Tubular and Hollow Items

duncan galvanizing,corrosion protection,hot dip galvanizing,galvanized steel

Tubular assemblies such as handrail, pipe columns, pipe girders, street light poles, transmission poles, pipe trusses, and sign bridges are commonly galvanized.

Cleaning

As with all steel to be galvanized, pipe and other hollow materials must be thoroughly cleaned before the molten zinc will alloy with the steel to produce the galvanized coating. Pipe commonly presents two special cleaning problems:

  1. The “mill coating” (varnish, lacquer, and similar materials) applied by the manufacturer costs extra to remove at the galvanizing plant. Further, some formulations, both foreign and domestic, are extremely difficult to remove with common cleaning solutions; blasting may be required. Removing this mill coat at the galvanizing plant can be avoided by ordering uncoated pipe from your supplier, for which there is usually no extra charge.
  2. Welding of mill-coated pipe burns and carbonizes the varnish in the surrounding heated areas. This “soot” must be removed by blasting or other mechanical means. The burned coating could be removed when blasting to remove weld flux, but if welding has been done with an uncoated rod, any blasting or other hand cleaning is expensive and highly impractical.

Venting

It is mandatory that tubular fabrications and hollow structures be properly vented.

Any pickling acid or rinse waters that might be trapped in a blind or closed joint connection will be converted to superheated steam and can develop a pressure of up to 3800 psi when immersed in molten zinc at 850~F (455~C). This is a serious potential hazard to galvanizing equipment and to personnel.

Since proper galvanizing demands that the inside, as well as the outside, be completely cleaned and coated with zinc, air and frothy fluxes must be allowed to flow upward and completely out; cleaning solutions and molten zinc must be allowed to flow in and completely wet the surfaces.

Simply stated, the structure must be lowered into the solution without trapping any air. It must be raised from the solution without trapping any solution. Consequently, ample passageways which allow flow in and out must be designed into the assemblies.

Since items, to be galvanized are immersed and withdrawn at an angle, the vent holes should be located at the highest point and drainage holes at the lowest point each member.

All sections of fabricated pipework should be interconnected with full open tee or with miter joints. Each enclosed section must be provided with a vent hole at each end.

Most galvanizers prefer to visually identify the venting from outside when the assembly is received. This is necessary to check the adequacy of the venting as well as to determine that it has not been omitted by mistake. Some galvanizers may hesitate to process complicated pipe assemblies (such as hand railing) unless all venting is visible on the outside and readily accessible for inspection.

Base plates and end plates must be designed to facilitate venting and draining. Fully cutting the plate provides minimum obstruction to a full, free flow into and out of the pipe. Since this is not always possible, the use of vent holes in the plate often provides a solution.

Vent holes can be closed with drive caps or plugs installed after galvanizing. To do this, pear shaped-shaped lead weights are often used. These can easily be hammered in and filed off flush with surrounding surfaces.

It is recommended that tubular structures be completely submerged in one dip in the galvanizing kettle. This may be difficult to discover during inspection because of the size and shape of the item.

Various methods of providing vent holes are acceptable but the subsequent plugging of these holes should be kept in mind when necessary.

Internal gusset plates and end flanges should also be provided with vent and drainage holes. In circular hollow shapes these should be located diametrically opposite to each other at opposite ends of the member.

In rectangular hollow shapes, the four corners of the internal gusset plates should be cropped. Internal gusset plates in all large hollow sections should be provided with an additional opening at the center. Where there are flanges or end plates, it is more economical to locate holes in the flanges or plates rather than in the section.

Handrail.

  1. Vent holes must be as close to the weld as possible and not less than 3/8” in diameter.
  2. Internal holes should be the full l.D. of the pipe for best galvanizing quality and lowest galvanizing cost.
  3. Vent holes in end sections on similar sections must be 1/2” diameter.
  4. Any device used for erection in the field that prevents full openings on ends of horizontal rails and vertical legs should be galvanized separately and attached after galvanizing.

Vent holes should be visible on the outside of any pipe assembly.

If full internal holes (the full I.D. of the pipe) are not incorporated in the design of the handrail, the following should occur:

  1. Each vent hole must be as close to the welds as possible and must be 25% of the l.D. of the pipe, but not less than 3/8” diameter.
  2. Vent holes in end sections or in similar sections must be 1/2” diameter.
  3. Any device used for erection in the field that prevents full openings on ends of horizontal rails and vertical legs should be galvanized separately and attached after galvanizing.

Vent holes should be visible on the outside of any pipe assembly.

Rectangular Tube Truss

Vertical Sections

Each vertical member should have two (2) holes at the top and bottom, l8Oj apart in line with the horizontal members. The size of the holes preferably should be equal and the combined area of the 2 holes at either end of the verticals should be at least 30% of the cross sectional area.

End PLates – Horizontal

  1. Most desirable - completely open.
  2. If H+W=24” or larger, area of holes. Clips should equal 25% of the area of the tube (H+W).
    If H+W less than 24” to and including 16” - 30%.
    If H+W less than 16” to and including 8” - use 40%.
    If H+W under 8” - leave open.
    Pipe Truss 3” & Larger

Vertical Sections

Each vertical member should have two (2) holes at the top and bottom, 180° apart in line with the horizontal members. The size of the holes preferably should be equal and the combined area of the 2 holes at either end of the verticals should be at least 30% of the cross sectional area.

End PLates – Horizontal

  1. Most desirable - completely open ‘same diameter.’
  2. Equal substitutes - openings should be at least 30% of the area of the inside diameter.

Box Sections

INTERNAL GUSSETS should be spaced a minimum of 36”.

Box Sections - H+W-24” or larger - the area of hole plus clips should equal 25% of the cross sectional area of the box (H+W).

Box Sections - H+W less than 24” but greater than or equal to 16” - use 30%.

Box Sections - H+W less than 16” but greater than or equal to 8” - use 40%.

Box Sections - H+W under 8” leave completely open; no end plate or internal gussets.

The following table is for square box sections only. For rectangular sections, calculate required area and check with your galvanizer for positioning of openings.

Box Size
H + W
Holes
A-DIM
Clipped Corneres
B-DIM
48” 8” 6”
36” 6” 5”
32” 6” 4”
28” 6” 3”
24” 5” 3”
20” 4” 3”
16” 4” 2”
12” 3” 2”


Tapered - Signal Arm

A. Small end open completely.

Pole Plate End

  1. Most desirable - completely open.
  2. Acceptable alternates - half circles or slot and round hole must equal 30% of the area of the I.D. of the pole end of the tapered arm for 3” and larger I.D. The opening must equal 45% of the area of the pole end of the tapered arm if the I.D. is under 3”.

Pipe Columns, Pipe Girders, Street Light Poles and Transmission Poles With Base Plates and With or Without Cap Plates.

Location of Openings

  1. Most desirable - completely open “same diameter” as section top and bottom.

Dimensions

Openings at each end must be at least 30% of I.D. area of pipe for pipe 3” and over and 45% of the I.D. area for 3” pipe or smaller.

The following is an example of sizes for 6” diameter section. Allow 30% of the area of the l.D. for hole sizes at each end.

#2 Half Circle A - 1-3/4” R.
#3 Slot B = 3/4” - Center Hole C = 3” Diameter.
#4 Half Circle D = 1-5/8” R.

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