Introduction

                  Intrinsic Safety in Potentially Explosive Industrial Environment

Intrinsic Safety in Potentially Explosive Industrial Environment
  • Client

    Sudip Sen

  • Services

    Safety in hazardous areas.

  • Technologies

    industrial Safety

  • Dates

    13/01/2018

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Description

Hazardous locations as those where Fire or Explosion hazards may exist due to the simultaneous presence of flammable materials, source of oxygen and source of ignition. A mixture of air and flammable material, in which after ignition, combustion spread throughout the unconsumed mixture, is called ‘Explosive atmosphere’. Hence all the explosion protection techniques operate by ensuring that either one of the above three ingredients are made absent or the resultant ignition is adequately controlled so as not to allow it to the extent of an explosion.

 

When the proportion of the fuel (in the form of a gas, vapour, dust) and the air is in between certain limits (UEL – upper explosive limit and LEL – lower explosive limit) they form an explosive atmosphere which can be ignited by an ignition source.

 

Outside the limits, the atmosphere will not ignite but will have the potential to do so if the proportion varies over a period of time. Auto Ignition Temperature (AIT) of a combustible material is the temperature at which that particular combustible material spontaneously ignites in air with or without any other source of ignition. The actual maximum body temperature that a device can attain under normal or fault/overload condition should be less than the AIT of the associated explosive material environment.        

                          

Intrinsic Safety (Ex ‘ i ’) concept of protection method is mostly used in Instrumentation & Automation applications in the hazardous area industries (Oil & Gas, Petrochemicals, Fertilizers, LPG Plants, Chemical & Pharmaceutical plants, etc), due to the advantage of live maintenance of equipment and no risk to the operators. The energy in the circuit is limited to values which do not result in illegally high temperatures or sparks but at the same time conventional Intrinsic Safety allows very limited power (approx < 2W) in the instrumentation loop. A circuit is intrinsically safe if no spark and thermal effect, which occur under defined experimental conditions (including normal operation and certain fault conditions), can ignite a certain explosive atmosphere.

 

To adhere to the maximum allowed energy level, < 24V and few hundreds mA current is allowed in this concept for handling all need of the field instrumentation. Hence, Intrinsic safety finds limitation with those application where long cable distance and higher power demand is needed.

 

Explosive gases are classified in two categories:

Class I gases as per NEC (or Group II as per ATEX/IEC), e.g., Hydrogen, Acetylene, Benzene etc., which are mostly found in Oil & Gas, Petrochemical and chemical industry environment. Few subcategories like IIA, IIB and IIC are there depending upon their explosive characteristics, IIC being the most dangerous category which require lowest level of energy to explode.

 

Class II gases as per NEC (or Group I as per ATEX/IEC), e.g., Methane, which are mostly found in coal mine environment.

 

Area Classification concept as per ATEX/IEC (mostly followed in Europe and other countries):

Zone 0 (or 20): in which an explosive gas (or dust)-air mixture is continuously present or present for long periods

Zone 1 (or 21): in which an explosive gas (or dust)-air mixture is likely to occur in normal operation

Zone 2 (or 22): in which an explosive gas (or dust)-air mixture is not likely occur in normal operation, and if it occurs it will exist only for a short time.

 

Area Classification concept as per NEC (mostly followed in North America, USA)

Division 1: Hazardous atmosphere is likely to be present in normal operation                    

Division 2: Hazardous atmosphere is unlikely to be present in normal operation

 

Intrinsic safety protection is indicated by Ex ‘i’ and further there are sub-categories like ia, ib and ic. ‘ia’ level protection is having triple redundant circuit having the best level of protection. On the other hand equipment protection level (EPL) has been defined as ‘Ga’, Gb’ and ‘Gc’. ‘Ga’ level being the ‘very high’ level of protection assigned to equipment based on its likelihood of becoming a source of ignition in an explosive gas environment.

 

Appropriate intrinsically safe apparatus can be used in all zones. In particular, it is the only solution that has a satisfactory history of safety for Zone 0 instrumentation. Equipment to be installed in Zone 0 locations should have EPL Ga certification as in those location probability of gas presence is very high and so ‘very high – triple redundant’ safety components should be used in the circuitry. Similarly equipment to be installed in Zone 1 & Zone 2 locations should have EPL Gb & EPL Gc certification respectively.

 

Another parameter causing an explosion is normally considered to be ignition by a hot surface. When a gas is heated above its ignition temperature it may spontaneously ignite. Intrinsic safety Apparatus is classified into temperature (‘T’) classes, as below, depending on its maximum permitted surface temperature.

 

                          

                                                         Temperature Codes

 

 

 

 

 

 

Temp in Deg C

IEC Code

NEC Code

Temp in Deg C

IEC Code

NEC Code

 

 

 

 

 

 

450

T1

T1

180

 

T3A

300

T2

T2

165

 

T3B

280

 

T2A

160

 

T3C

260

 

T2B

135

T4

T4

230

 

T2C

120

 

T4A

215

 

T2D

100

T5

T5

200

T3

T3

85

T6

T6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A typical Intrinsic safe circuit consists of an Interface module (associated devise) in which one portion of circuit contains normal power and the other portion contains restricted intrinsic safe power. Both the sides are normally having isolation either galvanic or optical. Voltage is limited by the zener diode, current is limited by the resistor and a too high input voltage causes the fuse to blow.

 

Following standards are predominantly being followed in India:

IEC 60079-11: Equipment protection by intrinsic safety

IEC 60079-25: Intrinsic safe systems (as per clause no. 3.1.3, this standard supersedes IEC 60079-11, in case of any difference of explanation or conflicting recommendation)

 

The components used in an intrinsic safe circuit need to have IEC Ex certificate and PESO approval (earlier name was CCOE) to be installed in industries in India.

 

n most process control applications, each piece of apparatus in a system is individually certified. A document that confirms the safety of the entire system is then produced using the information from the individual IS apparatus certificates, in accordance with the system standard IEC 60079-25. This IS system document also includes details of cable types and simple apparatus used in the system.

Author

Sudip Sen

Sudip Sen, MSc (Tech), PGDBA, is Country Manager (India) @ GM International srl, Italy, and Chief Resource Officer @ Uttaran ETC, Kolkata. He has 28 years of professional experience in instrumentation industries having worked in senior management positions in globally renowned multinational companies like GM International, DEHN+SOHNE, Pepperl+Fuchs, Turck Automation, MTL, Bartec, etc. He has delivered technical lectures and conducted trainings on Lightning & Surge Protection, Intrinsic safety & Ingress protection in various industries and professional forums

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