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A Pacific Northwest Extension Publication MAKING THE MOST OF IRRIGATIONSizing Irrigation Mainlines and FittingsPNW 290 Do you want to print a copy? We recommend downloading the print version (34 K). First download the free Adobe Reader, if you don't already have it. Cost categories When designing or retrofitting an irrigation system, one of the key decisions is picking the proper size pipes and fittings for the system. The best pipe size or fitting is not always the one with the lowest initial cost. The important consideration is the lowest cost of ownership. The objective is to minimize the sum of capital, pumping, maintenance, and energy costs during the life of the system. Cost categoriesIn general, costs can be classified under two categories. The first is capital or fixed cost of the pipe and fittings. This cost is determined by the initial installed purchase price of the pipe or fitting and the number of years of service that it is projected to last (usually 20 years). The annual cost of different-sized pipe or fittings is found by amortizing the purchase price, using the interest rate and service life. This amortization is similar to the repayments that are made on a loan. In fact, if all the money to purchase the pipe or fittings is borrowed, the annual cost will be the loan repayment. The second cost category is the operating or variable cost. These costs depend on the number of hours the irrigation system is operated and the expected friction loss in the pipeline and fittings. Smaller pipelines and fittings will have a greater friction loss and, hence, a higher operating cost than larger-diameter pipes and fittings. The total operating cost will depend on the number of hours the system is operated, the total friction loss, and the present and future cost per unit (kWh) of electrical energy. The friction loss depends on pipe size, pipe material, pipe length, and flow rate in gallons per minute. An economic analysis of a typical system is illustrated in Figure 1. This example assumes the following conditions: Flow rate (in gallons per minute) = 600 aThe cost per kWh for electricity is the average over the entire irrigation season and includes applicable demand charges prorated proportionately over all the kWh used. blnclude "equivalent length of pipe" for fittings from Table 3. The 8-inch pipe offers the lowest total annual operating cost. It is obvious that an undersized pipe has very high energy costs. If the pipeline is too small, the fixed costs may be higher than for the next larger pipe size due to the requirement and cost of a larger pump.
Friction losses in pipesAs a general rule of thumb, a water velocity
in the pipe of 5 feet per second is the maximum allowable in
most circumstances except suction pipes on centrifugal pumps,
which should be between 2 and 3 feet per second. Table 1 shows
the flow rate for various size pipelines when the velocity is
5 feet per second.
There also are differences between types of pipes. Smooth pipe has less friction loss and, hence, a lower operating cost than rough pipes. Plastic pipe, such as PVC, is the smoothest, followed by aluminum, steel, and concrete, in that order. Therefore, pipeline cost comparisons should be based on total operating costs rather than on only initial installed purchase price. Table 2 shows some typical friction losses in commonly used pipe; it can be used for estimating operating costs for pipelines and fittings. More precise figures from manufacturers' specifications should be used for design purposes.
Friction losses in valves and fittingsThe least cost of operating a system also extends to selecting the proper size valves and fittings. The economic analysis is similar to that of the pipeline. The friction loss in a fitting can be estimated by a technique called "equivalent length of steel pipe." To do so, the friction loss through the fitting is equated to the friction loss in an equivalent length of straight steel pipe of the same diameter as the fitting. Table 3 provides factors needed to calculate equivalent lengths of several types of fittings and valves. It generally is best to select fittings and valves of the same size as the main pipeline.
Estimating fixed costsA simple method of comparing the fixed costs of two different pipe sizes or two different fittings or valves is to compare the relative total costs of each. The fixed costs can be estimated by multiplying the purchase price of the pipe or fitting by the appropriate factor from Table 4. This gives the annual ownership cost. 
Estimating operating costsThe annual operating cost of a fitting or pipeline can be estimated with the following equation:
where $ = annual operating
cost For example, on the discharge of a centrifugal pump a butterfly valve is to be installed. The flow rate is 1,000 gpm, the pump discharge size is 6-inch, and the steel pipeline size is 10-inch. What size valve is most economical? Calculate the operating costs for each size valve. Assume 12 percent interest, 2,000 hours of operation, 70 percent pumping plant efficiency, and $0.04/kWh energy costs. Using the equation above, Table 5 can be generated. The lowest operating cost is for the 10-inch valve. A similar analysis can be made for pipe using the cost per 100 feet of pipe.
SummarySelecting pipeline sizes and fittings by this method provides the minimum cost for an irrigation system. It should be noted that if a system is not operated a large number of hours per year, smaller-size pipelines and fittings and higher energy costs are somewhat more attractive. Smaller-size pipelines and fittings also are more attractive with higher interest rates. One factor not easily accounted for, however, is that with increasing energy costs, larger-size pipes and fittings will become more economical with time. However, the 5-feet-per-second pipeline velocity rule is applicable over a very wide range of pipeline prices, interest rates, and annual operating hours. This lends a certain amount of assurance that a properly sized pipeline under today's economic conditions will tend to be properly sized even under future economic conditions that may change a great deal.
Prepared by Prepared originally for the Bonneville Power Administration by Walter L. Trimmer, former Extension irrigation specialist, and Hugh J. Hansen, Extension agricultural engineer emeritus; Oregon State University. Reprinted October 1997. |
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