting of safety valves

4. A failure of pressure parts within the boiler (burst tubes)

5. Inadvertent operation of blow down and/or drain valves

The ratio between the water content of a modern boiler and its rated output is approximately onethird. With this small quantity of water it is essential that any loss be attended to within seconds

by the operator.

It must be remembered that although the fuel supply may be turned off immediately on apulverized fuel or oil-fired boiler, there is still residual heat within the boiler. Thus, in the caseof boiler wall tube failure, the water level must if possible be maintained until the heat in the

boiler has been dissipated. What action is taken depends upon many factors, some of which can

only be decided in the light of prevailing conditions of the plant at the time.


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Loss of ignition:

Loss of ignition or instability of the furnace flames is a condition that can lead to dangerous

situations, particularly if the unburnt fuel continues to flow into furnace. All fuels should be shutoff as soon as loss of ignition occurs and the boiler purged with air before re-ignition.

When conditions exist that could lead to furnace instability, such as soot blowing, ashing, etc.then the ignition equipment such as oil burners, should be placed in service to assist inmaintaining combustion. A case occurred of a serious explosion in the combustion chamber of a

184,000 Kg/h boiler. The boiler was steaming at light load with one p.f. mill in service and four

auxiliary oil burners. Trouble had been experienced with one of the auxiliary oil burners withlighting up. Additional load was called for and the operator brought the second mill into

operation, still keeping the auxiliary oil burners in service. The operator then noticed that the

F.D. fan pressure had dropped and that it failed to respond to the adjustment of the controls.

Also the evaporation had not increased since the two mills had been brought in. the A.P.A.reported that the furnace looked dark and the operator sent for the foreman. However, before the

foreman arrived a severe explosion occurred in the combustion chamber.

The cause of the explosion was that the p.f. from the second mill had not been satisfactorilylightened, probably due to the unsuspected failure of the auxiliary oil burner which had given

trouble previously.

The operator had failed to act on thee standing instructions that when these conditions occur theboiler should be shut down immediately.

Automatically Controlled Boilers:The function of automatic control equipment is to provide precise control over combustion and

feed-water requirements in order to meet output fluctuations. The

Ordinance requires a steaming boiler being always attended by a "Competent Person" inpossession of a Certificate of Competency awarded for the type of boiler. It is essential, in the

interest of safety of the boiler plant that the operators are competent to carry out the normalduties in connection with such plants and it is the responsibility of the owners to ensure thatoperators employed by them are properly trained.

The automatic controls failure is the most frequent cause of boiler failure than other damages.

The automatic controls failure sometimes result in boiler overheating due to low water level,

boiler explosion due to low water level, boiler explosion due to unburned oil vaporize tohydrocarbon gases inside the furnace and fusing of heating element of the electrically heated

boilers.

Many cases of damage arising from failure of automatic controls have been caused by lack ofknowledge of controls on the part of the boiler attendants. Attendants with experience limited to

manually controlled boilers may be unfamiliar with modern automatic boiler controls. Before

they take charge of such boilers it is essential that they should be properly trained in the safeoperation of such plant the action to be taken in emergencies and to carry out the tests set out.

Explosions due to coal dust in mills and p.f. pipework:If any combustible dust in suspension, whether it be flour, coal, cork, etc. is ignited, an

explosion will occur. Coal mills and p.f. lines contain dust at all times, and if this dust is

disturbed and a source of ignition is present an explosion will follow, often resulting in damageto the paint and injury to personnel. The most likely cause of p.f. mill explosion is blowback


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from the furnace along the pulverized fuel pipe work to the mill and there is a hot spot in the

mill due to excess hot air being supplied.

Following a series of p.f. explosions in 1950/55 a Committee drew up a Code of Practice for the

Safe Operation of P.F. Plant. Included among the recommendations were the following:-

1. The training of operators

2. Cleanliness and care of operation

3. The temperature of standing mills

4. Automatic means of isolating standing mill fuel lines

5. Indication of loss of coal feed

6. Need of ignition equipment to be available and used if unstable furnace conditions

prevailed

7. Mill air scrolls to be kept clean

8. Need for air flow to be maintained through a tube mill when it is running light

Similar dangers exist when unburnt p.f. is carried through the furnace and deposited in hoppers

or heaters etc. Explosions have occurred when hopper doors have been opened and air has come

into contact with hot unburnt fuel. To prevent this, care must be taken when operating boiler

plant under light load conditions to ensure that the level of dust in hoppers is kept to aminimum. Oil fuel deposits can also lignite under similar conditions.

