Linemen who maintain transmission and distribution lines and towers often work in a fiberglass bucket or boom that stretches from 45 ft to 150 ft in the air. As such, safety is paramount. Any serious, undetected defects in aerial personnel devices, commonly known as bucket trucks, could have catastrophic results.
Acoustic emission (AE) inspection is part of a comprehensive preventative maintenance program for utility fleets to extend the life and reliability of their equipment. AE testing is designed to discover degradation of the insulated portions as well as detect defects in the metal components caused by fatigue or overloading, said Frank Petrasek, principal engineer of Georgia Power Co. AE can alert inspectors to the location of defects that are difficult or impossible to visually inspect, explains Keith McPherson, a North Carolina-based engineer with Altec Industries.
The concept of using the science of AE testing to evaluate the condition of fiberglass-reinforced plastic (FRP) has been around since the 1970s, but employing it to check the in-service condition of bucket trucks in the utility industry began in the 1980s.
AE inspection is based on the fact that materials under stress emit sound waves. When captured by the proper equipment, sound waves can be evaluated based on their strength, or amplitude, and their location within the bucket truck. Although additional methods can be used to assess the safety and soundness of other components of a bucket truck, AE is the only cost-effective, non-destructive method for checking FRP components.
Simulating Field Conditions
During an AE test, load, pressure and other stressors are applied to simulate the way a device or structure is used in the field. Inspectors stress the device slightly beyond a user-designated rating that is always less than the designed maximum capability engineered by the manufacturer but above what the device experiences in the field. Since the inspector's test load is always below the manufacturer's designed maximum load, there's no danger that the acoustic test will compromise the components of the device being tested.
If inspectors find evidence of any damage or defect that might be increasing, they evaluate the amount of emissions and/or their rate of growth based on requirements in published test standards, and decide whether the device or structure should remain in service, be repaired or be replaced.
Improving Vehicles' Reliability
Linemen appreciate the additional visual and acoustic inspections because it adds another set of “eyes” on the vehicles they depend on. As a result, they have more confidence in their vehicles and their own safety. The utilities that participate in this program inspect their vehicles on a regular basis. They often use load testing, which is monitored by AE under controlled conditions, along with thorough visual and other non-destructive inspections.
Once linemen understand the importance of the acoustic testing process for vehicle safety, they often are interested in what was found. Many times, they will give the inspectors clues about areas that need to be further investigated.
At the same time, the inspections help the fleet managers in their decisions on life extension of the vehicles and the attached aerial booms. In many cases, maintenance schedules of the vehicles benefit because problems may be discovered early. In addition, repairs can be scheduled at regular maintenance intervals, eliminating excessive downtime for the crews. Inspections are usually performed during off hours or scheduled downtime hours to eliminate disruption for the crews. AE inspections are usually performed during evenings and weekends when the crews are not scheduled. In areas where fleet mechanics are scheduled in shifts, most problems identified by the acoustic inspections can be handled before the vehicles are needed the next day.
The first test standard for field use of AE inspection of aerial personnel devices was F914, Test Standard for Acoustic Emission of Insulated Aerial Personnel Devices without Supplemental Load Handling Attachments, published in 1985. It was developed by Subcommittee 18.55 of ASTM Committee F18 on Electrical Protective Equipment for Workers, which includes representatives of the utility industry, safety consultants, manufacturers of aerial devices and AE testing organizations.
The first standard was followed in 1992 by F1430, covering material-handling insulated aerial devices; and in 1997 by F1797, concerning insulated digger derricks that bore holes and set posts. In subsequent years, each test standard has been reviewed and revised several times. In fact, AE data interpretation techniques, test equipment and field experience have evolved to the point where the word “insulated” has been removed from the standards, since AE is now used to evaluate the metal components of devices as well as insulating components. Additionally, ASTM Subcommittee F18.55 has developed acceptance/rejection criteria, which are now included in the latest revisions of test standards.
In accordance with ASTM standards for AE inspection, utilities and testing organizations have found cracks in the FRP booms of aerial personnel devices, damaged material-handling jibs, or winches on bucket trucks, and problems with hydraulic holding valves. They've also uncovered inadequate lubrication of pins and bushing and epoxy de-bonding at metal to FRP interfaces, resulting in repaired or replaced equipment and enhanced safety for workers.
Additionally, AE inspection has been used to inspect aerial devices after suspected damage. For example, if a bucket truck experiences an overload caused by using the winch to pick up an object beyond its rated load, AE testing can determine if any significant damage has been done, McPherson said.
Inspection results are commonly grouped by the severity and location of the acoustic activity, and are defined in three ways:
Being an acoustic activity or a visual sighting severe enough to remove the truck from service and requires further investigation and repair
Being an acoustic activity or a visual sighting usually caused by a known problem such as internal cables out of adjustment and can be scheduled for attention at a later time
Being no activity requiring attention.
To ensure a thorough inspection, ASTM standards also dictate the use of other non-destructive test methods that should be used in conjunction with AE, notes Jerry Tanner from Diversified Inspections/ITL. These include ultrasonic inspection, magnetic particle inspection, liquid penetrant and dielectric testing where high ac or dc voltage is used to ensure that the FRP section does not conduct electricity.
