Explosion protection Marijan Gorše, dipl. ing.

Transcription

1 Explosion protection Marijan Gorše, dipl. ing. 1/18

2 Flammable Gas Atmosphere We Can Control Them Explosion hazards mostly arise from flammable gases and vapours. Instead of avoiding their ignition by explosion protection measures..... it may be preferable to detect them before they become ignitable. 2/18

3 Flammable Gas Atmosphere The Explosion Triangle If flammable substances, ignition sources, and air or oxygen are simultaneously present at the same place there is a high probability of explosion. Flammable substance in sufficient amount (concentration) Source of ignition Sparks of sufficient energy Surfaces with sufficient high temperature Oxygen or air in sufficient amount (concentration) 3/18

4 Explosion Prevention and Protection ATEX-Directive: Prevention Is Of Highest Priority Prevention of the formation of explosive atmospheres Avoidance of the ignition of explosive atmospheres Mitigation of the detrimental effects of an explosion to ensure health and safety of workers 1 st priority 2 nd priority 3 rd priority Flammable substance 2 nd priority in sufficient amount (concentration) Source of ignition 1 st priority Sparks of sufficient energy Surfaces with sufficient high temperature Oxygen or air in sufficient amount (concentration) 4/18

5 Gas Detection Instruments Safety-related Feedback and Control into the Ex-Area Ex Ex area (Zone) Transmitter electronics Non-Ex area (Safe area) Controller with relay contacts Gas Gas-Sensor Alarm devices Ventilation Automatic counter measure Alarm plan Personell Reaction 5/18

6 Gas Detection Instruments How To Limit Gas Concentrations Safely %LEL Gas concentration nd Alarm 1st Alarm Ventilation on Power off %LEL 40 Gas detector signal 20 2nd Alarm 1st Alarm Gas concentration still rises - countermeasure not effective Gas concentration decreases - countermeasure is effective 6/18

7 Gas Detection Systems Decrease Hazardous Areas Zone Shrinking What are the definitions of zone 1 and zone 2? ZONE 1 A place in which an explosive atmosphere... is likely to occur in normal operation occasionally. ZONE 2 A place in which an explosive atmosphere... is not likely to occur in normal operation but, if it does occur, will persist for a short period only. Question: Are explosive atmospheres likely to occur if you have a reliable gas detection system? Answer: No, they are not likely to occur... as long as if gas concentration is effectively limited!... this is the zone 2 definition! 7/18

8 GAS DETECTION SYSTEMS DECREASE HAZARDOUS AREAS ZONES SHRINK With the exception of the close to the leak -area of the gas leak the probability of having a flammable atmosphere is reduced by gas detection systems with effective countermeasures. Thus the occurrence of flammable atmospheres can be assumed to be very seldom (e.g. if the gas detection systems fails without notice). Zone 1 reduces to Zone 2 where apparatuses of device category II 3G can be used. Customer s advantage! As the edging areas of Zone 1 are Zone 2 these will change to non-hazardous areas where apparatuses can be used without ex-approval. non-ex Zone 2 Zone 1 non-ex Zone 1 Zone 2 II 2 G II 2 G II 3 G II 3 G with certified gas detection system 8/18

9 LEL AND EXPLOSIVE LIMITS 28 Risk of Suffocation by O2-deficiency %V/V Gas (Methane) in Air 15 5 Measuring range of flammable gas detectors 10 optimum Upper Explosive Limit (UEL) Lower Explosive Limit (LEL) Methane/Air-mixture too rich, combustible with additional air access only Combustible range, combustion with self-content flame propagation 100 % LEL Methane/Air-mixture too lean, not combustible - not explosive 9/18

10 Flammable Gases and Vapours Temperature and Temperature Classes Surfaces of sufficient high temperature 450 C T1 Auto Ignition Temperatures (AIT) and Temperature Classes 400 C Ammonia 630 C T1, max. 450 C Hydrogen 560 C Methane 537 C Ethylene 425 C T1, max. 450 C T1, max. 450 C T2, max. 300 C 96 % 300 C T2 n-butane 372 C T2, max. 300 C Kerosine 210 C T3, max. 200 C Diethyl ether 160 C T4, max. 135 C Carbon disulfide 95 C T6, max. 85 C 200 C T3 Example: It is not possible to ignite even the most ignitable mixture of ethylene with air by a hot surface with a temperature of 400 C. 4 % 135 C 100 C 85 C T4 T5 T6 10/18 max. allowed temperature

