Jackman Penstock Replacement
Public Service Company of New Hampshire
Manchester, NH
Public Service of New Hampshire’s (PSNH)
Jackman Station is a 3 MW hydroelectric
facility in Hillsborough New Hampshire with
approximately 5,000 feet (ft) of wood stave
penstock extending from the Franklin Pierce
Reservoir to a surge tank upstream of the
power house. This 7.5-ft diameter penstock
was made up of 1970s, 1954, and original
1926 vintage wood stave sections. The downstream sections of the wood penstock leaked significantly, caused icing problems during winter, and required continual maintenance.
After PSNH decided to replace the remaining wood stave penstock, Kleinschmidt initially developed a feasibility study with options for replacing the penstock with five pipe materials; wood stave, concrete, HDPE plastic, steel, and fiber reinforced polymer (FRP). The corrosion resistance and relative stiffness of FRP pipe meant it could be supported on the existing grade and half buried, rather than placed on saddles or completely covered in a buried trench. This design flexibility, combined with a longer projected service life, and lower relative material prices lead to a competitive total construction cost for the FRP alternative. This newer type of pipe material is a composite comprised of fiberglass and resin extended with sand to provide a stable thick walled and corrosion resistant pipe that is lighter weight than other thick-walled alternatives.
After PSNH selected FRP as the pipe material, Kleinschmidt provided a final design package for replacing the downstream 2,400 ft section of the wood stave pipe. The lower frictional resistance of FRP compared to other materials allowed the replacement pipe to be reduced to a 7 ft diameter without additional head loss. The pipe was half buried in the shallow trench of the existing penstock, requiring minimal excavation costs and environmental disturbance. Approximately 300 ft of the new penstock was supported above ground on steel saddles to accommodate topography and minimize impact to the wetlands that the penstock crossed. At the end of the project, it was found to be approximately seven times cheaper per ft to partially backfill the penstock than to support it above ground.
FRP pipe is still a relatively novel material in hydropower conduit applications and can offer cost and technical advantages under site conditions suitable for partially and fully buried pipe. Final design of this project required extensive research into the manufacturing and quality control of both filament-wound and centrifugally-cast sand core FRP pipe. Kleinschmidt has gained a wealth of knowledge in utilizing the advantages of FRP as a penstock material.
For more information contact Keith Martin, P.E.