BOILER NOISE
Boiler manufacturers are required to design their appliances to meet the following modern requirements:
· space saving
· low cost
· high efficiency
As an almost inevitable result, the required amount of heat is passed through less heat exchanger metal than ever before in the history of the heating industry. This relatively new situation demands specific instructions on the part of the boiler suppliers, such as the Benchmark initiative, to prevent boilers becoming noisy. A boiler with kettling and knocking noises not only deprives the occupants of the house of part of their home comfort, but the localised overheating of boiler metal will certainly shorten the service life of the appliance. Minor to major boiler deposits are worth their weight in gold - to the fuel suppliers.
Boiler noises are practically always caused by deposits of either lime or hardened black iron oxide sludge (Magnetite - Fe3O4). The burst boilers we have investigated all had a history of earlier boiler noises, and all contained excessive scale of either lime or black iron oxide, often a mixture of layers of both, with oxide often predominating. However, where an installation had a hidden leak, and hard make-up water entering, the samples of boiler scale submitted consisted of mainly white lime, but identification by colour can be grossly misleading as the lime deposit adopts the black or red colour of the iron oxide content. Commercial boilers are too bulky and are usually too well insulated for the boiler noises to be heard giving their early warning.
Modern boilers require the following three essential conditions to be met and maintained in full for boiler protection, freedom from noises and for fuel economy.
· Total absence of even the slightest deposit of any kind.
· Good or high velocity of water through the heat exchanger.
· Complete avoidance of air blisters passing through the boiler.
Cast iron boilers are generally less critical than tubular boilers and can tolerate some variations from the ideal. The rough cast iron surface is usually covered by an initially firmly adherent skin of black iron oxide. Casting skin is normally only very thin and therefore has only minor thermal insulating properties but where this is removed by later descaling, the boiler can become more efficient than when brand new (See Fig. 1 line 5).
The two substances, cast iron and black iron oxide, have vastly different rates of expansion and contraction. The oxide film being hard and brittle by nature, eventually crazes and flakes, allowing localised boiling to occur within the crevices, even when the temperature of the circulating water is far below boiling point. In small domestic boilers this alone can lead to minor kettling noises, particularly at low water velocities. Detached oxide powder and flakes, and more seriously some previously adherent residue of casting sand, will form a sediment leading to localised over-heating.
The boiling of water under and within the sediment can produce louder noises and vibrations. Additional lime scaling and drifting oxide sludge from corroding radiators entering the boiler can lead to intolerably noisy conditions. The normal sequence of events being: lime film from hard water, oxide sludge from radiators, both increasing boiler metal temperatures. Boiler skin and sand become detached and form part of the scale and sediment. Boiler failures in untreated systems have been caused by this condition. (See Fig 1 column 4).
Tubular boilers (domestic) are sometimes described as ‘low water content boilers’. The arrangement could also be considered to be a continuation of the heating pipework, suitably fitted with heat absorbing fins, a burner and control gear. Most boilers of this type are manufactured from thin gauge, high thermal conductivity copper. Additional copper sections are brazed to the interior of the boiler for further improved heat absorption, or in an effort to spread the total heat intake over more metal. Others have inserts for the purpose of achieving a greater degree of water turbulence, also aiming at maximum heat absorption, i.e. high efficiency, as far as the basic design is concerned. Many low water content condensing boilers are manufactured from aluminium.
Local water conditions play an important part, more particularly the scale-forming solids dissolved in the supply water. It is quite incorrect to assume that medium hard water conditions can be ignored in a central heating system, even if the initial filling water is never drained and replaced.
Lime deposits become proportionally more troublesome with water changes or with make-up water entering. All boilers attract and deposit the scale-forming impurities from the entire circulating water content, causing it to become correspondingly softer. (This is of no benefit to the installation, as soft water is more corrosive than hard water). The substance generally considered as ‘lime’ is more often a mixture of mainly calcium carbonate (chalk), with smaller amounts of calcium sulphate (gypsum). The insulating properties of chalk and gypsum vary. Minor films can cause major temperature increases of the boiler metal and percentage efficiency losses as indicated in Fig.1.
