Distillation Columns Internal Inspection


1. Safety
Entry of a column can be made only after the approved, signed entry tags are in place on the column. Safety rules in effect at the vessel must be observed. Inspectors must not work alone in vessels but always in pairs to provide maximum safety. Do not attempt an internal inspection until the vessel has been adequately cleaned to permit a satisfactory inspection. If you cannot see it, you cannot inspect it.

2. Shell and Heads
Inspect the shell and heads for defects. Look particularly behind downcomer plates, on the shell opposite nozzles at points of impingement, under nozzles for rundown attack, at the liquid level on the trays, and at shell welds. Corrosion can appear in many forms, some of which are not readily apparent. A smooth general loss can look like the original surface. Observe for pitting in isolated areas. Measure the depth of the corrosion, if possible, with a depth micrometer and ultrasonically gage the remaining thickness of the shell or heads where corrosion is severe. Measure all fixed gage points using a depth micrometer, or ultrasonically gage at established locations.

3. Column Internals
Inspect all bubble cap trays for out-of-level, and for leaks and holes that could affect the liquid seal on the trays. Check the weirs at the edge of the trays to be sure they will maintain the proper level. Note the condition of the internal tray manways and the gasket surfaces on tray and cover.

Examine all tray support members for mechanical defects or corrosion. If corrosion is a problem, measurements should be obtained to establish corrosion rates.

Bubble caps, chimneys, bolts and holding members should be tight and in position. If they are loose enough to rattle, the tray will leak excessively. Note the condition of the caps. Check for corrosion.

Visually inspect and hammer-test all internal piping and spargers. Check spray holes in reflux headers to see that they are not plugged. Inspect the tray and shell carefully in the reflux area as corrosion is frequently severe in this area.

Inspect the reboiler baffle for leaks or holes, and ensure that the baffle manway cover fits tightly and is properly gasketed. Check that the vortex strainer is securely in place.

4. Fouling

a. Strip Lining
Special corrosion resistant linings may be installed in columns or vessels where corrosion rates are excessively high. These linings are usually of light gage strips of alloy material welded to the shell. Where lining has been installed it should be carefully inspected for cracks or corrosion. Where lining failures have occurred, representative sections should be removed for gaging of the shell.

Carefully check the condition of all shell and head lining for full protection. If there is any evidence of leakage or corrosion deposits between the lining and vessel wall, request removal of a section of the lining. Recommend abrasive blasting the exposed vessel wall and inspect for corrosion. Record the extent of corrosion observed. The lining must be maintained in such condition as to prevent any circulation of stock between it and the
vessel wall.

Inspect lined nozzles for bulges, collapse of the liner seams, and for torn or cracked weld seams.

Whenever lining is removed, inspect the metal surface of the column, particularly at the welds, connections, riveted seams, and vacuum bracing for corrosion.

In case of new lining installation or lining repairs, abrasive blast and inspect all column welds and joints before lining, and any which are exposed during repair. Inspect all new installations of lining for workmanship and condition of welds before returning the column to service.

b. Cladding
Some columns may be protected from severe corrosion by the use of alloyclad materials. Cladding is a thin alloy sheet, factory bonded integrally to the carbon steel plate.

In those corrosive services where cladding is used, the cladding must be carefully inspected, since penetration to the carbon steel usually results in vessel failure in a relatively short time. Such penetration is sometimes evidenced by rust stains on the cladding.

The remaining thickness of nonmagnetic claddings can be measured nondestructively by the use of the coating gage or the cladding gage. Cladding thickness can also be measured with a depth micrometer by grinding through the carbon steel. The interface between the carbon steel and the cladding material is found by using copper sulfate to plate copper on the steel. This requires welding up the test area, and is not recommended if measurements can be obtained with the coating gage.

c. Weld Overlay
Check weld overlay areas for signs of corrosion, cracking or leaks. Use copper sulfate solution to check the integrity of the alloy overlay if corrosion/erosion is indicated.

6. Hydrostatic Testing
After repairs have been made to the shell or heads of a column, it may be decided to hydrostatically test the vessel for strength or tightness. Engineering instructions define the procedure for such tests to provide safety and to prevent damage to the equipment.

Be sure that the test pump is equipped with a suitable pressure gage and a safety valve that is properly set and tagged. All air must be bled from the column and the pressure raised at the pump or the column top to the pressure stipulated by the engineer. All pressure-containing parts, particularly at welds, nozzles, reinforcing pad weep holes, as well as at the repaired area are then carefully inspected for drips or leaks or visible signs of weakness. At the conclusion of the test, the inspector should check to see that the vessel is properly vented to prevent a vacuum and the collapse of the vessel as the water is emptied.

A field hydrostatic test is seldom applied to a whole column due to the great weight of the water, which the foundation might not be designed to withstand. For nozzle replacement or repairs it is sometimes advisable to test the area affected only. This is usually done by applying a welding pipe cap over the inside opening of the nozzle, then pressuring up the nozzle-to-shell weld. Make sure that the pipe cap is removed after the test is completed.

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