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API RP 500 RECOMMENDED PRACTICE FOR CLASSIFICATION OF
LOCATIONS FOR ELECTRICAL INSTALLATIONS AT PETROLEUM FACILITIES
FIRST EDITION
Copyright Material IEEE
Paper No. PCIC-92-05
Paul S. Hamer, Senior Member, IEEE
Chevron Research and Technology Company
100 Chevron Way
Richmond, CA 94802-0627
David N. Bishop
Chevron U.S.A. Inc.
935 Gravier Street
New Orleans, LA 70112
Frederick Bried, Senior Member, IEEE
Shell Oil Company
Two Shell Plaza
Houston, TX 77252-2099
Abstract- API Recommended Practice 500 (RP 500),
Recommended Practice for Classification of
Locations for Electrical Installations at Petroleum
Facilities, provides guidelines for classifying
locations at petroleum refineries, production and
drilling areas, and pipeline transportation facilities
for the selection and installation of electrical
equipment. It has combined the previously separate
recommended practices RP 500A, RP 500B and RP
500C. This paper highlights the changes effected by
the new document. Particular attention is given to
the document sections on ventilation and the use of
combustible gas detectors.
Introduction
500" was formed. This task force was comprised of the
following six members, two from each of the API
Departments(Refining, Production,and Transportation):
David N. Bishop, Chevron (Production)
Frederick Bried, Shell (Transportation)
Paul S. Hamer, Chevron (Refining)
Floyd Lazenby, Exxon (Transportation)
Tom P. Pearson, Arc0 (Production)
John H. Rannells, Amoco (Refining)
The task force was charged with combining the three
documents into one that would be acceptable to all
three API departments -- a seemingly formidable task.
The initial and subsequent seven drafts of RP 500 were
edited by this group and the larger task forces originally
responsible for RP 500A, B, and C. The final document
was sent simultaneously to all three department's
standardization committees for letter ballot. The ballot
was unique in that each of the departments voted on
the general sections (Sections 1 through 5 and the
Appendices of RP 500)and its respective "Section" of
RP 500: "A" for Refining, "B" for Production, or "C" for
Transportation. The six-member task force was
responsible for resolving all ballot comments and
negative votes. Differences between each department
that could not be resolved meant that the unique
requirements each of the three departments be
integrated into RP 500 via Sections A, B, or C (Refining,
Production,and Transportation, respectively) in RP 500.
The approved RP 500 [6] was issued on June 1, 1991.
In order to comply with API policy, RP 500 must be
reaffirmed, revised, or withdrawn at least every five
years, or by June 1,1996. The procedures for updating
RP 500 will be similar to those initiated in 1988, and the
schedule could be accelerated if there are significant
changes in underlying codes or standards (e.g., the
NEC).
- History of the API RP 500 Series
While the National Electrical Code (NEC) (11 defines
rules on the selection and installation of electrical
equipment in classified areas, it provides only general
guidance on the extent of classified areas. The
American Petroleum Institute (API) RP 500 series (RP
500A, RP 5006, and RP 500C) have served for
decades as the petroleum industry's recommended
practices on the extent of classified areas. The first
document published by API on classificationof locations
also was designated RP 500, without a suffix, and was
issued by the Refining Department of API in 1955. This
document was further updated in 1957, issued as RP
500A in 1966, and updated in 1982 [2]. Since the initial
API document's scope included only refineries, the
Production Department of API created a document for
guidance in classifying production and drilling facilities.
Initially issued as RP 11J in 1961, it was designated RP
5006 in 1966 and updated in 1973 and 1987 [3][4].
Similarly, the Transportation Department of API issued
RP 500C covering pipelines in 1966, with updates in
1974 and 1984 [5].
The following includes highlights of RP 500. It is a
significant recommended practice for use by the entire
petroleum industry. Interpretationsof the recommended
practice are not intended.
