Planning Your Own Hydro System
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Planning Your Hydro System
How the Penstock Affects Head Pressure
Measuring Pipeline (Penstock) Length
The length of your pipeline (also known as the penstock) has a major influence on both the cost and efficiency of your system, as we'll discuss below. The measurement is easy, though. Simply run a tape measure between your intake and turbine locations, following the route you'll use for your pipeline.
Computing Net Head
In the section Measuring Head, you measured Gross Head – the true vertical distance from intake to turbine – and the resulting pressure at the bottom when no water is flowing. Net Head, on the other hand, is the pressure at the bottom of your pipeline when water is actually flowing to your turbine, and will always be less than the Gross Head you measured due to energy losses within the pipeline. Longer pipelines and smaller diameters create greater friction.
Net Head is a far more useful measurement than Gross Head and, along with Design Flow, is used to determine hydro system components and power output. This section will show you the basics for determining pipeline size and Net Head, but we suggest you work with your turbine supplier to finalize your pipeline specifications.
Head Loss
Head Loss refers to the loss of water power due to friction within the pipeline. Although a given pipe diameter may be sufficient to carry all the Design Flow, the sides, joints and bends of the pipe create drag as the water passes by, slowing it down. The effect is the same as lowering the Head; there will be less water pressure at the turbine.
Note that the effects of Head Loss cannot be measured unless the water is flowing. A pressure meter at the bottom of even the smallest pipe will read full PSI when the water is static in the pipe. But as the water flows, the friction within the pipe reduces the velocity of the water coming out the bottom. Greater water flow increases friction further.
Larger pipes create less friction, delivering more power to the turbine. But larger pipelines are also more expensive, so there is invariably a tradeoff between Head Loss and system cost. A good rule of thumb is to size your pipe so that not more than 10% to 15% of the Gross (total) Head is lost as pipeline friction.
Here is an example of how to determine the appropriate pipe (penstock) size. The chart below shows Head Loss for various sizes of PVC pipe at different flow rates.
Head Loss Chart
|
Design Flow |
||||||||||||||
|
GPM |
.25 |
.50 |
100 |
150 |
200 |
300 |
400 |
500 |
600 |
700 |
800 |
900 |
1000 |
1200 |
|
CFS |
.05 |
.1 |
.2 |
.33 |
.45 |
.66 |
.89 |
1.1 |
1.3 |
1.5 |
1.78 |
2.0 |
2.23 |
2.67 |
|
PVC pipe size and Head Loss per 100 feet |
||||||||||||||
|
2” |
1.28 |
4.65 |
16.8 |
35.7 |
60.6 |
99.2 |
|
|
|
|
|
|
|
|
|
3” |
.18 |
.65 |
2.33 |
4.93 |
8.36 |
17.9 |
30.6 |
46.1 |
64.4 |
|
|
|
|
|
|
4” |
.04 |
.16 |
.57 |
1.23 |
2.02 |
4.37 |
7.52 |
11.3 |
15.8 |
21.1 |
26.8 |
33.4 |
|
|
|
6” |
|
.02 |
.08 |
.17 |
.29 |
.62 |
1.03 |
1.36 |
2.2 |
2.92 |
3.74 |
4.75 |
5.66 |
8.04 |
|
8” |
|
|
|
.04 |
.07 |
.15 |
.25 |
.39 |
.5 |
.72 |
.89 |
1.16 |
1.4 |
1.96 |
Let's use an example site with these characteristics:
- Gross Head = 100 feet
- Pipeline length = 400 feet
- Acceptable Head Loss = 10% -15% (10-15 feet)
- Design Flow = 200 Gallons per minute
To determine what size pipe would be best, look up your Design Flow (200 GPM) in the Head Loss Chart above. Our maximum acceptable Head Loss is 15 feet (15% of our 100-foot Gross Head), which means we cannot exceed 3.75 feet loss for every 100 feet of our 400-foot pipeline. Reading down the column under 200 GPM, we find that a four-inch pipe would cause a loss of 2.02 feet per 100 feet – within our limits.
Using a four-inch pipeline, Head Loss for this example would be:
- Head Loss = 2.02 feet (per 100 feet) x 4 = 8.08 feet
Therefore, Net Head for this example would be:
- Net Head = 100 feet – 8.08 feet = 91.92 feet
Note the significant difference in Head Loss between 3-inch and 4-inch pipes. Likewise, a 6” or 8” pipe would cause even less Head Loss and deliver more power to the turbine, but the performance improvement may not be sufficient to justify the added cost.
Keep in mind that these Head Loss computations assume a straight pipe; they do not take into account bends in your pipeline that can rob significant power from your water. Your turbine manufacturer should be well versed in measuring head losses, and can be an excellent resource for pipe diameter recommendations.