1. System Installation Requirements
1-35
TI 33Q01J10-01E
Outline of G3 Environment Compatibility
The classification of the environment in which the process control equipment is installed is
determined by the ANSI/ISA S71.04 “Environmental Conditions for Process Control Systems”
standard. The environment having an atmosphere which contains steams and mists (liquids,
coded L), dusts (solids, coded S), or corrosive gases (gases, coded G) is classified into four
categories according the levels of these substances determined.
The four categories of the corrosive gas environment are defined as follows:
G1 (Mild):
A well-controlled environment in which corrosive gas is not the major cause
adversely affecting the reliability of plant equipment. The corrosion level on the
copper test piece is below 0.03 µm (see note below).
G2 (Moderate): An environment in which corrosive gas can be detected and it could be
determined that the gas is the major cause adversely affecting the reliability
of plant equipment. The corrosion level on the copper test piece is below
0.1 µm (see note below).
G3 (Harsh):
An environment in which corrosive gas is frequently generated to cause
corrosion and that it is necessary to provide special measures or employ
specially designed or packaged plant equipment. The corrosion level on the
copper test piece is below 0.2 µm (see note below).
GX (Severe): A corrosive gas-polluted environment that demands special protective chassis
for the plant equipment, specifications of which should be seriously determined
by the user and a power unit manufacturer. The corrosion level on the copper
test piece is 0.2 µm or more (see note below).
Note: Copper test pieces are used to determine the level of corrosion for the classification of the plant environment. The test piece is an
oxygen-free copper sheet, which is 15 cm
2
in area, 0.635 mm in thickness, 1/2 to 3/4 H in hardness. The test piece is placed in
the plant site for one month and checked for any change before and after the test to determine the degree of corrosion (see table
below). If the test period is shorter than one month, the result is calculated to obtain equivalent data using a expression defined
by the standard.
Table Classification of Corrosive-gas Corrosion Levels
Environment category
G1
(Mild)
G2
(Moderate)
G3
(Harsh)
GX
(Severe)
Copper corrosion level
< 300
(< 0.03)
< 1000
(< 0.1)
< 2000
(< 0.2)
≥ 2000
(≥ 0.2)
[Å]
( [µm] )
Group A H
2
S
< 3
< 10
< 50
≥ 50
[mm
3
/m
3
]
SO
2
, SO
3
< 10
< 100
< 300
≥ 300
Cl
2
< 1
< 2
< 10
≥ 10
NOx
< 50
< 125
< 1250
≥ 1250
Group B HF
< 1
< 2
< 10
≥ 10
NH
3
< 500
< 10000
< 25000
≥ 25000
O
3
< 2
< 25
< 100
≥ 100
Note: The gas density data indicated in the table are for reference only, with the relative humidity of 50 %RH or less. The category goes
up one rank higher every time the humidity increases 10 % exceeding the 50 %RH or over 6 % per hour.
The Group-A gases shown in the table may coexist and cause inter-reaction. Inter-reaction
factors are not known for the Group-B gases.
Sep. 30, 2008-00
