12 Power

Upstream Process Engineering Course 12. Power Generation and Distribution Systems

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Power Generation and Distribution

1

Contents • • • • • • • • • •

Power Generation Gas Turbines Gas Turbine Control Gas Turbine Emissions Noise Control Gaseous Fuel Specification Liquid Fuel Specification Gaseous Fuel Consumption Gas Turbine Ancillaries Gas Turbine Data

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• • • • • • • • • •

Emergency Power Generation Emergency Diesel Generator Internal Combustion Engines Gas Diesel Engines Electrical Heat Tracing Electrical Distribution Systems Load List Hazardous Area Classification Hazardous Gases Temperature Classification

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Power Generation •

•

•

The main power generation system on most installations comprises of gas or gas/diesel (dual fuel) turbines as the normal source of power and diesel engines for emergency power generation. Large individual consumers such as compressors often have their own power supply Gas turbines are normally fired by fuel gas taken from the production train, when this is not available, during start-up for example, the turbine can operate on diesel fuel The gas turbines are virtually self contained units, complete with their own controls, switchgear, lubricating and hydraulic oil system, compressed air system, air filters, inlet and exhaust silencers •

•

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A gas turbine compressor module being installed onto BP’s Cleeton platform (left) Power generation units located onshore, Saudi Arabia

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Gas Turbines Three-Stage High Pressure Turbine

Mechanical Drive

Eight Stage Axial Compressor

Exhaust

Combustor

Air Inlet Upstream Process Engineering Course

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Gas Turbines • • • •

Gas turbines can be either aero-derivative type, industrial or hybrid, which combine the best elements of both models Aero-derivative models were originally developed for aircraft service and have been adapted for industrial applications, they are light and compact and employ modular construction. Fuel specifications are more strictly controlled than for other turbines Industrial machines employ heavy, horizontal split casings, are very durable, have larger combustion chambers and can tolerate a wider range of fuels Aero-derivative machines are more maintenance intensive, with approximately 8000 operating hours between overhauls with a low down-time. In comparison, industrial models can run for around 16000 hours between overhauls, however the down-time is longer by several days. Also required (outside TAR): – –

•

Washing - 0.75 days every 60 days Boroscope - 2 days every 180 days

Gas turbines consist of a compressor section, combustor and turbine section (the Gas Generator) which supplies high pressure gas to the power turbine section. When the power turbine section is integral to the gas generator, the machine is called a Single Shaft machine • •

When the power turbine section is separate from the gas generator, the unit is called a Split Shaft or Two-Shaft machine The advantage of Split Shaft machines is that they can be operated at a different speed to the gas generator section

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Gas Turbines •

•

•

•

Split Shaft machines are preferred to Single Shaft types when the driver is used to power compressors, however for power generation purposes where a constant speed is required, single shaft machines are preferred Other configurations of gas turbines are available for combined heat and power generation, instead of using exhaust gas for power generation a combined cycle uses the gas to generate steam Waste heat recovery units in the turbine exhaust can be used to maintain the temperature in heating medium systems by passing hot exhaust gases over a bank of tubes containing the heating medium fluid The thermal efficiency (electrical power out divided by the fuel energy in) can be expressed at standard ISO conditions, for modern gas turbines this is around 30-36 % or at site rated conditions which take into account ambient humidity, temperature and pressure, height above sea level and with zero inlet and outlet pressure drops.

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A gas turbine generator being lifted into position after first removal following 40 000 hours operation on BP’s Ula platform

Power Generation and Distribution

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Gas Turbines High pressure air

Hot gases expand through the turbine

Combustion System

Compressor

Turbine

Load

Simple Cycle - Single Shaft Low pressure air inlet

Exhaust gases to stack

High pressure air

Hot gases expand through the turbine

Exhaust gases to stack

Combustion System

Turbines

Compressor

Load

Shaft speed varies with load

Low pressure air inlet

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Gas Turbines •

The operating principles of a gas turbine are as follows: –

– –

–

Air is drawn through inlet filters and silencers and adiabatically compressed in stages of axial compression, the air then passes to the combustion chamber Some of the air is fed directly to the fuel burners, the majority being used to cool the outer surfaces of the combustion chamber At higher combustion temperatures, the efficiency of the turbine increases however the the turbine blades would have a reduced operating life, therefore an economic compromise is required The hot gases then expand adiabatically through the power turbine section which drives the alternator

