GB/T 25285.1-2021 PDF in English
GB/T 25285.1-2021 (GB/T25285.1-2021, GBT 25285.1-2021, GBT25285.1-2021)
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Explosive atmospheres -- Explosion prevention and protection -- Part 1: Basic concepts and methodology
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Standards related to (historical): GB/T 25285.1-2021
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GB/T 25285.1-2021: PDF in English (GBT 25285.1-2021) GB/T 25285.1-2021
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 29.260.20
CCS K 35
Replacing GB 25285.1-2010
Explosive atmospheres - Explosion prevention and
protection - Part 1: Basic concepts and methodology
ISSUED ON: OCTOBER 11, 2021
IMPLEMENTED ON: MAY 01, 2022
Issued by: State Administration for Market Regulation.
Standardization Administration of PRC.
Table of Contents
Foreword ... 4
Introduction ... 6
1 Scope ... 7
2 Normative references ... 8
3 Terms and definitions ... 9
4 Risk assessment ... 15
4.1 General... 15
4.2 Identification of explosion hazard ... 16
4.3 Identification of ignition hazard ... 19
4.4 Estimating the possible effects of an explosion ... 20
5 Possible ignition sources ... 21
5.1 Hot surfaces ... 21
5.2 Flames and hot gases (including hot particles) ... 22
5.3 Impact, friction, grinding caused by machinery ... 23
5.4 Electrical equipment and components ... 23
5.5 Stray current and cathode anti-corrosion measures ... 24
5.6 Static electricity ... 25
5.7 Thunder and lightning ... 25
5.8 104 Hz ~ 3×1011 Hz radio frequency (RF) electromagnetic wave ... 25
5.9 3×1011 Hz ~ 3×1015 Hz electromagnetic wave ... 26
5.10 Ionizing radiation ... 26
5.11 Ultrasound ... 27
5.12 Adiabatic compression and shock waves ... 27
5.13 Exothermic reactions (including spontaneous combustion of dust) ... 28
6 Risk reduction ... 29
6.1 Basic principles ... 29
6.2 Avoid explosive atmospheres or reduce the amount of hazardous explosive
atmospheres ... 30
6.3 Hazardous locations ... 34
6.4 Design and manufacturing requirements for equipment, protective systems,
components to avoid effective sources of ignition ... 36
6.5 Design and manufacturing requirements for equipment, protective systems,
components to reduce the effects of explosions ... 50
6.6 Provisions for emergency measures ... 51
6.7 Principles of measurement and control systems for explosion prevention and protection
... 51
7 Usage information ... 52
7.1 General... 52
7.2 Information on preventing explosions during commissioning, maintenance, repairs 53
7.3 Qualifications and training ... 53
Appendix A (Informative) Equipment tightness ... 54
Appendix B (Informative) Relationship between equipment protection level (EPL) and
zones ... 57
Appendix C (Normative) Tools for explosive atmospheres ... 58
Appendix D (Normative) Verification procedure for ultrasonic threshold limits in
liquids ... 59
References ... 61
Explosive atmospheres - Explosion prevention and
protection - Part 1: Basic concepts and methodology
1 Scope
This document specifies methods for identifying and assessing hazardous situations that
may lead to explosions, as well as design and structural measures that are consistent
with safety requirements, through the following aspects:
- Risk assessment;
- Risk reduction.
Safety of equipment, protective systems, components can be achieved, by eliminating
hazards and/or limiting risks, i.e., by:
a) Proper design (no safety protection devices);
b) Safety protection devices;
c) Usage information;
d) Any other precautionary measures.
Explosion protection measures corresponding to a) (prevention) and b) (protection) are
covered in Chapter 6. Explosion protection measures corresponding to c) are covered
in Chapter 7. The explosion protection measures corresponding to d) are not covered in
this document. They are covered in GB/T 15706 (Chapter 6 in GB/T 15706-2012).
The preventive and protective measures, which are specified in this document, can only
provide the required level of protection, when the equipment, protective systems and
components are operated within the scope of their intended use and are installed and
maintained in accordance with the corresponding operating procedures or requirements.
