Page 52 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

2.2 CHAPTER TWO

PRESSURE AND TEMPERATURE RATINGS

The pressure rating for a wide range pipe and other components is provided by the ANSI
Pressure Classification System, which publishes qualitative performance standards. The
design pressure is the maximum sustained (steady-state) pressure that a piping system must
contain without exceeding its code-defined stress limits.
The design temperature of a component must be equal to or greater than the maxi-
mum sustained temperature that will be experienced during any normal or abnormal mode
of operation. Normal operating temperature is the temperature maintained by the system
while it is operating at steady-state, full-load, nontransient operation. It is the temperature
at which the allowable stress is calculated.

Cost

The total installed cost of the network includes the piping material cost, assembly of the
joint selected, handling due to the weight of the material, physical damage allowance, and
the support system required.

CORROSION RESISTANCE

Corrosion is the thinning of a pipe wall (usually) caused by a chemical reaction from a
corroding fluid or agent and is limited almost exclusively to metal products. Corrosion
resistance is the ability of a pipe to resist the internal corrosive effects of the fluid flowing
through it, as well as the external corrosive forces on the pipe, for example, soils (if under-
ground) or surrounding atmospheric conditions (when installed above ground). Corrosion
can be reduced or eliminated with the application of suitable coatings, linings, and cathodic
protection, depending on the nature of the problem. Since there is no universal corrosion-
resistant material, the selection of a specific grade or class of pipe material with specific
alloys to resist expected corrosion resulting from specific fluids within a pipe is a matter of
tradeoffs unique to each project.
Corrosion is separated into two basic types: general and localized. General corrosion
describes the potential dissolution of pipe over its entire exposed surface. Localized corro-
sion 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. This is shown in the gal-
vanic series of metals (Table 2.1).

Intergranular Corrosion. This type of corrosion occurs in the pipe wall when material
in the grain boundary of some alloys is less resistant to the corroding agent than the grains
themselves, and the bond between the grains is destroyed.

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