Invariably this time of year we are called onsite to respond to water hammer events. Water hammers happen mainly because of lack of awareness and improper Operation and Maintenance practices. In the case of a water hammer, the proverb of “prevention is better than cure” holds true. One such prevention action is to properly maintain your pipe supports to prevent sagging and localized low points where condensate can pool and flash on start-up.
Piping failures related to pipe support deficiencies usually involve through-wall leak conditions although in rare cases, they can involve catastrophic ruptures. Throughwall leaks have been traced, for example, to excessive bending moments caused by topped-out or bottomed-out spring hangers.
Spring hangers bottom out when the downward travel range has been exceeded. This results in a restriction of thermal movement which can produce extremely high thermal stresses. Throughwall leaks often occur due to excessively high thermal bending moments.
In some cases, extensive system deterioration precludes failure. Improper support which results in excessive dead weight deflections can, if left unattended, result in permanent pipe sag. Localized low points in the system occur which result in the collection of condensate during system shutdowns. During subsequent start-ups, water hammer and thermal transients can cause extensive damage to the piping and supports.
Water hammer events include the flashing of water to steam as well as the passage of water slugs through the system. With thermal transients, the passage of hot steam over cool water pockets causes hot and cold spots, wherein the differential in thermal expansion can be high enough to cause plastic deformation of the pipe lead.
Other upset loads include water hammer due to pumps stopping, steam hammer resulting from valve closure associated with turbine trips, flow-induced vibrations, seismic loading, and relief valve thrust loads. In addition to causing high pipe stresses, upset loads can damage hanger components and lead to further system degradation.
Another common problem associated with pipe supports is welded hanger lug failures. In addition to thermal differential stresses, the “bat-wing” type of hanger attachment lugs inherently involve high bending moments which are transferred to the attachment. This type of cracking can propagate through the weld or into the lug itself, eventually rendering the hanger ineffective. However, hanger lug cracking can also propagate into the piping base metal. If left unattended, a failure could result upon final propagation of the crack. The preferred design, in which cracking occurs less frequently, involves the use of welded shear lugs in conjunction with a pipe clamp.
High stresses associated with inadequate pipe supports can be compounded by inherent stress risers in piping. Stress intensifiers such as branch connections, reducers with an abrupt change in pipe size, etc. are normally calculated in accordance with applicable factors by the system designers, such as counterbores or defects associated with stress risers. In particular, there is little allowance for additional stresses caused by hanger problems.
In addition to piping failures, a number of equipment failures have been traced to improper pipe supports and/or piping system layouts. Several boiler tube failures have been caused, in part, due to excessive bending moments being transferred from piping systems through the headers. The cause of these failures is not always obvious to boiler inspectors and engineers, as their work is often associated with the boiler only. Unless properly informed, laboratories performing tube failure analyses may also be unaware of the actual loading conditions.
Other cases related to boiler supports involved the support of boiler fuel piping and ductwork. These components are typically supported by constant support hangers, variable spring hangers, and rigid supports. Excessive loads have been transferred to windboxes and onto waterwall tubes, also resulting in failures.
Inadequately supported piping systems have also imposed excessive loading on deaerators and other pressure vessels. Likewise, deaerators and other plant equipment can cause excessive loading on piping systems. Deaerator flashing can result during a unit trip due to a sudden reduction of inlet steam, drop in pressure, and/or flashing of water to steam. Conversely, the sudden filling of a deaerator with cold water can cause a vapor collapse due to the sudden condensation and reduction in the volume of the inlet steam. In either case, high reactions are imposed onto piping systems. If the supports are not designed to withstand the resultant transient loads, then support failures will occur which can lead to crack initiation in the piping system.
Turbine and pump manufacturers may specify allowable nozzle loading (i.e., forces and moments) to which piping system designers must adhere since the alignment of high speed rotating equipment is critical.
Piping analysts often utilize lateral restraints, snubbers, and limit stops to reduce end reactions on equipment. For example, a limit stop may be utilized to direct thermal growth away from turbine connections. Snubbers are often designed to absorb upset loads such as steam hammer loading associated with valve closure during turbine trips.
Annual pipe support system walkdowns can ensure that these critical components are properly functioning throughout the thermal cycle of a piping system. Additionally, stress analyses of piping systems, based upon information supplied by hanger walkdowns, can be used to resolve support related problems as well as locate areas of high stress which may be of concern.
When cracks develop in piping systems, boiler components or equipment, sections of the failed weld are often submitted to a laboratory for analysis. The results of the examination may determine that the failure was caused by fatigue or other mechanism. The cause of the failure mechanism is frequently overlooked and not investigated. An assessment of the pipe supports will often reveal the underlying cause and provide solutions for minimizing or preventing failures. Properly functioning supports are often the first line of defense in preventing untimely and costly failures.
Thielsch Engineering has extensive knowledge in performing hanger walkdowns and nondestructive testing to investigate possible damage in high-energy piping systems. For more information, please contact Mr. Peter Kennefick at Pkennefick@thielsch.com.
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