Hydrocarbon Processing - April 2021 - 60

Process Engineering
Grounding requirements. EO liquid is conductive. If EO
is stored in a metallic container that is grounded, the static
charge cannot accumulate. However, if the system is
not properly grounded, a static charge can be generIt must be emphasized that a detailed
ated and lead to ignition-owing to the low value of
the minimum ignition energy of EO, which is even
design and safety review should
lower than gasoline vapor.
be performed on the piping and
Therefore, all EO flare system components (including piping and equipment) must be properly
instrumentation diagrams of the
grounded to prevent the build-up of static electricity,
which could ignite EO and start a fire or explosion.
EO flare system before it is engineered.
Prevention of flashback. A flare system is usually equipped with a liquid seal drum to prevent
flashback. However, this method suffers from some drawbacks
sidering a binary mixture of EO and methane) or 40% nitrogen
due to the possibility of losing the liquid seal (e.g., if the seal
(considering a binary mixture of EO and nitrogen).3 However,
gets blown out following a peak release or if there are issues in
it is recommended that an appropriate factor of safety (around
establishing and maintaining the required liquid level). The use
2-3) should be imposed on these limits, owing to the limited
of two liquid seal drums in a series-one located at the base of
availability of data at higher temperatures.
the flare stack and another located between the outside battery
To improve the reliability of the EO flare system, the emerlimits flare knockout drum and the stack-can further enhance
gency natural gas purge should be automatically backed up by
the reliability of the EO flare liquid seal system.
nitrogen using a SIL-rated interlock. A pressure sensing sysIn the case of EO flares, the flare tip contains an antitem-consisting of two or more pressure transmitters placed
flashback device (velocity section), which must be designed
between the rupture disc and the PRV inlet-should be used
to minimize possible flame flashback initiated at the flare tip
for all PRVs, which could potentially release pure EO or EO-rich
by ensuring that the forward velocity of the flared gases exstreams to the EO flare. During an overpressure scenario, the
ceeds the flash-back velocity.3 Furthermore, an appropriate
rupture disc would rupture and the high pressure at the PRV inlet would be used to trigger the emergency purge. One or more
velocity seal would need to be provided to prevent air ingress
additional pressure transmitters can be located at each PRV disand conserve purge gas. Close follow-up with the flare vendor
charge to further increase the reliability of the system.
is recommended at every stage during the design of an EO
Flare gas analyzer. A flare gas analyzer (e.g., a gas chroflare system to increase the reliability of the system in view
matograph-based analyzer that is sensitive to 1 ppm of EO)
of the hazards associated with EO. Per flare design regulalocated on the main flare header can be programmed to trigger
tions followed in some countries, (e.g., Russia), the possibility
the emergency flare purge in case the concentration of EO or
of including a spare flare stack, liquid seal drum system and
oxygen exceeds a certain fixed value.
knockout drum may also be considered to further increase the
Materials of construction. Any piping and/or equipment
availability of the flare system; thus, ensuring uninterrupted
that can come into contact with the EO-rich stream must be made
operation of the connected units.4
of stainless steel (SS). The use of SS minimizes the potential for
Sampling of flare condensate. Flare condensate collected
rust formation. The 300 series austenitic SS has been widely used
in the flare knockout drum must be periodically sampled. Any
in EO service. Type 304L has been successfully used for the EO
EO-containing flare condensate is required to be routed to the
flare headers and sub-headers, while Type 304 and Type 316 SS
reabsorber column or elsewhere inside the EO unit for further
have been used for small tubing, which cannot be cleaned of rust.
recovery of EO. However, if the flare condensate does not conAustenitic SS can be used in those areas where EO liquid is likely
tain EO, it may be routed to wastewater treatment. This can be
to remain for long periods of time (suction and discharge piping
accomplished through an interlock based on the EO concenof flare knockout drum pumps, low point drains, etc).
tration as measured by an on-line analyzer.
Traces of rust on the internals of carbon steel piping or equipREFERENCES
ment would catalyze the disproportion of EO, which would
1
API, " Sizing, Selection and Installation of Pressure-relieving Devices, Part 1-
further raise the local temperature above the EO decomposiSizing and Selection, " API Standard 520, December 2013
2
tion temperature, leading to a hazardous situation. Furthermore,
API, " Pressure-relieving and Depressuring Systems, " 6th Ed., January 2014
3
American Chemistry Council, " Ethylene Oxide Products Stewardship Manual, "
even clean carbon steel could catalyze the polymerization of EO
3rd Ed., online: https://www.americanchemistry.com/EO-Product-Stewardshipbut at lesser rates than rusted carbon steel. Therefore, the use
Manual-3rd-edition/
4
of carbon steel piping and equipment in an EO flare network
Russian standard PB 03-591-03, " Regulations for the Design and Safe Operation
of Flare Systems, " 1992
should be prohibited.
Since EO attacks several non-metallic materials, including
DAVID A.G. SUARES is a process engineer with 29 yr of
several types of polymers and elastomers, proper care should be
experience. He earned an MS degree in chemical engineering
taken to select a proper material of construction for gaskets, Ofrom the Indian Institute of Science, Bangalore, India. He started
his career at Engineers India Limited and later worked for
rings, packing, etc. This would include rigorous monitoring and
Bechtel (India) Pvt. Ltd. and Fluor Daniel (India) Pvt. Ltd.
inspection programs before a material is deemed fit for use in EO
In the past, he has worked on a variety of flare and relief
service. Polytetrafluoroethane (PTFE) is resistant to EO even
system projects and has authored several articles based on the
same. He is now functioning in the capacity of a freelance consultant in India.
up to 260°C and has been used successfully in such applications.3
the system, the concentration of diluents required to keep the
system non-explosive must be more than 15% methane (con-

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Hydrocarbon Processing - April 2021

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