It is interesting to note that some fuel-saving innovations are available, at a price, which can only achieve a fraction of the savings that can be made by simply maintaining clean boiler conditions.
Initial scale deposits will form, in a hard water area, during the first test firing of a new installation, if the water velocity is low. Although precipitation of lime normally commences at approximately 60°C, and increases with rising water temperatures, deposition of lime has been demonstrated to commence from as low as 40°C, simply because with low water velocities the boiler metal temperature can rise to far above the boiling point of water. (See temperature increases of boiler metal given in Fig 1). Suitably increased water velocities will delay, but never avoid scaling.
Boiler noises are not caused by the formation of steam blisters, but are directly related to the rapid condensation or shrinkage (implosion) of steam in water. Implosive forces and the associated noise are naturally greatest if the surrounding water is cold. The same reaction of condensation is slower and less violent in warm or hot water. The faulty condition still exists within the boiler, but the effects are less audible at higher water temperatures.
The identical phenomenon has been observed by everyone who has heard cold water or milk being heated by steam injection in a cafeteria. The noise of implosion condensation reduces with liquid temperature and is replaced by mere bubbling when the liquid has reached boiling point, namely when steam hardly condenses at all, but passes through the liquid in its original state.
Clatter from radiators or non-return valves can be directly linked to the rapid changes of large volume steam to the lower volume water during the implosive shrinkage. Changing valves would prove futile. The origin of the problem being yards away, namely in the boiler.
Air blisters, like steam, temporarily displace their own volume of heat-absorbing water from the contact area with the boiler metal. The resultant localised temperature increase of the surface of the metal, can contribute to gradual fouling and noisy conditions. Large air blisters are worst, finely dispersed air is relatively harmless.Tracing the source of aeration can prove impossible where inlets are not apparent. Inward leaks, on the suction side of the pump, have been detected in a large number of otherwise mysterious cases. Joints, and particularly flanges, may prove to be water-tight, but not necessarily air-tight.
Latent heat is retained for a few seconds longer in the boiler metal, if the pump is stopped simultaneously with shutting off the flame. Parts of the boiler metal, and certainly the dry side of a heavy-walled cast iron boiler, is then at a temperature far in excess of the boiling point of water, probably even above the melting point of solder. The now motionless water, immediately in contact with the boiler metal will simmer or boil. This explains the hissing noises observed after turning a boiler off. The noise disappears with the dissipation of the excess heat into the water - Whilst a single occasion of this type would cause little damage, the true in-service condition is more serious. In a given situation, the thermostatic control gear will cause the boiler to fire and cut out 100 times in a 24 hour cycle, or 200,000 times over a period of 10 years. Boiling or simmering water will deposit its content of dissolved solids (mainly lime) onto the boiler metal. At the same time, the previously formed soft oxide sludge deposits will harden, crack, flake and lead to louder noises and further reductions of boiler efficiency. Circulation of the water should ideally be maintained until all latent heat has been dissipated.
Other debris that can form sediment and become part of hardened deposits may include sand, originating from the mould in which the cast iron was formed. Casting sand ought to be removed by the boiler manufacturer, but being hard pressed for manufacturing economies, the complete removal of sand seems not to be always achieved. Sand is gradually released from its bond to the casting skin. Flaking oxide skin and sand forming a sediment can produce intolerably noisy conditions. Sand and brick dust may also originate from carelessly passing open-ended tubing through walls and ceilings.
Iron oxide deposits burst boilers. Whereas the formation of lime deposits is limited by the total hardness of the initial and subsequent filling water, black iron oxide sludge is produced continuously in all installations, unless treated by the timely addition of a central heating protector to the circulating water. The several perfectly natural corrosion processes occurring in untreated systems, progress along the following chain of reactions: Iron goes into solution. Dissolved iron is unstable and precipitates first as iron hydroxide, later
as a higher oxide, forming very small particles. In the absence of generous supplies of free oxygen, the magnetisable iron oxide (Magnetite - Fe3O4) is formed. The near molecular sized oxide dust agglomerates and forms larger particles or granules and sludge. In radiators the soft sludge forms crusted, hard and brittle sediments, but much tougher baked-on deposits develop in boilers. The oxide is approximately 5 times as heavy as the water and has the natural tendency to drift to the lower parts of heating systems, where the boiler is usually situated. Lower water velocity leads to greater sludge retention in the boiler. Unless corrosion is retarded, or best completely prevented, oxide sludge accumulations should be removed periodically or the boiler will be at risk to buckling and cracking due to localised overheating. However, fuel economy suffers long before boiler damage occurs. A thin deposit of only 0.020" of iron oxide causes a fuel wastage of 2.5% (See Fig. 1.).