During pre-commissioning safety valve tests in a large unit, an explosion occurred in a pressure

type mill as a result of overheating of the mill casing.

An inspection showed that:-

1. There was no control of tempering air to the individual mills. (Tempering air is cold

air).

2. The temperature indication was faulty.

3. The alarm system has not been connected.

This incident is typical in that there were several factors which combined to lead to this

situation, and the absence of any of them could well have prevented the actual accident.


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Air Heater Fires:Air heater fires in pulverized fuel fired systems are generally associated with the frequent andprolonged use of lighting-up oil burners for pre-commissioning steam raising and low load

operation. Both in these and in oil-fired boilers the contributory causes can be summarized as:-

1. The carry over of unburnt oil due to delayed ignition or leakage

2. Inefficient combustion resulting in incomplete burning of oil which leads to the carry

over of oil vapour, coke and/or soot which collects in the cooler parts of the flue gassystem; the air heater is particularly vulnerable to these deposits.

In order to reduce air heater damage the following measures should be adopted.

1. Adequate instrumentation and equipment for the satisfactory control and operation of

both oil burners and air heaters must be available.

2. All air heaters should be provided with adequate inspection and cleaning access and a

high level of cleanliness should be maintained.

3. Fuel oil should be burnt with the highest efficiency. This may entail an adjustment of

burners, the use of low viscosity oil during commissioning, gas tight furnaces, and the

cleanliness of the air heater, gas side. When the unit is shut-down the furnace should bepurged and the fuel dampers closed. The air heater temperature should be observed at

frequent intervals during banked conditions.

Air receiver:

Compressed air is widely used in all industrial undertakings, e.g. pneumatic control systems andfor various pneumatic tools and cleaning equipment etc. Compressed air are either arranged torun continuously since they are fitted with unloading devices to allow running light when

desired supply line pressure is reached, or have pre-set cut in and cut out pressure switches.

Supply of clean, dry compressed air is required from the well-designed, correctly installed andmaintained air receivers. Any air receiver shall be examined by an appointed examiner within

26 months after the date of fitness issued.

Also it is contravention of the Boiler and Pressure Vessels Ordinance to "alter" the safety valvepressure set by the appointed examiners.

Air pressure vessels inside buildings should not be located near sources of heat, such as

radiators, boilers, or furnaces.

Dynamic Pressure Hazards:

The pressures in cylinders of compressed air, oxygen, or carbon dioxide are over 2000 psig

when the cylinders are full. A large cylinder, such as that used for oxygen for oxyacetylenewelding, will weigh slightly more than 200 pounds. The force or thrust generated by gas flowing

through the opening left when a valve breaks off a cylinder can be 50 to 20 times greater than

the cylinder weight. This can be compared to the propulsion system of a rocket or guidedmissile. Spectacular accidents have occurred when such charged cylinders were dropped or


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struck so the valve broke off. The cylinder would then take off, in some cases smashing through

buildings, rows of vehicles, and creating the tremendous havoc that a heavy steel projectile

traveling at high speeds can generate.

Excessive pressures from non-thermal sources:A vessel or container can be over pressurized so it fails. The simplest example is the bursting ofa childs balloon. Practically all pressure vessel failures occur at flaws in the material where

stresses are concentrated. If the flaw is serious enough, failure may occur at or below the normal

operating pressure. If the vessel is over pressurized, it will fail at a weak point, where a flawexists.

Safeguards against air receiver explosion:

The safeguards are:

Adequate cooling by provision of intercooler and aftercooler to restrict the operating

temperature.

Plant should be kept in good order; clean internally and externally and supplied with speciallycompounded compressor oil of the correct characteristics for the job.

