Gas Testing – Hazards, Precautions, Exposure Limits

INTRODUCTION:

Many fatalities have occurred because gas testing had not been conducted or the tests have been inadequate. Gas testing must be completed by trained competent personnel. However, it is important that everyone understands the basic requirements.

HAZARDS OF GASES:

In the oil/gas and petrochemical industries, gas hazards are of particular importance. These hazards can be split into 3 main categories:

  • The first are oxygen hazards, in the air we breathe if there’s too much or not enough oxygen you or your work mates may die.
  • Secondly explosive gases and flammable vapors, which, when mixed with air in certain proportions can lead to fires or explosions.
  • And finally harmful or toxic gas hazards; these cover everything from inert gases like nitrogen, used in purging operations, which, simply displace the oxygen out of the air we are breathing, right through to the nerve gases like Hydrogen Sulphide, which chemically interfere with the cells in our body.

Each one of these hazards are serious and life threatening.

 

THE FOLLOWING ABBREVIATIONS ARE USED:

Gas: any substance of very low density and viscosity. (No shape, fills the free space of container)

Vapour: mist or fumes suspended in air.

Liquid : any flowing substance with little tendency to disperse. (No shape, takes shape of container).

PPM: Parts Per Million.

TLV-TWA  : Threshold Limit Value – Time-Weighted Average

STEL : Short Term Exposure Limit

IDLH : Immediately Dangerous to Life and Health

LEL :  Lower Explosive Limit

UEL : Upper Explosive Limit

 

TO PREVENT AN INCIDENT 3 MAIN TYPES OF GAS TESTING ARE UNDERTAKEN:

  • Before entering a confined space or Excavation.
  • Before undertaking any type of hot-work.
  • During inerting.

GAS TESTING IS INVOLVED WITH THE FOLLOWING ACTIVITIES:

  1. Hot work of any type where heat is used or generated, e.g. by welding, flame cutting and grinding, etc.
  2. Work which may generate sparks or other sources of ignition.
  3. Work which may cause an uncontrolled release of hydrocarbons, other flammable or toxic materials.
  4. Electrical instrumentation work which may cause sparks.
  5. Entry into confined spaces and Gas alarm investigation.

 

WHY CARRY OUT A GAS CHECK?

  1. Confined Space and Vessel Entry Gas Testing:
    • Oxygen deficiency – suffocation / death.
    • Oxygen enrichment – fire / explosion hazard.
    • Gas or flammable vapour – fire or explosive hazard.
    • Toxic gas – poisoning hazard.

To certify that the atmosphere within a confined space is free from Flammable, Explosive or Toxic substances and that the Oxygen content is 20.9%

In other words, to certify that the atmosphere is safe to work in!

  1. Hot Work Gas Testing:

To certify that there are no leaks or accumulations of Flammable Vapours or Explosive gases at the worksite when hot work is being under taken.

Gas Detector: 
  • Gas detector should be calibrated – Calibration validity is 3 months as per KNPC.
  • Gas testing only to be carried out by Authorised Gas Tester.
MULTI GAS DETECTOR CAN SHOW 4 GASES:
    1. O2 (Oxygen)
    2. LEL (Lower Explosive Limit)
    3. CO (Carbon mono oxide)
    4. H2S (Hydrogen sulphide)

Exposure Limits of Gases:
  • O2 (Oxygen)- required in between 19.5 to 23.5%
  • LEL- shall be zero for hot work and 10% for cold work is allowed.
GAS TLV STEL IDLH
H2S 10 PPM 15 PPM 33 PPM*
CO 25 PPM 400 PPM 1500 PPM
Cl 2 0.5 PPM 01 PPM 10 PPM
SO2 02 PPM 05 PPM 100 PPM
NH3 25 PPM 35 PPM 100 PPM

IDLH of H2S : 100PPM as per NIOSH, 33PPM as per KNPC

  • CO (Carbon mono oxide)
  • H2S (Hydrogen sulfide)  – IDLH -33ppm as per KNPC & 100PPM as per NIOSH
  • Cl 2 (Chlorine)
  • SO2 (Sulfur dioxide)
  • NH3 (Ammonia)

TLV-TWA: Threshold Limit Value – Time-Weighted Average:

Limit which is set for exposure up to 8 hours per day which results in no short or long term ill effects

STEL: Short Term Exposure Limit:

Limit which is set for exposure for 15 minutes (limit 4 times per day with 1 hour between exposures)

IDLH: Immediately Dangerous to Life and Health:

Limit which is set for no exposure – Work not allowed

PROPERTIES OF EXPLOSIVE GASES AND FLAMMABLE VAPOURS:

All combustible gases and  vapours are  characterized by explosive limits between  which, the  gas  or  vapour mixed with air is capable of sustaining the spread of flame. These can be referred to as the Explosive or Flammable limits

Explosive ranges of other gases

SUBSTANCE  LEL (Vol%) UEL (Vol%)
ACETONE 2.15 13
ACETYLENE 2.4 88
BUTANE 1.5 8.5
ETHANE 3 15.5
ETHYLENE 2.7 34
HEXANE 1.2 7.4
HYDROGEN 4 75.6
METHANE 5 15
PROPANE 2 9.5
LEL – Lower Explosive Limit:

LEL of methane is 5% by volume i.e. 5 volumes of methane mixed with 95 volumes of air.  This is the minimum quantity of Methane in a methane/air mixture that will ignite to produce a self-propagating flame if exposed to a hot source or a spark from electrical or mechanical equipment. If there is less than 5% of Methane in the air by volume the mixture is  too  lean to  support combustion.

 UEL – Upper  Explosive Limit:

UEL refers to the  highest concentration of a gas in the atmosphere which results in a combustible mixture. For example, the UEL of Methane in air is 15% by volume, which means that if there is more than 15% of Methane in the air by volume the mixture is too rich to support combustion.

LOW OXYGEN EFFECT AND SYMPTOMS :

15 to 19%            : Strenuous work is difficult

12 to 14%            :  Respiration and pulse increases, co-ordination and judgment impaired

9 to 11%             : Nausea, fainting, blue lips, unconsciousness

6 to 8%               : Death within 8 minutes, recovery if resuscitated within 4 minutes

Remember! That rusting of steelwork and some chemicals in confined spaces can reduce the oxygen in the atmosphere to dangerous levels very quickly.

HIGH OXYGEN EFFECTS AND SYMPTOMS:

Above 23% oxygen serious fire risk causing some materials to spontaneously combust, for example, oily cloth and steel wool >(30%)

Prolonged exposure causes damage to breathe processes and intoxication which seriously disturbs judgment with (industrial 02 gas)

Most gas detection apparatus is uncertified if the oxygen content is increased above atmosphere levels

The minimum content of Oxygen required for a Catalytic Gas detector reading LEL is 13% or more to read accurately.     

Author: Afnan Tajuddin

2 thoughts on “Gas Testing – Hazards, Precautions, Exposure Limits

  1. Very interesting topic., it enhanced more of my knowledge with regards to handling of hazardous gases! Thank you very much.., and i want more more exciting topics.

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