Valves in action -
Pilot-operated control valve RP 840
Petrochemical plants become safer through tank blanketing
Tank farms are used for the interim storage of large quantities of liquid or gaseous products. Especially chemical and petrochemical industries maintain large-volume tank depots throughout the world, which are often located on shorelines so that the tanks can be filled or emptied by waterborne transport.
At a terminal in Southeast Asia petrochemical products and petroleum products are stored temporarily and forwarded by sea. Every year an average of 2,000 ships with a cargo capacity of up to 250,000 tdw transship totally 15 million tons of cargo. The petrochemical complex comprises six ship berths for loading and unloading various liquid goods including toluene. It is a volatile aromatic hydrocarbon compound with characteristic odour, which is amongst other things used as additive in motor vehicle fuels. The highly flammable substance is harmful to health and to the environment.
Pressurising the storage tanks with inert nitrogen prevents the emission of volatile compounds and simultaneously ensures flexible availability of the toluene. The inert gas forms a cushion in the tank head space and displaces air or humidity so that they do not get into the tank where they would activate undesired chemical reactions.
At an inlet pressure of abt. 6 bar, the pilot-operated pressure control valve RP 840 maintains a constant outlet pressure of at least 2.5 mbar for the inert gas within the tank. The pressure within the tank rises under solar exposure and falls when temperatures cool down. When the pressure falls below the value of 2.5 mbar, the valve opens and continues to supply nitrogen until the control limit has
been reached. The main and the pilot valve consist of deep-drawn stainless steel with excellent corrosion resistance and are arranged as a pre-mounted unit in a rack. The valve cone is soft-sealed. Large control surfaces, lever transmission and narrowly separated spring modules allow for small outlet pressures, very precise control and high reduction ratios. Thus the self-acting control valve considerably optimises the operating safety of the plant and reduces environmental pollution.
