Hydrocarbon Processing - February 2022 - 78

Environment and Safety
contact with the flammable atmosphere,
thereby mitigating the risk of ignition.
This mitigation considered during the
design phase assumes that the plant will
be timely inspected and well-maintained
to ensure that the insulation is continuous
and intact.
This discussion justifies disregardFIG.
1. Stoichiometric concentration.
TABLE 1. AIT of typical fl ammable fl uids
handled in the HPI
Fluid
Benzene
n-butane
Carbon monoxide
Ethane
Ethylene
Gasoline
Hydrogen
Hydrogen sulfi de
n-heptane
n-hexane
Kerosene
Liquifi ed petroleum gas
Methane
Methanol
Naphtha
n-nonane
n-octane
n-pentane
Propane
Propylene
Toluene
Xylene
AIT, °C
498
288
609
472
400
280
500
260
204
225
210
405
600
385
288
205
206
243
450
460
480
464
in air. Ignition does not take place at
the flashpoint unless there is an ignition
source. If the liquid is heated further, at
some point (AIT) it will ignite without
the application of an external ignition
source. This temperature is higher than
the flashpoint. AIT is the lowest temperature
at which a flammable gas or
vapor ignites by itself. AIT varies widely
for hydrocarbon, from 600ºC for methane
(CH4
(C7
) down to 204ºC for heptane
and ISO/
. Common fluids hanH16).
Values for typical flammable
fluids are given in NFPA 4975
IEC 80079-20-19
dled in the HPI are listed in TABLE 1.
78 FEBRUARY 2022 | HydrocarbonProcessing.com
Auto ignition of a fluid depends upon
numerous factors, including:
* How near a vapor/air
concentration is to the
stoichiometric concentration
* The temperature of surfaces
in contact with the fluid
* The contact time between
the surface and the fluid
* The area of the hot surface.
The stoichiometric concentration is
the concentration of vapor between the
LFL and UFL of that fluid in the air, also
known as flammable range, as depicted in
FIG. 1. The LFL of a vapor is the concentration
of that vapor in air below which the
vapor concentration is too lean to burn.
The UFL of a vapor is the concentration
of that vapor in air above which the vapor
concentration is too rich to burn. So, for
ignition to take place, the concentration
of vapor should be between the LFL and
UFL. Values of LFL and UFL for typical
flammable fluids are given in NFPA 4975
and ISO/IEC 80079-20-19
.
It is difficult to raise the temperature
of a gas or vapor to above the AIT under
open air ventilation conditions encountered
in the HPI. It is still a good practice
to avoid very hot surfaces in hazardous
areas, even with open ventilation. In areas
with less ventilation, it is easier for a
hot surface to heat a gas or vapor release
to above the AIT and, therefore, equipment
surfaces should not exceed the AIT
of any release.
For the HPI, based on the lowest AIT
encountered (that of heptane), a limit
of 200ºC is recommended. This means
that hot pipe and equipment surfaces up
to 200ºC cannot ignite any hydrocarbon
in open-air ventilation conditions.
Also, it is a normal practice in the industry
to insulate hot pipe and equipment
surfaces to prevent energy loss and for
personnel protection. This ensures that
the hot surface is not readily available for
LITERATURE CITED
Complete literature cited available online at
www.HydrocarbonProcessing.com.
BABANNA BIRADAR is a Principal
Engineer with Bechtel India. He has
30 yr of experience in oil, gas and
chemicals, infrastructure, metal and
mining front-end engineering and
detailed design and is a subject
matter expert in fire protection and
process safety. He holds an ME degree in mechanical
engineering from the University of Roorkee, India
(now Indian Institute of Technology, Roorkee).
He is a member of the NFPA technical committee
" Electrical equipment in chemical atmosphere
(EEC-AAA), " which is responsible for NFPA 496,
NFPA 497 and NFPA 499 standards.
ANIMESH AGARWAL is a Senior
Process Safety Engineer with
Bechtel India. He has 8 yr of
experience in petroleum refining,
petrochemicals, and front end
and detailed design. He holds a
BE degree in chemical engineering
from Thapar University, Patiala, India.
ing hot pipes and equipment as ignition
sources during hazardous area classification.
However, this is not captured in any
document-as standards are also generally
silent on this aspect, this becomes
a grey area for engineers designing,
maintaining and operating hydrocarbon
plants. It is recommended that this aspect
is clarified in the hazardous area classification
documents so that no ambiguity
exists in the later stages of the plant life.
Takeaways. All sources of ignition must
be controlled where flammable atmospheres
may form, and hazardous area
classification is a good basis for deciding
what equipment may be used at any
location. While the principal objective
of hazardous area classification remains
the classification of areas for the selection
of an appropriate type of electrical
apparatus, the design documentation for
hazardous area classification should provide
additional guidance to aid the location
and control of non-electrical sources
of ignition. It should include justification
for not considering the hot surfaces in the
plant, such as equipment and pipe surfaces,
as sources of ignition in the HPI.
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