The three documents were created and balloted by the
three separate departments within API, with minimal
liaison among their task forces writing the recommended
practices. In 1988, at the request of the three
Departmenttask forces, an "Advisory Task Force on RP
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92-CH3186-4/1/92/0000-0037 $03.00 @ 1992IEEE
Highlights of API RP 500 "General" Sections,
Sections 1 Through 5
Several methods of achieving adequate ventilation are
listed. All methods must assure that areas where
heavier-than-air vapors or lighter-than-air vapors may
collect are ventilated. Although not all-inclusive, the
methods listed follow:
Section 1
Section 1 emphasizes that the NEC [l] provides the
basic definitions and that RP 500 provides guidelines for
classifying locations at petroleum facilities for the
selection and application of electrical equipment. Its
electrical equipment applications.
scope includes&o
Specific recommendations for the degree and extent of
classified areas unique to refineries, production facilities,
and pipeline facilities are given in Sections A, B,and C,
respectively.
1) ventilation at a minimum rate of one cubic foot of
air volume flow per square foot of floor area, but
at least six air changes per hour;
2) natural
ventilation of enclosed areas
(recommended only for areas of 1000 cubic feet
or less) using the natural draft "stack effect" with
a safety factor of two, increasing the air flow
requirements to at least twelve air changes per
hour. Reference sample calculations in RP 500,
Appendix A.
Section 2
The "fire triangle" components are summarized in
Section 2. That is, for a fire to exist:
3) natural ventilation by the inherent construction of
the building or partially enclosed area: or,
1) the flammable gas or vapor must be present:
2) the gas or vapor must be mixed with air or
4) ventilation rates determined by the fugitive
emissions method described in RP 500,
oxygen in the proper quantities and
proportions; and,
3) the mixture must be ignited.
Appendix B.
Prior to this edition of RP 500, the general
recommendationwas to ventilate at a rate of twelve air
volume changes per hour for an area to be adequately
ventilated. In cold-weather climates, this imposed
severe requirements for enclosed areas where the
ventilation air had to be heated. The NFPA 30
Technical Committee considered the successful
experiences of the petroleum industry in making
changes to the 1990 edition of NFPA 30.
Section 3
Descriptions of flammable and combustible liquids,
highly volatile liquids, and flammable gases are
presented in Section 3. Also, the basis for the NEC [I]
"Grouping" of atmospheric mixtures is given.
With the new recommendations of RP 500, one has
choices based on the fundamental definition of
adequate ventilation rather than arbitrary or conservative
ventilation rates. The "fugitive emissions method"
provides a method of calculatingthe requiredventilation
rate based on the typical leakage rate from components
such as pump seals, valve packing, and flange gaskets.
Typically, this calculation method will result in required
minimum ventilation rates of less than three air changes
per hour for "adequate ventilation." For such low rates,
however, gas monitoring is recommendedto assure that
flammable gas concentrations of less than 25 percent
LFL are maintained.Applicationof the fugitive emissions
method requires an accurate count of all the potential
hydrocarbon leak sources in an enclosed area and
extensive calculations.
Section 4
One of the most significant parts of the document is
Section 4, "Classification Criteria." The definitions of
Class I, Division 1 and Class I, Division 2 are restated
from the NEC [I],except for editorial changes to
improve clarity for those involved with petroleum
operations. Considerations for classifying an area
Division 1 or Division 2 are included, and specific
locations that may be considered unclassified,
regardless of the degree of ventilation, are listed.
"Ventilation," Section 4.6, is significantly changed from
previous editions of the RP 500 series. A reason for this
change is the change in the NFPA 30, 1990 Edition [7],
definition of adequate ventilation. Reference Appendix
A of this paper. Although API RP 500 does not totally
embrace all the information in NFPA 30, particularly the
"sampling" provision of 5-3.3.1(b), RP 500 accepts the
methodology of using "fugitive emissions" to verify
adequate ventilation. Appendices B and C of RP 500
cover this subject.
The use of combustible gas detection equipment
(Section 4.8) can be the basis for classifying an
inadequatelyventilated areas containing equipment that
could release a flammable gas or vapor Division 2, but
never to make the area unclassified. Also, the interior of
vapor-tight rooms or buildings not containing a gas
source can be considered unclassified if detection
equipment is installed and maintained within the
stringent requirements outlined in RP 500. The
detectors are required to be of the stationary type,
approved or listed by a nationally recognized testing
laboratory, applied to ensure that all areas where
flammable gas might accumulate are monitored, and
The fundamental definition of adequate ventilation given
by RP 500 is as follows: "...ventilation (natural or
artificial) that is sufficient to prevent the accumulationof
significant quantities of vapor-air mixtures in
concentrationabove 25 percent of their lower flammable
(explosive) limit (LFL)..." [6].
