Table of Contents
The boiler house became integral part of the majority of cottages long ago. To bring to the remote building long-distance heating most often is represented impossible and it and is unprofitable. To warm up several floors during winter cold weather, to pump hot water in upper floors and in all batteries, to provide heating of system of heat-insulated floors – all this possibly only after construction of boiler installation.
However under conditions of non-compliance with some established practical rules except comfort such equipment can bear also serious danger. Failure of the boiler can lead to explosion with catastrophic effects. Lead some of the most common causes to accident:
- fuel explosion;
- water treatment shortcomings;
- abatement of the water level;
- pollution of boiler water;
- mechanical damage of pipes;
- non-compliance with regulations of warming up;
- violation of technology of purge;
- above-standard forcing;
- inadequate storage conditions;
- pressure reductions.
Let’s consider both dangerous factors, and ways of precaution which will allow not to be afraid and use safe operation of boilers.
At operation of boilers it is possible to come up against the most dangerous situation – explosion in fire chamber. Insufficient cleaning of fire chamber or oversaturation with fuel of gas mixture becomes the reason of the majority of explosions. Oversaturation of gas mixture turns out to be consequence of accumulation in fire chamber of not burned down fuel. It can occur for number of the different reasons: because of pressure fluctuations fuel of giving, damage of the equipment, failure of regulators.
Set of cases of explosions occurred after in work of torches there came interruptions. So, contamination of fuel atomizer brings to the low-quality spraying causing separation of flame or instability of burning. After there is the subsequent injection of fuel in fire chamber concentration of its vapors increases. Not burned down fuel collects also in cases of long work of torch with low-quality spraying.
The flash of not burned down fuel leads to explosion. It is possible to avoid it at observance of the following simple rule: never it is impossible to make injection of fuel in gas-polluted languid fire chamber. Before it is necessary to disconnect all torches manually and to carefully blow fire chamber air. And only after such simply operation and fault recovery with ignition of torch can be included again.
Abatement of the water level
The structure of carbon steel of which walls of boilers are made changes when exceeding temperature of limit in 427 °C – it loses durability. But the working temperature of fire chamber – is more than 982 °C therefore the boiler is cooled with the water proceeding through its pipes. If it long time works at scarcity of water, steel pipes in literal sense can melt as the burned-down wax candles.
For reduction of probability of the accidents arising because of this reason the shutdown of boiler coming at reduction of height of water has to be provided. Sensors of height of water of float-operated type or direct action carry out such task. The bypass of the starting device becomes critical link of system at the same time. Thanks to beypas the operating crew can blow the got littered sections, carry out their cleaning of scum and slime, to imitate emergency without stopping of boiler (the cut-off contour is so checked).
Water treatment shortcomings
In pipes, because of availability in water of magnesian or calcic rigidity, the scum is formed. Ions of rigidity are removed in the course of water treatment. Increase of scum leads to overheating of pipes which are intended for heat extraction from boiler. The scum reduces diameter of pipes, creates additional layer of heat insulation and worsens heat exchange. Local burn-out of pipe can become result.
To prevent this process, in boiler water the content of salts of rigidity should not exceed allowable limits. At the increased working temperature and elevated pressure of boiler installation also requirements to water treatment become tougher.
Lowering of calcic and magnesian rigidity happens to boilers of low pressure by means of ion-exchange installations. The boilers with steam-turbine installations differing in the modes of high pressure and temperature require full demineralization of water with removal of other impurity like silicates. If compounds of silicon are not removed, at evaporation they will mix up with water vapors and form deposit on blades of turbines and other equipment.
Water treatment for boilers includes also processing by chemical reactants. Reactants connect particles of pollution, transforming them to the slime which is not forming on surface of rainfall. Slime is removed when washing boilers. Insufficient water treatment serves as destructive force for boiler therefore in extension of its durability water quality plays large role.
Water of boiler installations consists of mix of the return condensate and feed. And the issue of its pollution very difficult, to it is devoted by the whole books. Pollution usually include oxygen and mix from pitch, oils, chemicals and metals.
The oxygen dissolved in water constantly threatens integrity of pipes. In boiler installations usually there is heater deaerator deleting oxygen from make-up water. In reservoirs of the deaerator of boiler installations which working pressure up to 7000 kPa, is usually added sodium sulfite – the absorber of free oxygen.
