Steam systems treatment chemicals

Water Treatment. Water and steam chemistry must be rigorously controlled to prevent deposition of impurities and corrosion of the steam cycle. Deposition on boiler tubing walls reduces heat transfer and can lead to overheating, creep, and eventual failure. Additionally, corrosion can develop under the deposits and lead to failure. If steam is used for chemical processes or as a heat-transfer medium for food and pharmaceutical preparation there are limitations on the additives that may be used. Steam purity requirements set the allowable impurity concentrations for the rest of most cycles. Once contaminants enter the steam, there is no practical way to remove them. Thus all purification must be carried out in the boiler or preboiler part of the cycle. The principal exception is in the case of nuclear steam generators, which require very pure water. These tend to provide steam that is considerably lower in most impurities than the turbine requires. A variety of water treatments are summarized in Table 5. Although the subtieties of water treatment in steam systems are beyond the scope of this article, uses of various additives maybe summarized as follows ... 

Makeup. Makeup treatment depends extensively on the source water. Some steam systems use municipal water as a source. These systems may require dechlorination followed by reverse osmosis (qv) and ion exchange. Other systems use weUwater. In hard water areas, these systems include softening before further purification. Surface waters may require removal of suspended soHds by sedimentation (qv), coagulation, flocculation, and filtration. Calcium may be reduced by precipitation softening or lime softening. Organic contaminants can be removed by absorption on activated carbon. Details of makeup water treatment may be found in many handbooks (22—24) as well as in technical Hterature from water treatment chemical suppHers. 

Steam. The steam system serves as the integrating energy system in most chemical process plants. Steam holds this unique position because it is an exceUent heat-transfer medium over a wide range of temperatures. Water gives high heat-transfer coefficients whether in Hquid phase, boiling, or in condensation. In addition, water is safe, nonpolluting, and if proper water treatment is maintained, noncorrosive to carbon steel. 

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. 

In addition, attention must be paid to the BW internal treatment program. Such programs involve the dosing of small amounts of polishing chemical treatments to specified points in the water-steam cycle system, often continuously and proportional to water flow or steam demand, to provide a desired inhibitory or protective effect. 

When steam in the cycle is lost or used in a process, the reduced volume of returning condensate is compensated for by introducing some level of MU water. The loss of water or steam from a steam system cycle may vary from 1 to 100%. The supply of MU (and to a lesser degree the addition of chemical treatments) provides a source of dissolved solid contaminants that can concentrate in the boiler until some predefined limit is reached. At this point, BD is required, the loss of which is also compensated for by the addition of further MU water. 

When operators believe that their HW systems do not lose water through pump mechanical seals or other places. They also commonly state that their LP steam heating systems return 100% condensate, so that no additional corrosion protection precautions need be taken above or beyond the initial fill of chemical treatment and an occasional top-up dose. 

When chemical treatment for both HW heating and LP steam heating systems is generally some combination of a nitrite/borate/silicate/ molybdate formulation, usually with TTA/polymer or phosphate but having no specific oxygen-scavenging capability. 

In practice, however, it is often the case that for small LP steam heating boiler systems with a very high percentage of CR, and in those regions of countries where soft water is supplied, no water softener is provided or deemed necessary. Instead there is merely a reliance on internal chemical treatments (or sometimes magnetic devices or other types of gadget ). A good rule of thumb is ... 

Ammonia, hydrogen sulfide, and sulfur dioxide may result from the decomposition of chemical treatments (although in large boiler plants, ammonia is often deliberately added to raise the boiler, steam, or condensate system pH). 

The Dual Auger system is an in situ treatment for soils contaminated with volatile organic compounds (VOCs), chlorinated volatile organic compounds (CVOCs), and inorganics. This process uses specialized equipment to mix soil and inject reagents. The system can add nutrients to promote bioremediation, inject steam to volatilize contaminants, install zero-valent iron to promote chemical treatment, or add a pozzolanic slurry to stabilize the contaminants. The injection and mixing process effectively breaks down fluid and soil strata barriers. Mixing also eliminates pockets of contamination that would otherwise remain untreated. 

