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Tubular Design

Tubular assemblies (handrails, pipe columns, pipe girders, street light poles, transmission poles, pipe trusses, sign bridges) are commonly galvanized to protect both the interior and exterior of the product. To provide an optimal galvanized coating, hollow products require proper cleaning and draining.

Cleaning

cleaning1.jpgAs with all steel, pipe and other hollow materials must be thoroughly cleaned before the molten zinc will bond with the steel. Pipe can present two special cleaning challenges:

  • First, the mill coating (varnish, lacquer and similar materials) applied by pipe manufacturers requires extra time and effort to remove at the galvanizing point. Some galvanizers cannot remove this coating. Some organic mill coating formulations are extremely difficult to remove with common cleaning solutions, so blasting may be required. Ordering uncoated pipe avoids costly attempts to remove these mill coatings. In some cases, it may be more cost effective to substitute tube for pipe.
  • Second, welding around mill coatings burns and carbonizes the varnish in the surrounding areas and cannot be removed by the normal cleaning process at a galvanizer. This soot must be removed by blasting or other mechanical cleaning methods prior to delivering steel to the galvanizing facility.

Venting

The primary reason for vent and drain holes is to allow air to be evacuated, permitting the object to be completely immersed into cleaning solutions and molten zinc. Proper sizing and location make it safer to galvanize and provide the optimal finish.

The secondary reason is to prevent damage to the parts. Any pickling solutions or rinse waters that might be trapped in a blind or closed joining connection when immersed in molten zinc will be converted to superheated steam and can develop a pressure of up to 3600 psi (1100 MPa). That endangers not only the fabrication being galvanized, but the equipment and any nearby people.

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. Proper galvanizing results when the inside and outside of a product are completely cleaned and zinc-coated.

The structure must be lowered into the solution without trapping any air, then raised from the bath without trapping any solution. Therefore, ample passageways allowing unimpeded flow into and out of the part must be designed into assemblies.

Because items to be galvanized are immersed and withdrawn at an angle, the vent holes should be located at the highest point and drain holes at the lowest.

venting10.gifAll sections of fabricated pipe work should be interconnected with full open tee or miter joints. Each enclosed section must have a vent hole at each end.

Most galvanizers prefer to visually identify the venting from the outside. This is necessary to verify the adequacy of the venting as well as determine that venting has not been mistakenly omitted. Some galvanizers may hesitate to process complicated pipe assemblies unless all venting is visible on the outside and readily accessible for inspection. (see Figure 10)

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. Because this is not always possible, using vent holes in the plate often provides the solution.

Vent holes are frequently left open but can be closed with drive caps or plugs after galvanizing.

venthole11.gifVarious methods of venting are acceptable (see Figure 11), but the subsequent plugging of these holes should be kept in mind, where necessary or desired.

We recommend that tubular structures be completely submerged in one dip into the galvanizing kettle. This minimizes potential internal coating problem that, because of the size and shape of the item, may be difficult to discover during inspection.

Handrail:
Figure 12 illustrates the most desirable design for fabrications of handrail for galvanizing. It shows internal venting as well as the minimum amount of external vent holes.

1.handrail12.gif External vent holes must be as close to the weld as possible and not less than 3/8-inch (9.5 mm) in diameter.
2. Internal holes should be full I.D. of the pipe for the best galvanizing quality and lowest galvanizing cost.
3. Vent holes in end sections or in similar sections must be ½-inch (13 mm) in diameter.
4. and 5. Ends should be left completely open. 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.

Handrail:
Figure 13 illustrates an acceptable alternative if full internal holes (the full I.D. of the pipe) are not incorporated into the design of the handrail.

handrail13.gif1. Each internal vent hole must be as close to the welds as possible and must be 25 percent of the I.D. of the pipe, but not less than 3/8-inch (10 mm) in diameter. The two holes at each end and at each intersection must be 180 degrees apart and in the proper location as shown.
2. Vent holes in end sections or in similar sections must be ½-inch (13 mm) in diameter.
3. and 4. Ends should be left completely open. 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.

Pipe Columns, Pipe Girders, Street Light, & Transmission Poles:
pipes16.gif(With base plates and with or without cap plates, Figure 16)

Location of Openings
1. The most desirable fabrication is to have the end completely open, with the same diameter as the section top and bottom.
2. and 3. and 4. This is an equal substitute if the full opening is not allowed.
5. This must be used when no holes are allowed in the cap or base plate: two half circles 180 degrees apart and at opposite ends of the pole.

