Module 9: Control Techniques - International Association of Fire ...

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IAFF Training for Hazardous Materials: Technician© Student Text 6. Determine the internal pressure of the cargo tank. The internal pressure must be determined before the tank can be moved or off-loaded in place. The pressure can be determined by: • Reading the tank’s pressure gauge • Reading the internal temperature gauge or taking the temperature of the contents and referring to vapor pressure/temperature graphs specific to the tank contents If neither temperature nor pressure can be measured, estimate the temperature based on ambient temperature. (Remember that the temperature of the tank’s contents may lag behind ambient temperatures by up to six hours.) Since tank contents may stratify into different temperature layers, pressure estimated from temperature readings may be lower than the actual pressure. Internal pressures in empty cargo tanks that contain residual vapors may be equal to pressures in loaded cargo tanks. Vapor pressure/temperature graphs are available from the Compressed Gas Handbook, the shipper, or the manufacturer of the material. When pressures are shown as absolute pressure, subtract 14.7 to convert them to gauge pressure. Many products are shipped under pressure of an inert gas (usually nitrogen) to preserve their purity. The inert gas prevents the material from reacting with air or moisture in air. Tanks used to ship materials this way are usually marked with a warning near the dome cover. Ambient temperature and low vapor pressure of the product may also indicate that the product is under pressure of an inert gas. When a product is shipped under pressure of an inert gas, vapor pressure/temperature graphs will not provide accurate estimations of internal tank pressure. Even if you have conducted a thorough assessment, never assume you have identified all the damage. Container weakness cannot always be seen. As in other emergencies, the most dangerous hazards are those that are not apparent. 9-34 Module 9: Control Techniques
Student Text IAFF Training for Hazardous Materials: Technician© Inspecting and Repairing Damaged Cargo Tank Fittings Many of the problems associated with cargo tank leaks involve valves and fittings. Once you identify the product involved, the source of the release, and the personal protective equipment needed, you can begin considering control measures. Often, releases associated with fittings can be stopped by tightening a fitting or re-closing a valve or cover. However, because the majority of cargo tanks are equipped with internal valves for product discharge, very little, if any, field repair can be performed. The tables on the following pages describe, in some detail, the likely locations of leaks, probable causes for these leaks, and basic control or repair methods. Module 9: Control Techniques 9-35
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