The graph of nominal output/specific heat consumption vs. inlet air temperature is unique to each model of turbine Upstream Process Engineering Course

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Gas Turbine Control •

•

•

• • •

The control system is capable of starting up the turbine bringing it up to normal running speed automatically and safely in response to one single start initiation signal Condition monitoring is provided locally and life prediction software calculates the time between maintenance based to the load delivered, number of start-ups, etc. The primary controls required are the speed governor (electrical frequency), voltage, shaft speed and turbine temperature which is measured at the inlet to the power turbine If extra load is demanded, the gas turbine frequency falls and eventually it will trip If more than one GT is being operated, the other(s) will attempt to take on the additional load Automatic load shedding will take place if the frequency continues to fall, that is users will be tripped in a predetermined sequence until the load has stabilised

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Gas turbine control panel

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Gas Turbine Emissions •

Low emission burner from an GEC Alstom Tornado gas turbine

•

•

•

Liquid fuels produce more exhaust gas pollutants than gaseous fuels, such as CO, unburned hydrocarbons (UBH) and SOx The target concentrations of exhaust gas pollutants for gaseous fuels are as follows: – – – –

NOx: less than 25 ppmv @ 15% O2 CO: less than 20 ppmv @ 15% O2 UHC: less than 20 ppmv @ 15% O2 Smoke: non visible

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•

•

The exhaust gas produced by gas turbines generally contains low levels of pollutants as the fuel is mixed with large quantities of excess air The most significant exhaust gas pollutant are oxides of nitrogen, NOx Historically, water or steam injection has reduced the amount of NOx emissions by reducing the flame temperature, however more modern gas turbines use dry, low NOx ‘lean burn’ combustors Another method of NOx reduction is Selective Catalytic Reduction (SCR) in which anhydrous or aqueous ammonia is injected into the flue gas upstream of a catalyst bed, which decomposes the NOx to elemental nitrogen and water

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DLE Gas Turbines - 1 The implementation of dry low emissions technology to gas turbines differs with manufacturer but broadly the approach is to achieve a lower and more uniform combustion temperature in the turbine by controlling the fuel:air mixture more tightly than in conventional gas turbines. This is difficult to do over a wide range of turbine loads and leads to the need for variable geometry in fuel nozzles, individual control of groups of fuel nozzles, bleed-off of compressor air at part loads or a combination of some or all of these. This leads to more complex fuel and engine control systems than non-DLE gas turbines which has implications both for operation and commissioning. This design approach is referred to as "dry" as opposed to controlling emissions by water or steam injection, or by injecting reactants such as ammonia in conjunction with exhaust catalysts all of which require complex auxiliary systems. Emission Performance (Ref LM2500): 25 ppm NOx from 60 - 100% load 40 ppm NOx from 40 - 60% 60 ppm NOx from idle - 40%

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DLE Gas Turbines - 2 •

DTI consider DLE standard practice and justification required for non compliance. Not all manufacturers & types available in DLE version e.g. Duel fuel machine.

•

Downside – Higher spec fuel gas required at higher pressure and dewpoint. Less variation in FG supply. – DLE run time and operating experience offshore is in its infancy and industry experience not be widely circulated at this stage. Onshore DLE operation well established. Need to focus on obtaining design, commissioning and operating experience direct from operating companies employing DLE engines as part of the engine manufacturer selection process. – Slight performance penalty but disputed by manufacturers. – Lower availability of spare engines. – CAPEX differences: about 5 - 10% of the engine cost.

