Hydrogen Sulphide

be used while sampling or gauging and during the commencement of cargo operations or on any other occasion where there might be risk of skin contact with the cargo or a release of vapour.

Chemical protective equipment must be readily available in case of spills or extensive amounts of cargo vapour being detected. The Chief Officer must not hesitate to order the use of such equipment, no matter how cumbersome its use may be, if there is a need for it.

1.5 Hydrogen Sulphide

Hydrogen Sulphide (H₂S) is a very toxic, corrosive, and flammable gas. It has a very low odour threshold and a distinctive odour of rotten eggs. H2S is colourless, heavier than air, has a relative vapour density of 1.189, and is soluble in water.

1.5.1 Sources of Hydrogen Sulphide

Many crude oils are produced with high levels of H2S, but are stabilised. This process usually reduces the level before the crude oil is delivered to the vessel. However, a cargo may be received on board with either a reduced level of stabilisation or one which has not been stabilised at all, in which case higher than normal levels of H₂S may be encountered.

H2S can also be encountered in refined products such as Naphtha, Fuel Oil, bunker fuels, Bitumens and Gas Oils. Cargo and bunker fuel should not be treated as free of H₂S unless the MSDS confirms it and the fact has been confirmed by monitoring.

1.5.2 Expected Concentrations

It is important to distinguish between concentrations of H2S in the atmosphere, expressed in ppm by volume, and concentrations in liquid, expressed in ppm by weight. It is not possible to predict the likely vapour concentration from any given liquid concentration but, as an example, a crude oil containing 7 ppm (by weight) H2S has been shown to produce a concentration of 700 ppm (by volume) in the gas stream leaving the tank vent.

Precautions against high H₂S concentrations are normally considered necessary if the H₂S content in the atmosphere is 5 ppm by volume or above.

H₂S concentration in vapour will vary greatly and is dependent upon factors such as:

• The liquid H₂S content;

• The amount of air circulation;

• The temperature of the air and the liquid;

• The liquid level in a tank;

• The amount of agitation of a liquid.

1.5.3 Exposure Limits

The TLV-TWA for H2S is 5 ppm over a period of eight hours. However, working procedures should aim at ensuring that the lowest possible gas concentrations are achieved in work locations.

H₂S concentration

(ppm by volume in air) Physiological Effects

0.1 – 0.5 ppm First detectable by smell

10 ppm May cause some nausea, minimal eye irritation.

25 ppm Eye and respiratory tract irritation. Strong odour.

50-100 ppm

Sense of smell starts to breakdown.

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 exposure.

150 ppm Loss of sense of smell in 2-5 minutes.

350 ppm Can be fatal after 30 minutes inhalation.

700 ppm Rapidly induces unconsciousness (few minutes) and death.

Causes seizures, loss of bowel and bladder control. Breathing will stop and death will result if not rescued promptly.

700+ ppm Immediately fatal.

Note: Persons over-exposed to H₂S vapour should be removed to clean air as soon as possible.

The adverse effects of H₂S can be reversed and the probability of saving the person’s life improved if prompt action is taken.

Typical Effects of Exposure to Hydrogen Sulphide 1.5.4 Procedures for Handling Cargo Containing H₂S

Precautions must be taken when handling all cargoes likely to contain hazardous concentrations of H2S. They should also be taken when ballasting, cleaning, or gas freeing tanks which previously contained a cargo having an H2S content.

In ship to ship transfers, particular attention should be paid to the difference in freeboards and the possibility of vapour not being adequately dispersed. Vent velocities should be kept high on the receiving ship and the vessels should be turned to allow the wind direction to carry the vapours away from the accommodation.

The following precautions should be implemented when handling cargoes with a high H2S:

• Closed loading procedures must be used;

• Check that all doors and ports can be securely closed to prevent any gas ingress;

• The cargo plan must contain specific details for the venting procedure, monitoring for vapour, personal protective equipment, accommodation and engine room ventilation arrangements and emergency measures;

• Venting to the atmosphere at a relatively low pressure should be avoided, particularly in calm wind conditions;

• Cargo loading should be stopped if there is no wind to disperse the vapours or if the wind direction takes cargo vapours towards the accommodation;

• Only personnel actively engaged in either vessel security or cargo handling should be permitted on open decks. Regular maintenance on deck should be limited or postponed until after the end of cargo operations;

• Visitors should be escorted to and from the accommodation and briefed on the hazards of the cargo and emergency procedures;

• The notice CCR47 - Hydrogen Sulphide must be posted at the gangway to warn visitors of the fact that high H₂S cargoes are being handled.

