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
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
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
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 and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.