New boilers in older systems can become excessively fouled by existing oxide sludge soon after installation. The work of replacing a boiler can disturb soft sludge and loosen sediment. Replacing pumps can produce different flow rates, redistributing soft sludge from the rest of the installation. All soft sludge should be removed prior to replacing a boiler.
COUNTER MEASURES
A) Preventative methods are relatively simple to apply. Available materials warrant some in-depth study.
Central Heating Protectors eliminate corrosion so that there can be no oxidic sludge or pitting, even under conditions of accidental aeration, or some over dilution. They prevent lime deposits at the same time, have a service life of many years, and may only require minor topping up at infrequent intervals.
B) Corrective treatments
Chemical cleaning is more costly and more labour intensive than the prevention of boiler noise and corrosion. However, Fernox produce a comprehensive range of cleaning agents and equipment to quickly and effectively restore system performance.
Oxide sludge can be removed by selectively dissolving it without attacking the metals, using a suitable inhibited acidic descaler such as Fernox System Cleaner. This treatment should be restricted to newer installations. Older untreated systems are more prone to have existing leaks, which are temporarily ‘plugged’ with corrosion debris. The removal of the latter would expose the leaks. Installations with a history of corrosion failures of radiators should not be acid cleansed, as other radiators are likely to have deep corrosion pits or complete perforations. The well stoved and hardened outer paint film can hold water for up to 2 years, - except if disturbed by acid from within or by a sharp knock externally, for instance with a vacuum cleaner.
Older installations can be substantially improved by dispersing the soft oxide sludge content with Fernox Restorer, followed by power flushing with plain water. This is an inexpensive method, and will remove soft sludge. However considering that crusted or hardened sediment is less likely to drift and to foul the boiler, this method of cleansing can be highly beneficial for the boiler.
Separate boiler de-scaling may be beneficial after removal of all soft sludge from older installations. The radiators should be isolated from the system leaving a restricted circuit comprising of the boiler header tank,
feed and vent pipes and hot water cylinder. The system, excluding radiators, can then be descaled over a period of up to 24 hours. This is followed by rinsing with plain water and neutralising to passivate the surfaces within the system. Again, Fernox System Cleaner may be used.
Sediment of sand is not soluble in normal descaling acids. Any sand remaining after descaling a domestic boiler is best removed by hard flushing. This means that one top and one bottom connection should be opened for water jetting from above and for speedy drainage from below. Control samples of flushing water should be taken at intervals to indicate the end of the hard flushing process. After carefully decanting off most of the water from the samples, the sediment should be inspected with a strong magnifying glass.
The cost of complete chemical remedial treatment may represent 5-25% of the replacement costs, according to the severity of a situation.
The cost of preventing corrosion, oxide sludge, lime deposits and boiler noises is in the region of 0.5% relative to the value of the system.
The prevention of corrosion, limescale and fuel wastage, using modern chemical compounds has made a valuable contribution towards the preservation of energy resources. More than 5,000 domestic and industrial heating and cooling installations are treated with a corrosion protector each week. Approximately 300,000 installations have been desludged and descaled, so far.
Fig.1 : Effects to common types of boiler scale
TYPES OF DEPOSIT THICKNESS OF DEPOSIT EFFICIENCY OF LOSS (approx.) TEMPERATURE INCREASE OF BOILER METAL
Calcium carbonate 0.003" 2.5% 31°C
Calcium carbonate 0.004" 5.0% 90°C
Calcium sulphate 0.009" 2.5% 31°C
Calcium sulphate 0.018" 5.0% 90°C
Ferrous oxide 0.020" 2.5% 31°C
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