Removing Uncertainty in Old HYDRO Power Plant

Pilgrim Admin


Arapuni Power Station is a hydro-electric power station on the Waikato River, in the North Island of New Zealand. It is owned and operated by Mercury Energy, and is the seventh and penultimate hydroelectric power station on the Waikato River. It is also the oldest currently generating, the first government-built, and the largest capacity single hydroelectric power station on the Waikato River.

During an upgrade of the 19MW Francis turbines to 24MW there was a discovery of a bolting issue. The problem was that the bolts working in their holes causing stressing and ovality of the bolt holes in the turbine to rotor coupling flanges. Pilgrim were contacted to help solve this issue.


PILGRIM selected its Radial Fit Bolts to solve the problem. The principle of the bolt is of a taper shank bolt drawn into a matching taper sleeve with parallel outer diameter which expands fractionally to produce a perfect interference condition in to the bolt hole. This condition, which “conventional” fitted bolts do not achieve, eliminates movement between mating components. Also with the expansion of the sleeve], exact flange alignment and concentricity is achieved. Axial clamp load is then created by a hydraulic puller. Once the puller draws the tapered bolt into its sleeve it is then used to tension the bolt once the nuts are fitted.

“We are truly impressed with the entire system, everything fitted perfectly and worked well. Everyone involved in the job were impressed with assembly times and methods of installation (I personally feel that it was the highlight of the reassembly) and we (ARAPUNI area Office) are sure that this is definitely the way to proceed for the remaining machines and we will be working very hard to gain approval from group office”

Tony Bevan – Technical Assistant


  • Retrofittable solution
  • Re-usable solution
  • No fretting
  • Elimination of seized bolts
  • Easy to install and remove.