Pipe Pressure Drop & Flow Calculations


Change in elevation, friction, fittings, length and bends are some of the items that need to be considered when doing flow calculations and pipe pressure drops. The physical properties of each of these items need to be factored into the many calculations necessary to calculate end pressure.

Friction Loss

  • There is friction between the wall of the pipe and the fluid. Additional friction is added to the flow as the liquid moves through valves, bends and various components. Other factors affecting friction include the density and viscosity of the fluid, changes in temperature and the length, diameter and inner surface roughness of the pipe. To calculate the friction factor it is necessary to know the fluid’s viscosity and density properties, the pipe diameter and the pipe’s roughness properties. These items are used to calculate the Reynold’s number. The friction factor is then calculated using the Colebrook-White equation. Once the friction factor is known, the friction loss can be calculated using the Darcy-Weisbach equation.

Pipe Fitting Loss Calculation

  • Pressure drops at fittings, valves and bends because of a disruption in the flow. The loss caused by a specific type of fitting is measured by running experiments with various types of fluid. The result is a K factor, the local loss coefficient, that is then used to calculate the loss caused by the fitting.

Pipe Component Loss Calculation

  • Pipe components such as a chiller or heat exchanger also can affect the flow pressure. Typically, the pressure drop varies with the liquid’s flow rate and most manufacturers supply a performance curve illustrating the products pressure loss characteristics at various rates of flow.

Other Factors That Affect Flow Rate

  • Other factors than can affect the flow rate are the pipe’s change in elevation. There will be pressure loss if the ending point is higher than the starting point and the reverse if the ending point is lower. The use of pumps also affects the flow rate. Pump performance is available from the manufacturer and must be factored into the pressure drop and flow rate calculations.

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