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Noise Control •

•

• •

•

The noise created by a gas turbine is considerable and must be reduced to protect plant personnel and minimise environmental impact The noise associated with the air intake and noise radiated from the casing is high frequency and in the range most damaging to the ear, exhaust noise contains more energy which results in a detectable pressure change, but the frequency is lower and less damaging Inlet noise is reduced by acoustic baffles installed just before an elbow in the inlet air ducting Casing noise is reduced by an acoustic enclosure over the turbine, with integral fire and gas detection and fire extinguishing equipment On the outlet, acoustic baffles are required and the exhaust ducting should be sound insulated

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Rolls-Royce/Allison electrical power package. Acoustic insulation is normally part of the equipment manufacturers scope of supply for the power generation package.

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Gaseous Fuel Specifications •

• •

At the system inlet, the gas must not contain any liquid hydrocarbons or water in either liquid or solid phase, no components in the gas should condense at the prevailing pressure within the system To achieve this specification, fuel gas is normally superheated by 20 ºC. Specification for fuel gas (typical, may vary depending on machine) – – – – – – – – –

30 - 100 MJ/m3 24.5  0.5 bara (40+ bara for DLE units) 120 ºC  5% (min. acceptable change is 0.5% per second) 5 ppmw (of solid particles should have a diameter less than 20m) Density, dew point, composition and sulphur content to be reported to the manufacturer Certain components within the machine are custom built to each individual specifications, therefore the gas specifications must not vary above the limits agreed by the manufacturer Density should be quoted for a dry gas For a low emission burner, it should be of the range 0.7 1.2 kg/Nm3 (0.044 - 0.075 lb./scf) Typically, the water content should be 1 lb/MMSCF Lower calorific value (range) Pressure,min Temperature, max Wobbe Index, variation Solids, max

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Liquid Fuel Specification • •

A variety of liquid fuels can be used to power gas turbines, however diesel is the most commonly used Specification for liquid fuel (typical, may vary depending on machine) – – – – – – – –

– – –

Viscosity depends on temperature and pressure, typical values are 5 - 7 cSt Free water 500 ppm max Sediment 100 mg/litre max Carbon residue 0.25 % max Ash content 300 ppm max Ca 10 Pb 1 Density, flash point, sulphur content cloud point and pour point are to be reported to the manufacturer Fuels with a flash point lower than 30ºC require special safety regulations The cloud point or pour point should be at least 10 ºC below the lowest temperature the fuel system may experience The fuel must be protected against contamination by water and sediment during transportation and storage

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Gaseous Fuel Consumption • •

The ISO rating of the turbine is the output power at 15 ºC, 1.013 bar and 60% relative humidity, with zero inlet and exhaust pressure losses The gas turbine will be site-rated according to the installed conditions: Psite = Piso  (1/1.02)  1/[1+0.008(t-15)]  1/(1+0.01H) Where t = ambient temperature (ºC) and H = height above sea level/100 (m)

•

The site-rated efficiency of the gas turbine can be estimated from the ISO power: – – – –

•

ISO Power 0 to 3500 kW 3500 to 15000 kW above 15000 kW

Thermal Efficiency (Conservative) 20% 23% 26%

A preliminary estimate of fuel consumption can be calculated as follows: – – – –

If the lower heating value and density of fuel gas is unknown, assume a value of 45 MJ/sm 3 and a standard density of 0.9549 kg/m3 For example, a 15MW gas turbine will require a fuel gas flowrate of: [(15 MJ/s / 45 MJ/sm3)]  100/23 = 1.45 m3/s (4.4 MMSCFD) A contingency of 10% should be added to the flowrate to allow for site rating of the turbine

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Gas Turbine Ancillaries •

The gas turbine power generation package comes complete with the following ancillary systems: – Compressed Air System • A dedicated compressed air system is provided for the various pneumatic actuators to enable the turbine to be started

– Ventilation • A forced draft system maintains a positive pressure within the gas turbine package

– Lubricating Oil • A 100% spared system supplies the whole unit with an emergency back-up to ensure the unit runs down safely

– Cleaning System • Fouling of the turbine and compressor blades by dust, oil, ash etc. can result in a loss of performance, therefore online or offline cleaning systems using a wet or dry agent are installed

