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INTRINSIC SAFETYPRESENTED BY
Hadrian Jones
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WHY DO WE NEED
INTRINSIC SAFETY?
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Class 1, Div 1 or 2 Location
Non-Hazardous
Location
The Explosion Triangle
I.S. LIMIT
GAS AIR
HEAT or SPARK
CONTROL SYSTEM
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DEFINING INTRINSIC SAFETY!
INTRINSIC:
SAFETY:
BUILT-IN, INHERENT, A NATURAL PART OF
THE STATE OF BEING SAFE
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Definition per Code... A system, compromising
apparatus and interconnecting wiring, in
which any spark or thermal effect in any part of the system
intended for use in hazardous areas is incapable of causing
ignition.
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WHERE DID
INTRINSIC SAFETY
ORIGINATE?
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Spark ! Bare Wires
BellBatteries
Winch
Mining Bare Wire Signaling System c. 1910
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Limit Voltage Limit Current Limit Stored Electrical
Energy
Basic Objectives of Intrinsically Safe Designs
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Plant and Installations are classified according to:
The nature of the Hazardous Atmosphere
Class 1………….. GassesClass 2………….. Dusts, PowdersClass 3………….. Fibers & Flyings
GAS CLASSIFICATION
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Division 1Division 1
Hazardous atmosphere is likely to be present in normal operation
Division 2Division 2
Hazardous atmosphere is unlikely to be present in normal operation
Area Classification in the Americas
The Probability that the Hazardous Atmosphere Will be Present
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NFPA 497 Table 2-1NFPA 497 Table 2-1
Representative
(Test) GAS
NEC 505
Zones 0,1 & 2
NEC 500
Divisions 1 & 2Spark ignition
Acetylene
Hydrogen
Ethylene
Propane
Group IIC
Group IIC
Group IIB
Group IIA
Group A
Group B
Group C
Group D
Ease of ignition
from spark energy
MTL,Inc.MTL,Inc.
GAS CLASSIFICATIONS
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Spark Ignition CharacteristicSpark Ignition Characteristic
LFL(Lower Flammable Limit)
UFL (Upper Flammable Limit)
Minimum Ignition Energy (MIE)
Ignition EnergyIgnition Energy(milli Joules)(milli Joules)
Volume concentration (%)Volume concentration (%)
1.0
0.1
0.01
Hydrogen-air(1 atmos.)
Flammable Range
Propane-air (1 atmos.)
Ethylene-air(1 atmos.)
0 10 20 30 40 50 60 70 80 90 100
180 microjoules
60 microjoules
20 microjoules
Group B
Group C
Group D
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20mA
10mA
50mA
100mA
200mA
500mA
1A
1 2 5 10 20 50
Matched power rating 1.3W for T4Hydrogenresistive curvewith 1.5 safetyfactor
Capacitiverestriction0.1µF
UseableArea
500µH (260mA)Inductive restriction
Open circuit voltage V
Short circuitcurrent
Practical Limitations of an IS circuit
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Area Classification Example
FlammableSubstance
Zone 1
Zone 0
Zone 2 / Div 2
Containment wall
Tank vents
Liquid surface
KeyArea Classificationfor a fixed-roof tank
Div 1
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T amb = 40 °C
Apparatus is marked with eitherT-rating or maximum surface
temperature
User has responsibility to ensure thatthe T-rating is below the Spontaneous
Ignition Temperature ( SIT )
T Class ºC
T1T2T2AT2BT2CT2DT3T3AT3BT3CT4T4AT5T6
450300280260230215200180165160135120100 85
“T” RATINGS
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Gas/equipment compatibility
Ammonia 630 Methane 595 Hydrogen 560
Propane 470 Ethylene 425
Butane 365
Gas IgnitionTemperature
700
600
500
400
300
200
100
T1
T2
T3
T6
T4 T5
ApparatusTemperatureClassification
Cyclohexane 259
Carbon Disulfide 100
ºC
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T4 for apparatus is usually OKExceptions are: Carbon Disulfide and Ethyl Nitrate,
both requiring T5
Ambient may be raised by process temperature (for instance, a solenoid valve mounted on a hot steam line)
(
NOTES TO TEMPERATURE CLASSIFICATION
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METHODS OF PROTECTION IN
HAZARDOUS AREAS
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AMERICAS IEC/Europe
Class 1Division 1
Class 1Division 2
Recognized in principleby some users, apparatussuitable for Zone 0 usually specified
Types of Protection:-explosion-proof-purging-intrinsic safety-oil immersion
Types of Protection:all types suitablefor Div 1 non-incendive
ZONE 0
ZONE 1
ZONE 2
Intrinsically safe Ex ia orspecifically approved for Zone 0
Types of Protection:(d) flameproof(p) pressurized(i) intrinsic safety
ia & ib(e) increased safety(q) powder filled(m) encapsulationTypes of Protection:all types suitable for Zone 0 & Zone 1(n) Type N(o) oil immersion
