Service Support

17

2013

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04

Safety barrier explosion-proof knowledge


Intrinsically safe

intrinsically safe circuiti   Electrical sparks and thermal effects under normal operation or specified fault conditions cannot ignite the specified explosive mixture.
Exia level: electrical equipment that cannot ignite explosive gas mixtures during one fault and two faults in normal operation.
Exib rating: Electrical equipment that cannot ignite an explosive gas mixture during normal operation and a fault.

According to the standards of the International Electrotechnical CommissionIEC60079-11:1991The provisions for the classification of hazardous locations are as follows: 

Zone 0: a place where a mixture of explosive gas and air occurs continuously or for a long time;
Zone 1: During normal operation, a mixture of explosive gas and air occasionally occurs;
Zone 2: During normal operation, there is almost no mixture of explosive gas and air.
The explosive gas components are defined as follows:
Ⅱ C component: acetylene more
Ⅱ C component: hydrogen easy
Ⅱ C component: ethylene point
Ⅱ C component: propane combustion
The definition of temperature level is as follows: (at ambient temperature40The maximum surface temperature generated when the electrical equipment is faulty)

T1

T2

T3

T4

T5

T6

450

300

200

135

100

85

Intrinsically safe explosion-proof combination conditions:
        Safety barrier andWhen "energy storage" instruments (such as transmitters, etc.) are connected, these instruments and connecting wires areSafety barrier,Operating end safety barrier,Safety barrier at inspection endThe load of such products must be calculated by relying on the explosion-proof parameters of the safety barrier and combined with its system:
System combination conditions: 
VZGiven Vi
       Imax≤ Ii
Lc≤ La-Li
        Cc≤Ca—Ci
Vz-maximum open circuit voltage
Vi-Maximum allowable input voltage of primary meter

 Imax-Maximum short-circuit current  
Ii-Maximum allowable input current of primary meter
Cc-maximum allowable distributed capacitance of the connecting cable
Ca-maximum distributed capacitance of safety barrier load
Ci-equivalent capacitance in primary instrument (given when the instrument is detected by explosion-proof mechanism)
Lc-maximum allowable distributed inductance of the connecting cable (given when the instrument is tested by the explosion-proof mechanism)
La-maximum distributed inductance of the safety barrier load (given when the explosion-proof mechanism detects the instrument)
Li-equivalent inductance in the primary instrument (given when the instrument is detected by the explosion-proof mechanism)
Use of Degraded Atmosphere: 
        The grade atmosphere applicable to the model marked in the parameter table of this manual, if the atmosphere used is reduced, such as hydrogen (ⅡC) reduced to ethylene (Ⅱ B), its La and Ca are 3 of the parameter table valuetimes, reduced to propane (IIA), its La, Ca is 8 times the value of the parameter table.
Calculation of cable length: 
The distribution parameters of the cable itself are generally given by the cable manufacturer. For instrument signal cables without distribution parameters, the cable length can be calculated according to Cc = 200 PF / m and L / R = 250 μH/Ω.

Intrue Safety Level Limits:   
According to the national standard, the limit of the safety level, KN9000 series Zener safety barrier:
        Grade C: 20 μJ
Class Ⅱ B: 60μJ

When the intrinsically safe limits of the primary instrument and the safety barrier are different, the explosion-proof system can only be degraded for use.

Explosion-proof level of high and low sequence:

ⅡA< ⅡB < ⅡC

T1 < T2 <T3 < T4  <T5   < T6

 

1.Explosion-proof concept

1,Three conditions necessary to cause an explosion,Three conditions at the same time.--Energy required for ignition of an explosion source of ignition ;Air or oxygen air or oxygen ;Flameable air dust

, To prevent the explosion must be considered from three necessary conditions, limiting one of the necessary conditions, it limits the explosion. 

In the industrial process, usually from the following three aspects of flammable,Dealing with explosive situations. 

1) Prevent or minimize the possibility of leakage of flammable substances;

2) Do not use or minimize the use of components that are easy to produce electric sparks;

3) Adopt methods such as nitrogen filling to maintain the inert state.