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Inlet Air Filtration •

•

AIR CONTAINS MANY SORTS OF POLLUTANTS: – DUST – SALT SPRAY – SAND – DRILLING CEMENT – RAIN – SNOW – INSECTS – SEA WATER – LEAVES AND OTHER VEGETABLE MATTER – SOOT AND OTHER ENGINE EXHAUST MATERIALS – INDUSTRIAL POLLUTANTS EVEN WITH SMALL POLLUTANT CONCENTRATION HIGH AIR MASS FLOW = LARGE AMOUNTS OF POLLUTANTS

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Gas Turbine Data MANUFACTURER

MODEL

TYPE

ISO RATING (MW) EFFICIENCY (%)

DIMENSIONS L:W:H (m)

WEIGHT (tonnes)

Solar

Centaur 40 Centaur 50 Taurus 60 Taurus 70 Mars 90

A/D A/D A/D A/D A/D

2.9 - 3.5 4.24 5.2 6.6 7.9 - 9.6

28 28 30 30 33

9.0:2.4:2.6 9.0:2.5:2.8 12.3:2.4:3.1 12.3:2.4:3.1 16:2.4:3.0

25 30 45 60 80

Alstom -formerly European Gas Turbines (Lincoln Design and Manufacture)

Hurricane

A/D

1.65

26

5.75:2.0:3.2

13.5

TA2500

A/D

3.5

-

9.5:2.4:2.8

25

TB5000 Typhoon Tornado Tempest Cyclone RLM2500

A/D A/D A/D A/D A/D H

3.92 4.35 - 5.25 6.45 - 6.75 7.7 12.9 22.5

30.5 31.5 35 37

9.7:2.4:2.4 8.0:2.4:3.2 * 11.5:2.4:3.2 ** 9.8:2.4:3.5 5.75:2.0:3.2 16.5:3.4:3.9

34 38.1 55 55 60 115

Kværner Energy

MS5001E (Frame 5)† MS6001E (Frame 6) MS9001E (Frame 9)

HDI HDI HDI

26.3 39.6 123.4

28.5 31.8 33.8

21.5:3.28:4.0 22.6:3.28:3.8 30.61:5.10:6.35

288 276 475

ABB Stal

GT35

H

15 - 20

32

-

-

GT10

H

25

34

-

-

GTX100

H

43

37

-

-

RB211

A/D

23.0 - 30.8

40

25:3.5:3.0

185

501-K

A/D

2.5 - 5.0

30

-

-

601-K

A/D

5.0 - 9.0

30

-

-

GEC Alstom (GE Gas Generator)

Rolls-Royce/Allison

• • •

HDI = Heavy Duty Industrial A/D = Aeroderivative H = Hybrid (mixture of industrial and aeroderivative)

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* Dimensions and weight shown for 5.25 MW model ** Dimensions and weight shown for 6.75 MW model † Frame 5 is a relatively old design which accounts for the lower power/weight ratio than the Frame 6

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Gas Turbine Data TYPE

ISO RATING (MW)

EFFICIENCY (%)

DIMENSIONS L:W:H (m)

WEIGHT (tonnes)

Centaur 40 Centaur 50 Taurus 60 Taurus 70 Mars 90/100 Titan 130 TA1750 TA2500 TB5000 Typhoon Tornado Tempest Cyclone GT35 GT10B GT10C GTX100 Avon RB211

H H H H H H HDI HDI HDI LWI LWI LWI LWI H LWI LWI LWI A/D A/D

3.5 4.6 5.5 7.5 9.4 - 10.7 14 1.3 1.9 4.0 4.35 - 5.25 6.75 7.9 12.9 15 - 20 24.8 29.1 43 14.58 23.0 - 30.8

28.5 29.4 30.5 33 31.5 33.5 18 20 25 30.5 31.5 31.2 35 32 34 36 37 27 36

9.8:2.5:2.6 9.0:2.5:2.8 11:2.5:3.1 12:2.8:3.1 15:2.8:3.8 14:3.1:3.8 9.0:2.6:3.1 9.5:2.4:2.8 9.7:2.4:2.4 8.0:2.4:3.2 11.0:2.4:3.3 10.75:2.4:3.6 13.5:2.7:3.9 25.0:4.0:4.2 20.5:4.5:5.3 20.5:4.5:5.3 22.0:4.5:6.0 -

