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Purpose of boiler water treatment: To prevent the following 3 main problems:

1) Scale
Build-up of scale creates an insulation layer on the tube, reducing the rate of heat transfer thus causing temperature of metal to rise which reduces its tensile strength. This can eventually lead to tube failures as a result of overheating. The effects of scale can be aggravated by other factors such as “flame impingement”.

2) Corrosion:
Oxygen pitting can lead to rapid tube failure, by wasting metal thus reducing overall strength.

3) Carry-over
Impurities (soluble salts and suspended solids) in feed water will concentrate in the boiler and eventually get carried over with the steam. For ships running on steam turbines, this can result in deposits in superheater and turbine blades with serious consequences.

 

Boiler corrosion

 

Corrosion is caused principally by complex oxide-slag with low melting points. High temperature corrosion can proceed only if the corroding deposit is in the liquid phase and the liquid is in direct contact with the metal. Deposits also promote the transport of oxygen to the metal surface. 

Corrosion in the boiler proper generally occurs when the boiler water alkalinity is low or when the metal is exposed to oxygen bearing water either during operation or idle periods. High temperatures and stresses in the boiler metal tend to accelerate the corrosive mechanisms. In the steam and condensate system corrosion is generally the result of contamination with carbon dioxide and oxygen. Specific contaminants such as ammonia or sulphur bearing gases may increase attack on copper alloys in the system.

Corrosion is caused by the combination of oxide layer fluxing and continuous oxidation by transported oxygen. 

 

There are two principle forms of corrosion.

 

Direct Chemical Attack: This can occur when metal is at high temperature comes into contact with air or other gases, resulting in oxidation or sulphidation of the metal.

 

Electro-Chemical Action: This covers most of the other forms of corrosive attack, usually taking place in the presence of moisture. One common form is galvanic corrosion. The others commonly occurring in boilers is namely hydrogen evolution, which causes general wastage of the boiler; and oxygen absorption, a form of attack which leads to pitting of the metal surface.

 

Corrosion control techniques vary according to the type of corrosion encountered. Major methods include maintenance of the proper pH, control of oxygen, control of deposits, and reduction of stresses trough design and operational practices.
Deaeration and recently the use of membrane contractors are the best and most diffused ways to avoid corrosion removing the dissolved gasses (mainly O2 and CO2). 

 

Cracking in boiler metal may occur by two different mechanisms.

First mechanism, cyclic stresses are created by rapid heating and cooling and are concentrated at points where corrosion has roughened or pitted the metal surface. This is usually associated with improper corrosion prevention. 

Second type of corrosion fatigue cracking occurs in boilers with properly treated water. In these cases corrosion fatigue is probably a misnomer. These cracks often originate where a dense protective oxide film covers the metal surfaces and cracking occurs from the action of applied cyclic stresses. Corrosion fatigue cracks are usually thick, blunt and cross the metal grains. They usually start at internal tube surfaces and are most often circumferential on the tube.

Reference: http://www.lenntech.com/applications/process/boiler/corrosion.htm#ixzz1dwfWzSJ9

 

Types of boiler tube corrosion

• Galvanic corrosion
  • This is when two dissimilar metals are placed in an electrode. The more noble of the two metals form the cathode to the base metal which, forms the anode, waste away.
• Caustic corrosion
  • Concentration of caustic (NaOH) can occur as a result of steam blanketing (which allow salts to concentrate on boiler metal surface) or by localized boiling beneath porous deposits on tube surface. Caustic corrosion occurs when caustic is concentrated and dissolves the protective magnetite (Fe3O4) layer, causing a loss of base metal and eventual failure. 
  • May take place at high pressures if there is high concentration of NaOH. 
  • The following conditions appear to be necessary for this type of cracking to occur :

1. The metal must be stressed,
2. The boiler-water must contain caustic,
3. At least a trace of silica must be present in the boiler-water, and
4. Some mechanisms, such as a slight leak, must be present to allow the boiler water to concentrate on the stressed metal.

