Tube support methods vary in
steam/methane-reforming heaters. Some designs require full support from the
top. In these designs the pigtail may be below the tube and unable to take any
load from the catalyst-filled tube. Counterweights are often used and may
support two or more tubes. The lever or pulley system must work as designed.
Interference from tube flange bolts, slipping of supports off tube flanges, and
other similar problems have led to pigtail failures.
Inadequate support also allows tube bending,
which puts a bending moment on a pigtail that exits the tube from the side,
thus causing localized high stress at the fitting on the tube or the outlet
headers.
Outlet headers grow, usually from a center anchor
point. Bottom tube supports on short pigtailed tubes must allow movement of the
tube bottom to minimize stress on the pigtail. If the tube is designed for
bottom movement, the upper tube supports must allow the tube to move at the
bottom end. To prevent a pigtail bending moment, the heater lining must not
press on the tube. Loose bricks are often used to help close openings. The
bricks must move freely if the tube presses on them.
If support springs are used, those that have
been stretched should be replaced. A stretched spring cannot support a tube.
When the tube is heated up after shutdown, the spring will no longer support it
as designed.
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