Boiler corrosion high-pressure

For some applications, notably feed-water treatment for high-pressure boilers, removal of oxygen is essential. For most industrial purposes, however, de-aeration is not applicable, since the water used is in continuous contact with air, from which it would rapidly take up more oxygen. Attention must therefore be given to creating conditions under which oxygen will stifle rather than stimulate corrosion. 

In the absence of sufficient hydrogen, the solubility of magnetite is markedly temperature dependent, which provides an explanation for some of the problems in high-temperature circuits. Most studies in boilers relate to high temperatures. Thus the work of Bloom " and of Potter and Mann has reproduced the types of corrosion found in high-pressure boilers. The relationship between corrosion rates and iron solubility and pH are given in Fig. 17.2. Note that the pH range about the neutral point (relative to 300 C... 

The three principal concentration mechanisms postulated as being responsible for on-load corrosion processes by Mann are dry-out, concentration in crevices, and concentration in porous deposits. (Clean boiler tube surfaces on which high-pressure water is boiled under forced convection do not develop concentration factors of more than about two.)... 

Deposit formation, due either to crud (suspended matter-mostly metallic oxides) transported into the boiler or to the products of corrosion in situ, is undesirable as, in many parts of the system, quite apart from the risk of overheating which they present, deposits are able to participate in a mechanism for concentrating solutes to unacceptably high corrosive levels, and are particularly dangerous in high-pressure plant (see Section 4.5). 

In high-pressure boilers, there are three types of on-load corrosion acidic chloride, neutral chloride/dissolved oxygen, and caustic attack. The first and second (once it becomes established) are brittle and thick-walled and are accompanied by hydrogen damage which can lead to failure within a few hundred hours. Caustic attack tends to produce a gouged appearance of the metal due to extensive wastage. The morphology is fairly characteristic of the failure type. Acidic chloride forms hard, laminated oxide, whilst, with caustic attack, the oxide is often soft, and, as it is easily removed, may be absent. 

As for the turbines, no steam-purifying equipment of the type used on drum boilers is feasible, so that the steam from super-critical boilers tends to be of inferior quality. Deposits have been reported of cuprous oxide on the extra high-pressure turbines and of cupric oxide on some high-pressure turbines of sub-critical plant. These deposits may lead to a loss of efficiency and to some risk of corrosion. At intervals, slugs of solute are carried over in the steam, which is therefore of fluctuating quality. This is countered by periodic water-washing of the boilers. 

Boiler scales and corrosion product deposits also may form from the thermal breakdown of treatment chemicals. Breakdown products of chemicals such as sulfite, amines, hydrazine, and chelants often are steam-volatile, and subsequent reactions may produce corrosion debris that forms deposits and causes blockages. This problem typically occurs when chemical treatments selected for use are applied under unsuitable operating conditions (as when the boilers are highly rated or raise steam at particularly high operating pressure or temperature). 

Although this chapter looks primarily at relatively common forms of corrosion, we also explore a number of often complex and esoteric boiler section corrosion mechanisms. Many of these types of corrosion will only ever seriously develop in larger high-temperature or high-pressure boilers, especially those requiring knife-edge control and operating under difficult thermal stress conditions. 

Corrosion processes in the boiler section generate hydrogen—this hydrogen release is regularly measured in very high pressure boilers as it can be indicative of the rate of corrosion taking place... 

The introduction of modem, very high pressure or temperature boilers has led to water chemistry control problems and cause-and-effect corrosion problems due to phosphate hideout. Under... 

Erythorbates are safe products and there are no harmful breakdown products, although when early formulations utilized ammonia as a PH buffer (and neutralizer for part of the carbon dioxide), copper corrosion problems resulted. However, erythorbates are not steam-volatile,and consequently there is no post-boiler oxygen scavenging potential available. Thus, in the event of complete breakdown of the product at high pressure, oxygen-induced, ammonia corrosion of copper may continue unchecked. 

It is important that the tube surfaces be kept clean to avoid the initiation of corrosion. Regular waterside inspections and, if necessary, chemical cleaning of high-pressure equipment is recommended. The level of chloride that may be tolerated in such boilers during steady operation depends on the type of treatment employed. Where all-volatile alkaline treatments (AVT) are used, then the chloride levels should be lower than where nonvolatile alkalis (NVAT), such as sodium hydroxide and sodium phosphate, are used. The value may vary, depending on whether the boiler is coal-fired or oil-fired. 

Romero, Matilde F. Pdrez, Orlando Navarro, Alfredo. Phosphate Attack as Caustic Corrosion in High-Pressure Boiler Tubes. Material Performance . NACE International, USA, March 1999. 

This pump is the same in principle as the piston type but differs in that the gland is at one end of the cylinder making its replacement easier than with the standard piston type. The sealing of piston and ram pumps has been much improved but, because of the nature of the fluids frequently used, care in selecting and maintaining the seal is very important. The piston or ram pump may be used for injections of small quantities of inhibitors to polymerisation units or of corrosion inhibitors to high pressure systems, and also for boiler feed water applications. 

The tail-gas exiting the expander at 290°C is used to preheat the high-pressure (HP) boiler feed water and also the low-pressure (LP) deionized water (prior to deaeration for high-pressure boiler-feed water use). The important design criteria in this exchanger is that the tail gas should not leave the exchanger at less than 65°C, otherwise problems with condensation/corrosion in the stack will result. The dew point of the tail-gas is approximately 17°C. 

Exceptions can exist since the corrosion in a wet solution of the interior boiler drum (steel) with dilute caustic soda at high temperature and high pressure and the reaction of high temperature water with aluminum and zirconium have been found to be best interpreted in terms of a dry corrosion mechanism.1... 

Chemical in South Korea in 1987.38 39 The process is based on 310 stainless steel, which resists corrosion in 98.5 percent H2S04 at temperatures up to 220°C. The intermediate absorber at Namhae takes 194°C gas from the converter third stage economizer and absorbs the S03 in 199°C, 99 percent acid. Recirculated acid from the absorber is cooled from 220°C in a 10-bar HRS boiler. The added energy recovery for this process is reported to increase the total recovery to 90 percent. Monsanto s proposed Monarch process combines HRS technology with the wet catalytic converter process (Lurgi) to increase heat recovery and shift it to high-pressure steam production for electric power generation.36... 

Excessive us of high-pressure steam soot blowers is a common source of tube erosion-corrosion. Other boiler cleaning methods less threating to boiler tubes are available such as mechanical rapping, shot cleaning, and compressed air soot blowing. 

Corrosion can be a serious cause of wear in boilers if suitable waters are not used. All feed water should be just alkaline to phenolphthalein, and sufficient caustic soda to create this degree of alkalinity should be added if necessary, using a device to introduce it continuously. Dissolved oxygen in the presence of carbon dioxide is a common cause of corrosion, especially in modem high-pressure boilers. The carbon dioxide reacts with the iron, forming ferrous carbonate which, in turn, tends to hydrolyse to ferrous hydroxide ... 

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