• • Stem blanketing is a condition that occurs when a steam layer forms between the boiler water and the tube wall. Under this condition, insufficient water reaches the tube surface for efficient heat transfer. The water that reaches the overheated boiler wall is rapidly vaporized, leaving behind a concentrated caustic solution, which is corrosive.
  • Boiler feed water systems using demineralized or evaporated make up or pure condensate may be protected from caustic attack through coordinated phosphate/pH control. Phosphate buffers the boiler water, reducing the chance of large pH changes due to the development of high caustic concentrations. Excess caustic combines with disodium phosphate and forms trisodium phosphate, by the following reaction:

Na2HPO4 + NaOH è Na3PO4 + H2O

• • This result in the prevention of caustic build up beneath deposits or within a crevice where leakage is occurring.

 

• Acidic corrosion
  • Low make up feed water pH can cause serious acid attack on metal surfaces in the preboiler and boiler system. Feed water can also become acidic from contamination of the system (process contamination of condensate or cooling water contamination from condensers).
    Acidic corrosion can also be caused by chemical cleaning operations (overheating of the cleaning solution, excessive exposure of metal to cleaning agent, high cleaning agent concentration).
  • In the boiler and feed water system, acidic attack can take the form of general thinning, or it can be localized at areas of high stress.

 

• Oxygen Attack
  • Without proper mechanical and chemical deaeration, oxygen in the feed water enters the boiler. Much is flashed off with the steam; the remainder can attack boiler metal. Oxygen in water produces pitting that is very severe because of its localized nature. Water containing ammonia, particularly in the presence of oxygen, readily attacks copper and copper bearing alloys. The resulting corrosion leads to deposits on boiler heat transfer surfaces and reduces efficiency and reliability.
  • Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems: iron oxide generated by the corrosion can produce iron deposits in the boiler. Oxygen corrosion may be highly localized or may cover an extensive area. Oxygen attack is an electrochemical process that can be described by the following reactions: 

Anode: Fe è Fe2+ + 2e–

Cathode: ½ O2 + H2O + 2e– è 2 OH–

Overall: Fe + ½ O2 + H2O è Fe(OH)2

• • In this reaction a temperature rise provides enough additional energy to accelerate reactions at the metal surfaces, resulting in a rapid and severe corrosion. 
  • The acceptable dissolved oxygen level for any system depends on may factors, such as feed water temperature, pH, flow rate, dissolved solids content, and the metallurgy and physical condition of the system. In general, the limit value of oxygen in make up water can be stared 0.10 mg/kg
  • For a complete protection from oxygen corrosion, a chemical scavenger is required following mechanical deaeration. Membrane contractors are also a possibility. 

 

• Hydrogen embrittlement
  • Hydrogen embrittlement of mild steel boiler tubing occurs in high-pressure boilers (above 10 bars), when atomic hydrogen forms at the boiler tube surface as a result of corrosion.
  • Hydrogen permeates the tube metal, where it can react with iron carbides to form methane gas, or with other hydrogen atoms to form hydrogen gas. These gases evolve predominantly along grain boundaries of the metal. The result increase in pressure leads to metal failure. Coordinated phosphate/pH control can be used to minimize the decrease in boiler water pH that result from condenser leakage. Maintenance of clean surfaces and the use of proper procedures for acid cleaning also reduce the potential for hydrogen attack.

 

• Carbon dioxide attack
  • Carbon dioxide exists in aqueous solutions as free carbon dioxide and the combine forms of carbonate and bicarbonate ions. Corrosion is the principal effect of dissolved carbon dioxide. The gas will dissolve in water, producing corrosive carbonic acid:
  • H2O + CO2 çè H2CO3 çè H+ + HCO3-
  • The low pH resulting from this reaction also enhances the corrosive effect of oxygen. 
  • In boiler systems, corrosion resulting from carbon dioxide is most often encountered in the condensate system. Because feed water deaeration normally removes carbon dioxide from the boiler feed water, the presence of the gas in condensate is typically due to carbonate and bicarbonate decomposition under boiler conditions. For an approximation is estimated that feed water with a total alkalinity of 100 mg/l as calcium carbonate could be expected to generate a carbon dioxide level of 79 mg/l in the steam (alkalinity multiplied by a factor 0.79). Such a high carbon dioxide level would create a very corrosive condensate. 
  • Carbon dioxide corrosion is frequently encountered in condensate systems and less commonly in water distribution systems. 

