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DNV-OS-A101 Safety Principles and Arrangements
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SECTION 4
Hazardous Area ClassificationSec.4
A. General
Sec.4
A 100 Objective
Sec.4 A
101 The provisions of this section are intended to avoid ignition
of potential flammable releases that may occur on the unit or installation
during normal operation. Release as a result of accidental events
such as blowout or vessel rupture is not addressed by area classification,
but shall be covered by emergency measures.Sec.4
A 200 Application
Sec.4 A
201 This section applies to all offshore units and installations
covered by this standard. The standards listed in Table A1 may be
applied or referred to for basic or supplementary information. The
code revision applicable is that valid at date of issue of this
standard unless otherwise agreed.
Sec.4 A
| Table A1 Reference
standards |
| Reference | Title | IEC 60079-10 | Electrical apparatus for explosive gas atmospheres
- Part 10: Classification of hazardous areas | | IEC 61892-7 | Mobile and fixed offshore units - Electrical
installations - Part 7: Hazardous area | | IMO MODU Code | Code for the Construction and Equipment of
Mobile Offshore Drilling Units, Consolidated Edition 2001 | | API RP 500 | Recommended Practice for Classification of
Locations for Electrical Installations at Petroleum Facilities Classified
as Class I, Division 1 and Division 2 | | API RP 505 | Recommended Practice for Classification of
Locations for Electrical Installations at Petroleum Facilities Classified
as Class I, Zone 0, Zone 1, and Zone 2 | | IP 15 | Institute of Petroleum; Area Classification
Code for Petroleum Installations - Part 15 | |
Sec.4 A
202 Hazardous area classification shall be documented by drawings
including location and selection of equipment, air inlets and exhausts.
Sec.4 A
203 A schedule of release sources shall be established.Guidance note:
IEC 61892-7 paragraph 10.1 and Table C2 provide further information.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4
B. Basic Principles
Sec.4
B 100 Definition of hazardous areas
Sec.4 B
101 Hazardous areas are all areas in which explosive gas or air
mixture may normally be expected to be present in quantities which
can require special precautions for the construction and use of
electrical equipment and machinery.
Sec.4 B
102 Hazardous areas are divided into zones depending upon the
grade (frequency and duration) of release:- Zone
0: in which an explosive gas atmosphere is continuously
present or present for long periods. (Typical for continuous grade
source present for more than 1000 hours a year or that occurs frequently
for short periods).
- Zone 1: in which
an explosive gas atmosphere is likely to occur in normal operation.
(Typical for primary grade source present between 10 and 1000 hours
a year).
- Zone 2: in which
an explosive gas atmosphere is not likely to occur in normal operation,
and if it does occur, is likely to do so infrequently and will exist
for a short period only. (Typical for secondary grade source present
for less than 10 hours per year and for short periods only).
Guidance note:
Note that conditions of ventilation may change the zone definition
for each grade of release. Also, the likelihood of detecting the
leak may influence the zone.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4 B
103 Non-hazardous areas are areas, which are not hazardous according
to the definitions in 101 and 102.Sec.4
B 200 Hazardous fluids (sources)
Sec.4 B
201 The following fluids shall be considered as sources requiring
area classification:- Flammable gas or vapour.
- Flammable liquids which are handled at or above their
flashpoint, or which could be heated to the flashpoint after release.
- Flammable liquid that could form a flammable mist.
- Fluids which satisfy the criteria in a), b) or c), and
which are present periodically within the plant for 100 hours per
year or more (e.g. during start-up).
- Unclassified, flammable liquids containing residual,
volatile materials and which are stored under confined, heated conditions
give rise to limited area classification.
Sec.4 B
202 Appendix A gives guidance
for general categorisation of sources of release based on definitions
in B100.Sec.4
B 300 General principles for area classification
Sec.4 B
301 Location of a continuous source within an enclosed area, or
in open areas with significant obstructions to ventilation, shall
be avoided.
Sec.4 B
302 The number and release rate of primary grade sources shall
be minimised as far as practicable. Location of a primary grade
source within an enclosed area shall as far as practicable be avoided.
