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Basically they are to provide maximum corrosion protection, and to allow the flow of molten zinc into and out of all parts of a fabrication.
This later provision is a necessary condition for the safety of personnel within the galvanising plant, to ensure a free flow of molten zinc within the fabrication, and thus a better quality galvanising finish.
It should be noted that any air or water trapped within a fabrication will be converted into superheated steam and can develop a pressure of up to 26.2mPa (3800 p.s.i.) when immersed in molten zinc at 455oC.
On successive pages we have set out fundamental design rules that will ensure the highest quality galvanising, at the lowest price, with the maximum safety.
Molten zinc must be allowed to free flow without impediment. Hence openings must be large enough – without impairment of the structural strength – for zinc to flow into the fabrication and out of it.
Strengthening gussets in the webs of columns and beams should have the internal corners cropped or holed to ensure that no zinc build up occurs in angles or corners, and that air locks and ash entrapment is eliminated.
Fabricators should first check with your local Galvanising Association of New Zealand (GANZ) member, before work goes to completion, to ensure that drains and vents are positioned correctly and of the appropriate size.
Your local GANZ member can galvanise most fabrications. If, however, a fabricator is uncertain of the most economical size or weight of steel that can be galvanised they should check with their local GANZ member.
Some long lengths of steel, not easily accommodated due to their length, may be double end dipped.
Alternatively a fabrication may be built in smaller modules that may be more easily accommodated.
In any event your local GANZ member has technical people who can advise on every stage of fabrication.
The following information on galvanising providing details on draining and venting, with appropriate drawings for enclosed fabrications such as tanks and rails. What follows is a synopsis of good galvanising practice and some basic information that may be helpful. In any event if further information is required contact your local GANZ member. Experienced advice is readily available to assist you.
Galvanising Hawkes Bay Limited is proud to be a GANZ member
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| DRAIN AND VENT HOLE SIZES (MIN.) FOR VARIOUS HOLLOW SECTIONS |
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1. The drain and vent hole size represent the minimum acceptable and it is preferred that the full tube cross section be provided.
2. The table is also applicable to hollow sections fabricated from channels and angles, etc. Use the table with the appropriate outside dimensions of the boxed sections.
3. For larger sections use the associated table for tanks.
4. Should the recommended hole size be unacceptable contact your local GANZ member for further advice.
5. Lifting eyes to be provided for the hanging of steel work (adjacent same side as vent hole0 or provide holes for lifting wires.
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| DRAINING AND VENTING |
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Tubular fabrications and hollow structures
Tubular assemblies such as handrail, pipe columns, pipe girders, steel light poles, transmission poles, pipe trusses, and sign bridges are commonly galvanized.
Cleaning
As with all steel to be galvanized, pipe, R.H.S. and other hollow materials must be thoroughly cleaned before the molten zinc will allow the steel to produce the galvanizing coating inside and out.
R.H.S. and pipe commonly presents two special cleaning problems
1. The "mill coating" (varnish, lacquer, 'black Japan' and similar materials) applied by the manufacturer costs extra to remove at the galvanizing plant.
2. Use only water soluble cutting fluids when drilling to avoid steel contamination that may incur extra cleaning costs.
Venting
IT IS MANDATORY that tubular fabrications and hollow structurals be properly vented. Refer to Fig. 2.
ANY PICKLING ACIDS OR RINSE WATERS THAT MIGHT BE TRAPPED IN A BLIND OR CLOSED JOINT CONNECTION WILL BE CONVERTED TO SUPER-HEATED STEAM AND CAN DEVELOP PRESSURE OF UP TO 26.2Mpa (3800 psi) WHEN IMMERSED IN MOLTEN ZINC AT 455 DEGREES CELCIUS.
THIS IS A SERIOUS POTENTIAL HAZARD TO GALVANIZING EQUIPMENT AND TO PERSONNEL.
Since proper galvanising demands that inside, as well as outside, be completely cleaned and copated with zinc, air and ash must be allowed to flow UPWARDAND COMPLETELY OUT, CLEANING SOLUTIONS AND MOLTEN ZINC MUST BE ALLOWED TO FLOW IN AND COMPLETELY WET SURFACES.
In all tables of draining, venting and gusset bevel calculations, allowance has been made for the speedy and total expulsion of entrapped air and ash produced during the galvanizing process.
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 that 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 IN EACH HOLLOW MEMBER. Refer Fig. 1.
All components of fabricated hollow sections should be inter-connected with full open tee or with mitred joints.
Each closed section must be provided with a vent hole and a drain hole.
Most galvanizers prefer to visually identify the venting from the outside when assembly is received. This is necessary to check the adequacy of venting as well as to determine that it has not been omitted by mistake.
YOUR GALVANIZER MUST BE NOTIFIED IN WRITING IF FABRICATIONS ARE INTERNALLY VENTED.
Base plates and end plate must be designed to facilitate venting and draining. Fully cutting the plate provides minimum obstruction to a full, free flow into and out of hollow sections. Since this is not always possible, the use of vent holes in the plate often provides a solution.
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| GUSSETS AND WEB |
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Welded gussets and webs on columns, beams, and channel sections should have corners cropped or holed,
1. To prevent the entrapment of air in pockets and corners,
2. To faclitate drainage during withdrawal from the galvanising bath.
WELDING
Use continuous welding all round to avoid moisture traps
DRILLING
Use only water soluble cutting fluids.
END PLATES
Provide holes in end plates for venting and draining or crop section webs
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| OVERLAPPING SURFACES |
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Avoid narrow gaps between plates, overlapping surfaces, and back-to-back angles and channels.
