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Table of Contents
                            DP MANUALS INDEX
MAIN DP's INDEX
SAFETY IN PLANT DESIGN TOC
SCOPE
DEFINITIONS
GENERAL
CONTROL OF HAZARDS IN PLANT DESIGN
	OVERSTRESSING
	FIRES AND EXPLOSIONS
	OPERATIONAL FACTORS AFFECTING SAFETY
	PROCESS FACTORS ASSOCIATED WITH SAFETY
	ENVIRONMENTAL FACTORS AFFECTING SAFETY
DESIGNING PLANTS TO MINIMIZE DAMAGE FROM FIRE OR EXPLOSION
SPECIAL CONSIDERATIONS IN SAFETY DESIGN
	SPECIAL FACTORS
	ADDITIONAL DESIGN SAFETY FEATURES
TABLES
	Table 1 Fire Hazard Properties of Some Gases and Liquids
	Table 2 Examples of Safety Critical Check Valve Applications
                        
Document Text Contents
Page 1

SAFETY IN PLANT DESIGN DESIGN PRACTICES

BASIC PRINCIPLES Section
XV-A

Page

1 of 13

PROPRIETARY INFORMATION - For Authorized Company Use Only
Date

December, 1998

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON
ENGINEERING

CONTENTS
Section Page

SCOPE .................................................................................................................................................... 2

DEFINITIONS........................................................................................................................................... 2

GENERAL................................................................................................................................................ 6

CONTROL OF HAZARDS IN PLANT DESIGN ......................................................................................... 7

OVERSTRESSING........................................................................................................................... 7

FIRES AND EXPLOSIONS............................................................................................................... 8

OPERATIONAL FACTORS AFFECTING SAFETY ............................................................................ 9

PROCESS FACTORS ASSOCIATED WITH SAFETY ....................................................................... 9

ENVIRONMENTAL FACTORS AFFECTING SAFETY......................................................................10

DESIGNING PLANTS TO MINIMIZE DAMAGE FROM FIRE OR EXPLOSION .........................................10

SPECIAL CONSIDERATIONS IN SAFETY DESIGN ................................................................................10

SPECIAL FACTORS .......................................................................................................................10

ADDITIONAL DESIGN SAFETY FEATURES ...................................................................................11

TABLES
Table 1 Fire Hazard Properties of Some Gases and Liquids......................................................12
Table 2 Examples of Safety Critical Check Valve Applications...................................................13

Revision Memo

12/98 Page 3: Expanded definition of Auto-Ignition Temperature.
Revised definitions of Blast Protection, BLEVE, and Critical
Exposure Temperature.

Page 4: Revised definitions of Deflagration, Emergency, Detonation and
Vapor Cloud Explosion. Added ethylene oxide and propylene oxide
to list of fuels that increase probability of a detonation. Added
oxygen enriched air to conditions that increase probability of
detonation. Added definition of Exposure Limits / TLVs.

Page 5: Revised definitions of Flammable and Combustible Liquids.

Page 6: Revised definitions of High and Low Flash Stocks, and Hazard.

Added definition of High Integrity Protective System.

Page 7: Expanded scope of Safety Critical Devices.

Added definition of Safety Integrity Level.
Included discussion of Inherent Safety under Managing Control of
Hazards.

Page 8: Modified pressure limitation for equipment at temperatures below
the CET for consistency with Section II.

Page 9: Revised reference to High Integrity Protective Systems.

Revised safe location criteria for the discharge of atmospheric vents.
Page 10: Added reference to oxygen under Control of Oxidants.

Included additional examples of potentially unstable processes.

Page 13: Updated references to NFPA 325 to reflect latest edition.

Added TABLE 2 - EXAMPLES OF SAFETY CRITICAL CHECK
VALVE APPLICATIONS.