This document specifies general design and manufacturing methods, to help designers
and manufacturers achieve explosion-proof safety, when designing equipment,
protective systems, components.
This document applies to any equipment, protective systems, components, which are
intended to be used in explosive atmospheres under atmospheric conditions. These
environments may be caused by flammable substances that are handled, used or
released by equipment, protective systems, components, OR by flammable substances
around equipment, protective systems and components and/or by the constituted
GB/T 3836.28. For other situations, risk assessment shall be carried out in accordance
with GB/T 15706 and/or EN 15198, unless there are other standards that are more
suitable for the situation, taking into account the following elements:
a) Identify explosion hazards and determine the possibility of hazardous explosive
atmospheres (see 4.2);
b) Identify ignition hazards and determine the likelihood of potential ignition sources
(see 4.3);
c) Estimate the possible effects of explosion after ignition (see 4.4);
d) Evaluate the risks and whether the expected level of protection is achieved;
Note: The intended level of protection is defined at least by legal requirements and, if
necessary, additional requirements specified by the user.
e) Consider risk reduction measures (see Chapter 6).
Risk assessment shall adopt a comprehensive assessment method, especially for
complex equipment, protection systems, components, devices that constitute
independent units, especially extended devices. This risk assessment approach requires
consideration of ignition and explosion hazards arising from:
- Equipment, protective systems, components themselves;
- Interaction between equipment, protective systems, components and the substances
they handle;
- Specific processes carried out within equipment, protective systems, components;
- The surrounding environment of equipment, protective systems, components, as
well as their possible interaction with adjacent processes.
4.2 Identification of explosion hazard
4.2.1 General
Explosion hazards are often associated with materials and substances that are handled,
used or released by equipment, protective systems, components, as well as with the
materials from which equipment, protective systems, components are made. Some of
these released substances undergo a combustion process in the air. The combustion
process is usually accompanied by the release of large amounts of heat and pressure, as
well as the release of harmful substances. Unlike combustion, an explosion is basically
the self-propagation of a reaction zone (flame) in a hazardous explosive atmosphere.
Once a hazardous explosive atmosphere is ignited by an effective ignition source, the
potential hazards associated with it are released.
The safety characteristics, which are listed in 4.2.2 and 4.2.3, describe the safety-
relevant characteristics of flammable substances. Material properties and safety
characteristics are used for explosion hazard identification.
It is important to note that these safety characteristics are not constants but are (for
example) dependent on the technology used for measurement. And for dust, the safety
data listed are only recommended values; because these values are related to the dust
particle size and shape, humidity and additives (even trace concentrations). For specific
applications, samples of dust present in the equipment should be tested; the resulting
data are used for hazard identification.
4.2.2 Combustion characteristics
Since the material itself is not used here, but the contact or mixing of dust and air is
used to illustrate potential hazards; therefore, it shall determine the characteristics of
the mixture of flammable substances and air. These characteristics give information
about the combustion characteristics of a substance and whether it is capable of causing
a fire or explosion. Relevant data include:
- The lower explosion limit (see EN 15794). If the lower explosion limit cannot be
obtained, it can be replaced by the flash point (with a certain safety factor);
- Explosion limits (LEL, UEL) (see GB/T 16425 and GB/T 12474);
Note: In GB/T 3836.12, the lower explosion limit (LEL) and upper explosion limit (UEL)
are named lower flammability limit (LFL) and upper flammability limit (UFL), respectively.
- Limiting oxygen concentration (LOC) (see EN 14034-4 and GB/T 38301).
4.2.3 Explosion characteristics
The characteristics of explosive atmospheres after ignition shall be described using the
following data:
- Maximum explosion pressure (pmax) (see GB/T 16426, EN 14034-4 and EN 15967);
- Maximum explosion pressure rise rate [(dp/dt)max] (see GB/T 16426, EN 14491,
EN 15967);
- Maximum experimental safety gap (MESG) (see GB/T 3836.11).
