Practical vapor control measures for the handler
Vapor recovery for the loading of volatile organic compounds (VOCs) in the maritime loading arena is a growing development, having been a requirement in the US and Norway for many years and is now becoming commonplace throughout Europe and other countries throughout the world. There are particular developments now in the Far East and Middle East.
Environmental pollution legislation is the driver behind the requirement to install vapor recovery systems, in terms of providing cleaner local environmental emission controls and safer working environments for the terminals and vessels being loaded. There are of course economic benefits from the recovery of a highly valuable product that cannot be ignored from any operating analysis of the installation of a vapor recovery unit. One aspect often over looked during the initial planning of such projects, is however, the handling of the vapor stream between the vessel and the vapor recovery system. During the loading of a vessel, whether the vessel is a barge or ship, vapors are displaced from the holds and either, as in the past, vented, or as is becoming more common, routed through a vapor manifold ashore to a nearby vapor recovery system. There are in some instances, vapor recovery units mounted aboard the vessel. These have particular requirements for recovery of vapors during, for example the loading of shuttle tankers at sea.
In handling volatile vapors the risk of a flammable incident is ever present. The safe design of the vapor handling system is critical issue not to be overlooked.
The displaced vapors will typically comprise of the VOCs in either an inerted atmosphere (nitrogen or engine exhaust gases), or in some cases air. Atmospheres in the holds can be very stratified, the layers directly above the product being loaded, having relatively high concentrations, this concentration reducing up through the hold. The result is that during the loading operation relatively low (lean) VOC concentrations flow from the vessel which, gradually increase as the liquid levels rise. The maximum concentrations, however, commonly occur as the product flows are reduced during the topping off of the vessel. Figure 1 provides an indicative picture of how the VOC concentrations may rise throughout a loading operation. Other significant factors affecting the vapor concentrations are the properties of the products being loaded.
During the loading of a VOC there is a natural degree of disturbance in the vessel. The effect of this is vapor growth. Simply put, the resulting flow from the loading operation exceeds the actual product loading rate. The difference between the vapor flow rate and the product fill rate is referred to as the growth rate. Other factors affecting the growth rates would be the temperature of the product and indeed, although to a lesser extent, the temperature of the vessel’s holds. Both vapor concentration and growth rates are important factors to understand when designing a vapor collection system.
Coast Guard regulations
Standards commonly referred to in respect to marine vapor recovery applications are the United States Coast Guard (USCG) regulations (33 CFR 154 Subpart E). The USCG issued the regulations with the primary intent of covering the safe installation and operation of vapor recovery applications for ship-loading applications. The regulations are quite prescriptive, although this should not relinquish the system designer from considering all aspects of the design in order to ensure a safe approach to handling the vapors is ultimately installed.
Although these regulations are not a legal requirement outside of the USA, they are to date the only set of guidelines governing these types of installations, and have over time been very well proven. A system designer would be well advised to consider at least a review of the regulations. To be fully compliant with the USCG regulations, such an installation would normally be certificated by the USCG through an appropriate certifying body.
There is a need however, to handle the vapors safely and in particular in moving the vapors from the vessel to the vapor recovery system. Issues that need to be considered are the volatility of the product being handled – that is, could the VOC vapor mixture fall within its flammable limits and adequately-sized vapor piping to ensure pressure losses in the vapor collection system to result in over- or under-pressuring the vessel, which could result in vapors being vented or air being drawn into the system?
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