Changes shown by ç

Page 2

DESIGN PRACTICES SAFETY IN PLANT DESIGN

Section

XV-A

Page

2 of 13

BASIC PRINCIPLES

Date
December, 1998 PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON
ENGINEERING

SCOPE

This section describes the basic principles which underlie the application of safety considerations in plant design.

DEFINITIONS

The following definitions cover terms which are commonly used in safety design. They are listed in alphabetic order. Further
definitions of terms particularly associated with pressure relief are included in Section XV-C.

Auto-Ignition Temperature

ç The Auto-Ignition Temperature, AIT (also Ignition Temperature, or Spontaneous Ignition Temperature or Self-Ignition
Temperature) is the lowest temperature required to cause self-sustaining combustion, without initiation by spark or flame.
Auto-ignition temperatures of the majority of hydrocarbons fall in the range of 400 to 1000°F (200 to 500°C) (see Table 1). In
certain phases of safety design, an arbitrary auto-ignition temperature of 600°F (315°C) is used (e.g., in layout, Section XV-G).
This is judged to be a conservative estimate in the absence of experimental data. When experimental data is available, the
actual value of the AIT or 600°F (315°C), whichever is lower, should be used as the governing criterion for equipment spacing
and any other design features where AIT is a consideration.

Hydrocarbon liquids or vapors can be heated to the AIT by coming into contact with hot equipment when released in an
uncontrolled manner. The auto-ignition mechanism may also work just by releasing a hot product above AIT. If the equipment
surface temperature is at or above the AIT, spilled liquids are likely to ignite. However, ignition of vapors is a function of
temperature and exposure time. Vapors are likely to ignite if they come into short contact with equipment whose temperature is
400°F (200°C) above the AIT. Only if vapors are allowed to contact equipment at lower temperatures for prolonged time is an
ignition probable. (See API PSD 2216 “Ignition Risk of Hot Surface in Open Air", July 1980.)

If insulation around piping or vessels becomes soaked with oil, ignition and fire may occur at a temperature considerably below
AIT. This phenomenon is called Wicking Action and results from the combination of fibers largely increasing the oil surface
area and prolonged exposure to heat from the equipment. It is therefore essential to maintain insulations free of oil.

Blast Protection

ç Blast Protection is adding design features to a building such that it will be capable of withstanding an external explosion of
defined magnitude. The strength of the potential explosion and the distance between the building and the potential explosion
domain are critical parameters in determining blast protection requirements. For more details see Section XV-H.

BLEVE

ç BLEVE is the acronym for a Boiling Liquid Expanding Vapor Explosion. This type of explosion occurs if a vessel containing
superheated hydrocarbon liquid fails catastrophically when fire exposure results in overheating and yielding of a pressure
vessel.

Contingency

A contingency is an abnormal event which causes an emergency. A single contingency is a single abnormal event causing an
emergency. A remote contingency is the result of a single extremely low probability event, or of two remotely related events
which may happen to occur simultaneously. A single contingency is part of the design basis for the facility. Remote
contingencies are not part of the design basis. However, since remote contingencies may occur, even though the probability of
such an occurrence is extremely low, equipment should be checked to ensure it will not fail if subjected to loads resulting from a
single remote contingency.

A double contingency would be the simultaneous occurrence of two or more unrelated events between which there is no
process, mechanical, or electrical inter-relationship. Double contingencies are not used as a basis for designing equipment.

Critical Exposure Temperature

ç Refer to Section II, DESIGN TEMPERATURE, DESIGN PRESSURE AND FLANGE RATING, for the definition of Critical
Exposure Temperature (CET) for pressure vessels, tanks and piping. When the metal temperature of equipment or piping is
below the CET, there is a risk of brittle fracture if the stresses from the operating pressure or other loads exceed some
percentage of the design allowable stress.

