Oil tankers operations at sea and while at port requires some basic safety procedure to be observed. Prior entering a space which contained or has a risk of the presence of any Toxic gases such as benzene, H2S, etc., the MSDS (Marine Safety Data Sheets) and other relevant information and precautions for Toxic gases as listed in ISGOTT should be referred to. Thorough gas checks using suitable Toxic gas detector tubes need to be carried out.
1. HYDROCARBON VAPORS Characteristic of H.C. Vapors
a). Hydrocarbon gases are heavier than sir (1.5 to 3 times), and tend to accumulate in the vicinity of the area where they are generated. A large amount of gas might exist sometimes in unpredictable locations. The bottom of the pump room is a typical example.
b). Gases flow to the leeward side, and are dangerous in that they may cause explosion at spaces other than where they are generated.
c). The explosion limits or flammable limits (LEL / UEL or LFL / UFL) varies according to the type of Hydrocarbon gas in question.Their proportionate mixtures present in the petroleum vapor in question. This is generally around 1.8% Vol. in Air (Min.) to 9.5% Vol. in Air (Max); whereas International Chamber of Shipping recommends a range of 1.0% to 10.0 Vol.%, to assume safer standards.
d). The danger for explosion is far greater in a lightly laden ship, while loading / unloading cargo, during ballasting operations or during tank cleaning, rather than when fully laden. This is because on a loaded vessel, the tank atmosphere contains hydrocarbons concentrations of well over above UEL.
e). The Company permits limit of Hydrocarbon (petroleum) gas mixture in air conducting man-entry as 1.0 % LFL as measured with suitable approved type gas detector.
Toxic Hazards of H.C. Vapors
a). Petroleum gas is noxious and harmful to the body. The table K-01-D-1 shows the concentration of petroleum gas and its effects on the human body.
b). Toxicity can be greatly influenced by the presence of some minor components such as aromatic hydrocarbons (eg. Benzene and Hydrogen Sulfide ie H2S).
c). Even if a tank is empty, gases might be regenerated from sludge in the tank. Precautions are also necessary when working in tanks because Petroleum Gas or Inert Gas from other tanks might enter the tank due to leakage from valves.
HC Gas concentration
(Volumetric Proportion in Air) Effects on the Human body 0.02% 300ppm Industry Permissible concentration (TLV-TWA for 8 Hrs) or 2% LEL 0.1% 1,000ppm Irritation in the eyes within an hour. 0.2% 2,000ppm Irritation in the eyes, nose or throat within 30 minutes, dizziness and unsteadiness. 0.7% 7,000ppm Signs of giddiness within 15 minutes. 1.0% 10,000ppm Sudden giddiness occurs and if the body is exposed to the same conditions continuously, unconsciousness results, and can sometimes lead to death. 2.0% 20,000ppm Sudden giddiness, unconsciousness, resulting in death.
HYDROGEN SULFIDE (H2S)
Characteristic of Hydrogen Sulfide (H2S)
a). H2S is a Highly toxic, corrosive and flammable gas that in low levels will smell like rotten eggs.
b). It may be present in bunkers in dissolved state or as a gas. In may also be found in certain Natural gases, Crude oils and certain Refined products such as Naphtha.
c). It is Colorless and Heavier than Air, having relative vapor density of 1.189.
d). Exposure to high levels of H2S can be fatal after a very short period of time.
e). H2S is a liquid soluble gas and produces vapor when the liquid is agitated or heated. It is not possible to predict the likely H2S vapor concentration present above a liquid in a tank, from any given liquid concentration but, as an example, an oil containing 70 “ppm by weight”? as concentration of H2S in liquid has been shown to produce 7000 “ppm by volume.
H2S Gas Concentration
(ppm by Vol. in air) Physiological Effects on the Human body 0.1 – 0.5 ppm First Detected by smell 10 ppm May cause some nausea, minimal eye irritation 25 ppm Eye and respiratory tract irritation. Strong odour 50 – 100 ppm Human sense of smell starts to break down. Prolonged exposure to concentrations at 100 ppm induces a gradual increase in the severity of these symptoms and death may occur after 4 – 48 hours of exposure 150 ppm Loss of sense of smell in 2 – 5 minutes 350 ppm Could be fatal after 30 minutes of inhalation 700 ppm RAPIDLY induces consciousness (few minutes) and death. Causes seizures, loss of control of bowel and bladder. Breathing will stop and death will result, if not rescued promptly.
Precautions for Hydrogen Sulfide (H2S)
a). In cases where H2S concentrations are known to be greater than 100 ppm in the vapor space and likely to be present in the atmosphere, Emergency escape Breathing Apparatus shall be made available to personnel working in the hazardous area, who, should already have a Personal (pocket-able) H2S gas monitoring alarm / instrument.
b). The presence of H2S in bunkers should not be ruled out. Empty bunkers tanks shall be tested for the presence of H2S prior to bunkering.
Health Concerns in connection with Benzene
a). Benzene is present in varying concentrations in some crude oils and the MSDS shall be consulted each time before cargo handling.
b). Benzene gas has poor warning qualities, as its odour threshold is well above the TLV-TWA limits. Exposure to concentrations in excess of 1000 ppm can lead to unconsciousness and death. Benzene can also be absorbed through the skin and becomes toxic when ingested.
c). If there is evidence that dissolved benzene is present in the liquid cargo in quantities of 0.5% by volume or more, respiratory protection is required when conducting tank “de-mucking” or standing up wind from tanks containing benzene are being vented;
Composite of Inert gas
After efficient scrubbing of the inert gas (to reduce the content of sulfur dioxide), the typical constituents of a flue gas are as showing the Table :
Inert Gas Percent Present (after scrubber)
Nitrogen N 83%
Carbon Dioxide CO2 12-14%
Oxygen O2 2-4%
Sulfur Dioxide SO2 50 ppm
Carbon Monoxide CO Trace
Nitrogen Oxides NOx 200 ppm
Water Vapor H2O Trace (high, if not effectively dried)
Ash and Soot C Traces
Density 1.044 (heavier than air)
Health Concerns of Inert Gas
a). The main hazard of inert gas is its low oxygen content.
b). The subsequent hazards such as the presence of traces of toxic gases in Inert Gas, inside cargo tanks and spaces of such encountering are reduced and controlled, by following the company’s designated “Procedures for Entry into Enclosed Spaces“ .
c). By Gas freeing from a “Purged condition (HC=2% VOL)” to the “Gas free condition(HC=1% LEL)”, sufficient dilution of such toxic gases to below their TLV-TWA limits will have been achieved.
Related article : Risk of gas freeing operation and precautions for oil tankers