30 32 32 55 70 75 23 25 34 36 56 57 75 160 185 185 275 165 185

501-K

A/D

3.95 - 5.27

29.5

-

27

601-K

A/D

6.5 - 7.92

31

-

63

LM1600

A/D

13.74

35

-

-

LM2500

A/D

22-30

38

-

-

LM6000

A/D

42

41.5

-

-

MS5001E (Frame 5)† MS6001B (Frame 6)

HDI HDI

26.3 42.1

28.5 32.5

21.5:3.28:4.0 22.6:3.28:3.8

288 276

MANUFACTURER

Solar

Alstom Power - (formerly Ruston and EGT) Lincoln Design and Manufacture

Alstom Power - (formerly ABB Stal) Finspong Design and Manufacture Rolls-Royce/Allison

GE

• • •

MODEL

HDI = Heavy Duty Industrial A/D = Aeroderivative H = Hybrid (mixture of industrial and aeroderivative)

•

LWI = Light Weight Industrial

† Frame 5 is a relatively old design which accounts for the lower power/weight ratio than the Frame 6

Emergency Power Generation •

• •

In the event of an interruption of the normal power supply, caused by a total main generation or distribution system failure, power to the emergency switchboard is provided by automatic start-up of the emergency power supply Emergency power generation can be either an Uninterruptible Power Supply (UPS) type, a standby diesel generator or both The emergency system supplies power via the emergency switchboard to the following consumers only, – – – – – – –

• •

Emergency Diesel Generator Set

communications: emergency radio and public address system emergency lighting emergency shutdown system (ESD) emergency depressurisation system fire and gas system diesel fire pump control and starting systems switchgear tripping facilities

The Emergency Power Supply is designed to supply power to these loads continuously for 24 hours A UPS system consists of a rectifier/battery charger, DC to AC converter and transfer or bypass switch

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Emergency Diesel Generator • • • • •

•

The diesel engine drivers are independent of other utilities and have their own cooling systems, starting mechanism and fuel supply Battery supplies are often provided to maintain power for essential equipment if the power supply from the diesel generator is lost Individual generator sets are supplied with diesel day tanks which are sized to hold enough fuel for 24 hours continuous operation A compressed air starting system consists of an electric motor driven compressor which maintains a pressure of 30 barg in the accumulator vessel The engine design includes some limited protection against operation in potentially explosive gas mixture atmospheres by reduced engine surface temperatures, exhaust gas coolers and an inlet air shut off valve The emergency generator is situated in an enclosure protected by fire walls, and has a dedicated CO2 fire protection system.

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Internal Combustion Engines •

There are three commonly available types of engines in use both offshore and onshore: – – –

• • • •

Spark ignition engines which operate using either LPG or gasoline and are usually referred to as gas engines Compression ignition engines which only operate using diesel fuel which must be of a consistently high specification Dual fuel (gas/diesel) engines operate in either liquid (diesel) only mode or gas mode with a constant pilot injection of diesel

Most spark ignition engines use a four stroke cycle however a two stroke cycle gives a higher output from the same engine size Two stroke engines are applicable to both spark ignition and compression ignition engines Supercharged engines use a compressor to increase the density of the combustion air before it is inducted into the cylinder Two types of supercharger are commonly used, a mechanical compressor driven by a separate prime mover or an exhaust gas recycle

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Gas/Diesel Engines •

• • •

•

• •

Gas/diesel engines use direct injection of fuel into the combustion space, only air is present in the cylinder during the compression stroke Pilot fuel is required to stabilise combustion Fuel can be natural gas, crude oil, LPG or diesel Engines are commonly used for start-up purposes where an independent, reliable power source is required Fire water and seawater lift pumps are often supplied complete with diesel engines for start-up and safety purposes Gas diesel engines are considerably heavier than gas turbines but have a lower initial cost Levels of availability, reliability and efficiency are comparable to gas turbines

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Air intake Air compression

Fuel injection

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Electrical Distribution Systems •

