Pepperell Rehab Progress

    Rehabilitation of a replacement for the T-2 turbine-generator set started with delivery of the used turbine and gatecase to the SRHOCO machine shop and delivery of the used generator to Pepperell (see picture of generator delivery) in February 2004.  The first task was to take the gatecase apart so that best wicket gate, link, and steady vane could be used as patterns so that new parts could be poured at a foundry.  In addition, new wicket gate pins, bolts and gate control ring operator arms could be machined in the SRHOCO shop.

T-2 replacement gatecase disassembly

Down to the bottom plate of T-2 gatecase                   Machining new wicket gate operators

The next task was to find a shaft for the restored T-2 turbine-generator set.  Fortunately, a shaft was found in the mill building where SRHOCO's shop is located that once powered all the drive shafts for the textile mill. It was rigged out, turned down on SRHOCO's giant lathe, shortened and a new coupling added to one end.

Rigging the old drive shaft from the basement of the mill

The 60' long steel shaft being machined to proper diameter and length in SHROCO's shop

    At the same time, new trashracks were fabricated at the shop and stockpiled for installation at the T-2 intake when the crane would be on site to install the new Atlas Polar trash rake ordered from Canada.  Next, the Pepperell power station had to be cleaned out and the old slate control panels removed to make way for new switchgear and to give room to laydown the T-3 turbine and generator while the broken T-3 runner was replaced by a runner fabricated by the American Hydro turbine design group in York, PA.

    First, the T-3 generator was uncoupled from the turbine shaft, so that the shaft and flywheel could be set aside in the turbine pit. The generator was then lifted off of its pedestal and placed on the T-2 generator mount for short-term storage. Then, the T-3 wicket gatecase was unbolted from its embedment and carefully lifted up and over the T-3 turbine shaft and set down in the turbine pit for later refurbishment of gate links with new bushings.  The rigging was done through the generator stator hole from the bridge crane and it was set in the powerhouse floor. The following pictures show the week-long rigging job as it progressed.

T-3 gatecase being lifted over turbine shaft and T-3 runner being rigged through generator mount

By design, the damaged turbine fits through the T-3 stator, as will the American Hydro replacement runner

    SRHOCO will now take the final measurements of the existing turbine so that American Hydro can fabricate the new runner with the exact dimensions required to use the gatecase that is being rebuilt in the shop.  The turbine is scheduled for delivery in mid October. In the meantime, the refurbished gatecase is being re-assembled.

    For the last 90 years, the 25 foot deep trashracks in the forebay have been raked by hand with equipment that is able to reach no deeper than 8 to 10 feet.  The result is that within a few weeks of being clean during the annual dewatering of the penstock and forebay, two-thirds of the racks are plugged with trash. This puts a premium on keeping the top portion of the racks very clean or else there is between 2 and 7 feet of head loss caused by the material plugging the racks.  Here is the process that prevailed in the past.

         Raking rack the old fashioned way with a 10' rake    Annual cleaning clears plugged racks for a few weeks

A new Atlas Polar trash rack being installed that has a 30 foot reach to keep the racks clear from bottom to top. The automatic rake will pay for itself in only a few leaf seasons which have no head loss due to plugged racks.

        The new rake spans all three sets of racks          The rake arm being calibrated to reach the bottom

It cost nearly $100,000 to replace the old turbines you see being returned to the Pepperell Hydro site. The old runner will be re-welded and place in storage as an emergency spare part.

Turbines arrive from American Hydro in York, PA

The new trashracks were swung into place by the crane hired at the time of the delivery of new T-3 turbine.  The forebay was dewatered down to the invert of the penstock, a beam was placed ready for welding in the intake gate slot of T-2, and the new racks were stacked up in the forebay ready for installation.

Forebay drained to weld in new support beams           New rack panel being lowered into the forebay

Once the support beam has been welded into the beam pocket, the racks will be raised and set into their positions and then welded to the support beam in order to stiffen the whole assembly to withstand the pressure of the trash rake.

Bill Fay prepares to install the support beams. There will also be a steel plate extender installed above the trashracks to permit the automatic trash rake to sweep the leaves up onto the deck without any falling back into the forebay.

Once the support beams were welded into place and hydraulic concrete packed into to fill the beam pockets, the individual trashrack panels were rigged into place and then welded to the support beams to increase stability against the force created by constant pressure of the automatic trash rake.