1.5.5 Vapour Monitoring

All analysers currently supplied to the Columbia fleet are capable of measuring H2S. High concentrations and the corrosive nature of the gas can have a damaging effect on the sensors of many electronic instruments, and therefore detector tubes should be used if it is necessary to monitor a known high concentration.

A personal analyser must be carried by personnel on deck whenever cargo operations are being undertaken involving a cargo with a known H₂S content. If the H₂S content is not known or is uncertain then a check must be carried out to determine the level of H₂S and precautions taken as necessary. Where high levels of H2S are anticipated then the Officers and ratings involved in the cargo operation must be briefed prior to the operation on what action to take in the case of their portable analysers alarming on high H₂S levels.

The company requires the alarm to be set at 5 ppm on both the MSA Altair 4 and the Draeger X-AM 7000 analysers.

If H2S is detected on deck then precautions should be taken to ensure that the entry of H2S into the accommodation, wheelhouse, engine room, and stores spaces is as far as possible avoided.

Over time, low concentrations of H₂S can cause discomfort to personnel.

For further information on the application and use of gas analysing equipment refer to Cargo Operations General, Chapter 6: Gas Detection and Personal Protective Equipment.

For the procedure on the removal of H₂S from cargo tanks refer to Cargo Operations Petroleum, Chapter 5: Cargo Tank Ventilation and Gas Freeing.

1.5.6 Personal Protective Equipment when Handling H2S

If, for whatever reason, personnel have to be involved in operations with products containing H₂S then the TLV-TWA of 5 ppm over 8 hours must be monitored and must not under any circumstances be exceeded.

Breathing apparatus must be used if exposure over the TLV-TWA of 5 ppm over 8 hours is anticipated. When working in areas where H₂S might be unexpectedly encountered, then consideration should be given to the use of breathing apparatus. Such occasions include:

• Open gauging and sampling;

• Removing blanks for connecting cargo hoses or loading arms;

• Cleaning filters;

• Draining lines to open containment;

• Mopping up spills; and

• Working in cargo tanks which have been cleaned but which previously contained a high H₂S content cargo.

If an H2S vapour-free environment cannot be guaranteed then breathing apparatus must be worn.

1.5.7 Corrosion

H₂S is very corrosive and enhanced inspection and maintenance regimes should be put in place if cargoes with a high H2S content are likely to be carried. P/V valve seats made of brass are more likely to fail than those of stainless steel. Mechanical tank gauges are more likely to fail since H2S has a damaging effect on stainless steel tension springs and metals such as brass and bronze.

1.5.8 General Nuisances

In addition to being a health hazard, H2S odour is also considered a public nuisance. Most terminals ban the release of H2S concentrations above 10 ppm to the atmosphere. Vapour return is normally provided.

Whenever a vessel is carrying a high H2S cargo, the notice CCR47 - Hydrogen Sulphide must be posted at the gangway warning visitors of the hazard.

1.6 Mercaptans

Mercaptans are colourless, odorous gases generated naturally by the degradation of natural organisms. Their smell has been likened to rotting cabbage. Mercaptans may occur on ships where seawater has remained beneath an oil cargo or where oil residues are left in tanks that contain water, such as in a dirty ballast tank after it has been incompletely drained. They are also found in water treatment plants and ballast treatment facilities.

Mercaptans may be present in the vapours of Pentane Plus cargoes and in some crude oils. They are used as an odourising agent in natural gas. They can be detected by smell at concentrations below 0.5ppm, although health effects are not experienced until the concentration is several times higher than this.

The initial effects of Mercaptans on people are similar to those caused by H2S exposure, i.e.

irritation to the lungs, eyes, nose and throat. Headaches, nausea, vomiting, and unconsciousness may result from exposure to Mercaptans.

Little is known about the dangers of Mercaptans, but it would be prudent to avoid prolonged exposure above concentrations of 0.5ppm. The presence of Mercaptans can only be measured by toxic gas detector tube.

All the current industry guidance indicates that the risk Mercaptans pose to personnel is minimal, if any, in the quantities which might be found on petroleum tankers. The company has therefore decided that the provision of Mercaptan detector tubes is unnecessary unless the vessel has specific information that the cargo to be carried contains significant quantities of Mercaptans. If this is the case Mercaptan detector tubes should be arranged to be on board prior to loading the cargo.