The Necessity of Onboard Gas & Vapor Detectors

^ Onboard Gas & Vapor Detectors Avert Disasters aboard Ships   – image Courtesy Nightman1965 at

Lest . . . Explosions & Deaths

December 29, 2011: Port of Fujairah, Dubai. Grayish smoke plumes clouded the sky above oil tanker MT Prem Divya as the air shuddered with the deafening blasts. The best place to be was somewhere else.

Two days earlier, the tanker had docked at the port for repairs. Investigations later zeroed in on the culprit that triggered the explosions: welding sparks. Three lost their lives. Bad experience. Sure. Sometimes, things get worse.

Texas was the scene of the worst industrial explosion in the United States on April 16, 1947 when French ship SS Grandcamp went up in violent flames while workers were loading ammonium nitrate fertilizer. The blast killed 600 and injured 3,500.

Then again, the shipping industry is migrating towards greener fuels such as LNG and LPG. LPG vapors are heavier than air and settle in the bilge area, the lowest in the ship. And, concentrations as low as one in seventy are explosive.

It only takes a spark to set off a fire. And there are countless such sparks waiting to ignite catastrophes. Fuels, oils, and cargoes release umpteen vapors, many of which are inflammable, or toxic, or both. Forewarned is forearmed, precisely why we need onboard gas and vapor detectors.

Vapor & Gas

Often used synonymously, there is a subtle difference between gas and vapor. Gas is any matter / substance at a temperature above its boiling point. In the context of normal pressure and normal operating conditions, a gas is any substance with a boiling point below 200C:

  • weighing 14 times less than that of air, hydrogen is the lightest gas
  • tungsten hexafluoride is the heaviest weighing 10 times as much as air
2 - Toxic GAs

Onboard Toxic Gas & Vapor Leaks can be Disastrous Image Courtesy John Lock at


Vapor is the correct term to describe gaseous matter below its boiling point. Vapors are the gaseous state of a liquid or a solid. They are created by changes in temperature and pressure. Please note, the change in state occurs without boiling of the liquid or the solid.

The Hazards & The Offenders

Overdoses of almost all gases and vapors can be lethal simply because people in the vicinity breathe them. The hazards include:

  • Explosion of flammable gases
  • Poisoning by toxic gases
  • Suffocation / Asphyxiation due to oxygen displacement
  • Flammability Escalation on account of oxygen enrichment

Even harmless, non-reactive gases such as helium, argon, and nitrogen are dangerous because they displace oxygen and promote suffocation. If oxygen concentration drops below 6% (by volume), it causes suffocation. But above 21%, it facilitates fires and explosions.

Human nose cannot detect an increase in oxygen concentration in the local ambience. Oxygen forms 21% (by volume) of our atmosphere and any increase escalates the possibility of fires and explosions via auto ignition.

At the same time however any decrease in oxygen concentration is equally lethal, for it aggravates the risk of suffocation. Industries use liquid nitrogen in large quantities and its leakage can create acute oxygen deficiency in the local ambience.

Toxicity: LC50 stands for lethal concentration or the concentration of the gas/vapor in air which will kill 50% of laboratory animals (usually mice or rats) when inhaled for a specified time (generally 4 hours).

Toxicity Values of Gases

Gas Threshold Toxicity
Carbon dioxide 5,000ppm (parts per million)
Propane, Butane 1,000ppm
Acetone 500ppm
Methyl ethyl ketone (MEK) 200ppm
Butanol 100ppm
n-Hexane, Toluene 50ppm
Acetonitrile 20ppm
Chlorobenzene 10ppm
Diethyl amine 5ppm 1ppm
Chlorine 500ppb (parts per billion)
Methyl Chloformate 200ppb
Chlorine dioxide 100ppb
Decaborane 50ppb
Phosgene 20ppb
Methylisocyanate 10ppb
MDI (Methyl-di-phenyl-di-isocyanate) 5ppb

Table Credit: Introduction to Gas Detection Systems at introduction-gds-fl-9046421-en.pdf

Gases with LC50 below 0.5gm/m3 are extremely toxic. These include:

