File:PVE-4687, Last Updated: 5-Nov-10, By: CBM

Pressure Vessel Engineering was contacted to help re-certify a series of 17′ Diameter 56′ tall digesters for Tembec Inc. which are currently in use for the pulp and paper industry. These digesters are filled with wood chips and mixed with acid in order to convert the wood chips to paper pulp.

Digester with Riveted Butt Seams

This digester has been in use since 1926. Vessels built in that time period were typically constructed with riveted butt joints.

Shell Model

A shell model of the entire digester was created and analyzed to determine the stress distribution.

Resulting Stress Profile

The resulting stress profile from the design pressure and static head. The highest stresses were observed at the bottom shell segments.

The next step was to analyze a small segment encompassing the bottom shell and cone and modeling in the actual butt straps with rivets. Rivets are installed in a hot state, so as they cool, they contract and generate a preload force that compresses the butt straps and the shell together. As the rivets cool, they plastically deform with preload stresses relaxing back to the yield point. Bolt connectors with the corresponding preload equal to the yield stress have been used to simulate the rivets.

Solid Model of the Digester

A solid model was created, incorporating the butt straps and the legs. Weight is applied to the model to generate stresses at the leg attachments.

Bolt Connectors

Bolt connectors are applied at each of the rivet locations with the calculated axial preload. No penetration contact sets are applied between all butt straps and shells.

Symmetry Conditions

The plane of symmetry cuts through the cylindrical shell. Symmetry is applied here. A seam is present at the conical shell thus no symmetry is applied. This forces the rivets to restrain the model.

Cross Sectional View of the Rivet

The rivet head is bonded on the inside and the outside butt straps. The rivet is restrained from moving through the hole in the conical shell.


The digesters experience radial expansion along with a bending load on the legs.

Von Mises Stress

Although the riveted areas are perforated, the butt straps provide additional restraint and actually reduce stresses at the seam. Peak stresses are generated immediately around the holes due to the high compressive preload stress.

Rivet Peak Stress

Higher stresses occur around the rivet holes. This is caused by the rivet preload being set to the yield strength. This causes a high compressive stress at the joint.

Outer Rivets

An outer row of rivets with a larger pitch was used in this design. Although this is still below the allowable stress, a concentration of stresses build up in this region.

Our FEA was successfully used to prove the integrity of the digesters in their current state to the local jurisdiction and the insurance company. Even though riveted boilers and pressure vessels have not been manufactured for many years, there are a number of them that are still in operation today. Although built to ASME code, many of these boilers were constructed at a time when no CRN requirement was in place. As inspectors come across these vessels, we expect to see more of this type of inspection and certification requirement.

We at Pressure Vessel Engineering Ltd are very grateful to Tembec for allowing us to post this analysis. Tembec has been acquired by Rayonier Advanced Materials and can be contacted through their website at