Case study: Bleach Washer at a Pulp and Paper Mill

October 9, 2020

A leading North American pulp and paper manufacturer in British Columbia observed cracks in the FRP panels of a bleach washer, one of five used in pulp washing and bleaching processes at the mill. Inspection and analysis by a UTComp engineer and licensee using UltraAnalytix® found additional cracks and loss of thickness in the FRP panels which could increase the risk of equipment failure. It was recommended that the cracked panels be repaired or replaced.


The pulp and paper industry uses a long list of equipment made from fiber reinforced polymers (FRP) and other composite materials that provide superior corrosion resistance, reliability, strength, long lifespan and low maintenance costs. FRP is used in a wide range of infrastructure — from pipes and ducting to process vessels and storage tanks — used to manage a wide range of chemical treatment, bleaching and washing processes where steel isn’t an option.

FRP used in pulp bleaching and washing equipment

Effective washing is a critical step in the chemical processes that convert wood chips and sawmill waste into kraft pulp used to manufacture paper. To make kraft pulp, the wood chips are broken down into their individual fibers using a multi-stage process of soaking the chips in a bath of sodium hydroxide and sodium sulphide, then cooking them in a digester. The pulp is washed and screened to recover process chemicals, then bleached to increase brightness and strengthen before it is pressed and dried into sheets that are bundled into bales for shipment to customers.

Bleaching is typically a five-step process of soaking and washing the pulp mixture in hydrogen peroxide, chlorine dioxide and caustic soda. The bleached pulp goes into a vacuum drum washer that uses water and pressure to rinse away chemical residues. FRP is used to make the large washer drum as well as large-diameter upflow tubes and retention towers where digested wood chips are first bleached in the pulping process. Effective washing is a critical stage that helps recover process chemicals, reduce bleaching chemicals, and lowers the amount of treatment required for the effluent.

Like most modern pulp mills, the client uses an elemental chlorine-free (ECF) process that includes chlorine dioxide rather than elemental chlorine as a key bleaching agent. However, while ECF pulp has a much lower environmental impact than traditional methods, chlorine dioxide is one of a very short list of harsh chemicals that can attack and degrade FRP and cause it to lose thickness/stiffness and strength over time. Therefore, regular monitoring and inspections are essential to avoid equipment failures or loss of containment which could result in serious injuries to workers as well as environmental hazards.

Inspection Summary

One of UTComp’s engineering analysts visited the site in October 2018 along with licensee Horizon Testing. An external inspection was completed following UltraAnalytix® guidelines and the principles of API 653 as adapted to specific experience with FRP vessels. The inspection revealed loss of thickness near visible cracks and detected more cracks in other panels using data collected by UltraAnalytix®.

The inspection was carried out in two stages:

  1. External / visual assessment:
  • Ensure that nozzles, pipes are properly supported with nothing out of place
  • Assess condition of the machine’s foundation
  • Look for other visible damage, including venting pipes, cracked flanges etc.
  1. Ultrasonic readings with UltraAnalytix®
  • Measurements taken at key points of the vessel to measure thickness/strength
  • Ultrasonic data is collected for analysis and reporting. The report predicts minimum service life that can be expected based on the loss of thickness over time

Ultrasound readings taken in a grid pattern

The UTComp team took readings in an approximate grid pattern, with readings taken in eight rows down, with four readings across each row. The images below show the gel residue where readings were taken on the two panels where cracking had been detected; cracks on on the rear (east) panel had not been detected prior to UTComp’s visit.

A plot of the Percentage of Design Stiffness (PDS) revealed that PDS was significantly lower in the area of the crack. Measured thickness also decreased near the cracked area. For these reasons, it was determined that the FRP was weaker in the areas near the crack, and should be repaired or replaced.

PDS and Thickness plots of UltraAnalytix® readings taken on the west panel with crack along the bottom are shown below. Values of both thickness and PDS are lowest in the bottom row of readings adjacent to the crack.

PDS Explained

Composites are different from steel in a number of important ways, including how they change over time in reaction to their environment. In general, steel loses thickness over time but not strength; composites like FRP won’t usually lose thickness but will lose stiffness or strength in ways that were difficult to measure without destructive testing until UltraAnalytix® came along.

UltraAnalytix® provides a proven, non-destructive way to calculate the loss of strength and stiffness of FRP over time as a percentage of its design stiffness (PDS).

FRP strength depends on a number of factors including the cure of the resin used to make the material and the workmanship that goes into its manufacture. New FRP is rated at a certain strength but will lose stiffness and strength over time as it is subjected to chemical corrosion and mechanical loads. FRP tanks, vessels and pipes include an inner corrosion barrier or liner made from layers of resin-saturated fibers designed to protect the structural outer layers from the corrosive action of the chemicals inside the container. The corrosion barrier erodes over time, and the traditional way to estimate the lifespan of FRP is by assessing the condition of the corrosion barrier.

With UltraAnalytix®, ultrasonic data is collected and analyzed using patented algorithms to quickly and accurately measure the thickness and calculate the PDS to determine the remaining lifespan of FRP equipment. Inspection and analysis can be completed in a matter of a few hours — while the equipment is operating with no time lost to shutdown — and a full report is sent to the client within three weeks.


The five bleach washer tanks at the facility are over 30 years old, and the cracks in one tank mark the first time there had been any problems with the FRP components. Properly maintained FRP equipment can be expected to last up to 50 years.

Analysis and review of the data collected by UTComp found no defects or damage to the support structures or pipe and nozzle connections of the tank. However, testing did reveal damage to two panels on the exterior of the tank, including cracks on the rear panel that had been previously undetected. UTComp recommended that the cracked panels be repaired or replaced.

Overall, the FRP vessel was suitable for continued service to the next recommended UltraAnalytix® inspection once recommended repairs are performed.

For more information on UltraAnalytix® or UTComp, please contact us:

UTComp Inc.
50 Fleming Drive, Unit 5
Cambridge, Ontario, Canada