Most hot bends are made on a bending slab (as
Post Pipe Bending Inspection
shown in fig. 16-15). To make the bend, exert the pull
in a direction parallel to the surface of the bending slab.
All piping, regardless of the method used to bend
The necessary leverage for forming the bend is obtained
the pipe, should be inspected after bending for defects
by using chain-falls or block and tackle. Or you can use
and to meet the requirements of MIL-STD-1627. The
surface should be free of pits, gouges, scratches, or tool
a length of pipe that has a large enough diameter to slip
marks. Defects are acceptable if they are less than 0.010
over the end of the packed pipe. Bending pins and
inch or 5 percent to the nominal thickness of the pipe,
hold-down clamps (dogs) are used to position the bend
whichever is greater. The depth of the defect should not
at the desired location.
reduce the pipe wall thickness below its minimum
Be sure to wear gloves when you are working on
requirement. You may remove defects exceeding the
hot bending jobs. You will occasionally need to move
depth limits by faring in, grinding, or buffing to a radius
the pins, clamps, and baffles during the bending
3 times the depth. The final pipe wall thickness after
defect removal must meet minimum thickness
operation. These items absorb heat radiated from the
requirements.
pipe as well as from the torch flame itself. You cannot
safely handle these bending accessories without gloves.
Wrinkles in the pipe wall are unacceptable.
However, buckles, bulges, and dents may be acceptable
Each material has its peculiar traits. You will need
if they meet the following criteria:
to know about these traits to get satisfactory results. The
following hints for bending different materials should
Buckles, bulges, and dents must blend in
prove helpful :
smoothly.
WROUGHT IRON—This material becomes brittle
The maximum vertical height of any buckle,
when hot, so always use a large bend radius. Apply the
bulge, or dent must not exceed 3 percent of the
torch to the throat of the bend instead of to the heel.
nominal pipe outside diameter.
BRASS—Do not overbend. Brass is likely to crack
The diameter-to-height ratio must equal or
or break when the bend direction is reversed.
exceed 12 to 1.
COPPER—Hot bends may be made in copper,
Out-of-roundness is another common defect found
although the copper alloys are more adaptable to cold
in pipe bends. To find out-of-roundness, use the
following formula:
bending. This material is not likely to give you any
trouble.
NOTE: All measurements should be taken with
either vernier or dial calipers.
ALUMINUM—Overbending and reverse bending
do not harm aluminum. However, there is only a small
OD max - OD min × 100 divided by OD nom.
range between the bending and melting temperatures. where:
Therefore, you will have to work with care. Keep the
heat in the throat at all times. You will not be able to see
OD max = Maximum measured OD at the bend
any heat color. You will have to depend on “feel” to tell
you when the heat is right for bending. You can do this
OD min = Mimimum measured OD at the bend
by keeping a strain on the pipe while the bend area is
OD nom = (OD max + OD min) divided by 2.
being heated. As soon as the bend starts, flick the flame
away from the area. Play it back and forth to maintain
The maximum allowable out-of-roundness for pipe
the ending temperature and to avoid overheating.
with a working pressure of 600 lb/in
2
and greater is 5
percent. For a pipe with a working pressure of less than
CARBON-MOLYBDENUM and CHROMIUM- 600 lb/in
2
, the maximum allowable out-of-roundness is
MOLYBDENUM—These may be heated for bending, 8 percent.
if necessary. However, use caution so you do not
Flat spots are the last defect you want to inspect the
overheat the bend area. These metals are easily
pipe for. Flatness is acceptable if it falls within the limits
crystalized when extreme heat is applied. Pipes made
established by a flatness limits graph (see fig. 16-18).
from these materials should be bent cold in either
By measuring the width of the flat and knowing the pipe
manual or power bending machines. size, you can determine if the flat spot is acceptable or
16-16
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