Page 6

DESIGN PRACTICES SAFETY IN PLANT DESIGN

Section

XV-A

Page

6 of 13

BASIC PRINCIPLES

Date
December, 1998 PROPRIETARY INFORMATION - For Authorized Company Use Only

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON
ENGINEERING

DEFINITIONS (Cont)

During storage, butadiene forms peroxides (which may be spontaneously explosive), and which in turn may lead to the
formation of plastic polymer and/or pyrophoric “popcorn" polymer. These reactions may be limited by inhibitors and by
controlling storage temperatures as low as practical, but precautions must be taken when opening equipment for inspection or
repair. For further information, refer to Exxon Chemical Company publication, Storage and Handling of Liquid Olefins and
Diolefins.

Reid Vapor Pressure (RVP)

The vapor pressure of a liquid at 100°F (37.8°C), determined by a standard laboratory procedure (ASTM Test D-323),
expressed in psia, is called the Reid Vapor Pressure. This test is applied only to crude oils, naphthas, gasolines, and materials
of similar volatility. The vapor pressure of LPG and similar materials is determined by ASTM Test D-1267. A listing of Reid
Vapor Pressures for some materials can be found in Table 1.

Risk

The combination of the probability of an abnormal event or failure and the consequences of that event on workers, the
community and the plant. There can be no risk without a hazard.

Safety Critical Device

ç A device or system is considered safety critical if it is the last line of defense to prevent an uncontrolled major breach of
containment, severe personal injury or death or a major environmental incident, or is otherwise essential in the control or
mitigation of such incidents. The term "safety critical" is usually applied to instrumentation, but any device may qualify as safety
critical if its failure could lead to serious consequences. For example, heat tracing systems (steam or electric) used to prevent
plugging of pressure relief devices due to solidification of process fluids are considered safety critical and should be identified
as such. Check valves can also be safety critical under certain conditions. Table 2 lists some examples of safety critical check
valve applications. Other examples of safety critical devices include restriction orifices that limit the flow rate to a pressure
relief device and Emergency Block Valves (EBVs).

Safety critical devices should be identified as such in relevant documentation such as Piping and Instrumentation Diagrams
(P&IDs), operating manuals, and equipment files. For safety critical instrumentation, reliability targets (Safety Integrity Levels)
must be specified, a testing and maintenance program must be in place to ensure that the reliability target is achieved, and a
system to control deactivation of the device must exist. All safety critical devices or systems must be subject to periodic
inspection and maintenance, and Management of Change (MOC) protocols must control their removal, alteration or
replacement.

ç Safety Integrity Level (SIL)

One of three possible discrete levels used to characterize the reliability of instrument-based safety systems as prescribed in
ISA S84.01. SILs are defined in terms of Probability of Failure on Demand (PFD). The PFDs for various SIL levels are as
follows:

SAFETY INTEGRITY LEVEL (SIL) PROBABILITY OF FAILURE ON DEMAND (PFD)

SIL-1

SIL-2

SIL-3

Between 1 in 10 and 1 in 100

Between 1 in 100 and 1 in 1000

1 in 1000 or better

S GENERAL

The basic principles upon which safety is incorporated into a plant design can be summarized in the three following steps:

1. Managing Control of Hazards
Hazards in process, storage and transportation should be managed by the following strategies.

ç Potential hazards that are associated with a process should be identified and evaluated by a thorough study during the
conceptual stage. Efforts should then be made to eliminate or reduce the hazard through the application of inherent
safety principles. The focus of inherent safety is on the avoidance of potential hazards rather than on their control by
the addition of protective equipment. The basic principles involved in designing for inherent safety are:

Page 7

SAFETY IN PLANT DESIGN DESIGN PRACTICES

BASIC PRINCIPLES Section
XV-A

Page

Page 13

SAFETY IN PLANT DESIGN DESIGN PRACTICES

BASIC PRINCIPLES Section
XV-A

Page

13 of 13

PROPRIETARY INFORMATION - For Authorized Company Use Only
Date

December, 1998

EXXON RESEARCH AND ENGINEERING COMPANY - FLORHAM PARK, N.J.

