PIAB Rope Tension Meters Case Study
Discover how PIAB’s rope tension meters helped to manage guy wire tension in refurbishment of guyed transmission towers at Dominion Virginia Power (DVP)
Background Information
The initial implementation of 500-kV transmission lines in the United States was established by Dominion Virginia Power, (DVP; Richmond, Virginia, U.S), in the mid 1960’s. The lines crossed a 350-mile course from DVP’s Mount Storm Power Station in the Allegheny Mountains of West Virginia to the Piedmont of central Virginia near Richmond. The new line design was based on the NESC heavy load parameters, with supplementary specifications for certain conditions. In mountainous areas, this included vertical loading determined by radial ice thickness of two inches on the conductors, as well as hurricane wind loading of 70 miles per hour.
In order to accommodate these design particulars, new fabrication of weathering steel, and lattice structures were created. To support the conductors in the mountainous sections of the lines, DVP used five types of suspension guyed-structures.
Transmission tower deterioration over time
In the late 1990s, line inspections began to show an increase in guy dead end grip strand failure incidents. This could have been a result of fatigue and cyclic loading, however there was no history of damage or failure to the actual wires. However, in December 2000 an upper guy grip on an FLT-GV tower completely failed, prompting DVP’s engineers to start an in-house study of the problem. The following year, on January 10, 2001 during a major windstorm, an FLT-GV tower collapsed on the side of a mountain valley.
Assessment of the guy wire tension challenges to be resolved
The guy grips that had failed were returned to the manufacturer, Preformed Line Products (PLP; Cleveland, Ohio), but their analysis determined that although there were indications of long-term Aeolian vibration damage, this did not contribute to the failure of the BG-4171s. The actual cause was fatigue from “longitudinal cyclical loading on guying members”. The PLP report addressed the importance of maintaining daily tension of at least 10% of the strands rated breaking strength (RBS).
The structural engineers of DVP, modeled the structures and transmission lines using PLS-CADD software. After analyzing the actual structural guy loading, a field climbing inspection, and in-depth file research they concluded:
- The original guy wires were pre-tensioned between 3500 lbs and 22,500 lbs each, depending on the tower type and guy wire size. The existing guy wire tensions were unknown. Guy grips had been replaced in the past, and no records could be found on the reinstalled tensions or the methods used for installation.
- The existing anchors, anchors rods and bottom adjustable hairpin anchor connections were in good condition and could remain.
- Most of the existing guy wire was in good condition and could be reused (guy wire samples were tested in an independent laboratory). Some guy wire had to be replaced due to the loads developed in the PLS-CADD analysis exceeding their design limits.
- All hardware should be replaced.
- The seat diameters for all guy grip connections, except those on the FLT-GV, would need to be increased.
The use of PIAB rope tension meters in the transmission tower refurbishment
DVP decided to replace all of the guy dead end grip assemblies and related hardware that was provided by PLP as the three 500-kV transmission lines with these structure types are necessary to DVP’s service reliability and power-transfer capability. After PLP suggested ways to accurately measure tensions in the existing guys and the reinstated tensions, DVP selected the PIAB RTM20D Rope Tension Meter from Sweden which was the only device that could achieve the required results. The PIAB RTM20D offers direct-digital readout for the wire tension, is easy to install, and allows calibration for 10 varying wire sizes, and is available for 2-, 5-, 10-, or 20-ton applications.
In order to help with DVP’s tight construction schedule, PIAB Sweden AB supplied one of the three units to be purchased as soon as possible, and shipped the other two within six weeks. When the PIAB RTM20D Rope Tension Meters arrived, they were field tested on several different tower systems. The PIAB RTM’s determined the guy tension values, and were then compared with a digital dynamometer’s reading. The range of tension of 300lbs or less was common between the devices, however DVP accepted this difference was reasonable due to the magnitude of the measured tensions.
DVP’s transmission maintenance crew did the replacement of the guying assemblies on the towers from “hurricane alley” in the fall of 2001. To measure tensions, the PIAB RTM 20D was used alongside a dynamometer, however after just a few guy grip installations the dynamometer was packed away. The similar tension readings on both devices, ease of use, and direct load readouts converted all skeptics to the PIAB RTM 20D.
A sudden wind while testing the tension on an existing guy with the RTM before changing out at a new site gave an impressive demonstration of the RTM’s capabilities. The readout on the PIAB RTM Rope Tension Meter began to change, showing the loading effect the wind was having in the tower and associated conductors. The guy replacement project had a second crew added, and while they exhibited initial skepticism of the PIAB RTM 20D at first, the second crew also saw the capabilities of the PIAB Rope Tension Meter and fully adopted its use.
The tension on a guy can be now known before trying to remove it, and provides the information needed to determine the correct rigging for the change-out. The PIAB RTM 20D was also used to re-tension the guy while the tension adjustment nuts were tightened. This project was completed in December 2003.
This case study was initially presented in an article published in Transmission & Distribution World Magazine, in the February, 2004 Issue.
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