Enhancing pressure vessel maintenance during decommissioning

Many pressure vessel accidents are often caused by neglecting maintenance during decommissioning. In a sense, an out-of-service period of poor maintenance of the pressure vessel even faster than the normal use of the vessel damage, this is because the out-of-service pressure vessel is not only not cleaned up by the container of residual media corrosion, but also by the corrosive effect of the atmosphere. In the atmosphere, the air is not saturated with water cooled to a certain temperature, water vapour will condense from the air and pooled into a water film covering the surface of the pressure vessel. If the metal surface is rough or there are dust and pollutants attached to the surface, or the corrosion protection layer is broken, etc., water vapour is more likely to precipitate and gather in these parts of the pressure vessel. Nitrogen, oxygen and other impurities in the air and carbon dioxide, nitrogen oxides, hydrogen chloride, etc. can be dissolved in the water film to form an electrolyte solution, and thus have the conditions for galvanic corrosion. Conditions affecting the corrosion of the first is the atmospheric temperature and humidity, followed by the composition of impurities in the air and its content, the chemical composition of the pressure vessel wall materials, container wall surface roughness and staining. On the other hand, if the medium inside the pressure vessel (such as synthesis system of nitrogen and hydrogen mixture, urea system of urea melt, etc.) on the vessel wall materials with corrosive, out of service is not cleaned up and residual in the pressure vessel in some corners, connecting parts or take over and other interstices, will also be dissolved in the water film to continue to corrode the wall of the pressure vessel.

For out-of-service pressure vessel maintenance measures are as follows:

① The internal medium must be excluded clean. Especially corrosive media, to be discharged, replacement, cleaning and blow-drying and other technical processing. Attention should be paid to prevent the dead space inside the container to prevent the accumulation of corrosive media.

② often keep the pressure vessel dry and clean to prevent atmospheric corrosion. Scientific practice has proved that dry air, carbon steel and other ferrous alloys generally do not produce corrosion, only in the case of humidity (relative humidity of more than 60%), and the metal surface with dust, dirt or old corrosion products exist, corrosion will begin to take place. Therefore, in order to reduce the corrosion of the atmosphere on the outer surface of the pressure vessel out of service, should keep the container surface clean, often scattered on the surface of the pressure vessel dust, ash and other dirt scrubbed clean, and keep the pressure vessel and the surrounding environment dry.

(iii) The outer wall of the pressure vessel should be painted to prevent atmospheric corrosion, and attention should also be paid to the anti-corrosion under the thermal insulation layer and at the support of the pressure vessel.