Boilers pretreatment

Pre-boiler pretreatment is concerned with providing higher quality MU and FW—that is, water with most, if not almost all, natural impurities removed. There are perhaps as many interpretations of what constitutes high-quality water as there are boiler designs requiring it, and consequently there are also many specifications available, each with minor variations on a similar theme. Trying to decide precisely what is required, above and beyond a basic good quality, as provided by the use of pretreatment equipment such as filters, softeners, and so forth is difficult. 

The oil and solids collected from the launders are then passed along to the oil-handling system the recovered produced water, providing it meets the necessary requirements, is either disposed of or reutilized in the oil recovery process. If the water is to be reinjected, cleaning by the IGF unit prevents formation plugging and reduced pump efficiency. If the water is to be used for steam generation, the IGF is used before the traditional boiler pretreatment equipment. 

DEMINERALIZER - A process to remove dissolved matter from boiler pretreated water by contacting the water with ion-exchange resins. 

Boiler feed water pretreatment systems have advanced to such an extent that it is now possible to provide boilers with ultrapure water. However, this degree of purification requires the use of elaborate pretreatment systems. The capital expenditures for such pretreatment equipment trains can be considerable and are often not justified when balanced against the capabiUty of internal treatment. 

Because of their compact size, packaged vertical boilers can be custom-designed as a complete boiler plant system and simply shipped to the customer on a steel skid or platform. This type of system may comprise a dual boiler arrangement, with a pretreatment unit (water-softening and chemical-feed system), boiler blowdown and condensate return facilities, and also possibly a dual stack containing an economizer. This type of packaged system may reach 85% GCV efficiency. 

External to the boiler itself are various pre-boiler and post-boiler water subsystems. These include the MU water pretreatment system, FW supply system, and steam-condensate recovery system, again of varying complexity depending on the design and size of the boiler. 

Typically, for any given pressure, industrial packaged boilers operate at higher heat-flux rates than field-erected boilers, This requires that the package boiler FW quality should be substantially better (i.e., lower overall TDS and lower levels of silica and sodium). Appropriate MU water pretreatment may, for example, necessitate the use of twin bed and mixed bed demineralization ion exchange, or RO and mixed bed (in addition to mechanical deaeration and other processes). 

However, the source of MU water for higher pressure steam-raising plants is important because it is generally the case that no matter the origin of the water, it will not be entirely suitable for the purpose and some form of water conditioning will be required. This conditioning requirement necessitates the provision of a (pre-boiler section) water pretreatment plant system (external treatment/external conditioning) to render the water fit for purpose. 

Most usually the pretreatment plant is a capital item fixed asset facility, but the trend is increasingly noticeable around the world today for the supply of properly conditioned MU water to be outsourced from a contractor, who either leases a mobile plant to the facility or generates a profit from the direct sale of water to the boiler plant. 

The FW supply system is part of the pre-boiler section, as is the pretreatment plant. It includes all necessary distribution pipes and valves, the boiler FW regulator, FW pumps, FW tanks, and various forms of FW heaters. 

Inspecting all associated pretreatment equipment, pumps, tanks, and valves also, all internal water surfaces whenever a boiler, condenser, FW heater, or other item of equipment is opened... 

Most HW and LP steam boilers (i.e., those that operate below 15 psig) and many smaller high-pressure (HP) steam-raising boilers employ, at best, relatively simple methods of FW pretreatment and a basic internal chemical treatment program. Many times no pretreatment is provided. 

The more extensive use of pretreatment equipment generally is a commendable approach, although it is difficult to externally overtreaf boiler MU and FW, it also is pointless to spend limited capital resources installing sophisticated equipment to provide high-purity water and then to subsequently add dissolved solids back into the FW or BW with an inappropriate, internal chemical program. Both external and internal water treatment programs must be compatible and mutually beneficial ... 

When considering the need for capital pretreatment equipment, sufficient care should be taken not to propose any particularly sophisticated external or internal water treatment regimen that of itself creates restrictive operational BW control limitations—where boiler design, steam usage, and common sense dictate no necessity for such limitations to be imposed. The overall water treatment program should not be overdesigned but should properly match the needs of the project. 

In many cases, boiler owners have run for years without appropriate pretreatment plants and therefore have unduly low expectations for boiler cleanliness, steam quality, and fuel consumption. But the fact is that recent changes in technology have brought down the costs of pretreatment equipment and increased quality and output so that even a relatively modest expenditure almost always results in significantly improved operational control and boiler plant efficiency. 

Given that the primary source of waterside deposition is almost always hardness salts, good operational practice dictates that (HW heating boilers aside) all boiler plants, wherever located and no matter how limited their output or pressure rating, require softened MU water as a minimum form of pretreatment. 

All boiler plants providing steam for electricity generation must be provided with silica removal pretreatment equipment. Pretreatment technologies for silica reduction and removal include ... 

In all cases, and irrespective of the types of pretreatment equipment employed and the degree of boiler operational control provided, additional and complementary support through the use of suitable internal chemical water treatment programs is always needed to limit waterside deposition. 

The opposite scenario also may occur, whereby a localized pre-boiler water chemistry problem can affect the downstream, mechanical operation of a boiler. As an example, difficulties in the control of pretreatment equipment may lead to treated MU water instability, causing downstream after-precipitation, leading to FW line blockages, and finally resulting in boiler cutout as a result of low water. 

