A wicket gate stem on Unit 1 at Holyoke Gas & Electric’s 30-MW Hadley plant sheared in two in January 2007. The utility quickly commissioned design and fabrication of a replacement gate. To avoid dismantling the unit to install the new gate, a contractor cut the new gate into thirds horizontally, installed it in the unit, and welded it back together. The unit has operated problem-free since the new gate was installed in May 2007.
Discovering the problem
The Hadley project began operating in 1951 on the Connecticut River in Massachusetts. Over time and operation of the units, the wicket gate end plates developed wear. In 1985, original equipment manufacturer Allis-Chalmers refurbished the wicket gates. This refurbishment included bringing the end plates back to the original specifications by capping the top and bottom portion of the wicket gates with stainless steel end plates. Since this work was completed, there were no apparent problems with the wicket gates.
On January 15, 2007, plant personnel shut down Unit 1 because it had developed excessive vibration and personnel could hear loud banging noises coming from the unit. Unit 1 contains 20 wicket gates. Personnel inspected the wheel pit and determined that the shear pin in the No. 7 wicket gate was broken. Plant personnel replaced the pin and started up the unit. Again, there was a problem with high vibration and loud banging noises. Personnel shut down the unit for a second time and dewatered it. Upon performing an inspection of the wicket gates in the scroll case, personnel discovered that the No. 6 wicket gate was open about 50 percent, while all the other gates were closed. The shear pin was not broken. Personnel then returned to the wheel pit and removed the gate arm. They determined that the key in the gate shaft was intact but that the upper stem had sheared completely off, about 23 inches from the top.
Investigating possible solutions
Holyoke Gas & Electric hired Devine Tarbell & Associates Inc. (DTA, now known as HDR|DTA) to determine the best method for replacing the broken wicket gate without disassembling the unit. In early February 2007, DTA personnel traveled to the Hadley plant.
In preparation for this visit, plant personnel removed the top plate from the wicket gate stems on this unit. Personnel also removed the center grease pipe on two wicket gates — No. 6 and an undamaged gate. The grease pipe from the undamaged gate was needed to provide dimensions.
DTA personnel first performed a visual inspection of the wicket gate stem, which revealed a fairly clean break. The inspection also showed that the drill hole for the grease line to the lower bushing was not centered in the stem. DTA personnel surmised this may have contributed to the break by reducing the cross-section of the stem on one side. The visual inspection did not give any evidence of where the crack started or ended.
|A wicket gate stem on Unit 1 at the 30-MW Hadley plant sheared in two in January 2007.|
A technician with DTA then performed an ultrasonic inspection of the broken wicket gate stem. The goal of this inspection was to arrive at some measure of the condition of the remaining 19 wicket gate stems to determine if any other damage was present.
The ultrasonic inspection revealed that two wicket gate stems (Nos. 3 and 14) contained anomalies at a distance about 23 inches from the top, the same distance at which the stem of the No. 6 wicket gate failed. Gate No. 3 is the first gate in the inlet to the spiral case, and gate No. 14 is almost 180 degrees from it. However, detection of an anomaly does not necessarily indicate that a crack is present.
To perform non-destructive examination of the wicket gate stems would require disassembly of the unit. Disassembling the unit and performing this examination would add two weeks to the schedule, depending on the availability of a qualified turbine erector. This was not an option because Holyoke Gas & Electric needed a way to get the unit back into operation quickly. This unit is the largest generator for the utility and would be needed during high spring flows and to provide electricity for the city of Holyoke.
Choosing the method used
To get the unit operating quickly, DTA designed a wicket gate that would be installed without dismantling the unit. DTA recommended that the replacement wicket gate be fabricated, stress relieved, and machined at a manufacturing facility. The gate would then be shipped to the Hadley plant. To get the gate into the turbine pit, it would have to be cut into three pieces horizontally through the leaf area, installed in the unit, and welded back together.
Removing the old gate, installing the new
In late April and early May 2007, personnel with Weld Mart Inc. (now known as Peak Hydro Services) cut the old wicket gate with a torch and removed it from the unit.
The replacement wicket gate was manufactured by Mountain Machine Works of Auburn, Maine, and shipped to the Hadley plant. There, Weld Mart personnel cut the wicket gate leaf into three sections and prepped them for welding. They then worked through the water passageway to install the upper section first, then the lower section, and finally the middle section. Personnel then aligned the sections and performed field welds under controlled heat to minimize distortion. Finally, Weld Mart personnel rotated the gate by hand to check for any binding.
The wicket gate was not painted, to allow better visual examination in the future and to avoid the problem of paint hiding surface distress.
Results of the work
Plant personnel returned Unit 1 to operation on May 18, 2007.
DTA recommended that gates Nos. 3 and 14 be closely examined when the replacement wicket gate was installed. To perform this examination, plant personnel closed the wicket gates and examined the contact point. The goal was to determine if there were any indications that the leaf to stem had moved (such as a twist in the steam or leaf). The inspection revealed no apparent movement.
In November 2008, DTA personnel returned to the Hadley plant to inspect the replacement wicket gate. Visual examination indicated the gate is holding up well, with no visible signs of distress. DTA recommended that the water passageway and gate No. 6 be inspected annually.
— By Paul Ducheney, hydro superintendent, Holyoke Gas & Electric; Maureen Winters, corporate consultant, hydropower division, Devine Tarbell & Associates (now known as HDR|DTA); and Scott T. Smith, marketing manager, Weld Mart (now known as Peak Hydro Services).