The storage of flammable liquids carries with it the need to put in place fire-protection mechanisms. Tank blanketing is an effective way of preventing fires in flammable-liquid storage tanks by means of controlling the formation of explosive vapour or air mixtures above the liquid surface.
Lavenda Sekwadi, process engineer at Energas Technologies, explains that blanketing can reduce the evaporation of the stored product and protect the tank from structural corrosion damage caused by air and moisture. He says that contaminants that may enter the vapour space to cause product degradation are blocked.
“It is a control practice that is widely used in the chemical, pharmaceutical, food-processing and petroleum refining industries, thanks to its potential to improve safety,” explains Sekwadi.
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 inbreathing cycles (vacuum cycles). An inbreathing cycle occurs when liquid is being withdrawn from a tank or when vapours condense in the tank as a result of a decrease in temperature,” he says.
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. 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.”
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.
The pressure setting of the vent is set slightly higher than the blanketing valve setting but below the maximum pressure that 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.
In conclusion, Sekwadi advises that safety systems, especially regarding the prevention of fires in flammable liquid storage tanks, are of paramount significance, and it is always advisable to source products of this nature from reputable suppliers.