11 Flammable Gases and Vapours Minimum Ignition Energy and Explosion Group Sparks of sufficient energy Minimum Ignition Energies (MIE) and Explosion Group Hydrogen mj IIC Acetylene mj IIC Ethylene mj IIB Diethyl ether 0.18 mj IIB Propane 0.25 mj IIA Methane 0.28 mj IIA Ammonia 14 mj IIA Dichloro methane 9300 mj IIA Example: It is not possible to ignite even the most ignitable mixture of ethylene with air by a spark of a capacitor discharge of E = 0.05 mj. 1 2 Capacitor C = 1 µf charged by U = 10 V: E = C U 2 MIE > 0.18 mj < 0.06 mj IIA IIB IIC MESG > 0.9 mm < 0.5 mm MIE: Min. Ignition Energy MESG: Max. Experimental Safe Gap MIE (for intrinsic safety i ) and MESG (for flameproof enclosures d ) correlate. A-B-C-grouping is important for i -, n - and d -marking. 11/18

12 Flammable Gases and Vapours The LEL Lower Explosion Limit Flammable gases and vapours in air are not ignitable as long as their concentration does not exceed the Lower Explosion Limit or LEL. LEL of Flammable Gases Ammonia 15.4 vol-% n-butane 1.4 vol-% Ethylene 2.3 vol-% Hydrogen 4.0 vol-% Methane 4.4 vol-% Propane 1.7 vol-% LEL of Flammable Liquids Acetone 2.5 vol-% Diethyl ether 1.7 vol-% Ethanol 3.1 vol-% Toluene 1.1 % by vol. Gasoline ~ 1.0 vol-% Octane 0.8 vol-% LEL Flammable Liquids Diesel oil 0.6 vol-% > 55 C Kerosine 0.7 vol-% 40 C Styrene 1.1 vol-% 32 C Xylene 1.0 vol-% 30 C Flashpoint (FLP) It is not possible to ignite e.g. 50% LEL of a flammable gas in air. Liquids can only form a vapour concentration above the LEL if their temperature is high enough for evaporation. This is indicated by the flashpoint. Examples: It is not possible to ignite a gas mixture of 2 % by vol. of methane in air. It is not possible to ignite Diesel vapours at room temperature. 12/18

13 Flammable Gases and Vapours Flammable Liquids - Flashpoint 70 C n-heptanol The flashpoint is the temperature which is necessary to form concentrations of higher than the LEL in a closed vessel. Below the flashpoint the liquid s vapour pressure is not sufficient to form ignitable concentrations in a closed vessel. The LEL cannot be exceeded. Example: o-xylene vapours cannot be ignited at 20 C. Example: Even below 40 C the vapour pressure of Diethyl ether is high enough to form concentrations if more than 1.7 Vol-% (LEL). Flammable Highly Flammable Extremely Flammable 60 C 50 C 40 C 30 C 20 C 10 C 0 C -10 C -20 C -30 C -40 C n-undecane Diethyl benzene Ethanol amine Hydrazin n-butanol o-xylene p-, m-xylene n-propanol i-propanol Methanol Toluene Acrylonitrile Benzene, MEK Butylene oxide Tetrahydrofurane Dichloroethene MTBE Diethyl ether Acetic aldehyde Highly dangerous Less dangerous 13/18

14 Flammable Liquids Spillages and Vapours 50 cm 20 cm nearly no vapour concentration (less than 2% LEL) lower vapour concentration (less than 20 % LEL) max. vapour concentration > 100% LEL but lower than saturated vapour conc. Normal indoor air movement is about 0.2 m/s up to 1 m/s. vapour concentration e.g. 40 %LEL Containment of acetone (20 C) In an open containment with low walls the vapours of a flammable liquid will rather creep on the ground than raising up. By doing this they will constantly be diluted and their concentration is decreased. Dilution can considerably be enforced by wind or electric fans. Electric fans are just used to ventilate fresh air into a hazardous area to avoid hazardous concentrations as a countermeasure. A spillage of acetone would behave similarly. Under stormy conditions spillages of flammable liquids / vapours might be undetectable! 14/18

15 Gasdispersion Four initial phases light gas neutral gas aerosol dense gas 15/18

16 Transmitter / analogue signal transmission DrägerSensor O 2 DrägerSensor O 2 -LS DrägerSensor H 2 O 2 DrägerSensor CO DrägerSensor H 2 DrägerSensor NH3 DrägerSensor H2S DrägerSensor NO DrägerSensor OV DrägerSensor NO 2 DrägerSensor HCl DrägerSensor SO 2 DrägerSensor HCN DrägerSensor Cl2 DrägerSensor AC-L DrägerSensor Hydride DrägerSensor PH3/AsH3 DrägerSensor O 3 DrägerSensor COCl 2 DrägerSensor Hydrazine DrägerSensor Hydride SC 10 ppb 100 ppb 1 ppm 10 ppm 100 ppm 1 1% 10% 100% 16/18

17 Safety analysis Failure statistics Reason for failures 17/18 Human cause Technical failure Process out of control Accidents

18 Indoor application Compressed gas threat Propane: LEL 1,7 %V/V 1 liter pressure liquefied propane equals 260 liter propane in gas phase generates liter combustible atmosphere also: 3L of gasoline in car garage -> very explosive 18/18