There are many types of storage tanks, each with particular design and manufacture specifications depending on the type of liquid or gas to be contained. Safety is paramount. Failure to ensure the highest safety standards can have disastrous results for employees working in close proximity to the tank, for people and property further afield and also the environment. Major incidents, such as the Boston Molasses Disaster in 1919 and more recently the Buncefield Oil Storage Depot Incident in 2005, demonstrate the dangers of failures in safety procedures but also provide lessons for the future.
Storage tanks can be situated above ground or underground with different regulations on their design and operation, depending on their location and use. To contain potential leakages, tanks will be surrounded by a containment dike or bunding, or will be bunded, offering a ‘second skin’. Bunded fuel tanks are required by law for any volume of fuel over 200 litres on a commercial property.
The type of liquid to be stored has implications for the design and manufacture of the tank itself. Tanks can have fixed roofs, floating roofs or both, with an internal floating roof and fixed roof above. Internal floating roofs are a safety requirement for use in many industries, such as oil refineries. They are used for liquids with low flash points, for example gasoline or ethanol, and have a cone roof with a floating roof inside that travels up and down along with the liquid level. This floating roof traps the vapour from the low flash point fuel to minimise the risk of ignition.
Tanks with only a floating roof and no fixed roof above, just an open top, are designed for medium flash point liquids, including kerosene, diesel and crude oil. And fixed roof tanks can contain high flash point fuels such as fuel oil, water and bitumen. These can have cone, dome or umbrella roofs.
Storage tanks perform essential functions in many industries, from refineries to agricultural settings, not just holding liquids and gases but in some cases storing them at specific temperatures. But any tank can be a hazard if not properly manufactured or maintained or if the correct procedures have not been followed. And damage to the tank, pipework or fittings can cause the contents to leak and create a dangerous situation, none more so than with highly flammable liquids. However, even molasses can be deadly, as demonstrated in Boston in 1919, when 21 people were killed when a tank failed due to poor design and quality.
More recently the Buncefield Oil Storage Depot Incident, in which two people were seriously injured, highlighted the essential role that flow equipment plays in ensuring the safe operation of storage tanks. The Buncefield Standards Task Group found that the two forms of level control both failed resulting in the overflow of large quantities of petrol. A large vapour cloud formed which ignited causing a massive explosion and a fire that lasted for five days.
In the task group’s final report the need for an overfill prevention system was highlighted. This should include equipment that provides sufficient separation between the maximum capacity of the storage tank, to avoid loss of containment, and the tank rate capacity, the level at which the alarm needs to be raised to prevent the maximum capacity, in other words the overfill level, from being reached or exceeded. This separation should take into consideration the possible need to operate a manual over-ride, which due to circumstances on the site may potentially be sited in an inaccessible position or which otherwise may require longer to operate.
Contrec’s 505 LM01 single tank level monitor is designed to give advanced warning as it monitors and measures the level of product in a single tank. The instrument uses the 4-20mA signal from a wide range of level sensors, including pressure transmitters, ultrasonic sensors and capacitance probes. The level monitor provides a 20 point strapping table and product density for level to volume and volume to mass conversions. It can display volume, percentage full and mass as well as level. Relay alarms are freely assignable as high or low alarms and an open collector output is provided for programmable level control.
A sub-menu gives full details of alarm status and can offer direct access to change the alarm setpoints. The instrument also has density correction available for pressure level sensors to cater for a deviation in product density. The relay alarms can be assigned to any of the main menu variables of a rate type. The alarms can be fully configured including hysteresis.
As with many Contrec instruments the 505-LMO1 can be further tailored to suit specific application needs including units of measurement, custom tags, second language or access levels.