Sec.4 B
303 It is not normally acceptable to locate open, non-hazardous
areas enclosed, or significantly enclosed, by hazardous areas.
Sec.4 B
304 Openings, penetrations or connections between areas of different
hazardous area classification shall be avoided, e.g. through ventilation
systems, air pipes or drain systems.Sec.4
C. Extent of the Hazardous Zone
Sec.4
C 100 General
Sec.4 C
101 The extent of the hazardous area depends on the rate of release,
ventilation conditions, and fluid properties.
Sec.4 C
102 The extent of the hazardous area shall be based on guidance
in recognised standards listed in Table
A1.
Sec.4 C
103 See Sec.7 for additional
requirements for production and storage units and Sec.8 for criteria for drilling units.Guidance note:
Where a standard quotes a discrete distance for the extent
of hazardous area, this should be evaluated taking account of the
source of release and the overall hazard involved.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4 C
104 Where there is a potential for large releases in operational
situations covered by hazardous area classification, e.g. process
vents, the extent of the zone shall be larger than the boundary
of 50% LEL concentration. This shall be determined by a
dispersion analysis. The resulting zone will be defined as Zone
1 or Zone 2 depending on likelihood of release.Sec.4
D. Openings, Access and Ventilation
Conditions
Sec.4
D 100 General
Sec.4 D
101 The level and extent of hazardous area classification is dependent
upon ventilation. Adequate ventilation is required to ensure that
releases are rapidly dispersed. The adequacy of ventilation conditions
shall be justified and documented.Guidance note:
Adequacy of ventilation implies that stagnant areas should
be prevented and that a minimum of 12 volumetric air changes per
hour should be achieved.
Open areas without significant obstructions are considered
to have adequate ventilation if air velocities are rarely below
0.5 m/s and frequently above 2 m/s.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4
D 200 General requirements for mechanical
ventilation systems
Sec.4 D
201 See DNV-OS-D101 for general requirements for ventilation systems
(e.g. capacity, functionality, penetrations etc.).
Sec.4 D
202 See Sec.3 for requirement
for location of air intakes.
Sec.4 D
203 See DNV-OS-D301 for requirements for gas detection in ventilation
air intakes and outlets.
Sec.4 D
204 Ventilation systems for hazardous areas shall be separate
from ventilation systems for non-hazardous areas.
Sec.4 D
205 Hazardous enclosed spaces shall be ventilated with underpressure
in relation to adjacent less hazardous locations. Fans shall be
interlocked to ensure outlet fan is engaged prior to inlet fan,
and ventilation failure shall initiate alarm at a manned location.
Fans shall be designed to minimise the risk of sparks occurring.
Sec.4 D
206 Inlet and outlet ventilation openings shall be arranged to
provide efficient ventilation in relation to the location of equipment
and sources in the area.
Sec.4 D
207 Ventilation inlet ducts passing through a more hazardous area
than the ventilated space shall be operated at overpressure in relation
to the hazardous area.
Sec.4 D
208 The outlet air from hazardous spaces shall be routed through
separate ducts to outdoor area which, in the absence of the considered
exhaust, is of the same or lesser hazard than the ventilated space.
Sec.4 D
209 The outlet ducts and the area in vicinity of the discharge
point shall have the same area classification as the ventilated
space. The dimension of the hazardous zone at outlet shall not be
less than the zone dimensions in open air for the largest single
source within the enclosed space.Sec.4
D 300 Supplementary requirements for
overpressure protection of enclosed spaces
Sec.4 D
301 Any enclosed non-hazardous space containing ignition sources
and located in Zone 1 and Zone 2 areas shall be maintained at overpressure.
Ventilation intakes shall be located in a safe area. Note certain
limitations on arrangement given in Sec.3.
