Pipe Support Performance as It Applies to Power Plant Safety and Reliability
George T. Mulvaney
Partner, Ocean State Technical Services
65th General Meeting in 1996
Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (4 printed pages)
When performed in an effective and meaningful manner, the inspection and servicing of pipe supports will enhance the continued safe and reliable operation of piping and related power plant equipment. Benefits include the detection and elimination of undesirable and, in many cases, unpredicted, stress. Untimely leaks that develop at flanged and welded joints can often be resolved and, ultimately, prevented. Additionally, general plant safety is improved by ensuring that critical station equipment is properly secured and positioned. All too often, however, the devices are taken for granted and are typically ignored once installed and placed into service. Ironically, this neglect is rooted in their reliability.
The primary purpose of a pipe hanger or a support is to provide a means of transmitting the deadweight of the suspended equipment into suitably placed structures. For this type of application, passive components such as structural members, rods, and associated hardware are utilized; however, where operating temperatures impact the geometry of the supported equipment, then the supports must not only carry the weight of the equipment, but also accommodate movement as well. To achieve this task, active or flexible-type supports, such as variable spring hangers and constant support hangers, are typically applied.
Whether they represent active or passive designs, the importance of the pipe supports is frequently overlooked, particularly with respect to how they influence the safe and continued operation of power plant equipment.
First, the reliability of devices themselves is a major factor that contributes to their neglect. Designed and constructed with significant safety factors (in some cases, up to five times their rated loads), hangers rarely break, become disengaged, and subsequently attract attention.
Second, the related problems that do develop at individual hangers, in the support scheme or in the connected equipment, are typically long-term or chronic in nature. This also tends to mask the importance of the devices, as well as the fact that they may have contributed to the ultimate problem.
Stress and Elevation Changes
Two areas of consequence which can evolve from improper support performance are the development of unpredictable stress and elevation changes in the suspended equipment. Furthermore, these two general problem areas are often the result of four broad categories acting independently or in various combinations. These categories, and some associated examples, follow:
- Design (product-specific deficiencies or an oversight in their application).
- Installation (supports improperly positioned or calibrated; for example, water-line supports set with the pipe empty or steam lines water-filled).
- Fabrication (poor workmanship/quality).
- Service (hostile environments, shock loading).
Of primary concern is the development of undesirable stress levels in piping systems. Typical problems involve cracking, or through-wall cracking from the bending moments that develop around the circumference of the pipe. The location of the cracking is usually at the smallest cross-sectional area next to a restraint; and a "restraint," in this case, is defined as a rigid or "tight" transition point that would also include a weld.
Unanticipated elevation changes are also a repercussion from poor support performance. One liability is the negative influence on pipe pitch that will render a drain line ineffective or obsolete, and rarely is this (or the resulting problems) recognized as hanger initiated. Clearly, the development of damage from water hammer and/or "flashing" is a major worry. These issues are of particular concern in systems that may periodically introduce water for the purpose of controlling steam temperature/pressure. Thus, the exposure to risk is amplified by the potential collection of the water in downstream locations. Examples of these systems include cold reheat steam leads, various auxiliary steam lines and even main steam leads.
Flexible-type supports are typically the primary cause for elevation changes, particularly where a "loose" piping system or segment is involved. The basic characteristic of a loose system is one where substantial dependency for loading and movement is placed on flexible-type supports. Additionally, the more dependent a piping system is on flexible-type supports, the more hanger problems one will tend to experience.
Discrepancies in hanger loadings will allow the suspended item to drift until restricted by an interference or by reaching the travel limits of the support(s). With respect to the loading discrepancy itself, it may result from damage to the support or simply a worn-out spring coil.
Another scenario having a negative impact on support loadings involves the replacement of piping accessories, such as a valve or insulations, and not taking revised weights (support loadings) into account. Rarely, however, are problems traceable to outright errors in the design process.
While the majority of hanger problems are associated with flexible-type supports, it is also important to emphasize that passive- and rigid-type hangers can generate trouble. Also, although most elevation changes occur gradually, they can be sudden and catastrophic in nature. Items such as structural members, welded attachments, clamps, U-bolts, or even rods all require a certain amount of attention. It should also be noted that deficiencies in most passive components are often masked. The cracking propagates slowly through the stock thickness without the development of distortion in the clamp halves.
Another problem that is often concealed, due to extreme temperatures and insulation coverage, is cracking in welded hanger attachments. Elbow lugs, shear lugs and side plates/"bat wings" are some of the more common attachments that are prone to outright failures, particularly on high-temperature (typically greater than 850 degrees F) steam leads.
Elevation changes can also be the primary initiating cause behind the development of significant and undesirable stress levels. The stress can lead to plastic deformation of the piping system and/or failures in selected components. One sensational example involves a main steam lead that developed a sag of 20 degrees over a span of approximately 40 feet. In this instance, a number of U-bolts incorporated into pipe clamp designs had failed and went unnoticed for years. Only when operations personnel could no longer walk under the pipe was the hanger failure identified.
Another example of stress problems related to elevation changes is often found in branch line connections to the significant run of pipe. Where the larger pipe is free to move, and the branch line binds at some point in the travel cycle, bending moments will develop at the connection and will often lead to cracking or outright failure. Again, the failure will typically occur at the smallest cross-sectional area of the connection/transition.
Safety and Servicing
Taking the above into account, it is evident that the adjustment and servicing of pipe support products should be addressed using extreme care. Planning, specific procedures development, and a review of the operating characteristics of the hangers are basic issues that must be considered. Above and beyond all else, the potential for sudden catastrophic failures must be anticipated and minimized through effective inspection beforehand. Additionally, the propensity for disengagement of the support should be minimized by thoroughly reviewing the length and condition of threaded components.
The overall integrity of the support must also be reviewed to ensure that it will be able to accommodate and endure the servicing. It is evident that an outright failure during adjustment would likely have a serious impact on workers.
To minimize the development of problems from poor support performance, operating companies should establish regular and routine inspections for pipe support systems. General recommendations in this area are also outlined in various codes, regulations and standards, including ASME B31.1 and ANSI/NB-23.
In its most basic form, an isometric sketch should be developed showing the configuration of the piping and the approximate location of the supports. An accompanying log sheet should also be used to list the design or nameplate data, readings at flexible-type supports, and general observations. The work would ideally involve inspections with the equipment in the hot or operating mode, supplemented by inspections in the cold or shutdown condition.
The fundamental visual examination process should also be supplemented by "programs," as warranted from inspection findings, site-specific concerns, or generic deficiencies with the designs or products installed. Furthermore, examples of programs include the following:
- Nondestructive examination of welded pipe attachments.
- Refined examinations of rods, bolts, clamps and other passive components.
- Selected engineering studies to guide corrective activities.
- Load testing of flexible-type supports.
- Refurbish, replace or upgrade the products used to suspend or restrain the suspended equipment.
- Adjustment of the supports.
- Inspection of piping segments and connected equipment (headers, boilers, steam drums, pumps, etc.).
Recognizing that power plant reliability and safety are long-term goals, vigilance is crucial. When performed in a meaningful and effective manner, the inspection and servicing of pipe supports will only serve to enhance these areas.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition of the ASME Boiler and Pressure Vessel Code for current requirements.