pipe vibration testing and analysis
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DESCRIPTIONPipe Vibration Testing and Analysis
37PIPE VIBRATION TESTING AND ANALYSISDavid E. Olson37.1 PIPING VIBRATION CHARACTERISTICScauses are rapid pump starts and trips, and also the quick closing or opening of valves such as turbine-stop valves and various types of control valves. Dynamic transients also occur as a result of rapid safety/relief valve (SRV) opening or as a result of unexpected events, such as water accumulating at a low point in steam piping during a plant outage. When the steam is returned to the line, a slug of water will be pushed through the piping, resulting in large axial loads at each elbow. Effects of transient vibrations are usually obvious; large pipe deections usually occur that damage the support system and insulation as well as cause possible yielding of the piping. Of course, damage can also be sustained by the associated equipment, valve operators, drain lines, and so forth. An example illustrating the striking nature of dynamic transients occurred in a fossil fuel plant cold-reheat line. There, the low-point drains had not been properly maintained, and water accumulated in the line after a turbine trip. When the turbine-stop valves were opened, a water slug was forced through the piping, resulting in a transient so severe that the 80 ft., 18 in. diameter pipe riser was lifted over 11 ft. in the air. When the piping came down, most of the hangers 2 were broken, and the piping had large deformations.
For the purposes of piping design and monitoring, vibration is typically divided into two types: steady-state and dynamic transient vibrations. Each type has its own potential causes and effects that necessitate individualized treatment for prediction, analysis, control, and monitoring .
Piping steady-state vibration can be dened as a repetitive vibration that occurs for a relatively long time period. It is caused by a time-varying force acting on the piping. Such a force may be generated by rotating or reciprocating equipment by means of vibration of the equipment itself or as a result of uid pressure pulses. Vibrational forces may also result from cavitation or ashing that can occur at pressure reducing valves, control valves, and ash tanks. Flow-induced vibrations such as vortex shedding can cause steady-state vibrations in piping, and wind loadings can cause signicant vibrations for exposed piping similar to that typically found at outdoor boilers. Steady-state vibrations exist in a range from periodic to random. The primary effect of steady-state vibration is material fatigue from the large number of associated stress cycles. This failure may occur in the piping itself, most likely at areas with stress risers such as branch connections, elbows, threaded connections, or valves. However, this failure can also occur in various piping system components and supports. Fatigue damage to wall penetrations can occur because of vibration in the attached piping, snubbers, and supports; premature failures of machine bearings are another potential consequence.
VIBRATION EXPERIENCE WITH U.S. NUCLEAR POWER PLANTS
The dynamic transient is the second, perhaps more dramatic form of piping vibration, differing from the steady-state vibration in that it occurs for relatively short time periods and is usually generated by much larger forces. In piping, the primary cause of dynamic transients is a high- or low-pressure pulse traveling through the uid. Such a pulse can result in large forces acting in the axial direction of the piping, the magnitude of which is normally proportional to the length of pipe legthat is, the longer the pipe leg, the larger the dynamic transient force the piping will experience ( pipe leg is dened as the run of straight pipe between bends). A common transient is water- or steamhammer. The usual
Piping vibration problems have been well documented for nuclear power plants. Fossil fuel power plants experience many of the same problems, but documentation of their problems is sparse. Problems in nuclear power plants are documented by Licensee Event Reports (LERs). An LER is a generic term for a reportable occurrencean unscheduled incident or event that the U.S. Nuclear Regulatory Commission (USNRC) determines is signicant from the standpoint of public health or safety. Kustu and Scholl performed a survey to identify the causes and consequences of signicant problems experienced with lightwater reactor (LWR) piping systems . The authors ranked the need for pipe vibration research as highest priority. Pipe cracking was identied as the most frequently recurring problem, the most signicant cause of which was determined to be piping vibration. Mechanical vibration was the cause of 22.3% of all reportable occurrences involving pipes and ttings. Problems with pipe and pipe ttings were found to be responsible for approximately 10% of all safety-related events and 7% of all outage time at LWRs.
