Find a penstock leak by closing the turbine gate and watching the pressure gauge, which should hold 1 bar for every 10.2 meters of head, and repair only after closing the intake and draining the pipe. A 3 mm hole at 30 meters of head passes about 0.1 L/s as a jet at up to 24 m/s, fast enough to injure, and it grows because the same pressure that forces the water out tears at the edges.
Part of Micro-Hydro Turbine
A penstock leak is a hole, a failed joint or a crack in the pressurized pipe; it wastes water that has already earned its head and can wash out the ground that supports the pipe.
A Small Leak Wastes Power In Proportion And Then Gets Worse
Leak flow through a small hole is Q = Cd × A × √(2 × g × H). Taking a discharge coefficient Cd of 0.6 (a recalled handbook value for a sharp-edged hole; the pipe’s real hole may differ), at 30 m the ideal jet speed is √(2 × 9.81 × 30) = 24.3 m/s:
| Hole diameter | Leak flow | Share of a 3 L/s plant | Power lost at 883 W hydraulic |
|---|---|---|---|
| 1 mm | 0.011 L/s (0.7 L/min) | 0.4% | 3 W |
| 3 mm | 0.103 L/s, about 0.1 L/s (6.2 L/min) | 3.4% | 30 W |
| 6 mm | 0.41 L/s (24.7 L/min) | 13.7% | 121 W |
Doubling the hole’s width multiplies the leak by four, since area goes with the square of diameter. The loss is a straight share of the flow because hydro power is proportional to flow (Power Calculation). The power matters less than the damage: washed-out soil under a support, a thrust block that settles, or a joint that pulls apart.
Method: Find The Leak
Step 1: Pressure-hold test. Close the turbine gate slowly, so as not to surge the line (Penstock Design & Materials). The gauge at the powerhouse should read the static pressure of the head: at 30 m, 30 ÷ 10.2 = 2.9 bar. If the needle falls with everything shut, there is a leak, or the gate itself leaks.
Step 2: Walk the line in daylight. Look for wet ground, green growth in a dry month, a mist or hiss, and soil that has slumped. Start at joints, then at the lowest sections where pressure is highest.
Step 3: Mark the spot with a stake, then shut the water off before touching anything.
Method: Repair It
Step 1: Shut the intake gate, open the drain at the low point and the air valves at the high points, and let the pipe empty before any hand goes near it.
Step 2: Dry the area and decide whether to patch or replace. A pinhole in sound pipe can be patched. A hole in thin, corroded metal cannot hold a patch because the wall around it is weak: cut the section out and replace it.
Step 3: Repair by material.
- Steel. Weld a patch on sound metal, or bolt a rubber pad under a split clamp. Check the surrounding wall thickness first.
- HDPE. Fuse a new section in with butt fusion or electrofusion, which needs tooling (Penstock Design), or replace the section with barbed fittings and two clamps at each joint (HDPE Pipe).
- PVC. Cut out the damaged length and cement in a coupling and a new piece. Follow the maker’s cure time before pressurizing; the PVC Pipe page warns that pressurizing early blows the joint.
- Emergency. A strip of inner-tube rubber under two hose clamps seals a pinhole until the pipe can be drained for a proper repair. Treat it as temporary.
Step 4: Refill slowly with the air valves open and watch the repair as the pressure rises (Installation describes a flood test of this kind).
Step 5: Re-run the pressure-hold test and compare the reading with the 1 bar per 10.2 m rule.
Fix The Cause, Not Only The Hole
A leak at a joint often means the joint moved: a thrust block that settled, a bend with no anchor, or a plastic pipe that shortened in the cold. A 100 m length of HDPE changes by about 0.4 m over a 20 °C swing (the figure on the HDPE Pipe page), enough to pull a rigid joint. A leak at a low point in steel often means corrosion from the outside; check the neighbouring pipe and recoat it.
Common Mistakes
- Patching a pressurized pipe — the jet is the hazard, and the repair will not seat.
- Opening the drain before the high-point air valves — a thin pipe can be squeezed flat by outside air pressure.
- Repairing the hole and leaving the settled support.
Safety
A pinhole jet at 24 m/s can cut skin
Water leaving a small hole at 30 meters of head moves at about 24 m/s, and at higher heads faster, since speed goes with the square root of head. Never put a hand over a leak, and never look along it. Shut the intake, drain the pipe, and confirm it is empty before touching it. Dig a trench around a buried leak with shored walls, and do not work alone in it. Keep the intake gate within reach.
What’s Next
- Learn next: Maintenance & Troubleshooting for the weekly penstock inspection
- Combine with: Penstock Design & Materials to remove the cause