A Study Of Site-Remanufacturing Technology Of LPG Spherical Tanks After Service

With the worldwide rapid development of petrochemical industry and the increase of liquid material storage and handling sites, pressure vessels have already become one of the essential and important facilities in liquid material logistics. Large scale, rapid construction and economic investment have become the trend of the storage and handling pressure vessels. Meanwhile, however, pressure vessels are confronted and imperiled by failures of various mechanisms. Because a tremendous loss to the state property or personal injury or death would be incurred in the event of any failure in any form due to the characteristics of inflammability and explosiveness and poisonousness of the material stored therein. Failure, especially a crack induced failure occurred to a liquefied petrochemical gas (LPG) spherical tank, is being more and more highlighted as an important safety issue by engineers and scientists from enterprises and academic circles. It has become an urgent task deserving further exploration and scrutiny to diagnose, predict, avoid or repair the defected pressure vessels in service in a purposeful, feasible, reliable and economical manner.The emerging of remanufacturing technology offers a new idea to the repairing and recovering of the pressure vessels in service. By using the advanced recovering and assessment technologies, the discarded pressure vessels could be put into use again with increased safety. Research activities are thus carried out in the scope of the dissertation aiming at healing the common cracking phenomena in the LPG spherical tanks in service. Some basic principles and technical methods are proposed to repair and recover the cracked LPG spherical tank in service.The inspection over the spherical tank is a critical step to diagnose the disease cause of the spherical tanks. The specific measures taken are as follows: physical inspection, wall thickness determination, non-destructive testing, corrosion inspection, chemical composition analysis, metallographical analysis, hardness measurement, residual stress determination and etc. With the measures being taken, the health status of the spherical tanks can be investigated and determined preliminarily. Afterwards the cause for the crack occurred to the spherical tank shall be analyzed in an attempt to seek a solution to well repair and recover the spherical tank in disease. The present paper, after doing much research work, especially metallurgy analysis and residual stress measurement, provides strong support to the feasibility of remanufacture.Stress corrosion and hydrogen-induced cracking are found to be the main causes for the crack occurred to the LPG spherical tank, which are attributable to excessive residual stress and off-specification of hydrogen and sulphide concentration. Stress corrosion and hydrogen-induced cracking are characteristics of dissolution at the anode and penetration at the cathode with certain compatibility, selectivity and delay-action to be behaved accordingly, which therefore may trigger severe injury to the spherical tank to the extent that the hydrogen damage and hydrogen brittleness and etc. be incurred or fracture failure finally incurred. In order to reduce the tendency of stress corrosion cracking, the approperiate remanufacture technology should be developed. The method to perform an overall heat-treatment shall be one of the most effective measures to build up the health of the spherical tank and shall also be an effective approach to remove the hydrogen and reduce the residual stress as a result ofbringing about certain benefit with regard to improving the structure performance and weldability of the material. Reheating cracking, reheating brittleness and excessive reduction in strength occurred in the weldment could be avoided by an overall heat-treatment with the temperature being controlled within 570 ± 100C. As the constraint due to the tank's supports can not be eliminated during the heat treatment, thermal stress analysis is thus carried out by using finite element method to ensure t...