Aromatic hydrocarbons in petroleum cargo- benzene, tolune, xylene and others

The aromatic hydrocarbons include benzene, toluene and xylene. These substances are components, in varying amounts, in many typical petroleum cargoes such as gasolines, gasoline blending components, reformates, naphthas, special boiling point solvents, turpentine substitute, white spirits and crude oil.

The health hazards of aromatic hydrocarbons are not fully established but it is recommended that personnel engaged in an oil tanker cargo operations take adequate precaution in order to minimise exposure to hydrocarbon gases. The TLV of an aromatic hydrocarbon vapour is generally less than that of other hydrocarbons.

Minimum standards for ships carrying liquids in bulk containing benzene are contained in the IMO MSC Circular 1095/2003. This establishes requirements for the carriage of cargoes with a benzene content of 0.5% or more.

This Circular contains requirements for:

  • The transfer of information on the cargo.

  • Occupational exposure limits.
  • Air quality monitoring.
  • Personal Protection Equipment (PPE).
  • Maintenance of PPE.
  • Enclosed space entry.
  • Training.
  • Medical monitoring.
  • Ship/shore connection.
  • Precautions during cargo operations.
  • Tank venting.
  • Cargo measurement and sampling.
  • Contaminated clothes.

Whilst the MSC Circular refers to benzene containing cargoes listed in MARPOL Annex I, some of the precautions to be followed are defined by MARPOL Annex II and the IBC and BCH Codes.

Benzene primarily presents an inhalation hazard. It has poor warning qualities, as its odour threshold is well above the TLV-TWA. Chronic exposure to concentrations of benzene vapours of only a few parts per million in air may cause leukaemia.

Exposure to concentrations in excess of 1,000 ppm can lead to unconsciousness and even death. Benzene can also be absorbed through the skin and is toxic if ingested.

Handling Procedures
Cargoes containing benzene should be handled using the closed operation procedures as this will significantly reduce exposure to benzene vapour. Where a vapour emission control system is available ashore, it should be used.

Operators should adopt procedures to verify the effectiveness of the closed loading system in reducing the concentrations of benzene vapours around the working deck. This will involve surveys to determine the potential for exposure of personnel to benzene vapour during all operations such as loading, discharging, sampling, hose handling, tank cleaning and gas freeing and gauging of cargoes containing benzene. These surveys should also be carried out to ascertain vapour concentrations when tank cleaning, venting or ballasting tanks whose previous cargo contained benzene.

Spot checks on vapour concentrations, using detector tubes and pumps, toxic analysers or an electronic detector tube, should be carried out by ship’s personnel to ascertain if TLV-TWAs are being exceeded and if personal protective equipment should therefore be worn.

Exposure Limits
The IMO MSC Circular gives the TLV-TWA for benzene as 1 ppm over a period of eight hours. However, working procedures should aim at ensuring the lowest possible gas concentrations are achieved in work locations.

Personal Protective Equipment (PPE)
Personnel should be required to wear respiratory protective equipment under the following circumstances:
  • Whenever they are at risk of being exposed to benzene vapours in excess of the TLVTWA.
  • When TLVs specified by national or international authorities are likely to be exceeded.
  • When monitoring cannot be carried out.
  • When closed operations cannot be conducted for any reason.

The respiratory protective equipment to be worn at any given time should be determined by the Company, but should not fall below that required by IMO MSC Circular 1095.
Operators should be aware that gas measuring equipment on board tankers will only provide spot readings and that personnel may experience concentrations of vapour in excess of the reading obtained. Therefore, careful consideration should be given to the type of respiratory protective equipment employed for specific tasks.

The need to use respiratory protective equipment may be extended by local regulations or company procedures to those personnel not directly involved in cargo operations.

Benzene in Fuel Oils

Chronic exposure to very low concentrations of benzene vapours in air may affect bone marrow and causeleukaemia. Similar to part (a) above for H2S, the MSDS for the fuel should be reviewed prior to the bunkering operation and the presence of Benzene considered.

In the event of Benzene detection during bunkering, the advice of the Ship Management Office is to be sought urgently and access to areas adjacent to and downwind of the loading area and tank vents, should be prohibited.

The Company provides equipment to monitor levels of Benzene onboard in the form of Draeger Multi-gas Detectors and tubes for benzene.

d) Threshold Limit Value of Benzene

It presents an inhalation hazard and has poor warning qualities as its odour threshold is well above the Threshold Limit Value (TLV-TWA). When inhaled, symptoms which may be experienced include nausea, convulsions, dizziness,sleepiness, headache, dyspnoea, and lossof consciousness.

The following guide, with the effects under various atmospheric concentrations on exposed persons, gives an indication of the dangers involved:

Measured In Air by Volume

50 to 100 ppm: headache and asthenia

500 ppm :aggravation of symptoms

20,000 ppm: death within 5 to 15minutes

The TLV-TWA for H2S is given as 1 ppm over a period of eight hours, however, working procedures should aim at ensuring the lowest possible gas concentration is achieved in work locations.

Tank Entry

Prior to entry into a tank that has recently carried petroleum products containing benzene, the tank should be tested for benzene concentrations.

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