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NON-INCENDIVE
PURGING
EXPLOSION PROOFING
INTRINSIC SAFETY
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NON INCENDIVE__(DIVISION 2 ONLY)ANSI/ISA S12.12 is the Standard
Falls Into two categories:
NON ARCING/NON SPARKING
Requires mechanical protection, hermetically sealed contacts.24VDC or 120VAC may be used
NON INCENDIVE(Energy Limited)
Similar to intrinsic safety including entity parameters,relaxed ignition curves,approved Div. 2 field devices, but
less well definedDesigned to eliminate hot surfaces or incendive sparks
under normal operating conditions
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Purging: a complex technique, but quite often the only solution
Pressure Switch
mains inlet
air inlet
initialpurge
X, Y, Z Purging
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Purging & Pressurization
AdvantagesAdvantages DisadvantagesDisadvantages
Sometimes the only solution
Can protect large volumes, panels and control rooms
Can have large margin of error before danger results
Clean air is not free; has to filtered, pumped, etc. Control system can be complex, includes other forms of pro- tection as well Live working is not
permissible Bulky
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Explosion-proof Enclosures/Conduits
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Gap
Flame Path Length
Explosion Proof Enclosures
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Explosion-proofing: Pros & Cons
Permits sparking apparatus in hazardous areasPROS
Older design of enclosures were difficult to weatherproof
No live working is permitted without gas clearance certification
Boxes tend to be substantial Special fittings are necessary Installation errors or faults are dangerous
C
O
N
S
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Intrinsic Safety…
the Better Way to
Prevent Explosions
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The The “Part s ” of a “Part s ” of a ZenerZenerproduction ZENERs have more production ZENERs have more
partspartsdual-redundant Zener diodesdual-redundant Zener diodesadditional resistanceadditional resistanceoften multiple fusesoften multiple fuses
+24V+24V
0V
Control Room
300Ω 20Ω
28V28V
MandatoryMandatoryinternal fuseinternal fuse
ReplaceableReplaceablefuse (option)fuse (option)
50mA
FT
HAZ
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Polarities--or + & -Polarities--or + & -
positivepositive
negativenegative
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Non-Polarized!Non-Polarized!
““a.c.” or bipolar (dual-channel)a.c.” or bipolar (dual-channel)
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Classified Area Control Bldg
Controller,
ROC, RTU
DCS, PLC,ROC, RTU or
SCADA
I.S Barrier or Isolator
Intrinsically Safe SYSTEMS
Certified I.S. Device
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Classified Area Control Bldg
Controller,
ROC, RTU
DCS, PLC,ROC, RTU or
SCADA
I.S Barrier or Isolator
Intrinsically Safe SYSTEMS
“Simple Apparatus”
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Simple Apparatus
"Devices in which, according to the manufacturer'sspecifications, none of the values 1.2 V, 0.1A, 20µJ or25mW is exceeded, need not be certified or marked "
Simple Apparatus Definition:
Hazardous Location apparatus can be either:
Certified, energy -storing (e.g instruments) or
Uncertified, "non-voltage producing, non-energy storing
Simple Apparatus"
Examples:
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Equi pment Rat i ngEqui pment Rat i ng
• Type of protectionType of protection e.g. Intrinsic Safetye.g. Intrinsic Safety
• Temperature ClassificationTemperature Classification
• Hazardous location classificationHazardous location classification
847efgdjeijruhnsjei4uuitim doir 7354378 ujmfnrieir ur785 e375463 834
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Compat i bl e Compat i bl e Part ners ?Part ners ?
safety parameters - is it a safe safety parameters - is it a safe combination?combination?
operational parameters - will the operational parameters - will the system actually work?system actually work?
FIELDFIELD
INSTRUMENTINSTRUMENT SAFETYSAFETY
BARRIERBARRIER
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SAFETY VALUES versus
OPERATIONAL VALUES!!
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SAFETY CHARACTERISTICS
28V 93mA 300R is a common barrier safety description
where V = 28 volts Current Limiting Resistor (CLR) = 300Rz
Barriers are usually described in terms of their safety parameters:
HazardousArea
Connection
SafeArea
Connection
4 1
2
3
∩∪
∩∪
< 1Ω
5
6
V+
Sig
0v
28V 300R93mA
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The Entity ConceptThe Entity Concept
250v max.
