Problem Statement

How is the pressure drop across a swage fitting in a pipe segment calculated in Aspen HYSYS?

Solution

In Aspen HYSYS, the pressure drop computed for swage fittings in pipe segments is calculated with the equations shown in the Pipe Segment | Rating Tab | Swage Fittings section of the Help Menu.

First, with the mass flowrate flowing through the pipe (mass in = mass out) and the mass densities at the inlet and outlet of the pipe segment, Aspen HYSYS computes volumetric flowrates. Then, it uses the pipe diameters to get areas and from there calculate velocities. Once Beta, K, in/out densities and in/out velocities are known, Aspen HYSYS calculates the swage fitting pressure drop. Depending on the case, the equations for either a reducer or enlarger will be used.

In the attached example simulation file, a swage fitting is used to model a reducer. A pipe segment has been set up with a feed stream that consists of pure water at 25 C and 116 psia. It consists of two pipe straight lengths with a 45-degree swage in between to account for the different inner pipe diameters of the inlet and outlet segments, 2.9 in and 1.28 in, respectively. Both pipe segments straight lengths are set to1E-6 m, so that the pressure drop calculation carried out for the pipe segment only accounts for the swage fitting pressure drop.

The pressure drop computed by Aspen HYSYS results in 22.85 psia.

Also, there is an Excel spreadsheet attached which shows the hand calculations for the swage fitting pressure drop. In the Excel spreadsheet, the assumption made is that mass density is unchanged, a valid one for an incomprehensible fluid that is not going through a phase change. The pressure drop results in 22.46 psia, which ends up being almost the same exact number as Aspen HYSYS.

Keywords

Swage Fitting, Pressure Drop, Pipe Segment, Diameter, Velocity, Density, Area, Volumetric Flowrate.