Sources of H2S Vapours - Practical Aspects
Environmental laws in ports of loading and discharging force the vessel to arrive in port with H2S content below certain levels. Before considering the ways of handling this issue it is pertinent to underline that origin of H2S vapours in cargo tanks may be of two sources:
a) vapours from remains of previous, H2S containing cargo, i.e. from ROB;
b) vapours from cargo which is being loaded is of high H2S content.
In the first case, to reduce H2S content up to required level it would be necessary upon completion of discharging operation to carry out purging of tanks with inert gas to dilute or displace H2S vapours from tanks. Some standard additional clauses in charterparties address this issue by way of obliging the owners’ to warrant that H2S shall have been reduced to below the certain levels prior to arrival to load port. Generally speaking, purging operation does not offer any difficulty but requires time, which depends on H2S content, outside temperature and characteristics of IG plant(Read more about IG plant here). It is also necessary to underline in this context, that any significant ROB of previous cargo in tanks will considerably complicate this task due to constant emission of H2S gas from remains of last cargo.
In the second case, presence of H2S gas in vapours of cargo being loaded give rise to a number of difficult legal issues. First of all, the shippers and terminal are usually reluctant to disclose, as required by ISGOTT, true H2S content in cargo to the master at the preliminary stage of loading operation. This reluctance has a natural explanation that the terminal likes to avoid to be exposed to risks related to safety of H2S cargo handling and, moreover, any such disclosure will also alert the charterers and/or cargo owners and may provoke a concern over the cargo quality and probably over its tradability as well. Subsequently, when the vessel is under way to the discharging port and presence of hydro sulphur has been asserted by crew, the owners may find themselves in difficult position when they have to comply with the laws and regulations of arrival port with respect of H2S content.
Unlike the situation with empty tanks containing some H2S vapours which can be managed by purging with inert gas, when tanks are full with cargo, such method, in most cases, will not give positive result. Due to volatile nature of crude oil and other petroleum products, they continuously vaporise gases, H2S including, from their surface. The rate of vaporisation or vapour pressure, as mentioned earlier, depends mainly on temperature and the ratio of gas to liquid by volume in tank. Obviously, introduction of inert gas in small empty volumes of loaded tanks will neither displace nor dilute H2S, but rather ventilate it out of tanks space. Temporarily some reduction of H2S content can be achieved, but this content will rise again quickly when introduction of inert gas will be stopped.
Apart from zero probability to reach any desirable reduction of H2S, such operation is obviously in direct contradiction with that interpretation of safety measures for handling H2S cargoes stated in ISGOTT. Their general idea is to retain or seclude H2S vapours liberated by cargo from contact with the crew and terminal staff as much as possible or, if such retention is not practically possible, to ensure safety of personnel by monitoring atmosphere and using personal protective equipment such as breathing apparatus. Dispersion of H2S gases by ventilation from ships tanks leads to dangerous exposure of personnel to poisonous vapours. See for example extract from ISGOTT sec. 11.1.9:
- Venting to the atmosphere at a relatively low tank 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.
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Posted by: Chris Noon, MNI, 28 November 2011
In addition to your comments above re purging H2S there is the real problem that prolonged purging will result in cargo loss.
I have a case where 9000 bbls of Murban Crude, out of 600,000 BBLS, have been lost apparently due to purging. Total Purging was conducted for 120hrs. I conducted a thorough inspection of vessel and found nothing to suggest anything other than purging to cause loss. Master advised Charterers that purging would result is some sort of cargo loss but they had no choice as port regs required H2S level below 100ppm. Levels on ship had been up to 1500ppm. MSDS just commented that that product has a smell of toxic H2S.
…By the time the vessel got to Australia and cooler climate the H2S levels stabilised enough to remain below 100ppm for the duration of the port visit. For the 2nd port vessel had to again purge tanks.