Pressure Drop in Pipes - Detailed Discussion

Depending on the pipe friction model selected, the calculation of pressure drop in a pipe differs.

Darcy-Weisbach Loss Model

Note: The pressure drop for compressible flow depends on several equations, as discussed in Review of Compressible Flow Theory. The effect of pipe friction is taken into account as part of the full compressible method. The Darcy-Weisbach pressure loss equation for incompressible flow is shown below for simplicity.

AFT xStream utilizes the Darcy-Weisbach loss model to relate the Darcy Friction Factor, the pipe geometry, fluid density, and fluid velocity to pressure drop in the pipe. The Darcy Friction Factor differs from the Fanning Friction Factor by a factor of 4.

This model requires the calculation of a friction factor, as described below.

• Roughness-Based Methods - This method calculates the friction factor based on the roughness of the pipe wall. Different equations are used based on flow regime - laminar flow uses the standard laminar relationship, while turbulent flow uses the implicit Colebrook-White equation. In the transition range between laminar and turbulent flow, a linearly interpolated value is used. The default transition Reynold's Numbers can be modified in the Environmental Properties panel of Analysis Setup.

• Absolute Roughness (default) - The absolute average roughness height ε is specified directly.

• Relative Roughness - The roughness is specified as a ratio ε/D.

• Hydraulically Smooth - The ratio ε/D is set equal to zero.

• Explicit Friction Factor - The friction factor to be used in the Darcy-Weisbach equation is specified directly

Frictionless

The pipe will not have any pressure drop across it. This is inherently unrealistic behavior, but can be useful for troubleshooting purposes.

Design Factors

In each pipe you can specify a Design Factor for the pipe friction. This is a multiplier that is applied to the friction factor calculated with the preceding methods.