Flowserve Edward Cast Steel Bolted Bonnet Valves - Manual (Page 5)

2. Leakage at the Gasket. The possibility of
a leak through the gasket itself, while
remote, should still be considered. This
may not be the result of external flaws
on the sealing surfaces of the gasket.
SEAT AND DISK JOINT LEAK
A leak existing between the seat and disk
of a closed valve might be indicated by
one of the following: a definite pressure
loss in the high-pressure side of the valve;
continued flow through an inspection drain
on the low-pressure side; or, in hot water
or steam lines, a downstream pipe that
remains hot beyond the usual length of
time and conductivity range.
Such a leak may be the result of a distort-
ed seat caused by uneven welding and
stress relieving temperatures that were pre-
sent in the body when mounting the valve
in the pipe line. It may also develop
because of the operator’s failure to close
the valve tightly. An increased velocity is
imparted to a flow forced through a very
small opening. This increased velocity sub-
sequently gives rise to the “cutting” of both
disk and seat, particularly by particles of
line scale or rust in suspension or normal
solids in solution; or, in spite of the fact
that the hard surfaced material on the seat
and disk is corrosion and erosion resistant,
grooves, pit marks, or other surface irregu-
larities may be formed on the seat and
disk joint surfaces when the disk is closed
against a foreign body on the seat. This
sometimes occurs during the initial start up
of a piping system.
Leakage of steam through a valve which is
badly steam cut has a whistling or
sonorous sound. If the valve is only slightly
steam cut, however, leakage is identified
by subdued gurgling or weakly popping
sounds. These sounds can be heard
through a stethoscope or by placing one
end of a stick against the valve body while
holding the other end between the teeth,
with hands over the ears. BODY WALL LEAK
This is a visual leak through the body wall,
welding end or end flanges and may be
the result of a shrink cavity or other void in
the casting. If small at first, such a leak
may go unnoticed for a time, particularly if
the valve is heavily insulated and the pipe
5
Flow Control Division
Edward Valves
Service Problems
(cont.)