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calibrated at a frequency in accordance with the
manufacturer's recommendation, but at least every
three months. An alarm should be activated upon
sensing 20 percent LFL and power should be
disconnected from devices in the area, if those devices
are not suitable for the area classification without gas
detectors, when 40 percent LFL gas concentration is
sensed. The use of combustible gas detectors provides
one with a new approach to design. However, if the
area being considered for the use of gas detectors
contains equipment that may release flammable gases
or vapors dugng normal operations, gas detectors are
not a feasible option unless some degree of ventilation
is provided. Otherwise, frequent alarms or equipment
shutdowns can occur.
Highlights of Section 8, Production
The last edition of RP 500B introduced the "limited
ventilation" concept. This was dropped in RP 500, and
the figures of Section 6 reflect this change, referring
only to "adequate" or "inadequate" ventilation.
Productionoperations, particularly offshore, have severe
space restrictions. For this reason, many specific pieces
of equipment are illustrated in the figures. Paragraphs
on sumps, drains, valves, and valve actuators were
added to RP 500. Recommendations for hydrocarbonfueled prime movers have been expanded.
Highlights of Section C, Pipeline Transportation
Section 5
In 1989, a task force was organized to review the
RP 500C in accordance with APl's five year review
cycle requirement. The formation of this task force took
place at about the same time as the "Advisory Task
Force on RP 500" initiated work combining RP 500 A,
B and C. Rather than revise the second edition of RP
500C (1984) for letter ballot, the RP 500C task force felt
that RP 500C should be reaffirmed, allowing the task
force to focus their efforts reviewing and commenting on
the various RP 500 drafts from a transportation
viewpoint.
Section 5 covers the extent of a classified location
based on the amount of release and whether the
location is outdoors or indoors. Statements on gas
density and the degree of air movement are included in
this section.
An outline of the procedure for classifying a location is
part of RP 500 (Appendix F) and is included as
Appendix B to this paper. It serves as a general
description of the steps required to classify an area.
The three categories or levels developed in the second
edition of RP 500C, were retained in Section C of
RP 500. Since the potential spread of a liquid, vapor or
gas release depends in part upon the operating
pressure and type of material involved, distances are
assigned for the difference in potential spread by using
separate dimensions for:
Highlights of Section A, Refining
Experience within refineries during the evolution of RP
500A had shown that occurrences of flammable material
liberation from some operations and apparatus is so
infrequent that it is not necessary to classify the
surrounding areas. An example cited in RP 500 is an
adequately ventilated location where flammable
substances are contained in suitable, well-maintained
(italics added) closed piping systems that include only
the pipe, fittings, flanges, meters, and small valves. This
area doesnoJ require classification. This is an exception
to the general Division 2 requirement that excludes only
"... all-welded closed piping systems without valves,
flanges or similar devices, or continuous metallic tubing
without valves, fittings, flanges, or similar devices..."
(italics added) and considers these locations to be
unclassified. Sound engineering judgement must be
applied to the "small valve" term, remembering that RP
500 is only a guide.
1) gas or liquids at an operating pressure of 275
psig or less,
2) gas or liquids at an operating pressure above
275 psig, and
3) highly volatile liquids (HVL) as defined by the
Department of Transportation (DOT).
Additional figures that were developed or added to
Section C are the tank carhank truck figures from NFPA
497A [8] and below-grade vaults.
The major changes to the figures include a drainage
path from tanks; expansion on the figure for separators,
dissolved air floatation units, and biological oxidation
units; and extension of Division 2 for cooling towers and
designating the pump pit as Division 1. Additional tank
carltank truck reference figures from NFPA 497A [8] are
included.
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Conclusion
References
The merging of RP 500A, B, and C into a single RP 500
is a major benefit to the petroleum industry. It required
the hard work and review of many people in the various
technical subcommittees and task forces of API. The
result is a document that can be used by the entire
petroleum industry in a consistent manner.
[ l ] NFPA 70, National Electrical Code. Quincy, MA:
National Fire Protection Association. 1990.
