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Minor Losses in Pipe Fittings

The water supply network of a municipality is a major part of its infrastructure. Each network consists of a system of inter-connective pipes creating a series of hydraulic connections that city engineers must constantly analyze. Pressure reduction results from frictional losses and minor losses. Maintaining the water pressure, or head, is essential in keeping the flow going to each residence and business that depends on a clean and adequate water supply.
  1. Minor Losses

    • Any incompressible fluid flowing through a pipe encounters frictional forces resulting from the roughness of the interior of the pipe and its length. Friction within the fluid itself converts kinetic energy into thermal energy that elevates the liquid's temperature. Other pressure losses occur in pipelines due to bend elbows, valves, joints, sudden contractions or expansions in the pipe diameter, and entrances and exits from the system cause minor pressure losses and slow down the flow.

    Head Loss

    • The pressure reduction of fluid flow is directly proportional to the pipe length. As the flow rate increases, the pressure will drop. The total head is the summation of the elevation head, velocity head and the pressure head of a fluid in motion through a pipe.

      A fluid contains three forms of energy: pressure head, kinetic and potential energy. The pressure head refers to the vertical distance or energy of the water above any specific point along the pipeline. It is measured in height, feet or pressure in pounds per square inch (psi). Multiply the psi by 2.31 to convert the head loss to to feet. This information can be determined with a pressure gauge.

      The measure of the reduction in the total head of a liquid as it moves through a system is head loss. It results from the friction between the fluid and the pipe walls and between the fluid molecules and minor losses due to the fittings and direction changes of the fluid and valves of the system as the liquid flows along the pipe.

    Calculating Minor Losses

    • Minor losses, taken individually, are often much less than the pressure loss due to friction, but they add up and in total, they disperse more pressure than friction losses due to pipe roughness. Loss of pressure due to friction mathematically adds length to the pipe. There are two methods to approximate the minor losses. One is the method of equivalent lengths and the other is the method of loss coefficients.

      To use the method of equivalent lengths, refer to the manufacturer's table to convert each valve and fitting into an equivalent length of straight pipe that would have the same head loss for the same discharge flow rate. Add this length to the total pipe length. The resulting value can be substituted into the Darcy equation, which relates the head loss due to friction along a length of pipe to the average velocity of the fluid flow.

    Local Losses

    • All pipe networks are subject to minor losses or local energy losses in addition to friction energy losses. Any hardware additions or fittings to the pipe between the entrance and the exit can change the pipe flow. The losses depend primarily on the layout of the pipe system and the geometry of the pipes themselves.


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