When small overlaps are unavoidable, seal edges by welding.
When left unsealed, small overlapping areas may trap pickle acid that can later escape to colour or damage the galvanised coating.
Larger overlapping surfaces
If contacting surfaces cannot be avoided, a hole 6mm in diameter for every 0.01m2 of overlap area should be placed in one of the members,
and the perimeter of the contacting area should be continuously welded.
The vent hole in one member will ensure the safety of the galvanizing personnel and prevent damage to the article.
If in doubt contact your local GANZ member.
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| GUSSET PLATE OR ABUTMENT MITRE SIZES |
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| BEVEL SIZES |
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Bevel cuts for angles and channels = 25% of flange width.
Bevel cuts for I Beams and columns = 25% of HALF the flange width.
Bevel cuts for RHS End plates = 25% of HALF the widest side.
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| EXAMPLE |
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1. All bevel sizes listed are 45 degress - e.g. 40 mm bevel = 40 x 40 x45 degress. (See Fig. 1).
2. Gusset plates or abutments in channels and beams have both ends bevelled.
3. If this is unacceptable contact should be made with your GANZ member.
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| DESIGN AND FABRICATION OF STEEL PLATE STRUCTURES FOR HOT DIP GALVANIZING |
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1. One hole or lifting lug required in plates where detailed in Fig. 1, 2 & 3.
2. Two holes or lifting lugs required in plates where detailed in Fig 4 (2 holes or lugs total / see below).
3. HOLE SIZES
A. Plates up to and including 10mm thick require 10mm dia. hole(s).
B. Plates over 10mm thick and up to and including 16mm thick require 16mm dia. hole(s).
C. Plates over 16mm thick require larger dia. holes or lifting lugs (refer to your local GANZ member).
4. Gauge or centers of holes/lifting lugs should be 1.5 diameter of holes required for lifting from edge of plate. (Fig 5).
5. LIFTING LUGS
A. Lifting lugs should be welded to plates in posistions detailed.
B. Lug thickness should equal thickness of plate being galvanized.
C. Lugs should have sufficient weld to support plate during the galvanizing process.
6. Stiffeners or attachments welded to plates at lifting points may be utilised as lifting lugs.
7. It is preferred that lugs be welded to the side of plates where their removal after galvanizing is NOT necessary. (e.g. floor plates).
8. CUTTING, SHAPING AND WELDING OF PLATES
If possible plates should be cut from one sheet to eliminate or minimise butt welds and stresses.
9. PLATFORM PLATE
Welds should be kept to a minimum when welding plates to heavy subframes to minimise weld stresses that cause distortion during galvanizing.
Bolting or screwing subframes is preferable.
It is preferable that fittings or attachment on plates be designed for bolted connection to plates to minimise weld stresses.
When cutting plates to shape it is preferred that all operations be as uniform as possible.
If plates exceed maximum sizes shown in Fig 4 refer to your local GANZ member for further guidance.
If large quantities of plates or flats of the same shape and size are to be galvanized,
refer to your local GANZ member and maybe holes can be eliminated through jigging techniques.
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| POSITIONS OF DRAIN AND VENT HOLES (MIN.) FOR HOLLOW VESSELS |
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When both internal and external surfaces are to be galvanized at least one filling and draining hole must be provided,
with a vent diagonally opposite to allow the exit of air during immersion.
Check over pages for minimum drainage and vent hole sizes for various tank capacities.
Internal baffles should be cropped as illustrated.
Manholes should finish flush inside to prevent trapping excess zinc.
1. Drain and vent holes are to be placed at diagonally opposiste locations typically as shown
in Fig 1, 2 & 3to facilitate complete venting and draining of the vessel.
Each mitred corner or scallop in internal baffle plates must be equal; in area to that of the drain
holes Fig 1, 2 & 3.
2. Provide suitable lifting lugs Fig 3.
3. Drain and vent holes must be flush internally and be located as close to corner welds as possible.
Fig 4. NB Figs. 5 & 6 unacceptable.
Should above drain and vent hole locations be difficult to include during fabrication contact your
local GANZ member for further advice.
When vessels and air receivers etc. are not to be galvanized inside, 'snorkel' tubes or extended vent pipes MAY BE FITTED
ONLY AFTER DISCUSSION WITH YOUR LOCAL GANZ MEMBER to allow air to exit above the level of molten zinc in the
galvanizing bath. Refer Fig 7. |
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| DRAIN & VENT HOLE SIZES (MIN.) FOR VARIOUS TANK CAPACITIES |
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1. Calculate cylindrical tank capacity as follows: -
Capacity (M cubed) = 0.785 x D squared x L (dia. & length in metres).
2. For tanks smaller than listed above the min. size for :-
drain hole = 50mm dia.
vent hole = 30mm dia.
3. refer Fig. 1 & 2 (Positions of drain and vent holes (min.) for hollow vessels) for drain and vent hole location.
4. Should above hole sizes be unacceptable contact your local GANZ member for further advice.
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| CLEARANCE FOR MOVING PARTS |
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Drop handles, hinges, shafts, and spindles require provision of minimum adial clearances as detailed in the table above, to allow for the thickness of the galvanizing coating.
Shaft or spindle size, Minimum hole clearance
Up to10 mm dia. , 1mm
10 to 30mm dia. , 2 mm
Over 30mm dia. , 2.0 to 2.5 mm
Australian standard 1214 specifies the following oversize tapping allowances:
Nominal internal thread diameters up to and including M24: 0.4 mm allowance.
Nominal internal thread diameters over M24 up to M36: 0.5 mm allowance.
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