4.2.4 Possibility of hazardous explosive atmospheres
The possibility of a hazardous explosive atmosphere occurring depends on the
following:
concentration exceeds the maximum value (explosion upper limit), the explosion
will not occur.
Note 3: Some substances have unstable chemical properties, such as acetylene and ethylene
oxide, which can undergo exothermic reactions even in the absence of oxygen; therefore,
the upper limit of explosion is 100%.
Explosion limits vary with temperature and pressure. Generally, the range between
the upper and lower explosion limits widens with increasing pressure and
temperature. When flammable substances are mixed with oxygen, their explosion
limit is much higher than that of air mixtures.
If the surface temperature of the flammable liquid is higher than the lower explosion
limit, a hazardous explosive atmosphere can be formed (see 6.2.1.2).
Note 4: At temperatures well below the lower explosion limit (LEP), mixtures (such as
aerosols and mist) may become explosive mixtures.
Compared with gases and vapors, the explosion limits of dust have different
meanings. Dust clouds are usually non-uniform. Due to the deposition, diffusion,
scattering of dust in the atmosphere, the concentration of dust fluctuates greatly.
When there is combustible dust deposition, it is generally believed that a hazardous
and explosive atmosphere may be formed.
d) Amount of hazardous explosive atmosphere
The assessment of whether the amount of explosive atmosphere presented presents
a hazard depends on the possible effects of the explosion (see 4.4).
4.3 Identification of ignition hazard
4.3.1 General
First, it shall be determined which types of ignition sources are possible and which are
associated with the equipment (or components, protective systems). Chapter 5
considers different sources of ignition. All sources of ignition, that may be exposed to
hazardous explosive atmospheres, shall be assessed for their significance.
Subsequently, the ignition capabilities of all equipment-related ignition sources shall be
compared with the ignition characteristics of flammable substances (see 4.3.2).
This step shall result in a complete list of all potential ignition sources for equipment,
components or protective systems. Subsequently, the likelihood of a potential ignition
source becoming an effective ignition source is assessed. Possible ignition sources,
which are generated during operations such as maintenance and cleaning, also need to
be considered.
- Hazardous substances released.
The above consequences are related to the following factors:
- Physical and chemical properties of flammable substances;
- The quantity, boundary, closure conditions of hazardous explosive atmospheres;
- Consider the geometry of the obstacle's surroundings;
- The strength of the housing and supporting structures;
- Personal protective equipment for persons at risk;
- The physical properties of the object which is compromised by hazard.
In order for the user to estimate the expected damage to people, livestock or property
and the size of the affected area, information on the consequences of the explosion is
required. Appropriate information shall be part of the user instructions.
Note: This procedure can also serve as a guide for users of equipment, protective systems,
components, when assessing the explosion risk in the workplace and selecting appropriate
equipment, protective systems, components.
5 Possible ignition sources
5.1 Hot surfaces
If an explosive atmosphere comes into contact with a heated surface, ignition may occur.
Not only the hot surface itself can become an ignition source, but also the dust layer
and combustible solids that are in contact with or ignited by the hot surface can become
an ignition source in explosive atmospheres (see 5.2).
The ability of a heated surface to cause ignition depends on the type and concentration
of the specific substance in the air mixture. As the temperature increases and the heated
surface area increases, the ignition energy increases. In addition, the temperature that
triggers ignition is related to the size and shape of the heated object, the concentration
gradient on the adjacent surface, the flow rate of the explosive gas around the hot
surface; to some extent, it is also related to the material on the surface. Therefore, for
example, in a fairly large heated space (approximately 1 L or more), an explosive gas
or vapor atmosphere can be ignited by a surface temperature, which is lower than that
measured in accordance with GB/T 3836.11 or other equivalent methods. On the other
hand, for a heated object with a convex surface rather than a concave surface, a higher
surface temperature is required for ignition; for example, a spherical or tubular object,
the minimum ignition temperature increases as its diameter decreases. When an
explosive atmosphere substance passes over a heated surface, higher surface
temperatures may be required for ignition, due to the short contact time. If the explosive
atmosphere is in contact with a hot surface for a relatively long time, primary reactions
(such as cold flames) can occur, thereby generating decomposition products that are
more easily ignited and accelerating the ignition of the original environment.