Vmax…...….> > …. ..…..Voc
Imax….........> > ....…….….Isc
Ci +Ccable...< <...Ca(allowed)
Li+Lcable....< <...La(allowed)
I.S. Field Device I.S. Barrier
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Rosemount? Transmitter
Vmax = 30V
Imax = 300mA Ci = 20nF
Li = 10 µH
MTL7787+Barrier
Voc = 28V
ISC = 93.0mA
Ca = .011µF
LA = 4.2mH
Is It Safe??
>><<
**If cable is less than 1,000 feet, DISREGARD
C & L of Cable!!**Over 1,000 feet,
ADDCi & C Cable < CALi & L Cable < LA
**If cable is less than 1,000 feet, DISREGARD
C & L of Cable!!**Over 1,000 feet,
ADDCi & C Cable < CALi & L Cable < LA
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Operational Characteristics
342R supply
V out V in
** End-to-end resistance is greater than the safety resistance
**Maximum permissible safe area voltage is less than thesafety (i.e hazardous area output) voltage
V wkg=26.0
V max=26.9
4 1
2
3
∩∪
∩∪
< 1Ω
5
6
43 +0.9V return
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250R (5V)
V +
0VHazardous area Safe area
Control System
1-5V
2 WIRE Transmitter with MTL 7787+
The Next Question That Must be Asked Then Is...
WILL IT WORK?IS Transmitter
12V (MIN) 24 V DC(NOM)
4
5
6
1
2
3
MTL7187+
.9V33R 10R
300R 42R
43R-0.9V
342R
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Loop voltage drops at 20mA are:Barrier (6.66+.9+.52) = 8.08Transmitter = 12.0DCS or PLC = 5.0
Lines = ?? _______________
TOTAL = 25.08V
Nom. working voltage = 24.0V
DOES THE MTL7787+ WORK HERE????
Answer: No, the System Does Not Work!!!!
Solutions: “Tweak” 24V power supply to 26V?
Use an “ACTIVE” ZEN--MTL7706!
Use a GIB--(Isolator)--MTL4000 or 5000 Series!
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Going t o Gr oundGoing t o Gr ound
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ANSI/ISA RP12.06.01-2003 STATES:
THE BARRIER-GROUNDING TERMINAL MUST BE CONNECTED TO THE GROUNDING
ELECTRODE…USING AN INSULATED CONDUCTOR NO SMALLER THAN 12AWG.
ALL GROUNDING PATH CONNECTIONS SHOULD BE SECURE, PERMANENT, VISIBLE AND ACCESSABLE. THE GROUNDING PATH
RESISTANCE FROM THE FARTHEST BARRIER TO THE GROUNDING ELECTRODE SHOULD NOT
EXCEED 1 OHM.
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O O
Field mountedinstrument
Plant bond
X
Controlsystem
L
E
N
Plant Grounding Scheme
Hazardous AreaNon-Hazardous Area
Barrier
E
Plant Power Ground Grid
One (1) ohm
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Instrumentsystem‘0v’ rail
PRIMARY 12 AWG GROUND
CONNECTION (LESS THAN 1 OHM!)
Earthing and Bond
HAZARDOUS AREA WIRING
HAZARDOUS ARA WIRING
SAFE AREA WIRING
SAFE AREA WIRING
Dual Conductor I.S. Ground
SECONDARY GROUND CONNECTION
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Instrumentsystem‘0v’ rail
Resistance meterchecks completeloop
Earthing and Bond
HAZARDOUS AREA WIRING
HAZARDOUS ARA WIRING
SAFE AREA WIRING
SAFE AREA WIRING
I.S. GROUND TESTING
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But, what happens if I simply don’t have a “good” ground at the location??
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Use the “other” I.S. approach:
“GIBS, TIBS, ISOLATORS, RELAYS.”
But how do THEY work?? Am I still intrinsically safe?? Are the rules still the same??
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Load
Power supply20-35 V dc
4-20 mA 17.5 V
300R
28 V
Isolation
Hazardous area Safe area
Typicallyup to 800R
Galvanic Isolator for 2-wire transmitters
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SIMPLE AND RELIABLE FAIRLY COMPLEX, LOWER MTBF
HIGH INTEGRITY GROUND FLOATING, ISOLATED
INEXPENSIVE MORE EXPENSIVE
GENERIC APPLICATIONS APPLICATION SPECIFIC
ZENERS ISOLATORS
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Advantages of Intrinsic Safety
Simple Apparatus: Permits the use of normal industrial devices if they are non-energy storing.
Safest Technique: Only method permitted in Zone “0” in Europe.
Fault Tolerant: Can have two failures in system and remain safe!
Live Maintenance: Can work on system with live power on.
No Explosion Proof Fittings or Conduit: Wiring is electrically--not mechanically--protected.
Personnel Safety: Lowest currents and voltages.
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This concludes the INTRINSIC SAFETY portion of the seminar.