[설계] Explosion Proof Grade (방폭등급) EP01, Useful information on ATEX

Explosions can cause loss of life, serious injuries and significant damage. Flammable gases, mists, vapours or combustible dusts, when present in sufficient concentration and mixed with air may explode if there is a source of ignition such as a spark from electrical equipment. Preventing releases of dangerous substances, which can create explosive atmospheres, and eliminating sources of ignition are two ways of reducing the risk.

This article provides an introduction to the European regulations, collectively known as ATEX, covering working practices in potentially explosive atmospheres. This includes an assessment of the environment, the location, the flammable substance, the classification of zones/areas, the type of protection required and the design and certification of equipment to be used in these hazardous environments.

What is ATEX?

ATEX refers to two European directives for ensuring safety in areas with an explosive atmosphere. The two ATEX directives are:

• The ‘Workplace’ Directive (1999/92/EC)
  provides the requirements for workplace safety, ensuring that workers are protected in environments where there is a risk of explosive atmospheres.
• The ‘Equipment’ Directive (94/9/EC, recently updated as 2014/34/EC)
  applies to all equipment used in potentially explosive atmospheres including protective systems. The name ‘ATEX’ is derived from the French title of this directive.

Pumps, their motors and ancillary equipment are often required to operate in hazardous conditions. As well as the equipment having to meet certain requirements regarding the pumped fluid, throughput, power requirements, operating pressure and reliability, it is essential that this can be achieved safely when operating in a potentially explosive atmosphere. Pumps are assessed to be compliant to operate in a specific environment either by the manufacturer or by a third party, and a Declaration of Conformity or ATEX certificate has to be provided to confirm compliance.

What is an explosive atmosphere?

An atmosphere can become potentially explosive when it contains dangerous levels of flammable gases, mists or dusts. With a source of ignition, combustion spreads in an uncontrolled and catastrophic manner. The risk of explosion can be reduced by preventing the release or build-up of flammable substances and by eliminating sources of ignition.

The ATEX directives distinguish between two different types of explosive atmospheres: gas and dust. Although it is not commonly appreciated, dusts such as flour or sawdust can be extremely dangerous and their handling requires special consideration. Areas within the two kinds of explosive atmospheres are each divided into three ‘safety’ sub-zones.

The following table shows the ATEX hazard zones and safety categories that equipment is required to meet to work in them:

ATEX Zone		Danger / Protection Level	Ex Safety Category Required
Gas	Dust
Zone 0	Zone 20	Constant danger/Very high.  Explosive hazard always present.	Category 1 equipment.
Zone 1	Zone 21	Potential danger/high. Explosive hazard occasionally present during normal working practices.	Minimum of Category 2 equipment.
Zone 2	Zone 22	Minor danger/normal. Explosive hazard not likely or only for short periods	Minimum of Category 3 equipment.

 

Equipment safety – how is ATEX applied?

Manufacturers must submit their products to conformity testing procedures by an outside agency known as a Notified Body. If equipment passes the tests carried out by the Notified Body and is deemed safe for use in one or more of the defined danger zones an Ex symbol may be displayed on the equipment to show that it has been approved to ATEX standards.

How is ATEX information displayed?

All ATEX equipment must bear the CE marking and this may be followed by the serial number of the Notified Body. The equipment will also bear the Ex logo and then a series of codes, to define its approval rating. An example for a hazardous area motor is shown below:

The ATEX designation consists of five elements:

• Equipment Group code and Safety category
• Atmosphere
• Protection type
• Gas group
• Temperature class

What is the Equipment Group and Safety Category?

There are two main Equipment Groups (designated by roman numerals) and these are sub-divided into Safety categories:

Equipment Group		Safety Category
I	The equipment is suitable for use in underground parts of mines as well as those parts of surface installations endangered by firedamp and/or combustible dust	M1	The equipment can be used in mines and powered safely when the explosive atmosphere is present
		M2	The equipment can be used in mines but only when an explosive atmosphere is absent. If this is not the case, the equipment must be de-energised
II	The equipment is suitable for use in all non-mining applications.	1	Protection level is very high and the equipment may be used safely when the explosive atmosphere is present continually or for long periods
		2	Protection level is high and the equipment may be used safely when explosive conditions occur occasionally under normal operating conditions.
		3	The equipment is safe for use when explosive atmospheres are not likely to be encountered under normal conditions and when they do, it will be for a short period only.

 

Pumps for use in explosive atmospheres will normally be classified under Group II, in categories 2 or 3. To be suitable for Group II, category 1 or Group I certification, pumps will require additional modifications.

When a pump is rated for use in more than one environment there may be an Ex code for each zone.

What is the protection code?