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A separate summary of LERs through Oct. 1979 documented 81 cracks in pipes less than 4 in. that were directly attributable to vibration . A more detailed review of the LERs found that cracks in tap lines (e.g., vents, drains, and pressure-tap connections) were a prevalent mode of pipe failure. The frequency of small tapline failures has also been veried by personnel familiar with start-up testing and operation of LWR plants. In addition, a Sept. 1983 Institute of Nuclear Power Plant Operations (INPO) Signicant Event Report (SER 64-83) noted that from April 1970 to Sept. 1983, 234 reported failures of small-diameter safetyrelated pipes have been caused by vibration-induced fatigue. The Operations and Maintenance (O&M) Reminder 424 (Small-Bore Piping Connection Failures, Jan. 7, 1998), another INPO report, stated that failures of small-bore piping connections continue to occur frequently and result in degraded plant systems and unit capability factor losses from unscheduled shutdowns. This INPO report also stated that of the 11 small-bore piping connection failures reported in 1997, 8 required plant shutdowns for repairs. Another study was completed by Bush to establish trends and predict failure mechanisms in piping . This study was primarily based on LERs and their precursors: Abnormal Occurrence Reports (AORs). Although this study dismissed failure in smaller pipe sizes as not having any major safety signicance, it did note that there was substantial failure data for small pipe sizes (diameter less than 4 in. and usually less than 2 in.). Such failures were attributed primarily to vibrational fatigue. Bushs study noted the large numbers of reported waterhammer and water-slugging events. Waterhammer is dened as a multicycle load induced by transient pressure pulsation in the uid, whereas water slugging is dened as a single load induced by accelerating a slug of water through the piping. Over 200 such events have been documented, ranging from the trivial to some that caused breakage of piping and signicant damage to the piping system. What can be concluded from this experience is that piping vibration has been a signicant source of problems in power plants. Not surprisingly, most pipe failures have been experienced in small piping; there is, after all, much more small-diameter piping than large-diameter piping in a power plant. In addition, small piping is often weaker than its support system; moreover, it is typically the weakest link that fails in the system. The structural vibrational modes of small-branch piping are often excited by the structural vibrations of the header piping. Frequently, pressure pulsations in the header piping or vortex shedding at the branch connection also excite acoustic resonances in the branch piping. Failure of large-bore piping has been less frequent. This is not surprising, for large-bore piping is often stronger than other components in the piping system. Although vibration of large-bore piping has resulted in pipe failures, failures of other weaker components are far more common. Snubbersboth mechanical and hydraulichave a history of failure when they are subjected to continuous piping vibration . Small-tap lines have failed because of vibration of largebore header piping; leaks have developed in anges and valves; and rotating equipment is adversely affected by piping vibration. Sudden failures can happen as a result of waterhammer or water slugs. Large-bore piping vibration can also create other problems, one example of which is a steam-bypass line in which steady-state pipe vibration caused failure of the piping weight supports. These failures went unnoticed until a 300 deg. circumferential crack formed in the line at the nozzle weld. The failed hangers resulted in a low point in the piping where water accumulated when the line was not used. The water slugging that resulted when the line was returned to operation contributed to the weld failure.
ALLOWABLE PIPING RESPONSE FOR VIBRATION
Nearly all piping in a power plant will experience some amount of vibration, and piping vibration problems in operating plants have resulted in costly unscheduled outages and backts. Vibration effects can be manifested in the gradual fatigue failure of the piping and its appurtenances, or in the more dramatic motions caused by dynamic-transient vibrations. The power industry has addressed these problems by using various Codes and regulations. The discussion that follows reviews the requirements of these documents, the allowable stress limits for piping vibration, and the effect of vibration on piping response.
Industry Codes and Standards
The governing Power Piping Codesthe ASME Boiler and Pr