The most dangerous type of oxygen corrosion — ulcer oxygen corrosion. Ulcer is called the corrosion concentrated on absolutely small site of surface. Even small distribution of corrosion in general can lead to through rust because of developing of such ulcer. Catastrophic effects of oxygen corrosion demand regular check of operation of absorbers of oxygen and deaerators and water quality control.
The pollution of returnable condensate which is not found timely becomes one more reason of pollution of boiler water. Pollution can consist of different parts: from iron and copper, to production chemicals and chalk. The metals getting to water — constructive materials of condensate lines and the equipment, and production chemicals and oils appear because of corrosion leakages of heat exchangers, glandsealings, pumps, etc.
Dangerous chemicals in large number can get to water because of failures of processing equipment. Therefore continuous monitoring of returnable condensate becomes guarantee of careful operation of boiler installation.
Serious pollution of boiler can be caused also by hit in water of ion exchange resin. It occurs at damages of auxiliary binding of ion-exchange installations or internal pipelines. Very effective and very cheap way preventing the similar phenomena – installation on communications of ion-exchange installation pitch of catchers. Pitch catchers will be able not only to protect boiler, but also in case of accident will prevent loss of ion exchange resins – very valuable material.
Pollution of boiler water proceeds and as gradual deterioration, and as instant accident. High-quality and constant service reduces possibility of troubles. Monitoring of make-up and boiler water allows to obtain timely information on pollution level.
Non-compliance with technology of purge
The constant purge of system and periodic washing of pallets leads to reduction of concentration of the weighed solid impurity which are contained in boiler water. Exceeding of concentration of pollution of boiler water is capable to create such problems as foaming of water in drum or instability of its level. Pollution of boiler superheaters, ablation of drop moisture steam, false drops of the alarm system of height of water can result.
At correctly designed system of purge there is monitoring of boiler water and maintenance of such intensity of the purge providing admissible concentration of impurity. Washing of mud collectors and pallets prevents slime accumulation. But the long purge of the sections forming screens of fire chamber is capable to cause their damage because of the overheating occurring owing to change of circulation of natural water. It is recommended to open instead at each shutdown of boiler valves of purge of sections until pressure loss in system to the level of atmospheric pressure.
Violation of regulations of warming up
The strongest testing to which the boiler — violation of the rules of warming up can undergo. At procedures of start-up and stop the equipment receives serious loadings. Work in constant duty of such loadings does not deliver therefore at frequent inclusions shutdowns observance of rules has to be more strict, than during the work in the rated mode. Step-by-step starting operations and correct regulations reduce accident risk and promote extension of service of the equipment.
The design of standard boiler means use of different materials: steel of different thickness (thick – for drum, thin – for pipes), fire-resistant and thermal insulation materials, massive pig-iron elements. Speed with which they get warm and cool down, it is different. The situation becomes even more difficult if material at the same time is affected by different temperatures. For example, the steam drum at height of water within norm contacts in different parts to water, air and steam. During cold start water therefore the lower part of drum tests thermal expansion bigger, than upper most quicker heats up. As a result the lower part becomes longer upper and the drum experiences deformation. Emergence of cracks of pipes between slurry and steam drums turns out to be consequence of serious deformation.
Very fast warming up during cold start can damage boiler brickwork envelope. At brickwork envelope low heat conductivity therefore it gets warm longer than metal. At not heated-up fire chamber material of brickwork envelope absorbs moisture from air. Slow warming up gradually dries brickwork envelope and does not allow moisture boiling up which could lead to cracking of bricks. According to the standard schedule of warming up of standard boiler the rise of temperature has to happen to speed not above 55 °C an hour.
Danger of the forced mode
Operation of boiler in the mode exceeding the maximum allowed long loading, according to the recommendations of manufacturers, cannot exceed on duration of 2-4 hours.
Physical restrictions of designs of boilers (the sizes of steam lines and fire chamber) can lead to the serious problems connected with pressure loss of steam and reduction of heat emission. Similar restrictions become the reason of the problems which are associated with overheating of boiler:
- erosion of pipes, ashes of cleaners, gas flues and screens;
- destruction of brickwork envelope, material of pipes, gas flues;
- corrosion of pipes of boiler superheaters and walls of fire chamber;
- ablation steam of firm suspended particles and drop moisture that leads to damage of blades of turbines, boiler superheaters, other processing equipment.