  • boron trichloride, boron triflouride;
  • hydrogen cyanide, hydrogen phosphide, hydrogen sulphide, hydrogen arsenide, hydrogen fluoride;
  • bromine, phosgene, diborane, ozone;
  • nitrogen monoxide, nitrogen dioxide;
  • fluorine, tungsten hexafluoride, and sulphur tetraflouride

Vector Concept Illustration of Open Fire @ Freighter Deck

Image Courtesy Adam Vilimek at

With LC50 values between 0.5gm/m3 and 2gm/m3, the following gases are moderately toxic:

  • acetronitrile, ammonia;
  • carbon monoxide, carbon disulphide, sulphur dioxide;
  • chlorine, hydrogen chloride;
  • benzene, dicyan, nitrogen triflouride;
  • methanol and methyl bromide

Flammability: Gases with low Lower Explosion Limits (LEL) and vapors with low Flashpoints are more likely to explode:

  • LEL is the least concentration (in volume%) of a gas or vapor in air at which it can create a fire or a flash in the presence of a source of ignition

Safety professionals often use LEL synonymously with Lower Flammable Limit (LFL). The LEL of most known flammable gases and vapors is between 0.5 and 15 (in volume%)

  • Flashpoint of a volatile substance is the minimum temperature at which it vaporizes to a level above its LEL

Gases do not have a flashpoint, only flammable liquids do. This is because gases do not have a liquid phase under normal conditions

Flammable Vapors: LEL and Flashpoint

Vapor LEL (Volume %) Flashpoint (0C)
Acetone 2.5 < -20
Acrylonitrile 2.8 -5
Benzene 1.2 -11
n-Butanol 1.7 35
n-Butyl acetate 1.2 27
n-Butyl acrylate 1.2 37
Chlorobenzene 1.3 28
Cyclohexane 1.0 -18
Cyclopentane 1.4 -51
1.2-Dichloroethane (EDC) 6.2 13
Diethyl ether 1.7 -40
1.4-Dioxane 1.9 11
Epichlorohydrin 2.3 28
Ethanol 3.1 12
Ethyl acetate 2.0 -4
Ethyl benzene 1.0 23
n-Hexane 1.0 -22
Methanol 6.0 9
1-Methoxy-2-propanol 1.8 32
Methyl-ethyl- ketone (MEK) 1.5 -10
Methyl methacrylate 1.7 10
n-Nonane 0.7 31
n-Octane 0.8 12
n-Pentane 1.4 -40
i-Propanol (IPA) 2.0 12
Propylene oxide 1.9 -37
Styrene 1.0 32
Tetrahydrofurane (THF) 1.5 -20
Toluene 1.1 6
Xylene (isomer mix) 1.0 25

 Table Credit: Introduction to Gas Detection Systems at introduction-gds-fl-9046421-en.pdf

 Flashpoint is a more comprehensive indicator of the explosivity of a flammable liquid because it is a combined measure of the volatility plus the LEL of the liquid.

Whether a liquid will vaporize to produce a concentration above its LEL depends on its vapor pressure that, in turn, depends on the temperature of the liquid.

Vapor Pressure is the pressure you have to exert on the surface of a liquid to prevent it from vaporizing. The greater the vapor pressure of a liquid, the greater is its tendency to vaporize. Vapor pressure increases with increasing temperature.

Flammable Gases: LEL

Gas LEL (Volume%)
Acetylene 2.3
Ammonia 15.4
1.3-Butadiene 1.4
i-Butane 1.5
n-Butane 1.4
n-Butene (Butylene) 1.2
Dimethyl ether 2.7
Ethene (Ethylene) 2.4
Ethylene oxide 2.6
Hydrogen 4.0
Methane 4.4
Methyl chloride 7.6
Propane 1.7
Propene (Propylene) 1.8

Table Credit: Introduction to Gas Detection Systems at introduction-gds-fl-9046421-en.pdf


It is always better to be safe than to be sorry. With a host of malicious gases and vapors waiting to ambush your crew and unleash grave destruction, onboard gas and vapor detection is the need of the hour.

Want to know more on safety practices aboard ships? Visit our blog. And if you are looking for exemplary marine fabrication services, marine pipe fitting, and large scale custom metal fabrication, contact Kemplon Engineering.