 2. Classification of hazardous areas and explosion-proof classification of electrical components and allowable temperature areas 

, Classification of hazardous areas Area ( Zone 0 ), explosive gas always or for a long time Area ( Zone 1 ), flammable gas may occur or exist during the normal operation of the instrument Area ( Zone 2 )。 In general, there is no flammable gas and its existence time is very short, even if it occurs occasionally.

Classification of explosive areas, comparison between international standards and American standards :

I.E.C.          N.E.C. 

Gas   

Zone 0 Class I,       Division I       

Zone 1 Class I,       Division I       

Zone 2 Class I,       Division II 

Dust   

Zone 10 Class II,     Division I       

Zone 11 Class II,     Division II

 I.E.C. : International Electrotechnical Commission ( International  Electrotechnical  Technology  Commission )

N.E.C.: American Electrical Code ( National Electrical Code, U.S.A. ) 

2. Explosion-proof classification of electrical components:

 1 General Protection EN50.014              

Oil immersion protection 0 EN50.015 

Charging protection facilities p EN50.016       

Powder fillingq EN50.017

2. Flame retardant shell d EN50.018            

Improve safety factore EN50.019

3. Protection i EN50.0120          

 8 Airtight protection h Not unified 

Pressure flower protectionnNot unified            

10 Special measures s Not unified 

Allowable temperature zone for electrical equipment Electrical element temperature zone grade number The maximum allowable value of element surface temperature (° C)

 Gas ignition point (° C)    T1                    200T450                  &nb

Classification standard of explosion-proof grade

This article contains the concept of explosion-proof, explosion-proof standards, the division of explosion-proof areas, the meaning of explosion-proof signs and some explosion-proof terms. 

The basic principle of explosion-proof 
The concept of explosion 
       An explosion is a physical or chemical change from one state to another, and releases huge energy. The energy released rapidly will make the surrounding objects suffer violent impact and destruction. 
       Explosion must have three conditions: 
1) Explosive substances: substances that can react with oxygen (air), including gases, liquids and solids. (Gas: hydrogen, acetylene, methane, etc.; Liquid: alcohol,Gasoline; solid: dust,fiber dust, etc.)

2) Oxygen: Air.

3) Ignition source: including open flame, electrical spark, mechanical spark, electrostatic spark, high temperature, chemical reaction, light energy, etc.
Why explosion-proof? 
       explosive substance: Many production sites produce certain flammable substances. About 2/3 of the underground coal mine places have explosive substances; in the chemical industry, about 80% of the production workshop has explosive substances.

Oxygen: Oxygen is everywhere in the air.

Ignition source: a large number of electrical instruments are used in the production process, and various frictional electric sparks,Mechanical wear sparks, electrostatic sparks, high temperature are inevitable, especially when the instrument, electrical failure. 
      Objectively, many industrial sites meet the explosion conditions. When the mixed concentration of explosive substances and oxygen is within the explosion limit, if there is an explosion source, an explosion will occur. Therefore, it is necessary to take explosion-proof.  
Principle of explosion-proof of instrument 
Hazard division of hazardous locations:  
      Definition of Explosive Material Zone: Chinese Standard    North American Standard 
Zone 0: Div. 1 Gasunder normal circumstances, where explosive gas mixtures exist continuously or for long periods of time
    1 Zone: A place where explosive gas mixtures are likely to occur under normal circumstances. 1 Area 
Zone 2: Div. 2 explosive gas mixtures cannot occur under normal conditions, only under abnormal conditions,Occasional or short-lived places 
    10 Area:Div. 1 Dust or fiber (CLASS II/III).under normal circumstances , A mixture of explosive dust or combustible fibers and air may be continuous,A place that appears frequently for a short time or exists for a long time .
    11 Area:Div. 2 Under normal circumstances, explosive dust or a mixture of combustible fibers and air cannot appear, only under abnormal circumstances, occasionally or for a short time.