[2] API RP 500A. ClassificationofLocationsfor Electrical
Installations in Petroleum Refineries.
4th Ed.
Washington, DC: American Petroleum Institute,
1982.
The document can be improved with input from those
who use it. For example, in future issues of RP 500,
one could expect to see similar figures from Sections A,
B, or C to be grouped in a section of common figures.
Figures unique to the refining, production, or
transportation segments of the petroleum industry would
remain in the existing Sections A, B, and C. Examples
that might be included in a "common figure" section
would be storage tanks, tank truck and rail car loading
areas, marine loading facilities, and compressor
buildings with lighter-than-air gas or vapor sources.
Comments or suggestions should be addressedto API.
Copies of RP 500 are available from: American
Petroleum Institute, Publications and Distribution
Section, 1220 L Street NW, Washington, DC 20005,
phone (202) 682-8375.
[3] API RP 5008, Recommended Practice for
Classificationof Locations for Electrical Installations at
Drilling Rigs and Production Facilities on Land and on
Marine Fixed and Mobile Platforms, 3rd Ed.
Washington, DC: American Petroleum Institute,
1987.
[4] R. J. Gobrick, M. W. Wedel, "The New API RP
5008 - One Year Later," in Conf. Rec. 1988 35th
Annual IEEE Petroleum and Chemical Industry
Conference, pp. 119-124.
[5] API RP 500C, Classificationof Locationsfor Electrical
Installations at Pipeline Transportation Facilities. 2nd
Ed. Washington, DC: American Petroleum Institute,
1984.
[6] API RP 500, Recommended Practice for Classification
of Locations for Electrical Installations at Petroleum
Facilities. 1 st Ed. Washington, DC: American
Petroleum Institute, 1991.
(71 NFPA 30, Flammable and Combustible Liquih Code.
Quincy, MA: National Fire Protection Association,
1990.
[8] NFPA 497A, Recommended Practice for Cla,ss$cation
of Locations of Class I Hazardous (Classified)Locations
for Electrical Installations in Chemical Process Areas.
Quincy, MA: National Fire Protection Association,
1986.
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APPENDIX A VENTILATION
(From NFPA 30 [7])
APPENDIX B
(Appendix F from API RP 500 [SI)
PROCEDURE FOR CLASSIFYING LOCATIONS
5-3.3 Ventilation.
5-3.3.1 Enclosed processing areas handling or using
Class I liquids or Class II or Class Ill liquids at
temperatures above their flash points shall be ventilated
at a rats sufficient to maintain the concentration of
vapors within the area at or below 25 percent of the
lower flammable limit. This shall be confirmed by one
of the following:
This Appendix is intended to provide an outline of the
basic procedures requiredto classify a location. It is not
all inclusive but should offer guidance to inexperienced
individuals.
Combined with sound engineering
judgement it will provide a good start toward area
classification.
F.l Introduction. The following procedure requires
answering a series of questions. An affirmative answer
to either question in Paragraph F.2 verifies the likely
existence of a hazardous (classified) location.
Boundaries of locations may be determined by applying
the recommendations of the preceding sections and
referring to appropriate figures in Sections A, B and C,
as applicable. Each room, section, or area should be
considered individually in determine its classification.
Initial planning should focus on grouping of sources and
allowing unclassified areas for electrical equipment
installation.
(a) Calculations based on the anticipated fugitive
emissions (see Appendix F for calculation method), or
(b) Sampling of the actual vapor concentration under
normal operating conditions.
The sampling shall be conducted at a 5-ft (1.5-m)
radius form each potential vapor source extending to or
toward the bottom and the top of the enclosed
processing area. The vapor concentration used to
determine the required ventilation rate shall be the
highest measured concentration during the sampling
procedure.
NOTE: Final &terminations of classification may
require coordination with process engineers, facility
design engineers,fire and safety specialists, instrument
engineers, and electrical engineers.
NOTE: Equipment in enclosed processing areas may
deteriorate over time, and periodic sampling should be
conducted to ensure that leakage rates have not
increased or that the ventilation rate is adequate for
any increase in leakage rates.
