Buckman Laboratories Inc.
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (4 printed pages)
A steam boiler is a specific type of pressure vessel into which water is fed and, when heat is applied, continuously converts (evaporates) that water into steam.
In a boiler, water is the life blood of the system, yet there are water-related "problems" that must be continuously addressed in order to maintain the integrity and performance of steam boiler components. Essentially, these problems can occur anywhere water or steam is present and may even continue to occur during off-line and lay-up periods.
Historically, water treatment of boilers has been in use for well over 100 years. During the Industrial Revolution, the need for steam rapidly increased in the operation of rotating machinery as well as locomotion and transportation. Advances in boiler design, which allowed higher-pressure steam generation, were marked with increased failure rates (many with catastrophic results).
Common threads linking the failures appeared to be the development of waterside deposits, called "boiler scale," and the destruction of metal, or "corrosion." As the sciences of metallurgy and chemistry advanced, water treatment technology evolved as a means to extend the life of the equipment, avoid costly downtime, and maintain system efficiency. To the operator of the boiler plant, "water maintenance" became an integral part of boiler operations. The boiler water maintenance program became one of:
- Preventing the formation of waterside deposits.
- Controlling the corrosion of metals.
- Limiting boiler water carryover (steam purity).
Water has often been described as "the universal solvent," as virtually all substances are soluble in water to some degree. Raw water, or untreated water (whether it is surface water from a lake, river or stream or subsurface water from a well) contains impurities. These impurities include suspended solid matter, colloids (clays), dissolved minerals, and atmospheric gases (such as oxygen, carbon dioxide and sulfur dioxide), as well as man-made contaminants. While raw water may be "fit to drink" in many cases, it will require "treatment" to meet the demands of modern steam generation or a continuous "water maintenance" program to achieve desired results. Stationary steam-generating systems can vary widely in their complexity. Typical systems can be categorized into four sections:
- Pretreatment - Chemical or mechanical processes are used to reduce or remove objectionable impurities from the supplied water (make-up water). Pretreatment systems may include clarification, filtration, zeolite softening, demineralization, reverse osmosis, etc. Higher-pressure steam systems have higher make-up water quality (purity) requirements, thus justifying the capital expense involved in the installation and operation of pretreatment water "purification" systems. Pretreatment will drastically reduce but not entirely eliminate the internal water treatment needs in the downstream sections of the steam system.
- Pre-boiler - This section includes the deaerating heater, piping, pumps, stage heaters, and economizers. Pre-boiler corrosion may not only result in metal failure, but also generate corrosion products which could restrict water flow. Corrosion products themselves can become the root cause of an aggressive type of metal attack known as "under deposit corrosion" either locally, or further downstream in the boiler. While the primary role of a deaerating heater is that of removal of gases in the water (notably oxygen, which may cause corrosion), its function is also to provide thermodynamic gain, as well as a vessel for feedwater storage. Considered a pressure vessel, the deaerating heater or deaerator can be the site of both precipitate mineral deposits, corrosive attack, and corrosion products, all of which will increase in potential as the water temperature is increased.
- Boiler - Defined as extending from the economizer outlet to the saturated or superheated steam outlet. The boiler traditionally receives most of the water treatment attention with regard to the control of corrosion and deposits. This is probably due to the potential economic impact corresponding to a premature boiler failure/loss, or downtime associated with tube cleaning, tube replacement, and repairs.
- Post-boiler - This part of the steam plant system includes turbines, turbine condensers, process equipment, piping, steam traps, condensate tanks, and pumps. Again, the complexity of this section can vary widely. The post-boiler system may have numerous pressure reducing stations and have a very extensive piping network or, conversely, be of relatively simple design.
Boiler water impurities that exit the boiler section along with the steam (called "carryover") can cause turbine deposits, and could also adversely impact steam purity requirements for any given plant process(es). Process-related impurities and condensate corrosion products can become a contamination concern as these impurities are transported back to the pre-boiler and boiler where they may contribute to deposits. Corrosion products and process contamination may also negatively impact upon steam trap performance. In addition to chemical treatment to minimize the corrosive effect of the condensate (due to carbonic acid formation and to a lesser degree, oxygen leakage), some plants employ condensate polishing as a technique to remove generated, soluble corrosion products from returned condensate.
Water treatment/maintenance decisions are an integral part of the overall successful operation of each section of the steam-generating system. Larger steam-generating facilities are often staffed with full-time, on-site technicians who routinely perform water testing at given intervals in order to maintain the water chemistry within defined acceptable limits.
Recordkeeping and statistical process control methods are essential tools that are employed to monitor overall operating performance/compliance for each part of the boiler system. A designated results engineer or plant chemist is then responsible for water management/maintenance decisions. Specialized, outside consultants are retained as needed, to address complex problems/issues, recommend system modifications, and suggest operational procedure changes.
Other facilities outsource their water maintenance needs to water treatment vendor companies that provide both chemical additives and value-added technical knowledge/services. Vendor selection criteria should include not only annualized chemical costs, but also the experience level of those providing the routine service, as well as technical resources available to the facility (buyer). Vendor treatment references should be requested and contact made to confirm both the results of the treatment along with a personal reference of the service representative. The local service representative's abilities should be viewed as that of a job applicant for a position within the buyer's organization. The water treatment representative must possess a predefined level of technical and communications expertise, have the ability to prioritize problems, and be able to generate action plans. He/she should have a positive attitude that is compatible with steam plant operating personnel. Periodic review with the vendor, in conjunction with equipment inspection results, treatment costs, and operating highlights, serves as an opportunity to uncover and address areas of mutual concern.
Problem-solving begins with a comprehensive system survey during which plant operating records are reviewed, previous inspections/results discussed with plant personnel, and appropriate samples of water and deposits collected for laboratory analysis. Metallographic failure analysis of system components may also be required. "What was tried, and why?" questions may hold the key in arriving at a course of action or action plan for future avoidance.
Today, reliable performance is the key to economical boiler operation, and the goal of modern boiler water treatment technology can be summarized as "to maintain the integrity and performance of steam boiler components." Specific water maintenance needs are dictated by the spectrum of impurities present in water that can lead to deposits and corrosion and their costly consequences if not adequately controlled. Only then can water be the true "miracle fluid."
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.