This typical example illustrates how the losses in a penstock are affected by flow rate.

**Penstock data:**

1,000 feet long

2 inches internal diameter

100 feet of vertical fall

We used a pipe friction calculator to determine the net head (shown in green) over the range of possible flows. This line also represents the pipe efficiency (in percent). We then used the rule below to estimate the available power in watts. This is shown in blue.

Power (W) = Net Head (ft.) × Flow (gpm) ÷ 12

At this site—with 100 feet of head and the valves closed—the pressure gauge should read 43 psi. The flow is controlled by the nozzle sizes fitted in the turbine. If all constrictions are removed so that the pipe runs “full-bore,” there will be no pressure on the gauge, and we will see about 70 gpm flow, but no power.

Maximum power for this penstock is achieved by using nozzles that reduce flow to 40 gpm. This coincides with a net head of 66 feet (the gauge will read 28 psi).

However, this 2-inch-diameter pipe may not be the best choice for a 40 gpm flow because the efficiency is only 66%. If 40 gpm is available for enough of the year then it’s worth considering the cost of larger pipe; this diameter is a good choice for a site with 20 to 30 gpm flows. Observe how the efficiency is much higher at these slightly lower flow rates.

Your choice of pipe diameter will be governed by the sizes that are available and by how much the pipe costs relative to the pressure loss it incurs. In most cases, a target of 80% to 90% efficiency is cost-effective. But if the pipe is the major cost, or you wish to benefit from short-term high flows, then 66% efficiency could be the right choice.