Valves in action -
Pilot-operated pressure reducing valve RP 840
Explosion protection for tanks containing nitric acid
Tank farms are complex installations where a large number of products with various chemical and physical properties is stored. Therefore, proper storage of the product in consideration of all technical possibilities is of particular importance in order to protect the plant itself, the environment and the operating personnel from damage.
In the port of Antwerp a Dutch company operates an area of large tanks with capacities of up to 8,300 m³. Ships are loaded and unloaded at seven berths. In addition, a railway connection and load/unload sites for road tankers, tank containers and flexitanks are available. Amongst others, mineral oils, biofuels and oils and grease are stored in the tanks, but also hazardous chemicals such as nitric acid. It is a mineral acid used for example for the production of fertilizers, colouring agents and explosive material. In high concentrations it accelerates ignition and has strong oxidising properties. It can cause severe burns on living tissues.
Tank blanketing with inert nitrogen ensures the safe storage of the nitric acid. Air is prevented from getting into the tank because a slight overpressure is permanently maintained with the aid of the nitrogen gas cushion. Furthermore, no nitrous gases, which may form by the decomposition of nitric acid, can escape from the tank. A pilot-operated pressure reducing valve RP 840 is used for tank blanketing. In this valve a millibar reducing valve as pilot valve controls a main valve adapted to the plant. Thanks to the large diaphragm of the pilot valve, very small pressures can be pre-set, thus combining high control accuracy at very small pressures with the volume flow required for big tanks and pumps. If the pressure within the tank falls below 0.014 bar, the valve opens and continues to supply nitrogen until the control limit has been reached. The self-acting pressure reducing valve is made from deep-drawn stainless steel with excellent corrosion resistance. This is of particular importance for its intended use in the highly corrosive atmosphere of the tank.
