PipeOpt: Pipeline Size Optimization
Developed by Steve Ainsworth, M.S., P.E.

An online version of PipeOpt is now available by clicking here.

Water Transmission Pipeline size selection is often based on “rule of thumb” maximum velocity constraints that are policy for a particular water agency. This method works well when power costs for pumping are not high enough to be required components in the pipeline size selection. With today’s emphasis on reducing power consumption, the old “rule of thumb” figures are not accurate enough. Pipeline size optimization is the process of including both installation cost and the time value of energy cost for a particular diameter of pipe, and selection of the pipe diameter that has the lowest life cycle costs if that diameter also works hydraulically in the system.

The energy requirement is computed from the energy required to overcome the friction losses caused by velocity and is a function of the flow and diameter. The total energy cost over the life span of the pipe is expressed in present worth dollars, and includes terms for both power rate and the inflation rate of power. The total life cycle cost for a particular pipe diameter is equal to the sum of the energy cost and installation cost. When the analysis is made for a series of flows and a series of pipe sizes, the engineer can build a table of the optimum velocity range for each pipe diameter. This velocity range can then be used with a water system network analysis program to select the optimum pipe sizes for water system network growth.

The variables to be considered when computing the optimum pipeline diameter are: construction cost, interest rate, power rate, beginning and ending friction coefficients, pump efficiency, design flow, initial flow, ultimate flow, pipe life span, yearly demand curve, and daily demand curves. The key advantage of the PipeOpt algorithm is that it allows solutions while analyzing all thirteen of these variables. It does not require any of the variables to be set to a constant value, and it always yields an actual pipe diameter as the optimum diameter. With the ability to process thirteen variables, the algorithm takes into account the fact that newly constructed pipes rarely are immediately used at design capacity. Also, most water systems have an annual peak as well as a daily peak. Given that energy consumed by pumping is related to velocity to the 2.85 power, energy costs are very sensitive to the demand peaks. Earlier algorithms required setting all flow variables to a constant.