The Connector - Spring 2008 - (Page 26)

Spring 2008 RWAU Advanced Distribution Chuck Jeffs, Compliance Technician What relationship does head loss have with pipe size and the flow of the water? 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 flows increase 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. 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 of (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 fi nd 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. What would be the purpose of installing two different sized pressure reducing valves in a parallel manifold? See “Equal Flow Volume” chart. Head Loss Chart Design Flow GPM CFS .25 .05 .50 .1 100 .2 150 .33 200 .45 300 .66 400 .89 500 1.1 600 1.3 700 1.5 800 1.78 900 2.0 PVC pipe size and Head Loss per 100 feet 2" 3" 4" 6" 8" 1.28 .18 .04 4.65 .65 .16 .02 16.8 2.33 .57 .08 35.7 4.93 1.23 .17 .04 60.6 8.36 2.02 .29 .07 99.2 17.9 4.37 .62 .15 30.6 7.52 1.03 .25 46.1 11.3 1.36 .39 64.4 15.8 2.2 .5 21.1 2.92 .72 26.8 3.74 .89 33.4 4.75 1.16 The graphic indicates that for a pressure control valve to work properly it needs 20–25 feet per second, maximum flow through it; where a normal maximum flow through a water line is 5–7 feet per second. This indicates that the normal maximum flow through a 12–inch pipeline would be approximately 1,761 gallons per minute. Then gal working the formula for area wo 1000 1200 to figure diameter, it indicates that the control valve should tha 2.23 2.67 be a 6-inch valve. Cavitation is one of the things that have a real impact thi on a pressure control valves. The two situations that cause cavitation are dropping too cav 5.66 8.04 high of a pressure across the hig 1.4 1.96 valve or flowing too small of val The Connector 26

Table of Contents Feed for the Digital Edition of The Connector - Spring 2008

The Connector - Spring 2008 Table of Contents President's Message Executive Director's Comments Letters From Readers Legislative Update RWAU Board and Staff Rural Water News Bees on the Pipeline, Jim Watts How Important are our Public Works People? Curtis Ludvigson A "What" Protection Plan? Erin Borger Meter Reading by Radio Communication, Terry Smith Advanced Distribution, Chuck Jeffs Where in Utah?

The Connector - Spring 2008

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