形状阻力公式大全(形阻公式).docx

Flow in valves and fittings Resistance coefficient K, valves and fittings head loss and flow velocity | Pipe equivalent length L/D Pressure drop or head loss is proportional to the velocity in valves or fittings. For the most engineering practices it can be assumed that pressure drop or head loss due to flow of fluids in turbulent range through valves and fittings is proportional to square of velocity. To avoid expensive testing of every valves and every fittings that are installed on pipeline, the experimental data are used. For that purpose resistance coefficient K, equivalent length L/D and flow coefficient Cv, Kv are used. These values are available from different sources like tables and diagrams from different authors and from valves manufacturers as well. Kinetic energy, which is represented as head due to velocity is generated from static head and increase or decrease in velocity directly is proportional with static head loss or gain. Velocity head is: where is: hL - head loss; v - velocity; gn - acceleration of gravity; The number of velocity heads lost due to resistance of valves and fittings is: where is: hL - head loss; K - resistance coefficient; v - velocity; gn - acceleration of gravity; The head loss due to resistance in valves and fittings are always associated with the diameter on which velocity occurs. The resistance coefficient K is considered to be constant for any defined valves or fittings in all flow conditions, as the head loss due to friction is minor compared to the head loss due to change in direction of flow, obstructions and sudden or gradual changes in cross section and shape of flow. Head loss due to friction in straight pipe is expressed by the Darcy equation: where is: hL - head loss; f - friction factor; L - length; D - internal diameter; v - velocity; gn - acceleration of gravity; It follows that: where is: K - resistance coefficient; f - friction factor; L - lengt; D - internal diameter; The ratio L/D is equivalent length in pipe