Steam condenser corrosion

Ronchetti, C., Buzzanca, G. and Diacci, E., A New System for a Continuous Monitoring of Steam Condenser Corrosion , CISE Report, CISE-NT 81.094 (1981)... 

Condensate Polishing. Ion exchange can be used to purify or poHsh returned condensate, removing corrosion products that could cause harmful deposits in boilers. Typically, the contaminants in the condensate system are particulate iron and copper. Low levels of other contaminants may enter the system through condenser and pump seal leaks or carryover of boiler water into the steam. Condensate poHshers filter out the particulates and remove soluble contaminants by ion exchange. 

Leaky valves are also a cause of erosion. Most turbine erosion-corrosion problems come from damage that takes place when the unit is not running. A shght steam leak into the turbine will let the steam condense inside the turbine, and salt from the boiler water will settle on the inside surfaces and cause pitting, even of the stainless blading. There must be two valves with a drain between them, i.e., a block valve on the header and an open drain in the line before it reaches the closed trip-throttle valve. 

This cooling water is usually arranged in a closed loop with the water being pumped through secondary coolers or over cooling towers and then returned to the jackets for reuse. Water quality must be good, with steam condensate being preferred, properly treated to prevent corrosion, etc. 

Most refiners employ continuous water wash as the principal mei of controlling corrosion and hydrogen blistering. The best sourc water is either steam condensate or well-stripped water from a water stripper. A number of refiners use ammonium polysulfat neutralize hydrogen cyanide and to control hydrogen stress crack... 

Alloys of high nickel content also have improved chloride stress-corrosion cracking resistance and Incolloy 825 has replaced type 321 stainless steel for steam bellows on some plants. Occasionally cracking of the latter was experienced due to chloride-contaminated steam condensing in the convolutions on shut-down and being re-evaporated at start-up. 

Pressure vessels and appurtenances should be constructed of stainless steel or other corrosion-resistant materials. Ideally, these steam generators should receive hot demineralized FW to minimize chemical treatment requirements. Alternatively, where a main boiler plant is installed, 100% steam condensate provides a good source of FW. In practice, it is very difficult to accurately control the correct amount of chemical feed. Chemicals are typically restricted to potable grade, deposit control agents such as polyacrylates, and other materials listed under the Code of Federal Regulations, CFR 21 173.310, or National Sanitary Foundation (NSF International) approval system. These boilers may be electrically heated or gas-fired. 

Where steam is used for sterilization, hospital duty, food and drug manufacturing processes, or steam humidification purposes, there generally is a reluctance (or mandated prohibition) to employ amines as a treatment for steam-condensate line corrosion protection. Clean-steam... 

Also, no specific amine-based inhibitor is generally provided to protect LP steam condensate pipework against the ravages of carbonic acid (H2C03)-induced condensate line corrosion. 

Where problems develop, there is always a cause-and-effect process. In this case, as oxygen infiltrates the CR system, enhanced condensate line corrosion results (i.e., corrosion over and above the level that may be caused by the carbonic acid formed during steam condensation). This enhanced corrosion, in turn, creates the potential for further downstream corrosion debris pickup by the returning condensate and transporting this material back to the FW system. 

The most serious pre-boiler contaminant is often copper, resulting from the ammoniacal corrosion of steam-condensate system components and its subsequent transport through the boiler plant. 

Ferrous bicarbonate may be transported to a point where little or no amine is available, which then provides the source for various secondary corrosion reactions. Corrosion mechanisms and deposition formation often take place at points in the system well downstream of the original points of steam condensation and initiation of corrosion. 

These corrosion scenarios show that simply sampling from one or two points in a large, convoluting steam-condensate system is unlikely to detect the real extent of any corrosion problems. Effective control starts with effective sampling, monitoring, and confidence in the validity of the test results. 

Although oxygen and carbon dioxide are the most important corrosive gases found in steam-condensate systems, small amounts of other contaminant gases are usually present. These may originate in various ways, for example ... 

Steam/condensate line corrosion control. Control over steam and condensate line corrosion requires the control of oxygen, carbon dioxide (carbonic acid), and ammonia. 

The corrosion risks to the steam-condensate system resulting from the decomposition of carbonates and the subsequent formation of carbonic acid. 

The FW oxygen scavengers mentioned earlier are volatile and can therefore provide additional protection against post-boiler section corrosion induced or enhanced by oxygen in-leakage. These particular scavengers also break down under pressure to produce some level of volatile ammonia, which can neutralize any carbon dioxide found in the steam-condensate system. 

