Stan Walker, EPRI
To prevent forced outages, utility maintenance crews routinely inspect boiler tubes and various piping systems during normal scheduled outages. However, the various nondestructive evaluation (NDE) techniques currently used for these inspections require expensive preparation and are time-consuming. As a result, they cannot be used for comprehensive inspection, but can only be used for limited inspection of sampled areas.
Figure 1. Long-range guided-wave examination of piping.
Recent laboratory studies and a utility field demonstration indicate that a new technology—called long-range guided-wave magnetostrictive sensor (MsS) technology—could be used as an economic inspection tool to compile comprehensive information on long lengths of power plant piping or tubing. The technology, which was developed by Southwest Research Institute, offers the potential to conduct complete inspections, rather than sampling, and without the need to remove insulation or perform other extensive preparations.
Guided-wave MsS technology
The guided-wave inspection technique, already in use for inspection of piping in refineries and chemical plants, provides a 100 percent volumetric inspection of a long length of pipeline (more than 100 feet [30 meters] in one direction) for inside and outside surface corrosion/erosion defects and circumferential cracks, from a single measurement location.
Guided waves refer to mechanical waves in ultrasonic and sonic frequencies that propagate in a bounded medium (such as a pipe or rod) parallel to the plane of its boundary. The wave is termed “guided” because it travels along the medium guided by the geometric boundaries of the medium. The probe consists of two elements: a coil that encircles the pipe, and a device—such as a permanent magnet circuit—that applies a magnetic field to the pipe. When alternating electric current is supplied to the transmitting coil, the coil produces an alternating magnetic field. This field expands and contracts the pipe via the magnetostrictive effect, generating guided waves in the pipe wall.
Figure 2. Guided-wave sensors being applied to piping at CPS’s VH Braunig plant fuel piping.
During a guided-wave examination, a short pulse of guided waves is launched along the structure being examined. Signals reflected from geometric irregularities in the structure—such as welds and defects—are detected in the pulse-echo mode. From the occurrence time of the defect signal and the signal amplitude, the axial location and severity of the defect are determined.
Field demonstration on fuel oil pipes
In a recent field demonstration, City Public Service of San Antonio (CPS) was able to use the guided-wave MsS technology to rapidly inspect more than 1,500 feet (460 meters) of fuel oil supply and return piping at the utility’s VH Braunig plant and detect areas of corrosion without the need to remove insulation. This capability allowed plant management to assess the cost of replacing the damaged piping without incurring the large expense of insulation removal and replacement. For portions of piping under roadways, the new technology also avoided the effort and cost of roadway excavation.
The Braunig plant has about three miles of fuel piping, including a mile and a half each of insulated, 12-inch (300-mm) diameter fuel oil supply and 8-inch (200-mm) diameter fuel oil return piping. During the two-day demonstration, several sections of piping were examined: straight runs, road crossings with elbows, and elbows and straight sections associated with expansion loops. The two-man crew examined the piping from a total of 18 access locations. This inspection consisted of approximately 830 feet (253 meters) for the 12-inch (300 mm) diameter supply line and 680 feet (207 meters) for the 8-inch diameter (200 mm) return line.
“The conventional approach would have been to strip off all the insulation and conduct a visual and ultrasonic inspection of the whole pipeline,” says Paul Barham, CPS’s Manager of Generation Control and Marketing. “We estimate we saved about $100K to strip the insulation and possibly more to replace it.”
Laboratory studies for boiler tube inspection
At EPRI’s Fossil NDE Center in Charlotte, North Carolina, researchers evaluated the use of guided-wave MsS technology to inspect long lengths of boiler reheater tubing. Results showed that this technique appears to be directly applicable to inspection of the entire length of boiler tubing between the outlet header and the inlet header from two elevation points in the boiler, one near each header.
A loss of 10 percent of tube cross-sectional area can be detectable over a distance of more than 200 feet (60 meters) from the test location for straight lengths of tubing. The presence of bends in the tube network reduces defect detection range somewhat. The amount of reduction varies with wave mode, wave frequency, and bend type. In this work, the amount of reduction was found to be no more than 25 feet (7.6 meters) for a 90-degree bend and 50 feet (15.2 meters) for a 180-degree bend (this occurred for 64-kHz T-waves), resulting in an inspection range of more than 100 feet (30 meters).
Walker manages EPRI’s Fossil NDE Technology & Training Program. He has 30 years of power-industry NDE experience and has been associated with EPRI’s NDE Center in Charlotte, N.C. for the past 20 years. He can be reached at 704-547-6081, firstname.lastname@example.org.