Recognizing the Benefits of Testing
Since the first ASTM standard was published, AE testing has uncovered defects that might have cost lives or injuries if gone undetected. It also has saved money by detecting defects that, going unnoticed, would have cost more to repair later. In addition, having testing standards ensures consistency in testing and data collection, allowing any consultant or testing agency to get similar results, said Tanner of Diversified Inspections/ITL.
But regardless of the inspection method used, inspectors should be certified and have years of experience testing and interpreting data for specific devices or structures. In fact, inspectors using ASTM standards are required to be certified under The American Society for Non-destructive Testing (ASNT) guide to employer-based personnel certification (ASNT TC-1A). ASTM International is also considering a program for personnel certification specific to its published testing standards for aerial devices.
AE testing has made great strides in its ability to locate and evaluate defects in both FRP and metal components of aerial devices for the utility industry. AE testing is now also used to detect leaks and other defects in underground piping, railroad tank cars, pressure vessels and aboveground storage tanks. In the hands of trained, competent personnel, it is an invaluable tool for maintaining the increasing complex inventory of equipment in utility fleets, the pulp and paper industry, power generating plants and many other industries.
William C. Veal ([email protected]) is a retired fleet test coordinator from Georgia Power Co. and has been working with acoustic emission since 1987. He is the chairman of ASTM F18.55 subcommittee, which developed the ASTM standards for aerial lift inspection using acoustic emission. He has been with ASTM since 1990.
ASTM Contemplates Certification Program for Aerial Device Inspectors
In order to assist owners and users of aerial devices in developing individuals proficient in the ASTM test standards relating to aerial devices, ASTM is considering developing a certification program that focuses on its three existing aerial lift standards. Here's what you need to know about the standards and proposed testing program.
- The applicable standards
The three standards are: F914/F914M-10 Standard Test Method for Acoustic Emission for Aerial Personnel Devices without Supplemental Load Handling Attachments; F1430/F1430M-10 Standard Test Method for Acoustic Emission Testing of Insulated and Non-Insulated Aerial Personnel Devices with Supplemental Load Handling Attachments; and F1797-09e1 Standard Test Method for Acoustic Emission Testing of Insulated and Non-Insulated Digger Derricks. These standards are under the jurisdiction ASTM subcommittee F18.55 on Inspection and Non-Destructive Test Methods for Aerial Devices.
- The difference between certified versus qualified employees
Generally, utility fleet, utility contractor groups and inspection service agencies have the ability to hire and maintain personnel certified for acoustic emission inspection. The American Society for Non-destructive Testing (ASNT) has a personnel certification program developed for employer-based certification in the recognized non-destructive examination (NDE) classifications for Level 1 and 2. This program, known in the industry as ASNT TC-1-A, contains minimum recommended requirements for each level of certification and a certificate of completion, furnished when all requirements are met. The actual certification level is administered through the employer as part of their quality assurance program. However, a “certified” individual using this process may not necessarily be a “qualified” individual. Verification that employees are qualified for specific NDE inspections still rests with the employer.
- The need for objective testing
Acoustic emission (AE) has always been recognized as an inspection discipline that is extremely operator dependent, especially as it relates to aerial lift inspection. Because of the inherent design of an aerial device, which includes internal cables, sheaves, pulleys, hoses, pins, hydraulics, and a host of noise-inducing equipment, it is difficult for the inspector to filter out non-relevant acoustic signals and focus only on relevant ones. Also, baseline acoustic signatures of one manufacturer's device compared to another may be different. These factors cause aerial device AE inspection to be more subjective and dependent on an operator's experience as he interprets the test data more so than in other types of AE inspection.
- Review of current training
The ASNT personnel certification process in TC-1-A for AE touches on all of the possible AE inspection methods. These methods include inspecting pressure vessels, underground piping, tank cars, and above ground storage tanks, etc. Because classroom instruction hours and length of examinations must be limited, this certification process lacks specific focus and detail on any one method. An individual is given an overview of how AE can be used, knowledge of the basic principles and theories, and it touches on data interpretation for various methods. The current certification process does not necessarily “qualify” an individual to inspect aerial lifts or interpret the type of test data received from such inspections. Developing qualified inspectors still remains an employer responsibility.
- The proposed program structure
If users and inspection agencies agree that a limited certification program would assist their efforts to develop qualified individuals in aerial lift inspection and would be interested in having their personnel attend ASTM training programs, the association could then move forward to create a program specific to the ASTM aerial lift standards. This will not replace the basic ASNT AE certification. In fact, the ASNT AE certification would be a prerequisite for attendance in the proposed training. ASTM would provide both classroom and hands-on field training, with both general and specific examinations. In addition, utilities could give the graduates a certificate of successful completion, which could be used through their quality-assurance procedure to create a limited certification in aerial device inspection.
Altec Industries | www.altec.com
ATSM International | www.atsm.org
Diversified Inspections/ITL | www.diusa.com
Georgia Power Co. | www.georgiapower.com