EXXON
ENGINEERING

ç TABLE 2
EXAMPLES OF SAFETY CRITICAL CHECK VALVE APPLICATIONS

1. Check valves for which credit has been taken for the prevention or reduction of backflow from high-pressure systems to
low-pressure systems when sizing the pressure relief device protecting the low-pressure system. Refer to EVALUATION
OF PRESSURIZATION PATH IN PRESSURE RELIEF DESIGN in Section XV-C.

2. Check valves used to separate portions of a system with different design temperatures, CETs, shock chilling potential or
construction materials if reverse flow or leakage through the check valve could lead to a loss of containment incident.

3. Check valves on the discharge of centrifugal pumps and centrifugal or axial compressors rated at or above 500 BHP. For
compressors with one or more interstage feeds, there should also be a safety critical check valve at the suction of the first
stage and at each interstage feed except the highest pressure one. For compressors with interstage products, the check
valve on each interstage product is also considered safety critical. The reason for this is that failure of a check valve in
these services can result in severe damage to the machine due to backspinning.

4. Check valves on the discharge of spared centrifugal pumps and centrifugal or axial compressors supplying a utility (e.g.,
cooling or boiler feed water, utility or instrument air) where failure of the check valve on a machine being taken off-line to
close could lead to total loss of a utility.

5. Check valves on the discharge of firewater pumps and in connections to firewater systems.

6. Check valves with drilled flappers for protection against thermal expansion of trapped liquids or water freeze-up.

7. Check valves intended for emergency isolation of fired heaters from downstream inventory in the event of a tube rupture.

8. Check valves intended to prevent the uncontrolled mixing of air with combustible or flammable materials. For example, the
check valve in the air injection line to Merox Sweetening or the check valve in the ammonia injection line to Thermal
DeNOx facilities, unless there is protective instrumentation such as a low-flow or low-pressure-differential cut-out valve.

9. Check valves in compressed air starting systems for diesel and gas engines (refer to ELIMINATION OF FLAMMABLE
MIXTURES IN COMPRESSED AIR SYSTEMS in Section XV-B).

10. Check valves in utility connections where the normal operating pressure of the process exceeds the normal operating
pressure of the utility. For example, the check valve in a steam line to a heat exchanger where the process pressure is
higher than the steam pressure. For cases where the operating pressure of the process exceeds the operating pressure of
the utility only during abnormal conditions, the need to treat check valves as safety critical should be based on a risk
assessment.

11. Check valves intended to prevent the backflow of vapor or mixed phase streams into atmospheric tankage.

12. Check valves in articulated pipe drains for floating roof tanks to prevent a large spill in the tank area.

13. Check valves in motive fluid accumulators for Emergency Block Valves.

14. Check valves intended to prevent a release of toxic material near grade, such as combustion air to a CO boiler or Claus
plant.

Installation Requirements for Safety Critical Check Valves
1. Safety critical check valves should be installed such that they will close depending only on gravity, not on flow reversal,

spring action or external actuators. For example, swing and tilting plate check valves should be installed in vertical upflow
or horizontal lines, straight-through ball and dual-plate check valves should be installed in vertical upflow lines, and globe-
type ball check valves should be installed in horizontal lines.

2. The use of "wafer-type" (flangeless) check valves is forbidden by IP 3-12-1 for service temperatures above 600°F (315°C).
3. Safety critical check valves should be installed such that they can be tested on line, without the need for removal or

disassembly. A bleeder should be provided upstream of the check valve for this purpose.

4. Where two or more check valves are installed in series for safety reasons, at least two different types of check valves
should be specified to minimize the risk of a common mode failure. Consultation with ERE's Mechanical Engineering
specialists to determine the optimum combination of check valve types is recommended.

5. Safety critical check valves should be clearly identified in unit Piping and Instrumentation (P&I) Diagrams and in the field to
minimize the risk of changes without appropriate review, and should be subject to regular inspection and testing programs
similar to those applicable to other safety critical devices.

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