     New rack sections are pulled up into place                    Then welded to the support beans behind      

A steel plate will be installed at the top of the trashracks so that the blades of the trash rake can slide smoothly off of the trashracks and up over the concrete section above the racks without dropping any leaves or branches. Today the rake deposits the trash on the deck where you can see the wooden cribbing that keep the leaves from dropping behind the racks if they were to fall into the holes where the gates and gate lifters are installed.  Next year a conveyor will be installed to carry the trash off the deck, depositing it in a dumpster, or a hydraulic sluice will be installed to carry the trash back into the river just above the tailrace.

Pepperell Hydro's new trashracks just before re-watering of the forebay. T-2 is now ready to be installed.

On December 1st, the new American Hydro Francis turbine arrived from York, PA.  Is was off loaded and rigged into the powerhouse, ready to be reassembled in the wicket gate case of T-3 (see its removal above). Now the sequence must be reversed in order to reassemble the new runner and reinstall the generator with new controls and switchgear.

Old and new T-3 turbines are delivered to site        The new Francis turbine ready for installation

Compare the design of the new turbine's blades with the design and condition of the Leffel turbine that Pepperell Hydro is replacing.  Gone is the bypass channel at the top of the turbine that Leffel incorporated to reduce turbulence in the draft tube.

Broken blades of old Leffel turbine    American Hydro blades are long, tapered higher up shaft

Instead of the bypass channel which gives no power, the new turbine's blades are much longer and therefore have a larger leading edge, providing greater lift and therefore efficiency.  The water is able to pass out of the turbine not only through the bucket area, but all along the rear edge of the blade due to the extended cut high up the shaft on the trailing edge of the turbine blade. There is very little "bucket" shape at the bottom of the American Hydro turbine blades so water must swirl out of the turbine into the draft tube without needing the bypass flow to reduce turbulence.

By January the new T-1 turbine had been lowered into the flume, fitted into its wicket gate case and the vertical shaft re-coupled.  The crew was cleaning the generator coils and windings when the great snow storm of 2005 arrived.  The deep snows quickly turned to ice on the dam and long penstock, making a fifth of a mile long icicle running into the Pepperell Hydro powerhouse.

Production was curtailed for 2 days to wire up the new control equipment, creating an icy image

The new control equipment and switchgear were delivered in January. Thus, the repositioning of all the control cabinets required installing new cable trays and pulling new circuits from the three existing generators. In addition, since the design includes new power packs in place of one of the governors and is to motorize the two other Woodward governors, new control circuits have to be installed throughout the powerhouse.  The SRHOCO crew has installed the new wiring and is at work installing the control cabinets.  The T-1 generator is now back on its pedestal and ready for testing as soon as the new controls are complete.

Pulling new wiring throughout the Pepperell powerhouse to modernize control circuits

T-2 is a much larger undertaking than simply replacing the old runner with a modern efficient runner and automating controls. The T-2 turbine/generator has not operated since the 1970's and was cannibalized for parts over the years. Breaking apart the old runner took several weeks, but finally the scrap runners were rigged out of the flume. Next the pedestal on which the gate case had been mounted had to be removed and poured at a lower level. The rehabbed

Leffel runner (before rehab) stored in flume over winter       Base of old gatecase being formed to pour new foundation

 Leffel B runner's wicket gate case had been completely rebuilt, all with new wicket gates, pins, links and shift ring in SRHOCO's shop over the last winter.  The Westinghouse generator had been scraped for parts, so a near-new Chinese generator with less than 10 years of operating use, was rigged out of a Vermont powerhouse and having the same speed, will be fitted to replace the T-2 generator.  Ingeniously, the spider from the old Westinghouse generator was saved and will serve to support the Chinese generator when it is assembled.

Burning the spider off of the Westinghouse shaft           Spider and other embedded parts ready for concrete

However, because it stands much higher than the former Westinghouse generator, its pedestal also had to be removed so the generator will sit closer to the floor level in order to make clearance room for the powerhouse gantry crane to move equipment overhead. The powerhouse was a jumble of embedded parts awaiting the concrete truck. 

Westinghouse stator rigged out of the powerhouse awaiting a crane to take it away for scrape

Largest among these is the newly refurbished Leffel B runner.  The crew welded its blades, polished and repainted it before lowering it into the flume.  Rigging the runner, gate case and especially the generator into the powerhouse has been a major challenge because the weight of the equipment exceeds the rating of the powerhouse crane. Most of the parts will therefore be assembled in place.  SRHOCO built a balcony by the door (see photo above) where equipment can be transferred from an external crane to the gantry crane in the powerhouse.

