1. Scope
APPENDIX A
Load Analysis
An analysis of the causes of discharge from the pressure relief valves that protect equipment, either
individually or in groups, helps to define the critical areas in process units. Major sources of flare loads are
thoroughly analyzed herein. However, sources that do not significantly affect the total load do not require
detailed analysis.
2. General
2.1 Flare Load Design
For closed pressure relief systems, flare load design is based on the total load from all contributors to the
single failure under consideration, rather than on the total load from all pressure relief devices.
2.2 Evaluation
Proper evaluation of simultaneous loads, which result from the single failures used to determine major flare
loads, requires considerable judgment, experience and a thorough knowledge of the operations and
equipment involved.
2.3 Causes of Discharge
The most common causes of pressure relief valve discharges that affect more than one process unit in an
installation are:
a. Loss of cooling water
b. Total or partial power failure
c. Loss of steam
d. Fire
2.4 Factors Affecting Emergency Relief Load
To establish the maximum emergency pressure relief load, factors to be considered are:
a. Layout of cooling water piping: this determines whether complete loss of cooling water to all
process units will occur (See ASME B31.3).
b. Types of drivers on cooling water pumps: standby drivers may be turbine driven or motor driven.
c. Use of air coolers: natural convection provides cooling when fans fail (see Section 4 in
Appendix A).
d. Use of steam-driven pumps or compressors or both, that remain in operation during power
failure.
e. Type of drive used on heat removal pumps or reflux pumps.
f. Use of proper instrumentation to keep heat removal circuits in operation during the emergency.
Note: Do not consider cases that result from double jeopardy, that is, two distinct events that are unlikely to
happen simultaneously, for example, a fire and a power outage. Multiple events with a common cause shall
be considered.
2.5 Two Phase Flow
Two-phase venting is the simultaneous flow of gas and liquid through a relief system. For a given relief
area when two-phase flow occurs, the volumetric discharge rate and the evaporative cooling of the vessel
contents decrease, even though the system’s mass-loss rate increases. Generally, two-phase flow requires
a larger relief area than all vapor or nonflashing liquid flow. The relief area requirement for homogeneous vessel venting can be as much as ten times that required for vapor-only venting. Special calculations are
required for sizing PSVs handling two phase flow. Recommended procedures for these calculations are
contained in Section 4.7 of API RP 520 PT I.
3. Heat Input
3.1 Effect on System
In general, heat input causes vapor generation and, in turn, increases pressure. This results in pressure
relief valve discharge.
3.2 Methods
Basic heat input methods common to most process units utilize:
a. Fired heaters
b. Heat exchangers
c. Reactor systems
3.3 Fired Heaters
a. Uses: The bulk of heat input for installation processes is provided by fired heaters. These
heaters, which are used for heat load services, include crude preheaters, reformer preheaters, coking
heaters and large column reboilers.
b. Heat Input to Equipment: To stop the heat input to equipment that is downstream of a fired
heater, the fuel flow shall be shut off. Where the oil charge is heavy and easy to crack, coke may form
in the heater tubes and prevent the complete shutoff of feed. Therefore, the oil charge shall be
continued at a reduced rate or steam shall be injected into the heater tubes to purge the oil. In either
event, after the burners are shut off, the heat input to the equipment that is downstream of the heater
shall continue at a reduced rate until one of the following occur:
(i) The oil or steam flow stops
(ii) The temperature of the fired heater drops to the oil or steam inlet temperature
3.4 Heat Exchangers
3.4.1 Heating Media: Heating media for two principal types of heat exchangers used for heat input in
process units are:
a. Steam: When steam is used as the heating medium, the continuation of heat input means a net
heat input to the overall process unit. To stop the heat input, either the steam or the flow of material
being heated shall be shut off. With no automatic action, it shall be assumed that the heat input will
continue at the normal rate until the material being heated is depleted. Following is a simplified
example of the use of steam.
(i) A steam preheater heats a column feed that comes from a surge drum. The preheater continues
heat input until the level control on the surge drum shuts off. Assume that feed continues at the
normal rate until the volume between the top and bottom level control taps is depleted. For steam
heated reboilers, compute the liquid holdup and composition in the column to establish feed duration.
Again, assume normal reboil rate during this time.
b. Hydrocarbons: Heat exchangers that use hydrocarbons from another section of the same unit as
a heating medium merely transfer heat from one section to the other. For example, a debutanizer
column reboiler may be heated by a heavy gas oil (HGO) intermediate stream from a fluid catalytic
cracking (FCC) main fractionator. The heat input to the debutanizer increases the flare load from the
gas plant and the load from the main fractionator decreases.
