1026 PREVENTION OF TOXIC CHEMICAL RELEASE
Skid-mounted units for applications in remote loca-
tions, or where onsite cryogenic facilities are too large to
be cost-effective, are available. These units provide nitro-
gen at purities of 97 to 99.5%. They operate by pressure-
swing absorption, in which a bed of activated carbon absorbs
oxygen from air at high pressure and desorbs it at reduced
pressure. The nitrogen produced by this cyclic process con-
tains 5 ppm of water and CO 2. 400 to 20,000 cfh of nitro-
gen can be supplied, at a pressure of 100 psig. Write Airco
Industrial Gases, Murray Hill, New Jersey.
Nitrogen for purging may also be supplied from cas-
cade systems or directly from “bottle” trailers under pres-
sure, normally 2000 psig. When drawn from large cascade
systems or reservoirs the nitrogen is piped into a “running
tank.” The “run” tank is then pressurized and its contents are
expanded through pressure regulators as desired for purging.
When pressurized trailers are used, the high pressure gas is
regulated down to the desired pressure for purging.ACKNOWLEDGMENTSThis article appeared in its original form in Chemical
Engineering, December 9, 1980, pp. 65–68. It has now been
redone to provide more useful detail in the designation and
use of the chart. A special section on checking points and
areas on the graph has been included. This author grate-
fully acknowledges with thanks the permission by Chemical
Engineering to update and republish.APPENDIX
STEPS IN SETTING UP PURGING CHART1) Draw right triangle ABC whose size is to accom-
modate ordinate and abscissa scales.
2) Fill in both ordinate and abscissa scales for the
combustible of concern.
3) From Table 1 superimpose the explosive range of
the combustible gas of concern on line AC, using
the abscissa scale values.
4) Using the experimentally determined value equiv-
alent of point, E, strike that value on line BC.
5) Draw line AE.
6) Draw lines LD and UD.
7) Draw line FDC.
8) Establish any point X below line FDC.
9) Draw lines AX and BX as shown.
10) Complete the chart by adding in the notes and
other embellishments.Draw chart carefully so that the various points can be
determined with accuracy. And remember to use a safety
factor in applying the chart figures, i.e. 25% of the chart
values (points E and F) when taking equipment out of ser-
vice and when putting equipment into service, respectively.PURGING CHART DESIGNATIONS AND USEPoint D No mixture of acetylene, air, and nitrogen which
contains less than 6.4% oxygen is explosive.
LDU All mixtures within area of triangle LDU are
explosive.
Point X Any mixture such as denoted by point X is not
explosive.
Line XA On dilution of mixture denoted by point X with
air, new mixtures along XA are formed.
LDE Any point to the left of LDE will not form
explosive mixtures on dilution with air.
FDC Any mixture of acetylene, nitrogen and air rep-
resented by a point below FDC may be diluted
with acetylene without forming an explosive
mixture when placing equipment into service.
Point F fe point reached after air displaced with nitro-
gen and ready to add acetylene (6.6% oxygen)
when placing equipment into service.
Point E Safe point reached after acetylene displaced with
nitrogen and ready to add air (4.8% acetylene).
This is the situation when taking equipment out
of service.CHECKING POINT AND AREAS ON GRAPHPurpose is to become better acquainted with the graph and to
check various points.UEL From UEL on AC read down vertically to 80% acety-
lene on BC. Then read horizontally left to YY and read 20%
air by volume, with Zero inert nitrogen. Then by calculation
and since composition of air is 21% oxygen and 79% nitro-
gen by volume: O 2 4.2%; N 2 15.8%.LEL Likewise: 2.5% acetylene and 97.5% air and zero
inerting nitrogen. In the air: 20.48% O 2 and 77.02 N 2 %.Point 1 This lies within area bound by LDU and should be
explosive. Now from point 1 drop vertically and read 30%
acetylene on BC. Move horizontally left to YY and read
air per cent 4.6. Then by calculation, O 2 content in air
9.66% and nitrogen content 36.34%. By difference inert-
ing nitrogen is 70–46 24% by volume. Since 9.66% O 2 is
greater than 6.4%, the mixture is explosive.Point X Should not be explosive. From X drop vertically
and read 21.5% acetylene as before. Move horizontally left
and read 19% air on YY. Then by calculation find O 2 in air
3.99% and N 2 in air 15.01%, leaving 59.5% inerting nitro-
gen by volume. Since O 2 3.99% and is less than critical
value of 6.4%, mixture is not explosive.Point 2 Should be explosive. Acetylene 30%, air 70%.
O 2 14.7% and nitrogen 53.3%. Zero inerting nitrogen.
Now since O 2 is greater than 6.4%, mixture is explosive.C016_011_r03.indd 1026C016_011_r03.indd 1026 11/18/2005 1:12:56 PM11/18/2005 1:12:56 P