A suitable temperature fusible plug could be incorporated, designed to melt and blow at a

temperature a little lower than the closed flash point of the lubrication oil

Regular inspection will help to minimize the risk of a failure by detecting the symptoms at an

early stage.

Whipping of Hoses and LinesWhipping of flexible hoses can also generate injury and damage. In one instance, the end fittingof a compressed air line was not tightened adequately when the line was connected. The line

separated when it was pressurized. It then began whipping about until it hit and killed a worker

by crushing his skull.

Water HammerWater hammer is caused by a sudden stoppage of liquid flow so a shock effect occurs which cancause the rupture of a line. The mass of liquid has momentum. If the flow is terminated abruptly

by closing a valve at the downstream end of a line, the momentum of the liquid is transformed

into a shock wave (water hammer) which is transmitted back upstream. The shock is transmitted

back through the liquid because liquids are practically incompressible. The energy shockinvolved may be adequate to break fittings and lines, especially if they are made of brittle

materials which do not stand shock well. To avoid damage to liquid lines, the use of quick-

closing valves should be avoided. If they must be used, the shock can be alleviated by a suitableair chamber or accumulator connected to the lines slightly upstream of the valve.

Testing of Pressure SystemEach pressure system should be tested prior to use, and pressure vessels should be tested

periodically after that to determine their adequacy for continued service. Wherever possible,hydrostatic testing, using water and not a gas such as steam or compressed air, should be used.

If the vessel being tested fails suddenly, the rapid expansion of gas might cause the rupture to be


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violent, possibly generating a blast wave with injury or damage. Hydrostatic testing, using water

as a fluid, has two major advantages. Leaks created by pressurization of a vessel can be detected

easily. The test can then be interrupted or continued with increased care. Because fluids expandlittle, in case of a vessels rupture, no shock wave will be generated.

ChlorineChlorine is used to prevent the growth of algae in C.W. systems in river water stations and to

control the mussel growth in sea-water stations. Chlorine is well known as a poisonous gas andone part per million is the maximum permissible concentration. A rule of thumb is that if you

smell chlorine it is time to evacuate from the area.

Operators concerned with chlorine plant should be conversant with and practiced in the use ofcompressed air breathing apparatus which should be readily available.

Chlorine is supplied in liquid form either in bulk or in drums. The liquid chlorine passes through

a vaporizer and the gas is dissolved in water before it is added to the C.W. system. The major

hazard occurs when changing over the connections on the drums, since an ill fitting connectionor a leaking valve can result in the discharge of the chlorine gas.

Liquid chemicals:The demineralization method of water treatment is in common use and the regeneration of the

various units associated with this type of plant involves the use of sulphuric acid and causticsods. These are both very corrosive liquids and can burn the skin. They should never be mixed

together and neither should water be added to them; the dilution of acid/alkali must always be

made by adding the liquid to the water. They are supplied in road or rail bulk tankers and care

must be taken that the connections are made to the appropriate tank. Any spillage should bewashed away with copious supplies of water and a douching shower should be available to

douche any person contaminated with these liquids. In addition eye wash bottles should bereadily available.Other chemicals used are hydrazine, compounds of ammonia and potassium, and certain volatile

alkaline.

No chemicals should be used in power stations until the precautions for its storage and handlinghave been laid down by the Station Chemist. It is particularly important to protect the face, eyes

and hands when handling these types of chemicals.

Entry into confined spaces:Entry into confined spaces, such as oil tanks, is forbidden unless the space has been certified

safe for entry without breathing apparatus or under conditions laid down. Where the person

enters a confined space wearing an approved breathing apparatus, he must be attached by a lifeline that is being held by another person outside.


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FOLLOW THESE SAFETY RULES

A better accident history can only result from the combined efforts of all. Carelessness accounts

for the majority of accidents. Go over this list and see how many apply to your job.1. Never wear frayed clothing which might catch in machinery or cause tripping.

2.

Never wear shoes that could cause slipping, or with worn soles that permit easypuncture.

3. Never fail to wear a helmet-type hat where there is danger of head injury from electric

wiring or falling objects.

4. Never expose your eyes to flying particles or the extended ray of a welding machine.5. Never leave your hands unprotected from burning, slivers, or abrasions.