3.4.2 Limiting Heat Input: The duration and rate of heat input for heat exchangers may be limited by either
the heating medium or the material being heated and both shall be checked. Plug flow of the heating medium inventory is assumed in closed systems. After the initial inventory is depleted, the heat input rate
shall be adjusted for the change in temperature of the heating medium.
3.4.3 Operating vs Set Pressure: When the heat exchanger input method is used, the difference between
the normal operating pressure of a system and the set pressure of the pressure relief valves shall be
determined. In some cases, this difference is large and therefore the heat input rate for a reboiler at
relieving pressure and temperature may change significantly from the heat input rate at operating pressure
and temperature. The heat input rate shall be adjusted for this effect if the system is a major contributor to
the flare load.
3.5 Reactor Systems
Reactor systems may be divided into two general groups:
a. Those that require heat (endothermic)
b. Those that release heat (exothermic)
These systems contain large heat sinks that can continue to supply heat input when the primary source of
heat is shut off. Examples of reactor systems are coke drums, hydrocrackers and FCC reactors.
3.5.1 Endothermic Systems
Catalytic cracking, coking and reforming systems are examples of endothermic types of reactor systems.
Although endothermic reactions require heat from the reactor system, they result in an increase in volume
and pressure, which requires pressure relief. The rate of vapor generation depends on the feedrate to the
system and the mass of the heat sink. Cases to consider are:
a. System feed shutoff: When the system feed is shut off, the rate of vapor generation usually is low
and can be neglected in establishing flare loads.
b. Continuing system feed: When the system feed stream continues, vapor generation is computed
by assuming that the heat of reaction is removed from the mass of the system and that the reaction
rate varies with the system temperature. In cases where the heat of reaction is supplied by the
incoming hot catalyst, it is assumed that the vapor generated is directly proportional to the feed rate;
that is, the source of heat to the system does not decay with time.
c. Reforming systems: In cases where the fresh feed is shut off but the recycle gas flow continues,
the time and rate of heat input (based on the time to purge the system of reactants) shall be
computed.
3.5.2 Exothermic Systems
The exothermic type of reaction results in a decrease of moles in the system and a rise in system
temperature. Hydrocracking, hydrotreating, polymerization and alkylation are examples of exothermic
types. Pressure changes are calculated in the same manner as in the endothermic system. For exothermic
system the reaction continues and the released system heat tends to increase pressure.
3.6 Compressors
3.6.1 Uses: Compressors may be used for refrigeration systems, reinjection, recycle compression and
booster compression.
3.6.2 Cryogenic Systems: When a refrigeration compressor trips, the refrigeration system warms up and
the pressure increases. Designing the refrigeration system for the maximum pressure (when the
compressor is down and the system has equalized) shall be considered. The remaining parts of the system
will also warm up quickly and will require emergency relief. The compressor trip may be interlocked with
blocking of the system feed to minimize flaring. If this is done, valve closure time becomes critical. Valve
operators (air, hydraulic or motor) shall be considered carefully.
3.7 High Pressure Ethylene Systems
3.7.1 Special consideration shall be given to relief of systems and equipment handling ethylene at high
pressure (>4500 psig [>300 bar]).
3.7.2 At pressures around 300 bar and temperatures less than 60 °C, ethylene will behave like a liquid as
it expands through a relief valve. Conversely, at similar pressures and temperatures greater than 60 C,
ethylene will act as a gas. This is because ethylene passes through the critical point when it expands from
~ 60 C and 300 bar. PSVs designed for vapor service will not “pop” even with system pressures as much
as 130 percent of the valve set point.
4. Heat Removal
4.1 Basic Methods
Some basic methods of consuming heat input to a given system are:
a. Use of air and water exchangers to completely remove heat
b. Use of heat exchangers to transfer heat to other systems
c. Raising the heat content of the system to absorb heat by such factors as quantity and
temperature of the oil inventory
d. Vapor generation to absorb heat by increasing system pressure
4.2 Air and Water Exchangers
4.2.1 Air Coolers and Condensers: In cases of fan failure, it shall be assumed that 10 percent of design
duty can be maintained through natural convection on forced draft air coolers. With induced draft air
coolers, 25 percent of the design duty may be assumed. Higher percentages of natural convection may be
available on induced draft air coolers and shall be calculated in accordance with the AIR PFR computer
program (MRDC).
