# Example 10: Steam pressure drop in pipe with valves and fittings comparing incompressible and compressible flow

Background

The examples provide a comparison of AioFlo results with published data from well known and respected references that are generally accessible to engineers. This will allow prospective AioFlo users to validate its accuracy against a range of typical calculations. The worked examples can also be run by new users as part of their learning process. To learn more about AioFlo click on "Home" in the menu above.

Description

Steam pressure drop and pipe sizing calculations are regularly required. This worked example calculates the steam pressure drop in a typical pipe with valves and fittings. The pressure drop is calculated using the incompressible and compressible models and the results compared.

Problem Reference

Flow of Fluids through Valves, Fittings, and Pipe. 1999, Crane Co., TP410M, Page 4-6, Example 4-10

Fluid Details

 Fluid : Steam @ 40 bar abs and 460°C Flow rate : 40 000 kg/h Phase : Gas (superheated) Density (upstream) : 12.3 kg/m³ Viscosity : 0.027 centipoise

Pipe Details

 Pipe Size : 6 inch Sch 80 Pipe ID : 146.4 mm Roughness : 0.046 mm Length : 120 m Fittings : 3 x 90° elbows r/d=1.5 1 x 6"x4" gate valve 1 x 6" Y-pattern globe valve

To be Calculated

Calculate the total pressure drop using the compressible and incompressible models

Comparison of Results

Calculated Item Reference AioFlo AioFlo
Fluid model Incompressible Incompressible Compressible
Flow Regime Turbulent Turbulent Turbulent
Pressure Drop (bar) 2.8 2.8 3.0

Discussion

In this example Crane have used the Darcy-Weisbach equation for incompressible flow, even though steam is clearly a compressible gas. This is in line with the rule of thumb discussed in Example 8 and Example 9. This rule of thumb states that for a gas flow where the incompressible model predicts a pressure drop of less than 10% of the upstream absolute pressure, the result will be sufficiently accurate for most purposes. The table of results above shows that the Crane and AioFlo results are identical when the incompressible model is used, but the end column shows that the pressure drop for the steam flow would be a bit higher if the Compressible model were used. The pressure drop calculated by the Incompressible model is only 7% of the upstream absolute pressure, and the difference between the two models is 6.7%.

When doing piping hydraulic calculations by hand there could be some justification for saving computational effort by using the incompressible model (especially when iterative solutions are involved), but with a computer solution like AioFlo it is just as easy to do the calculation rigorously using the compressible model.