A Pressure Reducing Valve (or Pressure Relief Valve) functions to divert the water and pressure from the turbine in the case of an untoward incident. It is designed to open instantly when tripped, which prevents serious pressure spikes in the penstock. The photos below from one of our projects show various boring bar setups used in the rebuild of these valves.
We also bolted in stainless liner plates below the seat ring of a Pressure Reducing Valve (PVR) where the wall of the valve had been largely destroyed by the forces of cavitation. This part of the valve was mostly spherical, and so it was necessary to build a special hydraulic drill that would adjust to all angles of elevation and rotation in order to get the holes radial to the center point of the sphere.