A water cooled blanket (and shield), ca. 1.5 m thick, made of vanadium alloy (e.g. V-lSCr-STi) and containing solid or liquid lithium. As much heat is produced, the lithium is used both for cooling and breeding and is pumped to heat exchangers for steam production followed by chemical treatment to recover the tritium. Cooling can also be achieved by He-gas, in which case the Li-system must be treated separately. 

The sulfuric acid used to dry the chlorine is also sometimes a source of ammonia compounds. Finally, the plant utilities are likely sources. Plant water that has not been thoroughly purified has many opportunities to enter the process. Examples of these opportunities are the use of impure water to prepare treatment chemical solutions, flushing water on rotating machinery seals, water recycle from waste-minimization systems, and direct-contact cooling water. Steam and its condensate are also potential sources. The culprits here usually are the amines used as corrosion inhibitors in the steam system (Section 12.3.2). 

Strict control of purity relies on the avoidance of corrosion in the steam-distribution system and on chemical treatment of the feed water or steam condensate [15]. Treatment chemicals may include ... 

Cold water systems usually cannot be steamed. Instead chemical disinfection or disinfection using ozmie is customary. An important benefit of ozone treatment compared to other chemical methods is that it can be executed automatically. 

There are a great number of lube and seal oil systems in these units. The fluid cokers may have up to three cyclones or venturis to separate the coke fines from the overhead products. Because the products leaving these units will have to be cooled, waterside problems will occur at these temperatures (formation of iron oxides and hardness deposits will require chemical treatment). The furnace itself will be steam/air decoked, but the fuel lines will have to be periodically cleaned to reduce burner fouling. 

The steam drum is the entry point for boiler feedwater and internal chemical treatment. It is also the withdrawal point for continuous blowdown, purging from the solute of suspended solids to maintain the minimum level of dissolved and suspended solids. If blowdown is not done deposits of solids would form continuously on the boiler tubes and/or contaminate the system. The solid contents of steam should, in principle, be kept to zero. 

Steam systems. Copper and copper alloys resist attack by pure steam, but if carbon dioxide, oxygen, or ammonia is present, condensates can be quite corrosive to copper alloys. Modem power utility boiler feedwater treatments commonly include the addition of organic amines to inhibit the corrosion of iron components of the system by scavenging oxygen and increasing the pH of the feedwater. These chemicals tend to release ammonia, which can be corrosive to some copper alloys. 

Secondary systems. Boiler water. Chemical treatment of the water in the steam side of a fluid fuel reactor heat exchanger is a major... 

Sodium alumiaate is used ia the treatment of iadustrial and municipal water suppHes and the use of sodium alumiaate is approved ia the clarification of drinking water. The FDA approves the use of sodium alumiaate ia steam generation systems where the steam contacts food. One early use of sodium alumiaate was ia lime softening processes, where it iacreases the precipitation of ions contributing to hardness and improves suspended soHds removal from the treated water (17). Sodium alumiaate reacts with siHca to leave very low residual concentrations of siHca ia hot process water softeners. Sodium alumiaate is often used with other chemicals such as alum, ferric salts, clays, and polyelectrolytes, as a coagulant aid (18,19). 

In lower pressure boilers a variety of additional treatments may be appropriate, particularly if the steam is used in chemical process or other nonturbine appHcation. Chelants and sludge conditioners are employed to condition scale and enable the use of less pure feedwater. When the dmm pressure is less than 7 MPa (1015 psia), sodium sulfite may be added direcdy to the boiler water as an oxygen scavenger. It has minimal effect on the oxygen concentration in the system before the boiler. 

Other types of regenerators designed for specific adsorption systems may use solvents and chemicals to remove susceptible adsorbates (51), steam or heated inert gas to recover volatile organic solvents (52), and biological systems in which organics adsorbed on the activated carbon during water treatment are continuously degraded (53). 

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