Creating NPT Connections for Piping Fittings

Last Updated: May 7, 2007, By:LB

Background:

The best way to analyze NPT threaded piping fittings using FEA methods is with the pipe included in the analysis. Pressurizing the pipe transmits realistic stresses to the fitting body. Anchoring a pipe instead of the fitting produces much more realistic stresses and deflections.

Manifold block with 2 attached pipes.

Manifold block with 2 attached pipes.

Manifold Block Section View

Section view

Construction of the Pipes:

Manifold block with NPT hole.

NPT Hole put into a block with the SolidWorks hole wizard.

Manifold block with reference geometry plane.

Reference geometry plane created through the center of the hole.

Manifold block with converted entities.

Edges of the hole “Converted Entities” (highlighted in magenta) used as the basis of a sketch. Use a pipe chart for the wall thickness. The outside diameter of the hole matches the outside diameter of the pipe.

Manifold block revolved image.

Part is revolved but not merged.

Manifold block with capped pipe.

Repeated for the other connection, only the pipe is capped.

Manifold block with pressurized inside surfaces.

All inside surfaces are pressurized. The end of the green pipe is anchored. The pipes and body are bonded.

Manifold block close up of pressurized inside surfaces.

Two commonly missed areas (magenta circles) these two surfaces are hidden, but must be pressurized to balance the fully pressurized face on the end of the pipes (blue circles). When all the internal areas are properly selected, the reaction forces will be correct.

Manifold block CosmosDesigner stress plot.

CosmosDesigner stress plot showing greatly magnified displacement of all components. The pipes are long enough because there is an area along each pipe that is straight (magenta ovals).

Manifold block with hidden pipes to show stresses.

Hiding the pipes allows the stresses and deflections for the body to be seen.

Any attempt to anchor faces or edges on this model would alter the deformed shape and the stresses measured. This method of including the pipes is simple and realistic.

This analysis method does not include the effects of thread shear stress on the connections. This can be calculated using Machinery Handbook formulas.

Riveted Vessels