Gross head: 100 feet
Pipeline length: 500 feet
Acceptable head loss: 10 to 15 percent (10–15 feet)
Design flow: 100 gpm
To determine what size pipe would be best, look up your design flow (100 gpm) in the head loss chart on page 44. In this example, the maximum acceptable head loss is 10 to 15 feet, which means we cannot exceed 3 feet of loss for every 100 feet of our 500-foot pipeline. Reading down the 100-gpm column, we find that a 3-inch pipeline would have a head loss of 2.33 feet per 100 feet of pipe—within our limits.
To determine total head loss, multiply 2.33 feet times 5 (for 500-foot pipeline), which equals 11.65 feet. To calculate net head, subtract the total head loss from the gross head (100 feet minus 11.65 feet). This gives us a net head of 88.35 feet.
Note the huge difference in head loss as pipe diameter gets smaller. Using a 2-inch pipeline, head loss for this example would be 16.8 feet per 100 feet, with a total head loss of 84 feet. Net head for this example would be 100 feet minus 84 feet, and result in only 16 feet of net head! This example shows how incorrectly sized pipelines can absolutely cripple a hydro system.
Choosing a 4-inch pipe would result in less head loss than 3-inch pipe, and deliver more power to the turbine, but the performance improvement is not sufficient to justify the added cost. Your turbine manufacturer should be well versed in measuring head losses, and can be an excellent resource for pipe diameter recommendations.