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Water Treatment for Boiler Plants

A. Bhatia, B.E.


Course Outline

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.

Learning Objective

At the conclusion of this course, the student will be aware of:

Intended Audience

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.


Course Introduction

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.

Course Content

In this course, you are required to study US Army Corps of Engineers document TM 5-650 "Central Boiler Plants":

US Army Corps of Engineers document TM 5-650 " Central Boiler Plants" Chapter 4; "Water Treatment for Boiler Plants"
(http://140.194.76.129/publications/armytm/tm5-650/c-4entire.pdf)


You need to open or download above document to study this course.

Course Summary

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.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.