Sec.4 D
302 Alarms and isolation of ignition sources shall be initiated
on detection of an explosive atmosphere adjacent to the ventilation
air inlets, in accordance with the shut down philosophy for the
unit or installation. The ventilation system shall be suitable to:| — | maintain at least 50 Pa overpressure
with respect to the external hazardous area when all penetrations
are closed |
| — | maintain an outward air flow through all openings (single
or multiple penetrations) of the enclosed space. |
Sec.4 D
303 Failure of overpressure ventilation shall be alarmed at a
manned location. Alarm delay of up to 30 seconds may be applied
to minimise spurious alarms when doors are intentionally opened.Guidance note:
The design of doors should take account of differential pressures
between spaces, such that personnel can easily open doors without
hazard.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4
D 400 Loss of ventilation in hazardous
areas dependent upon mechanical ventilation
Sec.4 D
401 Immediate remedial action to restore ventilation shall be
taken upon identified loss of mechanical ventilation in hazardous
areas.
Sec.4 D
402 Ignition sources shall be isolated where ventilation cannot
be restored within a short time, typically 30 seconds, or if gas
is detected either within the area or in the immediate vicinity
during ventilation failure. Sec.4
D 500 Dilution ventilation
Sec.4 D
501 Enclosed areas with internal source(s) of release may be defined
as non-hazardous provided that ventilation is sufficient to ensure
that the release is immediately diluted below flammable limits.Guidance note:
The ventilation rate should be based on calculation using
the total release rate from all primary grade sources together with
the rate from the largest secondary grade source. A safety factor
of 4 should be applied in the calculations, (i.e. aiming for < 20% LEL).
For turbine hoods a minimum of 90 air changes per hour will normally
be sufficient.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.4 D
502 The ventilation system providing dilution ventilation shall
be provided with 2 times 100% fan capacity. One fan shall
be driven from the emergency source of power, in areas containing
ignition sources that can not be removed instantaneously.Emergency power supply for main turbine hood fans is not required
if main power supply to the fans can be maintained from other turbine(s).
Sec.4 D
503 The ventilation system shall be suitable to avoid stagnant
areas, and flow of ventilation air shall be continuously monitored.
Sec.4 D
504 Non-Ex-certified electrical equipment and other ignition sources
shall be isolated immediately upon failure of ventilation, or upon
gas detection > 25% LEL, unless such shutdown
can cause escalation of the danger, in which case suitable alarms
shall be given in control room and other essential locations which
may need to provide immediate action.
Sec.4
D 600 Air locks
Sec.4 D
601 The requirements for air locks apply when they are installed
to prevent smoke or gas ingress.
Sec.4 D
602 Air locks shall consist of gas tight steel bulkheads and gas
tight self-closing doors.
Sec.4 D
603 The air lock shall be mechanically ventilated at a positive
pressure against the adjacent hazardous area or outside atmosphere.
Sec.4 D
604 The air lock shall as a minimum be classified as zone 2.Sec.4
D 700 Ventilation of battery compartments
Sec.4 D
701 Ventilation arrangement for battery rooms shall be in accordance
with DNV-OS-D201 Ch.2 Sec.2 I404.Sec.4
E. Electrical Installations in Hazardous
Areas
Sec.4
E 100 General requirements
Sec.4 E
101 Electrical equipment and cables installed in hazardous areas
shall be limited to that necessary for operational purposes.
Sec.4 E
102 All electrical installations in hazardous areas shall comply
with the requirements of DNV-OS-D201 Ch.2 sec.11.
Sec.4 E
103 Electrical equipment with temperature Class T3 (maximum 200°C)
shall be used when hydrocarbon gases give rise to hazardous areas.
Sec.4 E
104 Warning signboards shall be fitted, either easily visible
from the hazardous areas and of size about 600 x 400 mm with letters
of height about 30 mm, or smaller signboards at each socket outlet
in or adjacent to the hazardous areas, with text, e.g.:PORTABLE
ELECTRICAL EQUIPMENT SUPPLIED BY FLEXIBLE CABLES SHALL NOT BE USED IN
AREAS WHERE THERE IS GAS DANGER.
Sec.4 E
105 A warning signboard shall be fitted at each connection for
welding apparatus outside of the engine room, with text, e.g.:WELDING
APPARATUS SHALL NOT BE USED OUTSIDE THE ENGINE ROOM, UNLESS THE WORKING
SPACE AND ADJACENT SPACES ARE GAS-FREE.Connections for welding apparatus shall normally not be located
in hazardous areas.
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