To eliminate the risk of electrical sparks, any equipment containing circuitry may contain additional features such as a sealed enclosure. In the Ex designation, these features are shown by one or more lower-case letters. Some examples relevant to pumps, motors and seal support systems are shown in the following table:

Symbol	Type of Protection
d	Flameproof - The product incorporates mechanical features which are designed to contain an explosion and not allow any flame to escape. da and db versions are suitable for Zone 1 area use. The enclosure or junction box is constructed and certified as explosion protected according to the “Flameproof” standard. The “Flameproof” philosophy is that should an explosion occur within the enclosure, it will either be contained or the escaping flame will be cooled by a “flame-path” thus mitigating the risk of igniting an explosive outside atmosphere.
db	Flameproof, suitable for Category 2, Zone 1 area use.
eb	Equipment is designed and manufactured with components which prevent ignition sources being generated internally such as static discharges or high temperatures. Suitable for Zone 1 area use. The enclosure is constructed and certified as “Explosion Protected” according to the “Increased Safety” standard. The philosophy with Ex eb is to provide an impact resistant enclosure and also to ensure that its contents are certified such that they will never produce sparks, temperatures, or current creepage that could ignite the flammable atmosphere which may enter the enclosure. Ex eb rated enclosures are therefore explosion protected but are not “Flameproof".
ec	The enclosure is constructed and certified as “Explosion Protected” according to the “Increased Safety” standard, but to a lesser level than eb. Suitable for Zone 2 non-sparking (formerly nA) hazardous areas.
ia	Intrinsic Safety* (Permitted for Zone 0) depending on device category.
ib	Intrinsic Safety* (sufficient for zone 1 (+2))
ic	Intrinsic Safety* (sufficient for zone 2 (+2))

 

* ‘Intrinsic safety’ means that the circuitry is low power and cannot transfer sufficient energy to ignite an explosive atmosphere. It is the only protection type allowed in Zone 0/20 environments.

What is the gas group?

Chemical properties of explosive atmospheres such as molecular weight, upper and lower explosion limits and flame temperature affect the probability and severity of an explosion. Testing of these properties has resulted in the definition of design parameters including the maximum experimental safe gap (MESG), which is used to classify flammable gases for the design of electrical equipment in hazardous areas.

The MESG is a measurement of how easily a gas flame will pass through a narrow gap bordered by heat-absorbing metal. Every substance has its own combination of properties, however their MESGs rank in similar ranges, simplifying the selection of hazardous area equipment.

Each chemical gas or vapour used in industry is classified into a gas group.

Gas Group	Typical Substances
IIC	Acetylene & Hydrogen
IIB	Ethylene
IIA	Propane

 

What is an ignition source?

Potentially explosive atmospheres must be protected from sources of ignition. Examples of ignition sources are friction sparks and high surface temperatures. Aluminium components can be a cause of friction sparks so it is important that coupling guards are not constructed from this, and certain other sparking metals, if possible. It is also important that equipment does not exceed the auto-ignition temperature of the atmosphere it sits in. In a pump, high surface temperatures can result from, for example, mechanical friction in a bearing or simply because the pumped fluid is hot.

What is the Temperature class?

The Ex designation contains a code indicating the maximum possible surface temperature of the equipment. In some cases, the maximum surface temperature is written explicitly. Where it is dependent on the pumped fluid, it is marked with an X.

Since the surface temperature of a pump will also depend on the external, ambient temperature, the ATEX specification may also list an ambient temperature range for ‘safe’ operation, e.g. +1°C < Ta <40°C. Where this is not listed, it is assumed to be -20°C to +40°C.

ATEX temperature code	Maximum surface temperature °C
T1	450
T2	300
T3	200
T4	135
T5	100
T6	85
X	Maximum temperature class depends on the temperature of the pumped fluid – limitations and a range of allowed temperature classes should be given in the documentation accompanying the equipment.

 

Summary

It is your responsibility to identify the hazards and risks in your operating area and classify it, according to the ATEX Zone categories. In some cases, this is a job for an ATEX specialist. It may be necessary to designate an area where a pump will be working as potentially explosive even if it will be handling a non-flammable liquid. Conversely, with appropriate pump construction and installation, an area is not necessarily hazardous even if the pumped fluid is flammable.

Armed with this information, you can discuss your application with pump suppliers and manufacturers. In selecting a pump and motor, ensure that:

• It is suitable for use with the hazard type: gas and/or dust.
  
  
• It meets or exceeds the required ATEX Equipment Group and Category for the intended zone (for example, II 2).
  
  
• The self-ignition temperature of any atmosphere is substantially higher than the pump’s rated operating surface temperature (e.g. T4).
  
  
• It has sufficient safeguards against sparks or other ignition sources.
  
  
• After purchase, ensure that the manufacturer supplies a signed Declaration of Conformity for the pump.