TWOGETHER PAPER TECHNOLOGY JOURNAL
February 1998

After-market service saves fibre lines: Domtar, Potlatch Mills Use Service Provider Know-How for Pressure Washer Drum Replacement

In the early 1990s pressure washers were reintroduced in North America as a cost effective way to increase production. Pulp would be washed in an enclosed vessel, thus able to wash faster and more efficiently. Pressure washers were offered in huge sizes to make them even more attractive.

At first, the washers worked very well. Operating at high rotational speeds and throughput, customers were satisfied with ROI. Unfortunately, after about two years, structural problems developed in some of the washers. The washers began to experience severe fatigue cracking, causing long down times and expensive "patch-up" work. Some failed catastrophically. Over 100 of these pressure washers were built throughout the world.

Partnership for a Solution

Alternating stress summary for the existing design

Domtar Inc. has a fiberboard mill in Red Rock, Ontario. The mill has sought to at least achieve some economies of scale by installing a large pressure washer. The washer performed well for a year or so, but maintenance and inspections began to uncover stress fractures. Operational problems took down the line. A solution was required, since the mill faced further investment need in effluent treatment and other areas of the mill. Domtar needed an innovative, fast solution.

Their supplier of aftermarket service of thick stock pumps and other pulp equipment, Tristar Industries Ltd. of Vancouver, B.C., offered a solution using a unique relationship it had established recently.

Tristar has been working with Dr. Mohamed S. Gadala at the University of British Columbia's Mechanical Engineering Department for a year or so on new ways of using Finite Element Modelling to solve complex engineering problems. Tristar would use their own know-how on pulp washer design together with their work with UBC to solve the pressure washer drum problems. Domtar agreed and a project was born.

Alternating stress results of the new drum design

Using Finite Element Analysis

The finite element method is a numerical analysis technique for obtaining approximate solutions to a wide variety of engineering problems. UBC uses the NISA finite element analysis (FEA) package. To develop the new drum, approximately 20 intermediate designs were considered and 60 detailed models were constructed at UBC's Finite Element Modelling Laboratory.

Once the design concept was finalized the drum was remodelled using the ANSYS FEA package to cross check and develop submodels to fine tune the design in sensitive stress areas such as weld access holes ("mouse holes") and stiffness transition regions.

Building the New Drum

Domtar, U.B.C. and Tristar worked together on the problem drum during the design process. The team brain stormed potential solutions and physically measured all boundary conditions. Load cells and pressure sensors were installed on one washer during a mill shutdown to allow accurate measurement of all forces on the drum.

NISA Finite element alternating stress summary

Tristar has been a provider of aftermarket services for pulp equipment since 1974. The company has always rebuilt thick stock pumps, low and high pressure feeders, make-up liquor pumps, top separator screws and baskets, and vacuum drum washers.

Much of the company's vast manufacturing resources were concentrated on the Domtar project, including the latest in CNC boring machines, lathes and critical inspection work. Just six months after the model and testing were complete, Tristar was installing the new drum at the Red Rock mill.

The new drum design has significantly reduced the alternating stress levels. In the critical first weld near the drive journal, stresses were reduced from +/-10 ksi to +/- 2.7 ksi. (The design basis values used by Tristar are +/-4.5 ksi for full penetration weldment and +/-10 ksi for non-welded parent metal.)

The Results

In order to relate the alternating stress levels to equipment life expectations, crack propagation analysis was employed. Although this part of the analysis is somewhat crude, on a comparative basis it provides a very reasonable tool to assess the life expectancy impact on a reduction of alternating stress levels. By using the material properties of 316L stainless steel in black liquor and assuming an elliptical surface crack, the impact of reducing the alternating stress from +1- 10 ksi to +1- 2.7 ksi was comparable to increasing the drum life from 9 months to 90 years respectively.

To verify the accuracy of the finite element analysis, the Domtar drum was fitted with strain gauge rosettes mounted on the end plate at a critical stress area.

The strain gauges were carefully protected with layers of foil and rubber laminated in place. Wiring was run through the idle journal to a rotary electrical connector and fed through a data acquisition system to a computer monitor for real time viewing by the team The actual level observed was +1- 2.5 ksi -exceeding expectation for alternating stress.

End component detail for new Tristar drum design.

The three drums shipped to Domtar have been running well. No unscheduled down time has been recorded. Tristar and Domtar continue to monitor the drums' structural performance by making spot inspections.

Repeating Success

Tristar built on its Domtar success with another of its aftermarket service customers -Potlatch Corporation at their Lewiston, Idaho mill.

Potlatch also experienced reliability problems with their pressure drum washer line. Six months after the Domtar project completed, Tristar was awarded a project with Potlatch for two even larger drums.

The Domtar drums measured 3.5 m in diameter and 8.0 m face length. The Potlatch drums were considerably larger -4.0 m x 11.0 m. The scale up design required a concerted effort in modifying the models and subsequent design.

The Next Challenge

Implementation of the "Cluster Rules" will force mills to eliminate emissions. Today's rotary drum vacuum washers currently vent gas. The new laws will require mills to collect and scrub these gasses before emitting them to the atmosphere. Because of the high volume of gas and low concentrations of the contaminants, retooling mills with scrubbers is cost prohibitive. Pressure washers provide a solution. Tristar has applied for and received patents in the U.S. and Canada for their new technology.

Measured alternating stresses on Domtar prototype drum during field verification.

Because of their enclosed operation, pressure washers emit very little gas. Their designed throughput is excellent and washing efficiency is very good compared to conventional rotary drum vacuum filters. Replacement of rotary drum vacuum washers with pressure drum washers would eliminate air quality problems for the mill, while paying for the project with increased production.

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