Water Treatment for Boiler Plants
A. Bhatia, B.E.
There are several
problems which can occur within a boiler system. The major problems are; scale
and sludge deposition, corrosion and boiler water carryover. If left untreated
these problems can in extreme cases cause steam boilers to explode and it is
for this reason that standards have arisen on how to treat and maintain boiler
systems. This course specifies the chemical conditions required to be maintained
in the boiler water to avoid scale and corrosion and to ensure that the steam
produced is pure and dry.
This 3-hr course material is based entirely on the US Army Corps of Engineers document TM 5-650 " Central Boiler Plants" and covers Chapter 4; "Water Treatment for Boiler Plants".
The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.
At the conclusion of this course, the student will be aware of:
This course is aimed at students, engineers, designers, maintenance managers, H &S professionals, supervisors, shop foremen, technical and sales representatives and anyone who wants a basic understanding of boiler water treatment concepts.
In a boiler, energy in the form of heat is transferred from a fuel source to the water across a heat transfer surface. When subjected to temperature and pressure, the composition of the boiler water undergoes radical changes; Solubility's are exceeded, the effects of dissolved gases are magnified, and the efficiency of the heat-transfer process is affected.
The heat transfer efficiency of steam generation depends on the degree of fouling of the surfaces and wall. By keeping the surfaces clean both on the fire side and water side, the heat transfer coefficient can be reasonably maintained. Water treatment & conditioning alone becomes a very important aspect of energy conservation program.
Many factors determine
the type of boiler water treatment needed for any given system. Various water
treatment equipments based on ion-exchange or membrane technology are available
to improve the water quality. Besides wide range of chemical formulations are
available from water treatment companies to control individual parameters such
as alkalinity, conductivity, pH, dissolved gases etc.
In this course, you will learn about the Boiler Water Treatment Approaches.
In this course, you are required to study US Army Corps of Engineers document TM 5-650 "Central Boiler Plants":
You need to open or download above document to study this course.
Boiler is like
an industrial pressure cooker. A sealed cylinder containing water is heated
up by burning gas or oil. Steam is driven off through a regulating valve in
the boiler and used to heat up a process or application on the site. As steam
is pure water vapor, the boiler content is maintained and replenished with more
water called feed water. The steam once used in the application is either lost
or is condensed and returned to the boiler plant. This is called condensate
and has no solids dissolved in it. Even in the most efficient plant however,
it is not possible to recover all the steam as condensate thus the condensate
has to be added to in order to maintain the boiler water level. This water is
called make up.
The quality of make up (or feedwater) is very important in ensuring the safety, reliability and energy efficiency of the boiler system. There are numerous water treatment approaches in vogue each depending on the likelihood of makeup water impurities, the boiler design and its operating parameters. It is obvious that no single treatment can be prescribed for all water samples.
Make up water is usually pre-treated to remove some of the scale forming ions. To do this a Base Exchange softener is usually used. This takes calcium and magnesium out of the water and replaces it with non scale forming sodium ions. This softened make up water and condensate return make up the boiler feed water.
Scale and sludge deposition are the result of the precipitation of compounds that are no longer soluble. Calcium carbonate (chalk or limestone) is one of the most common of these compounds. When water dissolves calcium carbonate, calcium bicarbonate is formed. When this solution is exposed to heat, as in a boiler, the calcium carbonate re-precipitates out and forms a scale within the boiler. Much of the calcium carbonate is removed from the make up water by the base-exchange softener but some hardness may remain. The water should be regularly tested as boilers cannot tolerate large amounts of scale.
Other compounds may remain dissolved even under high temperatures and pressures (e.g. calcium sulphate) until the point of saturation when they precipitate out as suspended solids. These solids can bake onto heat transfer surfaces and affect efficiency. The solids can be conditioned with the use of phosphate and polymers which create an amorphous sludge which can be removed through blowdown.
There are many types of corrosion that can occur in a boiler. Two common types are oxygen corrosion and carbon dioxide corrosion. All natural water contains oxygen. This dissolved oxygen can cause oxygen pitting but can be removed using deaerator and/or an oxygen scavenger, which absorbs the oxygen from the water. The gas carbon dioxide can also be dissolved in water and can be neutralized using amines.
Boiler water carryover is where water droplets are carried over into the steam line thus creating wet steam. Wet steam has less energy and introduces impurities into the condensate return and thus the boiler feed water. Carryover can occur when the boiler pressure reduces or when the water in a boiler foams. It is important, therefore, that boiler pressure is maintained and that foaming is minimized. Foaming can be caused by a high alkalinity in the boiler water (a boiler should maintain alkalinity between >350ppm P-alkalinity and <1250ppm total alkalinity). High alkalinity is caused primarily by a high level of dissolved solids. Controlling dissolved solids should serve to maintain alkalinity within the guidelines.
A chemical addition program in addition to the external water treatment is the best way to insure, that deposit formation or corrosion does not become a problem. The requirement of chemicals goes down substantially if proper water treatment system exists. Other than the routing water treatment program, condensate return must be maximized. The condensate water has high level of purity and it recovery is critical to the cost of water treatment.
It is very important
that careful operation and maintenance practices along with regular checks &
testing procedures be incorporated as a part of the treatment program. No treatment
can be better than the way it is applied.
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.