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F.2 Step 1 Need for Classification. The need for
classification of a location is indicated by an affirmative
answer to either of the following two questions:
An acceptable alternate is to provide ventilation at a
rate of not less than 1 cu ft per minute per sq ft of solid
floor area (0.3 m’ per min per m*). Ventilation shall be
accomplished by natural or mechanicalventilation, with
discharge or exhaust to a safe location outside the
building, without recirculation of the exhaust air.
1. Are flammable liquids, gases, or vapors handled,
processed or stored in or adjacent to the area?
2. Are combustible liquids likely to be handled,
processed or stored in or adjacent to the area at
temperatures above their flash points?
Exception: Recirculation is permitted where it is monitored
continuously using a fail-safe system that is designed to
automatically sound an alarm, stop recirculation, and
provide full exhaust to the outside in the event that vapor-air
mixtures in concentration over one-fourth of the lower
flammable limit are detected.
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F.3. Step 2
Assignment of Classification.
Assuming an affirmative answer from Step 1, the
question in F.3.1 and F.3.2 should be answered to
determine the degree of classification (Division 1 or
Division 2).
Provision shall be made for introduction of make-up
air in such a manner as to avoid short-circuiting the
ventilation. Ventilation shall be arranged to include all
floor areas or pits where flammable vapors may collect.
Where natural ventilation is inadequate, mechanical
ventilation shall be provided and shall be kept in
operation while flammable liquids are being handled.
Local or spot ventilation may be needed for the control
of special fire or health hazards. Such ventilation, if
provided, can be utilized for up to 75 percent of the
required ventilation. (NFPA 91, Standard for the
Installation of Blower and Exhaust Systems for Dust, Stock,
and Vapor Removal or Conveying, and NFPA 90A,
Standard for the Installation of Air Conditioning and
Ventilating Systems, provide information on this subject.)
F.3.1 Division 1 locations normally are dictated by an
affirmative answer to any one of the questions that
follow:
1. Is an ignitible atmospheric concentration of gas or
vapor likely to exist in the location under normal
operating conditions?
2. Is an ignitible atmospheric concentration of gas or
vapor likely to occur in the location frequently
because of maintenance, repairs, or leakage?
3. Would a failure of process, storage, transfer or
similar equipment likely cause an electrical system
failure that would create an ignition source (e.g.,
arcing) simultaneously with the release of ignitible
concentrations of gas or vapor?
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4. Is flammable liquid or gas handled, processed or
stored in an inadequately ventilated location?
NOTE: Specific piping and tubing systems
described in Paragraph 4.5.1 are excluded
from this consideration.
5. For flammable liquids with heavier-than-airvapors,
is ventilation inadequate to ventilate all areas
(particularly floor areas) where flammable vapors
might collect?
6. For lighter-than-air gases, are roof or wall
openings inadequately arranged to ventilate all
areas (particularly ceiling areas) where gases
might collect?
F.3.2 After Division 1 locations have been
determined, Division 2 locations usually may be
distinguished by an affirmative answer to any one of the
following questions:
1. In a system containing flammable liquids or gases
in an adequately ventilated location, can the liquid
or gas escape from potential sources (such as
valve packing, flanges, or pump seals) as a result
of an abnormal condition?
NOTE: Specijic piping and tubing systems
described in Paragraph 4.5.1. are excluded from
this consideration.
2. Is the location adjacent to a Division 1 location
without separation by vaportight walls or barriers?
NOTE: In some cases communications ofjlammable
gases or vapors between adjacent locations can be
prevented by adequate positive-pressure ventilation
from a source of clean air. Reference Par. 4.43.
3. If a positive mechanical ventilation is provided,
could failure or abnormal operation of the
ventilating equipment permit ignitible
concentrations of gas or vapor to enter or
accumulate in the location?
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F.4 Step 3 Extent of Classified Locations.
Reference Paragraph 4.7 and Section 5. Reference
also Sections A, B or C, as applicable.
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F.5 Step 4 Determination of Group. Reference
Paragraph 3.5 to determine the proper group.
F.6 Documentation. It is recommended that area
classifications be documented.
Electrical area
classification drawings are used as a guide by
designers, constructors, and inspectors of electrical
installations, and all classified locations should be
indicated. This documentation will serve as a record of
the original classifications and will serve as a guide
when future additions or revisions are considered.
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