2.5 Trap Failures. B y failing closed, there is a backup of air and
condensate that floods the equipment. Flooding prevents the equipment from
p e r f o r m i n g its heat transfer function. By failing open, air, condensate, and
steam continue through the trap into the condensate system. This pass-through
wastes steam and affects heat transfer equipment by excessive pressures and
t e m p e r a t u r e s in the return lines. Major causes of trap failure are residue
buildup and wear. Industry has found the combination ball-float and
thermostatic steam traps to be the most reliable and cost effective. The
t h e r m o s t a t i c part of the trap is to pass condensate should the ball-float fail
c l o s e d , and to release air on initial start-up.
( a ) Residue. D i r t , rust, and foreign particles can build up in steam
traps, because when the valve is closed the trap body forms a natural pocket.
Dirt pockets should be installed on all steam header drip legs and the
strainers should be opened periodically for blow-down. S t r a i n e r s , whether
installed before the trap or included in the body of the trap, should have
blowdown cocks installed. I n s t a l l e d , operating strainers are one of the most
i m p o r t a n t protections for traps, but are only as good as their care. Figure
3 - 9 shows typical trap installations with strainers and blowdown valves. The
following conditions should be checked.
All strainers should be blown down every inspection.
Residue between the seat and disk may cause a trap to fail
open; residue buildup in the trap body may cause a trap to fail
P i s t o n impulse traps, disk traps, and orifice traps have small
holes and should have a fine mesh strainer.
(b) Wear. W e a r of internal parts, l i n k a g e s , and seals will cause trap
F a i l u r e can be in either the open or closed position. The
following are some typical wear conditions.
(1) When the mating surfaces of valves and valve seats wear out,
t h e r e is a tendency for an initial leak to enlarge by a process called
wire-drawing, which shows up as a small gully worn across mating surfaces.
Valves that are partially open because of residue lodged between the valve and
s e a t can initiate wire-drawing, since high-speed steam will follow the
condensate and cut the mating surfaces.