Deposits and scales found in boilers

• May occur anywhere
• Wall and screen tubes most heavily fouled , superhtr has deposits formed elsewhere and carried with the steam or carryover. Economisers ( non-steaming) contain deposits moved from there original site.
• Tube orientation can influence location and amount of deposition.
• Deposits usually heaviest on the hot side of the steam generating tubes. Because of steam channelling, deposition is often heavier on the top portion of horizontal or slanting tubes
• Deposition occurs immediately downstream of horizontal backing rings.
• Water and steam drums can contain deposits, as these are readily accessed then inspection of the deposition can indicate types of corrosion. e.g. Sparkling black magnetite can precipitate in stm drums when iron is released by decomposition of organic complexing agents.
• Superhtr deposits ( normally associated with high water levels and foaming ) tend to concentrate near the inlet header or in nearby pendant U-tubes
• Contaminated attemperating spray water leads to deposits immediately down stream with the possibility of chip scale carried to the turbines.
• At high heat transfer rates a stable thin film boiling can occur, the surface is not washed ( as it is during bubble formation ) and deposits may form
• Thermal stressing can lead to oxide spalling ( the exfoliation of oxide layers in areas such as the suphtr). These chips can pass on to the turbine with severe results. Steam soluble forms can be deposited on the turbine blades , If chlorides and sulphates are present , Hydration can cause severe corrosion due to hydrolysis.
• As deposits form on the inside of waterwall the temperature increases. This leads to steam blanketing which in turn leads to reduced heat transfer rate , long term overheating and tube failure.

Effects on tube temperature of scale deposit

DEPOSITS

Iron oxides

Heamatite (Fe₂O₃)
is favoured at low temperatures and high oxygen levels can be red and is a binding agent and tends to hold over materials in deposition. This is an indication of active corrosion occuring within the boiler/feed system

Galvanic corrosion associated with copper deposition is very rare in a well passivated boiler. Zinc and nickel are very often found near copper deposition , nickel being a particularly tenacious binder

Rapid loss of boiler metals can occur. Copper can appear in various forms as a deposit in the boiler. As a copper coloured metallic deposit, usually in a corrosion pit, as a bright red/orange tubercules on the boiler metal surface or as a brown tear drop shaped formation.

Copper is generally an indicator of corrosion (or possible wear) occuring in the feed pump whether in the condensate lines or in the parts of a feed pump. A possoble cause of this is the excessive treatement of hydrazine which decompose to ammonia carrying over with the steam to attack suc areas as the air ejectors on condensers.

Copper oxide formed in boiler conditions is black and non- metallic.

The least soluble salts deposit first

Calcium carbonate-effervesces when exposed to HCl acid

Calcium sulphate-Slightly less friable then CaCO₃

Magnesium Phosphate-Tenacious binder, discoloured by contaminants

Silicates-Insoluble except in hydroflouric acid E.G. Analcite

Water soluble deposits can only be retained if local concentration mechanism is severe. Prescence of NaOH , NaPO₃ Na₂SO₃ should be considered proof of vapouration to dryness.

Calcium and magnessium salts exhibit inverse solubility. As the water temperature rises their solubility reduces, at a temperature of 70'C and above they come out of solution and begin to deposit. Feed water must be condition to remove the hardness salts before the water enters the boiler. The purity of the water is related to the steam conditions required of the boiler.

Hydrolyzable salts such as MgCl can concentrate in porous deposits and hydrolyze to hydrochloric acid

Scaling mechanism examples

Ca(HCO₃)₂ + Heat = CaCO₃ + H₂O + CO₂

Magnessium Silicate
Tor form requires sufficient amounts of magnessium and silicate ions coupled with a deficiency in OH^- alkalinity

Mg²⁺ + OH- = MgOH⁺

H₂SiO₃ = H⁺ + HSiO₃^-

MgOH^- + HSiO₃^- = MgSiO₃ + H₂SO₄

Thus this rough tan scale can be prevented by the maintenace of alkalinity levels

Calcium Phosphate (hydroxyapatite)
Ca₁₀(PO₄)₆(OH)₂

Found in biolers using the phosphate cycle treatment method this is a tan/cream deposit. This is generally associated with overdosing a boiler but can occur where insufficient disperseing agent reduces the effects of blow down.

In anouther form Ca₃(PO₄)₂Ca(OH)₂ it is associated with correct treatment control

Scales forming salts found in the boiler

Calcium Bi-Carbonate 180ppm

• Slightly soluble
• >65^oC breaks down to form CaCO₃ +CO₂, remaining Calcium carbonate insoluble in water
• Forms a soft white scale

Magnesium BiCarbonate 150 ppm


• Soluble in water
• at more than 90^oC breaks down to form MgCO₃ and CO₂ and then Mg(OH)₂ and CO₂
• Forms a soft scale

Calcium Sulphate 1200 ppm

• Worst scale forming salt
• > 140oC (sat. press 2.5bar) or >96000ppm will precipitate out
• Forms a thin hard grey scale

Magnesium Sulphate 1900ppm

• Precipitates at high temperatures and about 8 bar
• Forms sludge

Magnesium Chloride 3200ppm

• Breaks down in boiler conditions to form MgOH and HCl
• forms a soft white scale Rapidly lowers pH in the event of sea water contamination of the boiler initiating rapid corrosion MgCl2 + 2H2O---> Mg(OH)2 + 2HCl HCl + Fe --->FeCl + H 2FeCl + Mg(OH)2 ---> MgCl2 + 2FeOH This series is then repeated. Effective feed treatment ensuring alkaline conditions controls this problem

Sodium Chloride 32230 to 25600 ppm

• Soluble <225000ppm
• forms a soft encrustation
• Free irons promote galvanic action

Other deposits-

Amorphous Silicon dioxide (SiO₂) - trace
• at high tempos and pressures (>40bar) silica can distill from the bioler as Silicic acid and can sublime and pass over into the steam system as a gas. Here it glazes surfaces with a smooth layer, which due to thermal expansion crack and roughen the surface. Troublesome on HP blading. Can be removed only by washing with Hydroflouric acid.

  Magnessium Silicate 3MgO.2SiO₂.2H₂O (Serpentine) is formed in water with proper treatment control

Nucleate Boiling

Scale built up as a series of rings forming multi layers of different combinations. Much increased by corrosion products or prescience of oil, even in very small quantities.
Oil also increases scale insulatory properties.

Departure form nucleate boiling (DNB) Under normal conditions steam bubbles are formed in discrete parts. Boiler water solids develop near the surface . However on departure of the bubble rinsing water flows in and redissolves the soluble solids