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Baker Hughes
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XV. Trouble Shooting
Certain troubles may develop through use or damage to
working parts. The most common are leakage, chattering,
hang-up, and set pressure variation.
Leakage is the constant escape of liquid at normal
operating pressure below the leak tightness pressure
of the valve. It is caused by either damage to the
seating surfaces or to foreign matter being trapped.
If during pressure testing or manual lifting, wide open
does not give relief, the valve should be repaired at
the first opportunity to prevent further damage. Another
cause of leakage is a bent spindle that causes hang-up
and improper installation of discharge piping so as to
introduce undue strain upon the valve.
Chattering is a hammering action of a vibratory nature
of the disc on the nozzle seat and must be stopped
immediately to protect the seating surface. Chattering
is caused by excessive back pressure from undersized
discharge piping, insufficient flow compared to the rated
flow and an undersized or closed vent hole in the bonnet
outlet.
Hang-up occurs upon closing and is defined as leakage
from the valve failing to shut-off tightly. Mechanical
interference is the primary cause. Disc and disc guiding
area in the bonnet should be examined.
Set pressure variation may be caused by spring
relaxation, damage to the seats from foreign matter,
from chatter or difference in thermal conditions between
various test runs.
The drop off in opening pressure due to spring relaxation
may be caused by excessive valve lift in the event of
excessive discharge beyond rated flow. The compression
screw can be adjusted to provide desire set pressure.
However, this should be done for temporary relief. The
relaxed spring should be replaced with a new one.
Between initial and subsequent test runs, the soft seat
settles a little bit providing a drop off in opening pressure.
However, three consecutive tests runs within opening
pressure tolerance should indicate settling of the soft
seat. Any drop in opening pressure can be corrected by
turning compression screw clockwise and by verifying the
opening pressure by test.
XIV. Setting and Testing
To change opening pressure of the valve, loosen the
locknut. Turn the compression screw clockwise to
increase pressure or counter-clockwise to decrease
pressure. The opening pressure of the valve is indicated
by the first continuous discharge of the liquid through
outlet. Before opening, in a number of cases, liquid will
discharge one drop at a time. This is not to be confused
with valve opening pressure. This should happen beyond
the leak tightness pressure of the valve and before its
opening pressure.
