The storage of flammable liquids carries with it the need to put in place fire protection mechanisms. Tank blanketing is deemed one of the best and effective ways of preventing fires in flammable liquid storage tanks by means of controlling the formation of explosive vapour or air mixtures above the liquid surface.
The threat of fires in flammable liquid storage tanks is not far-fetched. Fires can occur at any time for many reasons. Therefore, it is vital to safeguard both human life and material assets from the destructive effects of fires. Tank blanketing valves provide an effective means of preventing and controlling explosions in flammable liquid storage tanks.
Lavenda Sekwadi, process engineer at Energas Technologies, explains that blanketing can reduce evaporation of the stored product and protects the tank from structural corrosion damage caused by air and moisture. He adds that contaminants that may enter the vapour space to cause product degradation are prevented.
“It is a control practice that is widely used in the chemical, pharmaceutical, food processing and petroleum refining industries due to its potential to improve safety,” explains Sekwadi.
Understanding tank blanketing
The operational principle of the blanketing valve is simple; a blanket of inert gas in the tank’s vapour space prevents atmospheric air from entering the tank. “Inert gas is admitted only during the in-breathing cycles (vacuum cycles). An in-breathing cycle occurs when liquid is being withdrawn from a tank or when vapours condense in the tank as a result of a decrease in temperature,” explains Sekwadi. “The amount of inert gas required for a specific application is dependent on the maximum in-breathing demand and the maximum emptying rate.”
Nitrogen is the most commonly used gas for blanketing, because, as Sekwadi explains, it is inert, widely available and relatively low-priced. “Other gases such as carbon dioxide or argon are also used on occasions. However, carbon dioxide is more reactive and argon is generally more expensive. The selection of an inert gas for a particular process is based on several properties, including flammability, non-contamination, chemically inactive, non-toxic, availability of large quantities of use and cost effectiveness.”
How it works
Typically, a blanketing valve is mounted on top of a storage tank along with a pressure/vacuum vent and an emergency pressure relief vent. The blanketing valve provides primary vacuum relief for the tank. It opens and supplies gas to the vapour space when pressure decreases to the valve’s set point.
“When vapour space pressure increases, the valve reseals. The pressure/vacuum relief vent is sized to take care of overpressure and vacuum conditions when nitrogen blanketing valve is out of service,” Sekwadi explains, adding that the pressure/vacuum relief vent is sized to take care of overpressure and vacuum conditions when the tank blanketing valve is out of service.
The pressure setting of the vent is set slightly higher than the blanketing valve setting, but below the maximum pressure the tank can withstand. Similarly, the vacuum setting is set higher than the normal operating vacuum but below the maximum vacuum pressure the tank can withstand.
Since the failure of an inert gas poses the danger of a fire, a flame arrester can be mounted upstream of the pressure/vacuum vent to prevent fire propagation into the tank. An emergency pressure relief vent should be considered for the relief of excessive pressure in the case of fire as per API 2000.
Energas Technologies supplies high-end and specialised equipment to the oil and gas industries in southern Africa and has stocked Protectoseal’s range of blanketing valves for the past 12 years.
“There are two types of blanketing valves depending on the actual process requirements: the 1-inch pilot operated (for high flow rate applications) and the 0.5-inch spring operated (for low flow applications) blanketing valves,” says Sekwadi.
Energas has now also added Protectoseal’s Series 10 2-inch Tank Blanketing Valve to its range, which provides capabilities that the existing Series 20 1-inch could not offer in larger flow applications. The valve is specifically designed for tank blanketing and its pilot-operated design offers a very tight operating band. It has the most compact design and fewest external connections of any other pilot-operated valve on the market. With uptime in mind, it is also field serviceable.
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