 

Boiler chemical treatment and what chemical to be used and what value should it be.

 

Boiler coagulant

Boiler Coagulant prevents the formation of deposits on boiler internal surfaces. Sludge is kept dispersed in small particles and conditioned to be removed by normal blowdown. In this way tube overheating due to deposits is avoided.

Boiler Coagulant can also be used where minor oil contamination has been experienced, the oil being required to be coagulated for removal by blowdown. However, it must be noted that if oil contamination is continuous and excessive, off-line cleaning will be required. The source of oil contamination must be stopped immediately.

Dosage and control

The initial dosage is 20 ml of treatment daily for every ton of boiler capacity. Daily bottom blowdown is required when using Boiler Coagulant.

Combitreat

The main attributes of Combitreat fall into the following categories:

 

Control of alkalinity:
The correct level of alkalinity ensures that optimum conditions exist for:

• Precipitation of hardness salts in conjunction with phosphate.
• Neutralisation of acidic conditions.
• Avoidance of caustic corrosion.
• Control of magnesium and calcium salts.
• Control of hardness: Combitreat provides a phosphate reserve to effectively react with and precipitate the
  hardness salts introduced with the feedwater.
• Conditioning of sludge: Boiler sludge can only be removed by blowdown if it is free flowing, Combitreat will ensure this by preventing the sludge from adhering to metal surfaces. The resulting sludge is composed of small particles flowing towards the bottom of the boiler.

 

P-Alkalinity*	0	50	100	150	200	250	300	350
Dosage Kg/ton	0.4	0.3	0.2	0.1	0	0	0	Blowdown

 * as ppm CaCO3

Our recommended control limits are:
p-Alkalinity: 100-300 ppm CaCO3, Chlorides: 200 ppm Cl max.  Condensate pH 8.3-9.0

These are recommended values based on experience, and are in no way intended to replace the boiler manufacturer’s specifications, or company regulations.

Excessive Chlorides are removed by blowdown.

 

Oxygen control

Oxygen Control is a catalysed liquid hydrazine hydrate based oxygen scavenger for boiler and steam line corrosion protection. As an additional benefit it will assist to neutralise dissolved carbon dioxide. It provides the required conditions for the establishment of a passivating layer (magnetite) in the boiler and condensate system.

 

The treatment combines with dissolved oxygen to form water and inert nitrogen gas, thus effectively removing O2 from the water. No solid materials are produced, so there is no contribution to the increase in total dissolved solids -a critical factor in high pressure boilers. The removal of dissolved
oxygen is vital for preventing oxygen pitting and corrosion in boilers. Oxygen Control reacts with ferrous and non-ferrous oxides to prevent general corrosion.Ferric oxide (red rust, Fe2O3) is converted to magnetite (black iron oxide, Fe3O4), which is a tough corrosion resistant oxide which seals the metal surface. The term for this is ‘passivating’ the surfaces, so that they are protected from further corrosion.

 

Dosage and control

The objective is to maintain a hydrazine hydrate residual between 0.05-0.2 ppm depending on operating pressure and boiler design.

Actual consumption is determined under operating conditions. A normal dosage is approximately 1 ltr. per day, depending of system layout.

 

	Hydrazine Hydrate Test Result In PPM
Pressure	0-0.05	0.05-0.10	0.10-0-15	0.15-0.20	0.2 +	Range*
0-40 Bar 0-588 PSI	Increase dose 25%	Increase dose 25%	Satisfactory Maintain dose	Satisfactory Maintain dose	Decrease dose 25%	0.1-0.2

 

• This topic was modified 4 years, 10 months ago by Admin.
• This topic was modified 4 years, 5 months ago by icedcappucino.

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