6. Never use a flashlight unless it gives suitable light and is dependable, so as not to leave

you in the dark in some remote area of a large vessel or boiler.

7. Never use an extension light with a weak or frayed cord, or without a cage around thebulb.

8. Never experiment with electrical hook-ups beyond your knowledge.

9.

Never take a step in a dark area if you cant see where you are putting your foot down.10.Never enter the furnace of an automatically-fired boiler without making sure that the

firing mechanism is in the locked out position and is tagged to this effect. Never enter

the furnace of a water-tube boiler without watching for falling slag.11.Never enter a boiler installed in battery without making sure that all stop valves in the

connecting pressure lines are securely closed and tagged. If there are bleed-off

connections between the stop valves, open them.

12.Never drain a vessel or boiler without opening a suitable vent. This will preventcollapse and permit complete drainage.

13.Never remove a handhole or manhole cover plate from a vessel or boiler unless you are

sure that it is empty.

14.

Never enter a boiler or pressure vessel unless a reliable person is stationed nearby tonote any personal mishap or weakness.

15.Never use a ladder without checking for worn rungs and loose side rails.16.Never use a ladder that is too short. In trying to reach up, the angle of the ladder and

user becomes such as to cause falling backward. When trying to enter an inspection

opening, the ladder should extend to or above it. Climbing from the uppermost rung cancause the ladder to tip out at the top or kick out at the bottom. In placement watch out

for electric wiring.17.Never use a ladder not properly adapted to the job. The bottom ends of the rails should

be fitted with spikes or cleats to prevent slipping. If a ladder is not so equipped, or is

placed on a metal floor, secure the bottom to some fixed object. Do not rely on someone

to hold the ladder, as he may be called away on another mission or emergency.18.Never risk your life by removing fuses from high-voltage electrical circuits with your

bare hands. Use tongs or fuse pullers.

19.Never use ladders as bridges.20.Never enter a pressure vessel that contained a toxic agent or may contain dead air

without thorough purging. If necessary, wash and clean the surfaces.

21.Never use a rope ladder or bosuns chair without first inspecting the rope. Also, be surethat you are physically able to make an inspection from such flexible equipment.


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22.Never inspect a boiler or pressure vessel while it is under hydrostatic pressure test

without using due care. Always make sure a vessel is completely vented of air before a

hydrostatic test is applied.23.Never operate soot blowers unless the burners are operating at a high firing rate. Air

flow should be adjusted to insure a high carbon dioxide but low oxygen content in theflue gases.

24.Never enter a building or other enclosure in the presence of ammonia fumes, sewer gas,

natural gas or any other toxic gas without proper clothing and respiratory equipment

and then only with extreme caution.25.Never at any time stand in front of the discharge opening of a relief valve particularly

attempting to test it.

26.Never blow down an appliance on a boiler without first observing the point of discharge

your foot may be under it.27.Never blow down a boiler under pressure by opening the slow-opening valve first.

Always open the cock first and then the slow-opening valve. After blowdown close the

slow-opening valve first. This prevents a sudden back pressure wave on the connectingpiping.

28.Never shut a valve off suddenly in a pressure line except under extreme emergency. The

sudden change in flow may result in rupture at some weakened location. Also, neveropen a valve suddenly (particularly in a steam line) where water hammer might result.

29.Never operate a pressure vessel with only part of the head or cover plates, lugs or

clamps in place.

30.Never continue to use any bolt, pin, lug, or clamp after it becomes worn, sprung,stripped or otherwise weakened.

31.Never release the holding mechanism of any quick-opening door or end closure until

sure that the chamber is void of pressure.

32.

Never depend on automatic controls as safety devices unless they are kept clean and arechecked, tested and otherwise regularly maintained.

33.Never operate boilers, pressure vessels or machines at pressures or speeds above rated.34.Never take someone elses word for something you should have checked yourself.

35.Never resort to horse-play.

36.Never neglect your fire protection. Check fire buckets and extinguishers regularly, andnever leave a water line valve shut off to a sprinkler system through lack of

maintenance. Red tag it and stay with it until corrections or alterations have beencompleted; and then return all valves to their proper positions for service.

CH-13 Boiler Safety

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