AE Inspection of Seam Welded Hot Reheat Pipe

With the recent release of EPRI's Acoustic Monitoring Guidelines for Hot Reheat Piping (EPRI TR-105265-V1, Nov 1995), standardization of his promising and cost-effective inspection technology is coming to fruition. A full-scale test of a reheat line structure using the guideline approaches was completed at PG&E's Potero plant in early 1994, with excellent correlation between AE and conventional NDE methods achieved. Additional tests using the guideline approach have been conducted at PP&L Montour in 1995, CP&L Nueces Bay and AEP Gavin in 1996. The recommended methodology employs both online monitoring (during normal plant operation) and a full cooldown as the best approach to enveloping the stress environment of each piping system. The cost savings potential of AE inspection is realized when its powerful source location capabilities are used to determine likely defect locations. Localized inspections with other standard NDE methodologies (ultrasonics, radiography, magnetic particle) can then be applied more economically to suspect areas, eliminating the need for complete removal of insulation.

Testing and development programs sponsored by EPRI and individual utility companies date back to 1987, and currently encompass some 60 test experiences. Program efforts conducted independently by PG&E and jointly with EPRI funding have resulted in the current recommended strategy. PG&E conducted an extensive research program from 1990-94 that explored many facets of the application problem, including waveguides, sensors, instrumentation, and methods of stimulating (loading) the structure. Offline hydrotesting, live steam injection, startups, cooldowns, and online monitoring during normal plant operation were explored. Superior results were demonstrated with online monitoring for a period of normal plant operation (1-2 weeks), combined with a full cooldown. 'The fundamental reasons for the combined testing methodology relate to the nature of the technology and the types of defects in piping structures.

Acoustic emission separates itself from other NDE methods by the fundamental nature of its application. It is a passive monitoring technique, detecting the sound energy from emitting sites well away from sensor locations. It is therefore a global inspection technique, capable of detecting and locating a variety of emitting sources on the piping.

Piping structures are a combination of structures and materials: welded and bolted attachments, seam and circumferential welds, cast elbows, tees and branches, reducers, drain plugs, steam vents. etc. Predominant defect growth mechanisms are high temperature creep and thermal fatigue. Line pressure, bending loads, thermal excursion rates, weld geometries and structural configuration all lay roles in the type and rate of growth of different defect types.

The PG&E work demonstrated that a variety of defect types and locations could be more effectively evaluated by "enveloping" the stress environment for the piping system. Some sites are found to emit primarily under transient thermal conditions, responding to thermal shifts of as little as 25°-30°F during normal load cycling. Other sites have been found to emit under steady--state conditions of temperature and pressure, as might be expected of creep-related growth mechanisms.

Online monitoring during normal plant operation proved remarkably consistent over 1-2 week monitoring cycles. Online monitoring requires instrumentation capable of defeating the flow noise found in varying intensities at different locations and plant operational profiles. Active source location, high frequency sensors, and an effective "floating threshold" are required. Online monitoring is typically conducted over several weeks time to completely characterize the piping system under operational profiles. 8-16 channels of data acquisition can be rotated between different waveguide positions, which are typically spaced at 15-18 ft. on the structure. The waveguides are preferably welded to the piping: but mechanically attached waveguides with high temperature couplant between the waveguide and pipe have also been used in temporary monitoring programs (cooldowns and short duration online monitoring) Remote monitoring via modem reduces the cost of data acquisition for this mode of testing.

Cooldown monitoring is typically conducted as the plant is going into a planned outage AE instrumentation must cover the entire piping system for the cooldown period from operational temperatures to about 600°F, typically requiring 30-35 channels for 500 ft. of piping. Sources which are primarily responsive to axial thermal contraction are revealed in this testing model These include defects in circumferential welds, at tees, hanger attachments, elbows, drain plugs, etc. This mode is not considered adequate for stimulating creep-related defects in longitudinal seam welds due to low hoop stresses – pressure is rapidly declining, and thermal differentials produce insubstantial hoop loading (<1 KSI).

The recommended reinspection interval for AE testing of fossil hot reheat lines is three years according to the EPRI Guidelines. Typical fully loaded inspection costs - including conventional NDE backup inspection - are about $200/ft. for AE inspection vs. $650/ft. for conventional NDE (UT, MP, RT). Twenty year cost management models based on the Net Present Value method show that utilization of AE as the primary inspection method will save 50% of the cost of continued conventional (NDE) inspection of reheat lines.u