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Emergency Shutdown Devices and Relief System Sizing and Design in Oil Refineries
Figure 1: Applicable Standards
Advantages of Taking Credit
Clearly, the biggest advantage of taking credit for ESDs is minimising
API RP-521, Section 5 To flare
the size of the relief system required to handle the PRV discharges
from a unit or the entire refinery. Relief and flare headers are typical
API RP-520, Part II, Section 5
API RP-520, API RP-521, §5.4.1.3 of other safety-related equipment in a refinery – they cost a great
Part I, Section 3
deal of money to design and install to begin with, and then take up
API RP-520, Part II, Section 4
a significant portion of the regular maintenance effort. Consider the
API RP-521, §5.4.1.2
flare system shown in Figure 2. The main flare is designed to take
discharges from four crude distillation units, a crude gas recovery
ASME Section VIII, Division 1: UG-125 to UG-136
unit, a visbreaker, a kerosene re-run unit, a hydro-desulphurisation
unit, a liquefied petroleum gas (LPG) treater, a naphtha re-run
Figure 2: A Typical Refinery Flare System complex and several drip drums in the refinery gas circuit. The second
flare (the fluid catalytic cracking unit [FCCU] flare) is connected to the
1–4 crude units, visbreaker, kerosene re-run
To refinery flare FCCU, a crude unit associated with the FCCU, a polymerisation plant
and a gas compression unit. During FCCU shutdowns for
LPG treater, naphtha re-run KO drum 1
maintenance, the main refinery flare can also be taken out of service
2 polymerisation plants
for maintenance by diverting its flare load to the FCCU flare. The
FCCU flare
FCCU
third, independent flare system – the low-sulphur fuel oil (LSFO)
flare – serves the requirements of a hydrogen plant, a hydro-
KO drum 2 KO drum 3 KO drum 4 desulphurisation unit and a sulphur recovery unit. The foul water
From foul water stripper
stripping unit is normally routed to the LSFO, but can be directed to
To refinery flare header
the main refinery flare header if needed (this is to enable other units
To low-sulphur fuel oil unit flare in the refinery to remain on-stream when the LSFO complex is down
From HDU and SRU
for maintenance). The presence of a platformer/unifiner unit brings
KO drum 5 into question the requirement of a fourth flare.
FCCU = fluid catalytic cracking unit; HDU = hydrodesulphurisation unit; KO = knockout;
LPG = liquefied petroleum gas; SRU = sulphur recovery unit.
Header sizes of 36 inches, 42 inches and larger are needed to handle
the maximum possible flows from the units. In a large, well-spaced
standards and other related topics need to be made in order to better refinery requiring flare stacks to be located at a safe distance, the cost
define the scope of the concerns and the real, underlying issues. The first of installing large headers and associated equipment can be
issue is process vessels designed in accordance with ASME ‘Boiler and substantial. Furthermore, if some sections of the header system call for
Pressure Vessel Code’ Section VIII, Division 1.
4
The need for pressure relief special metallurgy, the costs escalate further.
devices is included in Parts UG-125 to UG-136 of the code. Similarly,
British standard BS-5500 ‘Unfired fusion welded pressure vessels’ If credit is taken for the unit/equipment ESDs in the belief that these
specifies the need for PRVs. In terms of relief header sizing, no distinction will tend to reduce the expected flare load, considerable savings in the
is made between PRVs installed for code compliance purposes and those investment costs can be realised by installing smaller headers and
installed for other reasons. Once a decision is made to install a PRV at a ancillary equipment (valves, knockout drums, seal drums, etc.). The
given location in a refinery unit, its inlet piping needs to be designed as space required for the system would be smaller, as would the
per API RP-520, Part II, Section 4.
5
Similarly, the design of other parts of civil/structural work. In some cases, a smaller system results in lower
the relief system – such as PRV sizing, individual discharge piping and the regular maintenance costs (cleaning, inspection, etc.).
header piping – can be carried out on the basis of the various API
recommended practices (RPs). Applicable sections of the API RPs are A major advantage of ESD credit taking is the use of an existing relief/
illustrated in Figure 1. flare system for the purpose of permitting additional discharges into it.
In the example illustrated in Figure 2, if ESD credit taking is not
Some designers in this field will argue that the API RP-520 Part I (January permitted, a fourth, new flare system will have to be engineered and
2000), Part II (August 2003) and RP-521 (March 1997)
6
are merely RPs, installed to accommodate the relief load from the platformer/unifiner
implying that they need not be adhered to as meticulously as warranted unit. If, on the other hand, credit is taken for the existing ESDs in
by codes of practice or standards. However, it should be noted that various units of the refinery, the existing flare headers can be modified
these two RPs are extensively used by designers worldwide in order to to take in the relief load from the platformer/unifiner unit at a fraction
identify the minimum requirements necessary for an acceptable design. of the costs associated with a new flare system. The need to
For all practical purposes, the status of these two RPs is on a par with accommodate additional relief load is not just a hypothetical case.
that of any other internationally recognised standard or code. In Many refineries have faced this problem, with the need arising from a
addition to being connected to various PRV discharges, the unit variety of reasons, such as: changes in product slate requiring
manifold may be connected to piping carrying excess gas that needs to alterations in process parameters; recommendations arising from a
be directed to the flare header from time to time as part of the normal major incident, for example BP Texas City;
1
revamping or
operation of the refinery or as part of a controlled flaring activity debottlenecking of existing units; technology upgrade; addition of new
following a minor plant upset. A utility failure scenario at a time when units; seeking compliance with more stringent environmental
such flaring is taking place has not been considered in this article. regulations; and capacity increment.
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HYDROCARBON WORLD VOLUME 4 ISSUE 1
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