4.2.2 Water Coolers and Condensers: It is assumed that water coolers and condensers maintain a duty
proportional to the cooling water circulation rate. For total loss of circulating water, the exchangers are
assumed to have no heat removal capacity. Some decay time will occur on loss of circulating water while
the water in the exchangers is heated to hotside temperatures. Normally, this decay is not taken into
account and is assumed to be negligible. The decay time shall be checked in exchangers that have large
water volumes or in which steam will be generated.
4.2.3 Overhead Condensers: The overhead condenser is the most critical service for both air and water
exchangers. When part of the overhead heat removal capacity is lost, the new exit vapor-liquid
composition and temperature are estimated from the condensing curve. This is particularly important in
columns with overhead material of a wide boiling range, for example, crude columns, FCC main
fractionators and coker fractionators.
4.2.4 Liquid Filled Air Coolers: The relief rate for fire exposure of liquid filled air coolers shall be based on
a spill fire under or adjacent to the air cooler. The fire area is based on the drainage system layout
(typically 3,000 square feet). Assume that 100 percent of the surface area of all rows of tubes (without fins)
is exposed to the fire, whether or not the fans are in operation. If the exchangers are located under the
high point of the drainage area, with adequate equipment spacing and no fire potential equipment located
under the coolers, consideration could be given to reducing the tube surface area assumed to be exposed
to fire to no less than 50 percent of the surface area of all rows of tubes (without fins).
4.3 Heat Exchangers
4.3.1 The heat exchanger method covered in Section 3 of Appendix A is applicable here. Heat transfer
circuits are merely a means of transferring heat from one section of the system to another. As an example,
consider an FCC reactor system in which the debutanizer is reboiled with an HGO stream from the main
fractionator. When a failure occurs, the total feed to the FCC reactor stops and, thus, the heat input to the
main fractionator stops. If the debutanizer reboiler circuit continues, the level of the oil in the main
fractionator is substantially decreased and the heat is used to generate a flare load in the debutanizer. If,
however, the debutanizer reboiler circuit is stopped, neither the main fractionator nor the debutanizer
relieves to the flare.
4.3.2 The action of all heat transfer circuits on the occurrence of a failure shall be carefully considered to
minimize the total release from the system.
5. Heat Content of System
5.1 Analysis
5.1.1 Not all the heat input to a system goes to vapor generation. When a system is analyzed for flare
loads, a part of the heat and material input can and, normally, does remain in the system. This can best be
illustrated by an example of heat content.
5.1.2 Assume a column is being fed from a surge drum through a steam preheater. The preheater heats
the feed from 38 C (100 F) to 121 C (250 F). At tower conditions, the feed flashes to 30 percent by
weight vapor. If all streams to and from the tower (except the feed) were to stop, only the incoming vapor or
an equivalent number of moles will have to be condensed to prevent pressure buildup. The heat associated
with the liquid portion of the feed remains in the system.
5.1.3 This concept is important in analyzing large columns, for example, crude columns and FCC main
fractionators, where the heat input is supplied by vaporized feed. The approach to this type of system is as
follows:
a. Flash the incoming feed at the expected flash conditions to determine the vapor feed to the
column.
b. Assume the product fractions of the feed are condensed at the draw temperature of the column.
Light fractions that boil at a lower temperature than the top temperature are assumed to go overhead
as vapor.
c. Determine the enthalpy difference between the incoming vapor and the condensed fractions.
This represents the portion of heat to be removed by vaporization of column inventory or heat
exchanger circuits. The balance of heat is used to heat the system.
d. Repeat this procedure for small increments of time, taking into account the increase in system
temperature and pressure levels. The time increments will depend on heat input rate, duration and
liquid inventory in the system.
5.2 Vapor Removal
5.2.1 The net vapor generation within a given system, after all methods of heat removal have been
considered, will cause the pressure to increase. Vapors may be removed from the system by any of the
following methods:
a. Compressors that continue to operate
b. Pressure control valves that vent to other units, to the fuel gas system or to the flare
c. Pressure relief valves that relieve to the flare or to the atmosphere
5.2.2 In addition, the system itself shall have a capacity to absorb vapors while increasing pressure to the
various set pressures of the relieving elements. For large systems, this surge capacity can be a substantial
aid in reducing peak flare loads and shall be computed.