Page 176 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.12 CHAPTER FOUR

Microbial control consists of sterilization, disinfection, and sanitation. Sterilization is
defined as the lethal disruption of all bacteria, molds, and yeasts and the elimination of
biofilm and spores. Numerically, it is a 12 log reduction in bacteria. Disinfection is a 6 log
reduction of microbials. Sanitation is generally considered to be the killing of the vegetative
organisms and it minimizes the presence of bacteria and endotoxins. Numerically, it is a
3 log reduction in bacteria.
There are different methods of controlling biofouling. The method selected depends on
the intended use of the treated water and the proposed materials of the system components.
Chemicals, ultraviolet radiation, heat, filtering, and ozone are the most commonly used.

CORROSION

Corrosion is the loss and eventual failure of metals and alloys from the electrochemical
reaction between water and the pipe material. It is separated into two basic types: general
and localized. General corrosion describes the potential dissolution of pipe over its entire
exposed surface. Localized corrosion affects only a small area of the pipe surface.

General Corrosion
This is a breakdown of the pipe material at a uniform rate over its entire surface by direct
chemical attack. It is caused by the loss of the protective passive film that forms on the
surface of the pipe coupled with a chemical reaction occurring between the pipe material
and the chemical in the fluid.
Galvanic Corrosion. This type of corrosion occurs in a liquid medium (called an electro-
lyte) when a more active metal (anode) and a less active metal (cathode) come in contact
with one another and form an electrode potential. When this occurs, the more active (noble)
metal will tend to dissolve in the electrolyte and go into solution.
Intergranular Corrosion. This occurs in the pipe wall when material in the grain bound-
ary of some alloys is less resistant to the corroding agent than the grains themselves, and
the bonds between the grains are destroyed.

Erosion Corrosion. This is caused by a wearing away of the pipe wall, usually as a result
of excessive fluid velocity or constant wearing away by solids in the water striking the
walls of the pipe.
Localized Corrosion

This takes place on small areas of the surface, usually at high rates, and takes various
forms.
1. Stress-corrosion cracking is the physical deterioration and cracking of the pipe wall
caused by a combination of high operating temperature, tensile stress on the pipe, and
chemicals in the fluid stream.
2. Pitting is characterized by deep penetration of the metal at small areas of the surface,
concentrating in small cells, without affecting the entire surface.
3. Crevice-attack corrosion occurs at junctions between surfaces (often called crud traps)
where a crack exists that allows an accumulation of a corroding agent.

Conventional corrosion treatment of feedwater for boilers and cooling water systems
consists of pH control and chemical corrosion inhibitors. Dissolved gases are removed by
deaeration.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com)
Copyright © 2009 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.