|
|
POINTS TO CONSIDER WHEN DESIGNING FABRICATIONS FOR HOT DIP GALVANIZING
-
More extensive guidance on design for hot dip galvanizing can be found
in BS EN ISO 14713 and other Galvanizers Association publications.
 
Extenal stiffeners, welded gussets and webs on columns and beams, and
gussets in channel sections should have their corners cropped.
(IMAGE)
Cropping the corners of these brackets will aid access and drainage of molten zinc and a cleaner coating will be obtained.
Angle bracings should, if possible, be stopped short of the main boom
flange. This will allow the free flow of molten zinc across the surface
of the flange, enhancing drainage from the structure. This will assist
the development of a smoother galvanized coating, reduce the potential
for retention of ash on the surface of the flange and help to avoid air
traps within the structure, which could lead to uncoated areas.
On structural hollow sections, provision must be made for venting and
draining. With vertical members, drilled holes or V notches should be
provided diagonally opposite each other at top and bottom.
Every sealed section of a fabrication must be vented for reasons
of safety and to allow access and drainage of molten zinc. Holes
diagonally opposite each other should be as close as possible to the
sealed end.
Overlapping or contacting surfaces are potentially dangerous as
pretreatment solutions trapped between surfaces is converted to
superheated steam in the galvanizing bath and can lead to an explosion.
If contacting surfaces can not be avoided, as with these channels, then
the edges of the contacting areas should be continuously welded. One
hole should be drilled through both members for each 100cm² of
overlap, the minimum hole diameter being 10 mm, or the thickness of the
section, which ever is greater in order to eliminate the danger of an
explosion in the galvanizing bath.
If this is not possible, then welding should be intermittent: in
service there may be some weeping of trapped pretreatment solutions
from between the plates leading to brown staining but this will not be
detrimental to the protection given by the coating. It will not
generally be necessary to make any provision if the enclosed area is
less than about 100cm² (e.g.10 cm x 10 cm).
Internal diaphragms in large box sections should have cropped corners
and a 'manhole'. Internal diaphragms on small box sections should have
cropped corners. Where any hollow section is vented internally it is
essential, for safety reasons, that the galvanizer is able to view such
venting.
Alternative designs for venting sections fixed to base plates.
External stiffeners for beams or channels should have the corners cropped.
Welded joints should be continuous if they are not enclosing an
otherwise unvented surface. Bolted joints are best made after
galvanizing.
Examples of how
weldments should
be designed in order
to avoid acid traps
(narrow crevices).
To minimise the risk of distortion, flat panels should be braced, e.g.
dished or ribbed. Openings should be provided in the corners.
Large open top tanks should be stayed to minimise distortion. Where
angles are used to rim the tank, appertures must be provided in the
corners. Angles or flats used as stays should be as close as possible
to the tank wall thickness.
Vents should be diametrically opposite and at least 50mm in diameter.
Internal baffles should be cropped top and bottom. Lifting lugs are
required as indicated. It should be possible to view the baffles
through either the vent holes or an inspection hole - the placement of
the inspection hole should be discussed with the galvanizer.
|
|
|