These coils are subject to a variety of aggravating problems that may develop over time and ultimately lead to coil replacement. The frequency of replacement may be as often as every 2 to 3 years in some cases but perhaps only every 7 to 10 years in others. The timespan depends primarily on the particular source of water used for boiler MU and domestic supply and any pretreatment provided. Usually the first time that developing problems may be seriously investigated is when users complain of an inadequate HW flow, reduced pressure, or low temperatures. 

There are a variety of waterside operational problems that may potentially develop in HP plants. Some of these occur in the pre-boiler section and mostly concern the boiler feedwater (FW) system. For any particular boiler plant, the steam output, operating pressure, types of pretreatment, and daily operating procedures are all considerations that factor into the scope and severity of these problems. 

Pretreatment (and internal treatment) is often limited for smaller HP FT boiler steam systems but gradually becomes more extensive with larger and higher output FT and WT plants. 

Pretreatment for FT boilers designed for industrial duty becomes slightly more exacting because these typically operate at about 85 to 100 psig/114.7 psia (338 °F/169.9 °C) perhaps up to, say, 250 to 300 psig (264.7-314.7 psia). 

If, in any boiler plant, the installed treatment processes are unsuitable or inadequate for the needs or if operational problems persist, downstream operational problems certainly will develop in a short time. The extent of these problems and their associated costs are likely to be serious if the external treatment problem is not rectified at the source. In almost all cases, internal chemical treatment cannot totally and economically compensate for lack of pretreatment. 

Where downstream boiler plant problems do result, these may often be traced back to inappropriate FW quality. This does not mean that the FW is necessarily contaminated in some way, but rather that the most suitable or necessary type or capacity of pretreatment equipment (such as a water softener, dealkalization plant, or deaerator) has most probably not been installed. 

The use of FW that has been inadequately pretreated makes the mechanical operation of a boiler and the control of boiler water conditions innecessarily difficult. Additional quantities of internal treatment chemicals and higher rates of blowdown (BD) are usually required, which reduces boiler plant efficiency and raises the cost of generating steam. 

Therefore, to minimize the certainty of waterside carbonate deposition, essentially all types of steam generating boilers should be provided with a water softener or some other equally effective form of pretreatment equipment. 

Where hardness removal is required, the simplest pretreatment method for smaller, lower pressure boiler plants (below 200-300 psig) is to use a cation-exchange softener. This removes the calcium and magnesium at source and converts the bulk of temporary hardness salts into sodium bicarbonate (NaHC03), which decomposes to form sodium carbonate (soda ash) but does not scale under normal boiler operating conditions. 

Today, in an era of water reuse and higher costs for premium-quality water, condensate line corrosion may be an inevitable and serious drawback when using naturally high-alkalinity water as a MU supply source. Some form of dealkalization pretreatment process generally is provided to high MU demand boiler plants but unfortunately not so often to smaller facilities. 

Because of the problems relating to silica and silicates, silica levels in the FW and BW must be strictly controlled. This normally is not a problem in higher pressure boilers because various methods of pretreatment are typically employed to reduce the concentration of silica in the FW as much as possible. Automatic monitoring and alarm systems are also provided. 

It should be noted that, in practice, where lower pressure industrial process boiler plants are operated, the problem of overcoming risks of silica deposition is seldom tackled by the installation of external silica removal pretreatment equipment. Instead, control centers primarily around ensuring an adequate ratio of caustic alkalinity to silica in the BW and limiting the TDS. 

In larger HP boiler plants, excess alkalinity in the MU water is generally removed by means of (ion-exchange) dealkalization, demineralization, or other pretreatment processes, so that the bicarbonate and C02 potential is lost at source. 

Modifying the mode of boiler operation to reduce boiler startups and wide load swings. Boilers are far less prone to silica vapors when operating at continuous load (provided, of course, that the basic pretreatment and water chemistry controls are in place). 

A further consideration is that boiler plants operate most effectively when the FW and BW chemistry is constant and predictable. Thus, irrespective of a facility s size (or lack of size), a key objective should be to produce FW of a consistent composition. Pretreatment equipment processes can often be particularly useful in ironing out variations in the quality of both MU and CR. 

Most raw water sources considered for use as boiler MU have been treated or conditioned either by a water utility (providing city water) or in-house (providing industrial water). They are supplied to the boiler plant clean and relatively free of suspended solids, colloidal material, organics, and iron. In hard water areas there also may be some reduction in hardness and alkalinity provided. Where boiler plant raw water (RW) quality is still unacceptable for the particular boiler plant needs, additional pretreatment pre-boiler conditioning or external treatment) may be required. 

Where RW of basic good quality is supplied as MU for HW heating boilers, no other form of pretreatment is usually necessary. However, an internal, corrosion inhibitor treatment-based program should be provided, and periodic boil-outs may still be necessary. 

Where RW of basic good quality is supplied for LP steam boilers (that is, firebox, Scotch marine, cast-iron sectional boilers, etc. at operating pressures below 15 psig) and where the MU water volume demand exceeds 5% of the FW, pretreatment by ion-exchange softening should be additionally provided. This rule also applies to electrical resistance boilers, electrode boilers, vertical boilers, and coil boilers. 

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