Pressure Vessels Inspection
1. External Corrosion
Horizontal drums resting on concrete support saddles frequently corrode externally in the area of the saddle. Inspect the water seal at the edge of the concrete If evidence of corrosion exists, it might be advisable to lift the vessel off the support or cut away the concrete for more complete inspection. Look for corrosion on the external surface of vessels where salt water drips on them from overhead coolers or condensers.
Rock, pall rings, or Raschig rings, etc., used to provide large surface area and good mixing in some vessels should be inspected for excessive breakage and fouling. Fouling or plugging of the bed can cause undesirable channeling of the flow through the vessel. Corrosion is usually more severe in a vessel in the area of the packing.
3. Demister Pads
Demisters fabricated from wire mesh pads are installed in vessels to prevent a carryover of liquids. Inspect carefully to see that the supports, ties, and retainers are securely fastened and that they cannot be bypassed. Severe fouling or plugging of a demister pad can cause it to be blown out of position.
4. Impingement Plates
Impingement or wear plates in vessels should be inspected for corrosion or erosion, and the attachment welds or bolting checked to prevent loss of the plate. Inspect the shell of the vessel carefully for corrosion adjacent to the impingement plate to be sure that the plate is large enough to cover all of the affected areas.
5. Nonmetallic Lining Materials
Vessels in highly corrosive service may be internally protected by many materials. Among those commonly used are glass, various grades of fiberglass, rubber, plastic and numerous paint-like products.
Glass linings are highly effective to protect against unusually severe corrosion, but are subject to damage by impact or localized temperature changes. Inspection usually consists of a careful search for cracks or chipped areas. Cracks, pinholes, or gas pockets in the glass can result in a leak in the vessel.
Extreme care must be taken during the entry and inspection of glass lined vessels to protect the glass from impact damage. In addition, the vessel shell and nozzles must be protected from external blows or heating. No welding, flame cutting, or hammer-testing is permitted, and the vessel should be so stenciled.
Temperature limitations may be imposed for steaming, washing, and placing the vessel in service to avoid thermal expansion strains.
All other types of protective coatings should be carefully inspected to be sure that the bond to the shell is tight and for surface deterioration. Fiberglass, plastic, and rubber linings sometimes pull loose from the shell, permitting a corrosive attack on the vessel. Loose pieces of lining material may plug off the outlet of the vessel and cause a shutdown. It is advisable to take a few thickness readings on the coatings of all lined vessels while performing an internal inspection. Most of these coatings will deteriorate at elevated temperatures.
Categories: In-Service Inspection | Tags: Demister Pads, Raschig rings | Leave a comment