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Emergency Shutdown Devices and Relief System Sizing and Design in Oil Refineries
Invariably, economic considerations must – and do – enter into decision-
Table 1: Parameters for Furnace Shutdown Systems
making on issues such as those illustrated in the example above. Consider
High tube-skin temperature
a scenario in which ESDs are installed in the platformer/unifiner unit and
High individual pass outlet temperature
credit is taken for these ESDs in terms of a reduction in the expected relief
Low total heater pass flow
load; here, it may be possible to accommodate this reduced load into one
Low fuel gas pressure
of the existing flare systems. If the life-cycle cost estimated for the
Low pilot gas pressure
Low fuel oil pressure
installation and maintenance of the ESDs turns out to be greater than that
Low atomising steam pressure
associated with a new flare system, the question of ESD credit-taking is of High and low combustion air pressure (for forced draught)
only academic interest to the decision at hand. It can be argued that there
Low combustion air flow (for forced draught)
High pressure in firebox
may be other reasons for considering the installation of ESDs, such as the
Low percentage of oxygen in flue gas
need to meet existing (or foreseeable future) environmental regulations or High percentage of combustibles in flue gas
as part of an overall safety enhancement recommended by a hazard and
High smoke density in flue gas
Low flue gas temperature (for air pre-heaters)
operability (HAZOP) study team. Under these conditions, it may not be
possible to maintain independence between these reasons and those • level 3 (high-level) is a fully redundant system in which a high degree
related to the flare system. of reliability is achieved by means of redundant components,
enhanced self-diagnostics and avoidance of common mode failures
A reduction in relief load means reduced flare stack diameter and length (by selecting different types of sensor, etc.).
and reduced header and sub-header sizes, and hence lower investment.
In addition to the effect on installation costs, and perhaps of greater In addition, the term ‘triple-modular-redundant’ (TMR) has been used to
significance, is the impact of relief load reduction on the following key describe systems in which the objective is to achieve both high reliability
parameters associated with the performance and siting of a flare stack: and high availability (these are more popularly known as ‘two out of three
voting’ systems or simply as ‘2oo3’).
• in-plant thermal radiation at grade;
• radiation received at adjacent equipment; Auto-lockout Devices
• radiation level at refinery fence-line; Auto-lockout devices refer to non-normal automatic instrumentation that
• combined radiation from more than one flare; trips or shuts a power or heat source. It is actuated by an abnormal
• dispersion of combustion products; condition and results in the stoppage of a process stream, a utility stream
• dispersion on flame failure; and/or a piece of equipment that adds to a relief load. Examples are:
• compliance with environmental regulations; automatic steam supply shut-off (valve closed) to a tower reboiler on high
• health impact on immediate area; tower pressure; and fuel gas supply shut-off to the burners in a fired
• health impact on surrounding communities; and heater on high-pass flow temperature.
• quantity of product sent to flare.
Auto-start Devices
Types of Emergency Shut-down Device Auto-start devices are those that attempt to reduce the flare load by
As mentioned earlier, the term ‘ESD’ has been used generically in this starting some equipment. Examples are: automatic start-up of a spare
article. However, before discussing various ways in which credit could be reflux pump (steam turbine driven) on electric power failure; and
taken for ESDs, there is a need to briefly describe the types of ESD under automatic start-up of a cooling water circulation pump.
question and the different terms used in the literature to refer to them. The
emphasis is on brief descriptions rather than an elaborate set of definitions. Shutdown Systems for Fired Heaters
Shutdown systems for fired heaters can consist of several levels, e.g.
Safety Interlock Systems individual main fuel trips, total ‘heat-off’ and emergency shutdown of the
Safety interlock systems (SIS) is a term favoured by the American entire unit or a complex within the refinery. Process parameters that need
Institute of Chemical Engineers’ Center for Chemical Process Safety to be brought into the ESD design logic can be determined by carrying out
(CCPS) guidelines.
3
Subsequent to the joint publication of the a quantitative risk analysis (QRA) of the costs in relation to the degree of
functional safety standard by the American National Standards desired availability and/or reliability of the installation. Typical process
Institute (ANSI) and the International Society of Automation (ISA),
7
the parameters commonly considered in a QRA study for fired-heater
acronym is more commonly known to represent safety instrumented shutdown systems are shown in Table 1. The number of parameters from
systems. It consists of a dedicated controller (programmable logic this list that can be cost-effectively brought into the ESD design logic
controller [PLC]) taking input from instrumentation installed for depends entirely on the particular circumstances of a furnace installation.
normal operating process control and/or sensors installed exclusively Hence, results from a QRA study identifying these parameters for one
for the SIS. The output is in the form of dedicated alarms, an event furnace installation cannot be directly used for a different furnace.
logger and field actuation (automatic valve, motor starter or motor
trip, etc.). Three integrity levels can be considered for SIS design: It should be noted that not all ESDs necessarily reduce the expected flare
load. There can be situations in which an automatic trip can actually
• level 1 (low-level) is a single-path design with no redundant increase the expected flare load. An example of this is a steam-turbine-
components; driven reflux pump that is expected to continue to work in the event of
• level 2 (medium-level) consists of some redundancy (especially of an electric power failure but gets cut out by a steam-load-shedding
components with known low reliability); and system acting to prevent failure of the overall plant steam supply system.
HYDROCARBON WORLD VOLUME 4 ISSUE 1
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