•

Electrical switchgear is a form of switchboard in which all the equipment required to control an individual circuit including bus, circuit breaker, disconnecting devices, current and potential transformers, controls, instruments and relays, is assembled in one metal cubicle and the circuit breaker is provided with means for ready removal from the cubicle Circuit breakers can be of the oil or air type, although the air type is more popular

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•

•

Power from the main generators is distributed via high or low voltage switchboards to each individual load The purpose of switchgear is to minimise the risk of short circuits and to ensure personnel and operational safety during all operating conditions, inspections and maintenance

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Load List • • •

• •

It is important to identify the utilisation voltage of each piece of equipment in order to properly design the electrical system An electrical load list is the best way to determine the overall system requirements in terms of the operating load The load list should include the power rating of each individual consumer and whether it is run continuously, intermittently or as standby for another consumer Once this has been accomplished, local loads may be grouped to be served from switchgear which in turn are served from transformers When generating power, the starting loads need to be considered in addition to the total steady state load

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Load List Description MOL pumps Crude export pumps Drilling Separation Compression Gas dehydration Water injection Seawater lift pumps Seawater treatment Fresh water Electrochlorinator Fuel gas Cooling medium Heating medium Relief system Closed drains Instrument air Fuel oil Chemical injection Miscellaneous Total (MW)

Power (kW) Intermittent 1800 3200 3160 382 5000 600 350 2500 380 190 50 50 80 250 150 160 150 25 25 20 360 180 50 100 150 2000 1000 13.727 8.635

Continuous

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Hazardous Gases •

USA Standard Gas Groupings

•

Group A

Atmospheres that may contain acetylene

•

Group B

•

Group C

•

Group D

• •

Group E Group F

Atmospheres that may contain hydrogen such as manufactured gases with more than 30 vol% hydrogen Atmospheres that may contain ethyl ether vapours, ethylene, cyclopropane, carbon monoxide, hydrogen sulphide and others Atmospheres that may contain vapours of hydrocarbons and natural gas, alcohols, ketones, solvents, ethers, esters and others Atmospheres that may contain dust of aluminium, magnesium or similar metals and alloys Atmospheres that may contain carbon, coal or coke dust

•

Group G

Atmospheres that may contain flour, starch or grain dusts

•

British Standard Gas Groupings

• • • • •

Group 1 Group 1A Group 2A Group 2B Group 2C

•

Methane (Mining only) Methane above ground (e.g. sewage or LNG plants) Propane Ethylene Hydrogen, Carbon Disulphide and Acetylene (N.B. equipment marked suitable for H2 may not be suitable for Carbon Disulphide or acetylene, check with manufacturer) Note: Group 2 sub groupings required for Exd (flameproof) and Exi (intrinsically safe) apparatus only

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Temperature Classification •

Classification

Maximum surface temperature –

–

–

–

The highest temperature which is attained in service under the most unfavourable conditions (but within the tolerances) by any part of the surface of electrical apparatus, which is able to produce an ignition of the surrounding atmosphere The most unfavourable conditions include recognised overloads and fault conditions For some items of apparatus the surface temperature is external, e.g. Exd (flameproof) For other types of apparatus, the internal surfaces are equally important if an explosive gas/air mixture has access to them, e.g. Exe (increased safety)

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Maximum Surface Temperature

EU

US

ºF

ºC

T1

T1

842

450

T2

T2

572

300

T2A

536

280

T2B

500

260

T2C

446

230

T2D

420

215

T3

392

200

T3A

356

180

T3B

329

165

T3C

320

160

T4

275

135

T4A

248

120

T5

T5

212

100

T6

T6

185

85

T3

T4

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Electrical Heat Tracing • •

• • •

The most common type of electrical heat tracing is the resistance type heat cable The resistive material is generally nichrome wire which comes into contact with the bus wires (one hot, one neutral) at predetermined intervals along the length of cable The nichrome wire creates heat (I2R) as current passes through it A temperature switch is used to turn the heat trace on and off when required Another type of heat trace cable is the current limiting type cable which is similar to the resistive cable except that a special property or alloy is contained within the cable such that the current is limited in proportion to the change in temperature

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