June
2016
HYDROCARBON
ENGINEERING
32
tied into a radially projecting nozzle at an angle less than the
vertical, in order to lower the flare header and pipe rack
elevation, therefore reducing cost.
Generally, two flare knockout drum pumps are provided,
one in operation and one on standby; however, it should be
ensured that at least one of the pumps is on emergency power
or, alternatively, on a steam turbine drive.
Flare stack
The height of the flare stack should be fixed based on the
specific project flare radiation and pollutant ground level
concentration criteria. Rigorous thermal radiation and
dispersion calculations need to be carried out to arrive at an
acceptable stack height, keeping in mind the requirements of
the design standard being used and the requirements of the
local regulatory bodies.
The provision of a system for continuous monitoring of
the operational flare stack is of vital importance. Several
options listed below are available for the purpose:
n
n
The provision of dedicated closed circuit television
(CCTV), which visually monitors the operational flare tip
from the main control room, is the simplest and most
commonly used system.
n
n
Opacity meters may be installed to continuously monitor
the flame, which can also be programmed to automatically
inject a calculated quantity of steam, thus ensuring that the
flare is smokeless within a certain range of capacity (the
generally acceptable smokeless load is around 10 - 15% of
the peak flare load from economic considerations).
n
n
Burn-back thermocouples provided on the flare tip, with
alarms on distributed control systems (DCS), would ensure
that the flame is not ‘burning back’ and causing potential
damage to the flare tip. For increased reliability, dual
thermocouples with a separate thermowell for each pilot
may be considered. Alternatively, retractable
thermocouples may be provided as these can be taken
out for maintenance without shutting down the flare.
n
n
Provision of a flame failure alarm on the DCS, which alerts
the operator in case of ‘flame out’.
n
n
The provision of an infrared (IR) video camera is another
reliable flare monitoring tool as it permits visual monitoring
of the flare tip and is not affected by adverse weather
conditions. It can also be programmed to automatically
control the quantity of assist gas, which needs to be injected
into the flare tip when flaring gas with a low heating value.
Table 1 (continued).
Relief network checklist
20
Check that there are no velocity violations based on data
entered.
21
Check that there are no back pressure violations for any of
the relief valves in a given scenario.
22
Check that there are no slug flow violations for any pipe
segments.
23
Check that the back pressures at the unit battery limit/PSV
outlet flange are within the limit specified in the project
relief system design basis.
24
Check that there are no temperature violations for any
segment of piping (based on the maximum acceptable
temperature for carbon steel/stainless steel piping – as the
case may be).
25
Check that there are no noise violations for any of the
piping segments.
Flare header purge
Flare header purges should be provided at the stagnant end of
each header/sub-header to ensure that a small positive flow
through the network exists at all times. Low pressure refinery fuel
gas is the generally accepted purging medium. However, it should
be ensured that a back up purging medium is available, in the case
of failure or unavailability of the primary purge. Furthermore, an
emergency nitrogen purge should also be provided, which sweeps
the main header following a hot release and subsequent cooling
of the hot flared gases, to prevent shrinkage and the development
of a vacuumwith possible air ingress into the system.
Smokeless flaring
The opacity of the flared gases is measured using the Ringlemann
number (#1 Ringlemann is 20% opacity while Ringlemann 0 is
clear).
2
The flared gas composition and heating value are the two
inherent factors that are responsible for the production of smoke
during flaring. Unsaturated hydrocarbons, which contain double
and/or triple bonds, are responsible for the smoky flame.
Smokeless flaring over a range of capacity (usually around 10 - 15%
of design capacity from economic considerations) is
accomplished by the injection of a calculated quantity of steam
or air into the tip. Steam assist is more effective as steam can
supply more momentum, enhancing ambient air-fuel mixing, as
well as ambient air entrainment. Air may be used in cases where
steam is unavailable.
Provision of assist gas
In cases where the heating value of the flared gases is particularly
low (less than 200 Btu/ft
3
for unassisted flares and 300 Btu/ft
3
for assisted flares),
2
incomplete combustion of the flared gas
could occur and, so, a calculated quantity of assist gas (usually
refinery fuel gas or LP natural gas) is injected into the flared gas in
order to boost its heating value, thus ensuring complete
combustion. The flared gas is analysed and its heating value
estimated by the use of a Btu analyser, which is then used to
compute the quantity of assist gas required to boost the heating
value of the flared gas to the target value of 200 or 300 Btu/ft
3
.
Conclusion
Flare designs could range from simple utility flares to enclosed
ground flares, which may be required to operate alone or in
combination with an elevated flare, and frommultiple injector
steam assisted flares to air assisted flares. The exact flare header
configuration to be employed needs to be examined considering
the specific requirements of the project, the overall economics of
the system, as well as the layout of the units in the complex with
regard to the flare design regulations being followed. A rigorous
analysis of all possible relief scenarios is required to check that
the relief network has been adequately sized. Furthermore,
particular care should be taken to ensure that the stack radiation
criteria and pollutant ground level concentration are within the
limits specified in the project design criteria/flare design
regulation being followed. One should ensure that a reliable stack
monitoring system is in place and adequate purges have been
provided at all dead ends of the flare header and sub-headers.
References
1.
Russian PB-03-591-03, 'Rules for the Design and Safe Operation of Flare
Systems', 1992.
2.
API Standard 521, 'Pressure Relieving and Depressuring Systems',
6
th
Edition, 2014.
