Site Plumbness Tolerances for Erected Distillation Columns

This article is about Site Plumbness Tolerances for Erected Distillation Columns and Cold Boxes Containing Distillation Columns. This worldwide fabrication and erection technical specification defines the requirements for the vertical alignment of distillation columns and boxes containing distillation columns when erected on their foundations at site.

Site Plumbness Tolerances for Erected Distillation Columns

This technical specification applies to all distillation columns regardless of type (e.g., bubble cap, sieve tray, packed), diameter, height, process (e.g., air separation or HYCO), or material of construction (e.g., aluminum, stainless steel, or 9% Nickel).

TOLERANCE CLASSIFICATIONS

1. The tolerances are intended to achieve optimum operating efficiency and economy in erection.
2. Tolerances are classified according to the source of their necessity.
3. “M” tolerances are set by mechanical considerations only. Components not complying with these tolerances may only be used on a variation note basis, subject to the Vessel Engineer’s approval.
4. “MP” tolerances are set by mechanical considerations but have some impact on the process. If a “MP” tolerance is not met for a component part, then a “P” tolerance may be exceeded on final assembly. Components not complying with these tolerances may only be used on a variation note basis subject to the Vessel Engineer’s approval. The Vessel Engineer’s approval shall only be given when compliance with the “P” tolerance can be assured.
5. “P” tolerances are set solely by process criteria. Components not complying with such tolerances shall be subject to review and approval by a Process Manager.

SITE-ERECTED COLUMNS

1. Columns erected onto foundations at site (i.e., site-built cold box) will be fitted with two pairs of “bulls eyes” alignment decals. One pair of decals will be attached near the top of the column, while the other pair will be attached near the bottom of the column. The two bulls eyes forming a pair will be orientated at 90 degrees apart circumferentially to each other.
2. The column vertical alignment shall be checked using a theodolite to view each pair in turn (see Figure 1).
4. The maximum vertical misalignment of the bulls eyes shall not exceed 7 mm. This is classified as an “MP” tolerance.

COLD BOXES CONTAINING COLUMNS

1. Cold boxes containing columns erected onto their foundations at site will be fitted with one or two pairs of plumb line lugs. Before erecting the box vertically, piano wire shall be stretched between the lugs.
2. The column vertical alignment shall be checked using a theodolite to view the plumb line or plumb lines as shown (see Figure 2).
   • Note: For cold boxes with two plumb lines, the direction of sighting each line is important; they are only set plumb in one plane.
3. The maximum vertical misalignment of the plumb lines shall not exceed 7 mm. This is classified as an “MP” tolerance.
4. Cold boxes may be supplied with “bulls eyes” fitted in place of plumb line lugs. In this case, for alignment purposes, the cold box shall be treated as a site-erected column (see Section 5).

COLUMNS FITTED WITH LEVELLING BOSSES

Some columns may be fitted with top head levelling bosses. These are rarely fitted to new columns, because they are not considered an accurate method of setting the column plumb. However, they occasionally are fitted on “exact” repeat columns. If fitted, they shall not be used for setting the initial alignment.

INSPECTION

The column or cold box alignment shall be checked by either the Air Products representative (if experienced in measuring vertical alignment) or by an independent surveyor.

RECORD OF MEASUREMENTS

All measurements made in connection with Sections 5, 6, or 7 shall be recorded by the Air Products representative and forwarded to the Vessel Engineer for review. The records will be retained by Air Products.

Figure-1 Vertical Alignment of a Column (before the cold box is built) 

Note: Dimensions are in mm.

Figure-2 Vertical Alignment of a Cold Box Containing a Column

Note: Dimensions are in mm.

Appendix A Bosses on Existing Columns

A.1 While bosses are not suitable for aligning new columns, bosses on existing columns do, however, provide a useful means of checking changes in column plumbness, provided the original readings are known for comparison.

A.2 When bosses were used for setting columns plumb, the maximum out of level between any two of the bosses was limited to the value of “Z.” When “Z” was determined as shown in Figure A1.1, “L” was the distance between bosses in meters and “H” was the column tan to tan height in meters. This was classified as an “MP” tolerance.

Figure A1.1 Vertical Alignment Using Top Head Levelling Bosses (NON-PREFERRED METHOD)

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