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ES&H
Manual Flammable
Gas Supplement |
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Chapter
2 – Examples of Gas System Class
Determination |
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1.0
Purpose
This appendix provides examples of Gas System Class determination using Figure 1 in Flammable Gas Supplement Chapter 1 Storage/Use of Flammable Gases and requirements of this chapter. The first step in such an analysis is to determine the inventory in terms of hydrogen content and then to follow the Figure 1 flowchart to determine the Gas System Class. In many cases, the presence of flow and/or pressure restrictions may permit the facility to be separated into constituent parts which may be assigned different Classes.
2.0
Subdivision of a
System
Figure
1 below is an illustration of a typical facility amenable to such separability. The
storage area is an attached building separated from a processing area which is,
in turn, separated from the experimental area.
The processing area could, for example, contain mixing
apparatus or temperature regulation equipment.
One could, of course, have the processing area included within either
the storage area or the experimental hall.
Each installation will differ, however both solid walls with appropriate
ventilation controls and limitations on the gas flow to render areas separable
are generally required.
In
this figure a system is shown in which two different gases (designated by
different cross-hatching) are used to supply various particle detectors. Important details such as bubblers,
check valves, orifices, shutoff valves, and gas detectors are not shown. Note that the storage area contains several
cylinders in “off-line” storage. After
passage through the particle detectors, the gases are vented to the outdoors at
the right of the figure. The precautions
of this policy are dependent upon the nature and size of the entire complex
including all flammable gases present, even if there are independent systems supplying
different particle detectors, or even different experiments in the same
building.
Figure 1. Typical Facility Amenable to Separability
3.0
Examples of Gas System Class
Assessment
3.1
Example 1
Two 81 SCF cylinders of a 50-50 mixture (by volume) of
argon-ethane will be used in a room whose volume is 9 x 15 x 20 ft3
(2700 ft3). This room, inside
a larger building, contains no obvious fire hazards such as welding
operations. The gas is to be supplied to
drift chambers.
First, to determine Q, it is recognized that only 40.5 SCF of a
given cylinder is ethane. Thus, from Flammable
Gas Supplement Chapter 4 Electrical Classification Guidelines
and Chapter 5
Electrical Installation:
Q = 2 x 40.5 ft3 x 0.028(m3/ft3)
x 1.26(kg/m3) x 0.36 (H2 equivalence factor)
Q = 1.03 kg hydrogen equivalent inventory
Thus by box 1 in the flowchart, we exceed the limit for Class 0
and must go to box 2. Continuing to box
2, we find the answer to be yes but the answer to the question in box 3 is
negative. Doing the calculation
prescribed in box 4 we find that 5% of 2700 ft3 is 135 ft3. Dividing 81/135 finds a maximum concentration
of 60%, which exceeds the flammability upper limit. Thus, any concentration below this limit is
reachable with the available inventory, since no inventory controls have been
specified. Therefore the answer to this
question is yes and the system Class II.
If only a single cylinder was needed, the 0.5 kg hydrogen equivalence
would have rendered a Class 0 determination.
3.2
Example 2
This example is the same as that explored in example one except
that these two cylinders are used to test a drift chamber in an open
experimental hall 60 x 200 x 30 = 360,000 ft3. The nearest ignition source is a temporary brazing
operation at a distance of 40 ft (12.2 m).
Following the flowchart, the same path is found until box 4 is
reached. Five percent of this much
larger room volume is 18,000 ft3.
Thus the maximum concentration in this volume would be 1.5%, so that
this question is answered negatively. At
box 6, we determine that objects or operations presenting an ignition hazard
(the brazing operation) are more distant than the 2.0 m required by the formula
based on the hydrogen equivalent quantity.
Thus the Gas System is Class I.
3.3
Example 3
A large system having an inventory of 15 cylinders such as used in
the first two examples is stored in a separate “gas room” and is connected to a
drift chamber system in the same experimental hall as in example 2. The same brazing operation is
continuing. There is no processing area,
only a storage area and the experimental hall.
The inventory of the storage area is 15 x 0.5 kg/cylinder = 7.5 kg
hydrogen equivalent. For the storage
area a “yes” is encountered at box 2 while a “no” is encountered at box 3. The volume of the storage room is only 6 x 10
x 8 = 480 ft3 (13 m3), however the gas inventory
corresponds to a volume of 608 ft3.
Thus, without considering ventilation, the storage area will be Class II
after the query of box 4. Continuing on
to the experimental hall the interconnection is considered. If appropriate flow restrictions exist, one
may only have to consider Q to be the volume of detectors plus piping in the
hall. Thus it may be appropriate to
declare the experimental hall to be Class I, if the condition on the distance
from obvious ignition sources is met at box 6.
If the detector volumes are large, then box 4 may indicate Class II.
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TECHNICAL POINT-OF-CONTACT |
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