The problems connected with overheating of boiler in many respects depend on type of the used fuel. But irrespective of fuel forcing of operation of the boiler increases the speed and volume of combustion gases and their pressure that has impact on erosion. There is rise of temperature of partitions and walls of pipes that affects metal durability. The forcing of fire chamber can cause distribution of flame on screens, and it also becomes the reason of localized corrosion.
Mechanical damage of pipes
The boiler practically does not contain identical elements. Especially it can be carried to pipes of which sections of convective heating and screens of fire chamber consist. Damage of one of them leads to stop of all equipment. And considering that thickness of such pipes does not exceed 2-3 millimeters, it becomes clear that they can be easily damaged. The reason of damage can become:
- blows at assembly or in the course of production;
- incorrect orientation at purge for removal of soot;
- the blowing of soot of wet vapor leading to erosion of pipes.
Design of new boilers provides increase in wall thickness of pipes. It leads to increase in the cost, but provides margin of safety. Besides in places of bend thickness of wall becomes less and at initial small thickness in the place of bend it can not conform to the allowed standard.
Negligent storage of boiler can lead to corrosion of surfaces both from water, and from gases. Corrosion of the gas party happens if earlier in boiler sulphurous fuel was used. There are such sites of fire chamber from which ashes at normal purge cannot be removed. First of all, it is gaps between brickwork envelope and pipes and between partition on entrance and pipes. At the warmed boiler corrosion cannot appear as on surface there is no moisture. But after stop of surface of brickwork envelope and ashes begin to absorb moisture that through time leads to the beginning of corrosion. The localized ulcer corrosion can be established by knocking and to the changed sound.
One of ways to avoid such effects – warm storage. As the heater the slurry drum or purge by the heat carrier going from another, working, boiler can be used. It is enough for temperature maintenance of the surface exceeding dew point of acid solution.
One more storage method of small boilers – dry storage. For this purpose blow nitrogen into boiler, and its inlet openings are condensed with absorbent-dehumidifier.
Failure in vacuum
Designs of boilers can work with excessive pressure, however do not provide possibility of pressure loss up to one level lower atmospheric — vacuum. Its emergence is possible during boiler stop. When cooling there is abatement of the water level and vapor condensation. As a result pressure can decrease to level below atmospheric. As a result the vacuum will lead to leak through the ends of pipes beaded in such a way that their consolidation happens with excessive pressure. It is rather simple to avoid problem — it is necessary to slightly open in steam drum air vent still when in it there is excessive pressure.
Necessary safety precautions
Recommendations which will allow to avoid emergence of problems during operation of boilers:
- to check flame timely to notice malfunctions with burning;
- at extinction of torch to define the reason, but not to try to light repeatedly it;
- before lighting torches, to carefully clean fire chamber. It is especially important to make it if in fire chamber liquid fuel has been spilled. The excess of combustible gases which concentration can become dangerous is removed purge. It should be made at the slightest doubts.
- not to apply the raw water. To carry out inspection of the equipment of water treatment, water quality has to meet the standards accepted for this pressure and temperature;
- for avoiding of accumulation of slime in deadlock sites of water coolers, water contour, etc. Their regular washing is necessary. Water circulation never has to be stopped.
- its constant purge is necessary for removal from the deaerator of not condensed gases. It is also necessary to control the content of the free oxygen which is contained in the water which is going out of deaerators, the working pressure of deaerators and water temperature in supply tanks;
- to carry out monitoring of returnable condensate. In case of its pollution because of failure of processing equipment to provide immediate draining in the sewerage;
- it is constant to blow boiler for maintenance of required quality of boiler water, to periodically wash out drum mud collector. Surfaces of fire chamber should not be blown at the time of operation of the boiler;
- it is regular to carry out inspection of internal surfaces of the deaerator on corrosion. Corrosion of the deaerator can lead to the fact that it will rust through. It will lead to rough boiling up of water and filling steam of all room of the boiler house;
- if on water surface there are signs of adjournment of scum, it is necessary to adjust water treatment;
- always to adhere to the standard schedule of the warming up of water providing growth of temperature with speed not above 55 °C an hour. If the boiler long time was operated with minimum load, the warming up can proceed with speed above of stated. Therefore for the normal rate of warming up work of torches with breaks has to be provided in the starting mode;
- at shutdown of boiler for long time, it is necessary to support him in dry and warm state. To use sodium sulfate – it will allow to absorb oxygen from boiler water and to fill with nitrogen. At storage in dry state together with nitrogen to place moisture absorbent in drum;
- if pressure falls below indicator of 136 kPa, to open air vent in steam drum.