In addition to easily identifiable hot surfaces such as radiators, drying ovens, heating
coils and other products, machinery and machine processes can also cause dangerous
temperatures. These processes also include equipment, protective systems, components
that convert mechanical energy into thermal energy, namely various friction clutches
and mechanically operated brakes (e.g., on vehicles and centrifugal separators). In
addition, all moving parts such as bearings, shaft channels, sealing glands, etc. can also
become sources of ignition, if they are not adequately lubricated. In the sealed housing
of moving parts, the intrusion of foreign objects or axis deviation can also cause friction,
which in turn causes the surface temperature to rise; in some cases, the temperature
even rises very quickly.
Hot surfaces may also be produced by heating the absorber from other ignition sources,
such as electromagnetic waves (see 5.8 and 5.9) and ultrasonic waves (see 5.11).
Temperature increases due to chemical reactions, such as with lubricants and detergents,
also need to be considered.
See 5.2 for ignition hazards during welding and cutting work.
For protective measures against ignition hazards caused by hot surfaces, see 6.4.2. If
the hot surface is fully or partially covered more specifically by other clauses, then these
clauses shall apply, see 6.4.4, 6.4.9, 6.4.10, 6.4.12.
5.2 Flames and hot gases (including hot particles)
Combustion reactions at temperatures above 1000 °C are usually accompanied by
flames. Hot gases are the products of the reaction; in dusty and/or soot-laden flames,
hot solid particles are also produced. Flames and their thermal reaction products or other
high-temperature (unburned) gases can ignite explosive atmospheres. Even a small
flame is the most effective ignition source.
If an explosive atmosphere exists inside and outside equipment, protective systems or
components, or in adjacent parts of the device, AND if an ignition occurs in one of these
places, THEN, the flame can spread to other places through openings (such as
ventilation ducts). Preventing the spread of flame requires specially designed protective
measures (see 6.5).
Welding chips produced during welding or cutting are particles with a large surface area;
therefore, they are also the most effective ignition source.
For protective measures against ignition hazards caused by flames and hot gases, see
6.4.3.
5.3 Impact, friction, grinding caused by machinery
As a result of friction, impact or abrasive processes such as grinding, particles can be
produced that are separated from the solid material and heated by the application of
energy during the separation process. If these particles contain oxidizable substances,
such as iron or steel, they can undergo oxidation processes, that reach higher
temperatures. These particles (sparks) can ignite flammable gases and vapors as well as
certain dust/air mixtures (especially metal dust/air mixtures). In the deposited dust,
sparks can cause simmering and become the ignition source of explosive atmospheres.
It is necessary to consider that foreign objects such as stones or stray metals may enter
the equipment, protective systems, components, thereby causing sparks.
Sliding friction, even between similar ferrous metals and between certain ceramics, can
produce hot spots and sparks similar to grinding sparks. These can cause explosive
atmospheres to ignite.
When stainless steel is impacted, rubbed, or ground, it can easily create hot surfaces
that can become an effective source of ignition. The application of high contact pressure
in the context of friction or grinding can also produce sparks.
Impacts between rust and light metals (such as aluminum and magnesium) and their
alloys can cause thermite reactions and can also cause ignition in explosive atmospheres.
When the light metals titanium and zirconium collide or rub against sufficiently hard
materials, they can produce ignition sparks, even without rust.
See 5.2 for ignition hazards during welding and cutting work.
For protective measures against ignition hazards, which are caused by mechanical
sparks, see 6.4.4.
5.4 Electrical equipment and components
Sparks and hot surfaces (see 5.1) of electrical equipment and components can become
sources of ignition. Sparks and hot surfaces can be generated under the following
conditions:
- When the circuit is open and closed;
- Loose connections;
5.6 Static electricity
Under certain conditions, static electricity can produce ignition discharge. Charge
discharge from charged insulated conductive parts can easily lead to ignition sparks.
