Paper No. 59
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.