Vibration
Troubleshooting of Existing Piping Systems
J. C. Wachel/D. R. Smith, July 1991.
When a vibration problem is reported, the vibration analyst/engineer
must determine whether or not the high vibrations represent a problem.
The evaluation of whether or not the high vibrations represent a
problem has to be based primarily on the vibratory stresses introduced
into the piping; however, the vibration levels can be high based
on the psychological effect that it has on the operators. In addition,
many times apparently high vibrations may not cause excessive stresses
in the piping, but could cause excessive stresses to piping systems
or appurtenances that are attached to the vibrating pipe.
The vibrations
are obviously too high if there has been a piping fatigue failure
or if the piping has extremely high vibrations. Since it requires
approximately ten million cycles of stresses in excess of the
endurance limit for a pipe to fail, it is desirable to correct
high vibrations before failures occur, if at all possible. As
a rule of thumb, excessive vibrations at 5 Hz required 20 days
to reach ten million cycles, therefore, for low frequency vibrations
failures may not occur for a month. If the vibrations are at 100
Hz, it would only require one day of operation at the excessive
levels. This means that extreme care should be exercised in evaluating
piping systems in low speed reciprocating machinery systems or
any system which has low frequency vibrations.
In addition,
vibrations are sensitive to engine speed and loading conditions,
therefore, the vibrations that are present during the initial
survey may not be the highest that will occur. The point of maximum
vibration is sometimes difficult to establish in an initial survey
and some safety factor is necessary to allow for these factors.
If the high
vibration levels have been occurring for many months without failures,
the vibrations may not be so severe that failure is imminent but
may need to be reduced to increase the margin of safety.
This paper
will discuss the types of problems that occur in typical piping
systems and will try to direct the analyst to determine the cause
of the problems and give input into possible solutions. A set
of diagnostic charts has been developed to aid in the assessment
of the severity of the vibrations and to help evaluate possible
solutions. The following steps will help the vibration analyst/engineer
diagnose and solve the problem.
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