Brush discharges can also occur with charged parts, which are made of non-conductive
materials (mostly plastics and other materials). In special cases, propagating brush
discharges may also occur during rapid separations (for example, when the film passes
over rollers, drive belts, or due to a combination of conductive and non-conductive
materials). Cone discharges and electron cloud discharges from bulk materials may also
occur.
Depending on the discharge energy, sparks, propagating brush discharges, cone
discharges, electron cloud discharges can ignite various types of explosive atmospheres.
Brush discharge can ignite almost all explosive gas and vapor environments. According
to the current knowledge, it can be ruled out that brush discharge ignites explosive
dust/air environment.
For protective measures against ignition hazards caused by static electricity, see 6.4.7.
Note: For more information on electrostatic hazards, see GB/T 3836.26 and GB/T 3836.27.
5.7 Thunder and lightning
If lightning occurs in an explosive atmosphere, it will usually cause ignition. In addition,
there is also the possibility of ignition when the arrester reaches a higher temperature.
Powerful electric currents flow where lightning strikes, these currents can generate
sparks near the point of shock.
Even if there is no lightning strike, thunderstorms can cause equipment, protective
systems, components to generate high induced voltages and cause ignition hazards.
For protective measures against ignition hazards caused by lightning, see 6.4.8.
5.8 104 Hz ~ 3×1011 Hz radio frequency (RF) electromagnetic wave
All systems that generate and use radio frequency electrical energy (RF systems) emit
electromagnetic waves, such as radio transmitters or industrial RF generators used for
melting, drying, quenching, welding, cutting, etc., or medical RF generators.
All conductive parts located within the radiation field function as receiving antennas.
If the field strength is large enough and the receiving antenna is long enough, these
conductive parts can cause ignition in an explosive atmosphere. For example, received
RF energy can cause thin wires to heat up or spark, when they come into contact with
or disconnect from conductive parts. The amount of energy gained by the receiving
antenna that can cause ignition depends primarily on the distance between the
transmitter and the receiving antenna, as well as the size and RF power of the receiving
antenna at a specific wavelength.
See 6.4.9, for protective measures against ignition hazards caused by electromagnetic
waves in the radio frequency spectrum.
5.9 3×1011 Hz ~ 3×1015 Hz electromagnetic wave
Radiation in this spectrum range (optical radiation), especially when focused, can be
absorbed by explosive gases or solid surfaces and become an ignition source. For
example, sunlight can cause ignition, if an object concentrates the radiation (e.g., a
bottle acting as a lens, focusing reflector).
Under certain conditions, the radiation from strong light sources (continuous or flashing)
is absorbed by dust particles in large quantities, causing these particles to become
ignition sources for explosive gases or deposited dust.
In the case of laser radiation (e.g., in communication devices, distance measuring
devices, surveying instruments, optical instruments), the energy or power density of the
unfocused beam can be high enough, to cause ignition even at great distances. In
addition, heating processes also occur when the laser beam strikes a solid surface or
when the laser beam is absorbed by dust particles in the atmosphere or on contaminated
transparent parts.
It shall be noted that radiation-generating equipment, protection systems, components
(such as lamps, arcs, lasers, etc.) themselves are ignition sources defined in 5.1 and 5.4.
For protective measures to prevent ignition hazards caused by electromagnetic waves
in this spectrum range, see 6.4.10.
5.10 Ionizing radiation
Ionizing radiation from, for example, X-ray tubes and radioactive materials can ignite
explosive atmospheres (especially those with dust particles), due to absorbed energy. In
addition, due to the internal absorption of radiant energy in the radioactive source, the
temperature of the radioactive source itself can rise beyond the minimum ignition
temperature of the surrounding explosive atmosphere.
Ionizing radiation can cause chemical decomposition or other reactions, resulting in the
production of highly reactive radicals or unstable compounds, which can cause ignition.
Note: This radiation can also create an explosive atmosphere through decomposition (for
example, the decomposition of water by ionizing radiation produces a mixture of oxygen and
......
Source: Above contents are excerpted from the PDF -- translated/reviewed by: www.chinesestandard.net / Wayne Zheng et al.
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