Dimensions
Opening at each end must be at least 30 percent of the I.D. area of the pipe for pipe three inches and greater and 45 percent of the I.D. area for pipe smaller than 3 inches (7.6 cm).

Allow 30 percent of the area of the I.D. for hole sizes at each end.
Illustration 1, end completely open.
Illustration 2, Slot A= ¾-inch (19 mm), Center hole B = 3 inches (7.6 cm) in diameter.

The following is an example of sizes for a 6-inch (15 cm) diameter section. Illustration 3, half circle C = 1¾-inch (4.5 cm) radius
Illustration 4, oval opening = 1¾-inch (4.5 cm) radius
boxsections17.gifIllustration 5, half circle D = 1 5/8-inch (1.9 cm) radius

Box Sections:
Figure 17 shows the location of holes and clipped corners, which must be flush. Using the following formulas, Table 1 shows typical sizes of holes.

Internal Gussets - space at a minimum of 36 inches (91 cm).
Box Sections
- H + W = 24 inches (61 cm) or larger, the area of the hole, plus clips, should equal 25% of the cross-sectional area of the box (H x W).
Box Sections
- H + W = less than 24 inches (61 cm) but greater than 16 inches (40.6 cm), the area of the hole, plus clips, should equal 30 percent of the cross sectional area of the box.
boxsecttable1.gifBox Sections - H + W = less than 16 inches (40.6 cm) but greater than or equal to 8 inches (20 cm), the area of the hole, plus clips, should equal 40 percent of the cross sectional area of the box.
 Box Sections - H + W = less than 8 inches (20 cm), leave completely open, no end plate or internal gusset.
Table 1
is for square box sections only. For rectangular sections, calculate the required area and check with your galvanizer for positioning of openings.

Tapered Signal Arm:
(Figure 18)
A. The small end should be completely open.

Pole Plate End:
1. The most desirable fabrication is to have the end completely open.
2. and 3. and 4. For acceptable alternatives, the half circles, slots, and round holes must equal 30 percent of the area of the I.D. of the pole end of the tapered arm for 3 inches (7.6 cm) and larger I.D.s.

taperedsignalarm18.gif

The opening must equal 45 percent of the area of the pole end of the tapered arm if the I.D. is less than 3 inches (7.6 cm).

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

In rectangle 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 a section.

Designing for Proper Venting & Draining of Enclosed & Semi-Enclosed Products

Tventing19.gifanks and enclosed vessels should be designed to allow cleaning solutions, fluxes, and molten zinc to enter at the bottom and air to flow upward through the enclosed space and out through an opening at the highest point. This prevents air from being trapped as the article is immersed (see Figure 19).

The design must also provide for complete drainage of both interior and exterior details during withdrawal. The location and size of fill and drain holes are important. As a general rule, the bigger the hole the better the air and zinc flow.

When both internal and external surfaces are to be galvanized, at least one fill/drain hole and one vent hole must be provided. The fill/drain hole should be as large as venting20.gifthe design will allow, but at least 3 inches in diameter for each cubic yard (10 cm in diameter for each cubic meter) of column. The minimum diameter is 2 inches (5 cm). Provide vent holes of the same size diagonally opposite the fill/drain hole. This allows the air to escape.

In tanks, internal baffles should be cropped on the top and bottoms or provided with suitable drainage holes to permit the free flow of molten zinc. Manholes, hand holes and openings should be finished flush inside to prevent trapping excess zinc (see Figures 20-22).

venting22.gifOpenings must be placed so that the flux on the vessel can float to the surface of the bath. These openings also prevent air pocket formations that may keep solutions from completely cleaning the inside of a vessel.

Items such as vessels or heat exchangers that are galvanized on the outside only must have snorkel tubes or extended vent pipes. These openings provide an air exit from the vessel above the level of molten zinc in the galvanizing kettle (see Figure 23). Consult your galvanizer before using these temporary fittings, because special equipment is needed.

venting23.gifYour galvanizer should always review the drawings of enclosed or partially enclosed vessels before fabrication. Galvanizers may recommend changes that would provide a better galvanized product. If a change is needed to facilitate galvanizing, the least expensive time to make the change is before fabrication.

 

--Courtesy of the AGA