Whether ammonia arises from its use as a FW pH level adjuster or from adventitious provision as a result of DO scavenger breakdown, it should be recognized that any excess ammonia will clearly end up in the steam-condensate system. Although the benefit of carbon dioxide neutralization may be legitimately claimed, unfortunately, excess ammonia also may permit the corrosion of copper and its alloys, especially if some oxygen persists. 

In lower pressure commercial and industrial boilers, handling and safety are issues of more concern than relative chemical effectiveness, and the fact is that because of such problems, ammonia simply is not used in these smaller plants. Caustic generally provides the necessary FW alkalinity and amines are then employed to overcome the problems of carbonic acid corrosion in the steam-condensate system. 

Distribution Ratio (DR) The DR relates the concentration of an amine present in the steam phase to that concentration in the condensate phase (vapor-liquid distribution ratio). Consequently, it identifies in which condensate production region of a steam-condensate system any particular amine will concentrate and thus provide protection against corrosion. It also helps to indicates the portion of amine loss due to vaporization in a condenser or venting of a deaerator. The expression for DR is shown here ... 

Consequently, it is important to devise an amine blend that will be present in the condensate, wherever in the system it forms. Thus, it is common practice to blend two, three, or even four amines to achieve a balance of DR values and to improve corrosion protection throughout the steam-condensate system. For most general types of commercial or industrial application, a minimum blend of two amines is necessary, while three may be particularly beneficial. 

The primary reason for employing vapor-phase or neutralizing amines in steam-condensate systems is to reduce the level of corrosion of both ferrous and nonferrous metals. A further beneficial consequence of this function is the reduction of metal transported back to the FW system. 

A volatile, amine-based chemical treatment usually added to the boiler FW, designed to neutralize the corrosive effects of carbonic acid in steam/condensate and raise condensate pH. 

Angus. TB 41 AMP-95 Corrosion Inhibitor in Steam-Condensate Lines. Technical literature. Angus Chemical Company, USA, 1998. 

Morpholine is a synthetic organic liquid used mainly as an intermediate in the production of rubber chemicals and optical brighteners, as a corrosion inhibitor in steam condensate systems, as an ingredient in waxes and polishes and as a component of protective coatings on fresh fruits and vegetables. Occupational exposure may occur during the production of morpholine and in its various uses, but data on exposure levels are sparse. It has been detected in samples of foodstuffs and beverages (lARC, 1989). 

Oils, greases and waxes are used for the temporary inhibition of corrrosion. They act as a barrier, preventing the condensation of water on the metal surface. Inhibitors such as piperidine (59) and morpholine (60) are often incorporated to give added protection. Volatile amines are also used in boilers they function by reducing the acidity of the steam condensate, thereby making it less corrosive. 

Venting the channel head through the balance line shown in Fig. 8.6 will prevent an excessive accumulation of C02. This is done by continuous venting from the top of the condensate drum. For every 10,000 lb/h of steam flow, vent off 50 lb/h of vapor through a restriction orifice, placed in the condensate drum vent. This is usually cheaper than controlling reboiler steam-side corrosion, with neutralizing chemicals. 

The hardness deposits coat the inside of the boiler s tubes, interfere with heat transfer, and overheat the tubes. The carbon dioxide, which is also generated from the dissolved solids, creates more serious corrosion problems in downstream heat exchangers. When the steam condenses, the carbon dioxide may remain trapped in the reboiler or preheater as a noncondensable gas. Actually, there is no such thing as a noncondensable gas. Even C02 is somewhat soluble in water. As the C02 dissolves in the condensed steam, it forms carbonic acid, a relatively weak acid (pH typically between 5 and 6). Strong acids will have pH values of 1 to 2. Pure water has a pH of seven. Carbonic acid is particularly corrosive to carbon steel heat-exchanger tubes. 

In most cases, condensate does not require treatment prior to reuse. Makeup water is added directly to the condensate to form boiler feedwater. In some cases, however, especially where steam is used in industrial processes, the steam condensate is contaminated by corrosion products or by the inleakage of cooling water or substances used in the process. Hence steps must be taken to reduce corrosion or to remove the undesirable substances before the condensate is recycled to the boiler as feedwater. 

The material of construction in HTS reactors is usually low-alloy steel containing 1% Cr and 0.5% Mo. If CO2 and HjS are in the steam condensate that is fed to the shift conversion reactors, they should be made of 304 stainless steel to avoid corrosion problems88. 

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