Ken Smith and sons inspect the runner before it is lowered into the T-2 turbine pit

All of the gate case parts have previously been lowered into the pressure flume for assemble in situ because the diameter of the gate case is larger than the diameter of the generator foundation hole.  Once securely suspended in the draft tube, the wicket gatecase was reassembled.  The runner can then be raised and blocked off inside the gatecase. The crew then turns to assembling the generator over the turbine flume and installing the shaft and intermediate shaft.

T-2 refurbished Leffel B gatecase  being assembled in the flume before the shift-ring and operator as installed

Bill Fay after cutting the shaft to install a new coupling on T-2 rotor

T-2's generator is refurbished before the rotor is installed

T-2's rotor being lowered into the stator of the new generator

The shaft of the new generator was shortened to fit with the Leffel B runner and gatecase installed in the turbine flume. By mid-September 2005, most of the components of the new T/G set were in place awaiting alignment. The oil cooling system was assembled and the oil cooling system checked to be ready for testing the T-2 generator.

Late night landing of the LEM on the T-2 rotor shaft, ready for alignment

Lining up the gearing for the oil pump

There had been very little flow in the Nashua River during the 2005 summer.  Finally, rain arrived on October 11th so that testing of T-3's new runner and switchgear was proceeding.  On October 13th, flows increased such that T-1 was conditioned and returned to service. That night, a fault on the National Grid 69 kVA line caused both generators to trip, T-3 shutdown properly, but T-1 while off line ran in overspeed all night -- eventually causing the T-1 generator bearing to melt, setting a powerhouse fire that burned out the windows and firemen cut 3 holes in the roof.

On October 13, 2005, Pepperell's T-1 generator caught fire, melting its thrust bearing

The T-1 generator dropped down when its bearing melted, causing damage to both the generator pole windings and allowing the runner to cut a hole in the bottom of its gatecase making rigging the runner back up into place difficult.  The T-1 generator had to be completely rebuilt and rewound by Leppert-Nutmeg in Bloomfield, Connecticut.

Rigging T-1 generator out of boarded up powerhouse to send for rewind/rebuild by Leppert-Nutmeg

The Pepperell Hydro powerhouse was boarded up all winter in 2006.  Crews replaced the roof during the fall rains and slowly over the winter months, new windows were hung and re-glazed, powerhouse walls painted, and a new roof installed. The powerhouse was refurbished and all new wires pulled during the winter, ready for 2006's spring flows.

Pepperell Hydro powerhouse benefits from the post fire facelift: new windows, roof and substation rehab

Powerhouse interior with new windows, roof and controls, waiting for T-1 generator's return

With the electrical equipment reconnected  to the grid by March 2006, T-3 was put back on line.  Much of T-1's generator had to be re-manufactured by Leppert-Nutmeg. The new generator was returned to Pepperell Hydro's powerhouse near the end of April 2006.

Swinging the rebuilt/rewound T-1 rotor back into powerhouse

Slipping the new rotor into place in refurbished T-1 stator

With T-1 back, Pepperell Hydro can begin to test the new switchgear and automatic PLC that will control each of the three units to provide continuous run-of-river operations, putting an end to cycling of the headpond. This change to an automatic run-of-river operating mode has qualified Pepperell Hydro to sell Class I RECs in Connecticut.

Threading the shaft in through the spider and T-1 generator's steady bearing

The three reconditioned turbine-generator units will bring Pepperell Hydro back to its original 1,920 kW capacity. However, now it will be capable of automatic operations, and have environmental mitigations such as ramping flows up and down to eliminate surging flows downstream, stable headpond elevation eliminating the cycling of the head pond, and installation of a downstream herring passage facility to help restore anadromous fish to the Nashua River.

T-3 new runner commissioning shows capacity right at its 640 kW specification

    In the spring, the Nashua River continued to flow in near flood stage until the beginning of July. It was a phenomenal spring run which allowed the two turbine-generators that had been rehabbed to operate much longer than normal, and Pepperell Hydro was able to collect business interruption insurance for T-2 until it was ready to put put back on line following the repair of damage from the fire. Here is proof of the overflow at the dam and high water in the forebay in June 2006.

The wood stave penstock was flooded all spring and water spilled over the forebay overflow continuously.

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