Stud-tension Instrument Speeds Nuclear Plant Maintenance

High-pressure applications throughout the process industries depend on carefully adjusting, and then monitoring, the tension on stud bolts on pipe and fittings. Perhaps nowhere else is this monitoring more critical than in nuclear power plants. There, plant maintenance teams must adjust the bolts, record measurements and review the process with quality control personnel.

Now, an electronic meter is available to speed up this maintenance procedure. At the James A. Fitzpatrick Nuclear Power Plant (New York Power Authority, Lycoming, NY), a new Stud Elongation Measurement System (SEMS) supplied by Biach Industries, Inc., of Cranford, NJ, is being used successfully. The system allows a single operator-plus the Quality Control supervisor-to take accurate elongation measurements more quickly, and provides clear, automatic documentation of the measurements. There is no need to manually record the readings, and therefore less chance of error.

Refueling turnaround
Until recently, the procedure at the Kirkpatrick plant has been to check the status of the reactor pressure vessel (RPV) head flange by manually measuring and then analyzing the stud elongation. These measurements have been performed with a dial depth indicator and laborious hand-calibration. The lead mechanic took the measurements, a second mechanic recorded, and someone from Quality Control was there to verify. One pass around the RPV head required at least an hour - usually more.

For the most recent outage, maintenance management decided to employ the SEMS system. The system incorporates a simple three-step measurement process:

• The mechanic engages the hand-held measurement pod in the top of the stud to meet the datum rod. The pod is designed so it automatically positions itself correctly with the center of the measuring rod.
• The stud number is verified and entered into the portable control unit. The pod then measures the distance from the top of the stud to the top of the measuring rod. Upper and lower limit ranges were preset to ensure that measurements are within an expected range. If the measurement was out of range, the operator is warned.
• The distance, reflecting the elongation of the stud, is read out on a digital display. The mechanic presses an enter key to record the measurement for printout. After storing the data, the operator is prompted to move to the next stud.

In the initial pass measurements, the mechanic and QC technician, accompanied by the maintenance supervisor and a Biach representative, went around the vessel in only seven to eight minutes. Since a measuring pass that required more than an hour using the dial depth indicator now requires less than ten minutes, Fitzpatrick maintenance management estimates that tensioning time has been reduced from sixteen to eight hours. In addition, all manual calculations are now done automatically using the SEMS unit.

Details of the SEMS system
The stud-elongation measurement system is hand-held, allowing a single operator to make, record and average stud-tensioning measurements. The system utilizes LVDT, (linear variable differential transformer), or rectilinear technology, and is battery-powered for five hours of portable use. It has room for calculation and number storage for as many as ninety-nine studs and an internal printer for immediate documentation. After use, data stored in the EPROM may be downloaded via an RS232 port to a PC.

The quality control department was also eager to confirm that the system would remain in calibration for the entire time of its use. After the first pass, the Biach representative was able to show the QC inspector the recalibration block, which continued to read within ±1/1000 in. of its initial setting.

One very useful function of the measurement system is its automatic flagging of measurements which are out of range - a glance at the digital display or printout shows which studs need attention. The system also allows its operator to calculate or verify how many turns of the nut handle on the tensioner are required to achieve target elongation.

The Biach SEMS is used wherever a bolting problem may occur and the studs have measuring rods. Applications include petrochemical plants, refineries, offshore pipelines and power plants.

James Bathie is a New York Power Authority Mechanical Maintenance Supervisor with the James A. Fitzpatrick Nuclear Power Plant in Lycoming, NY.

Edited by Nick Basta