Case study: NDT inspection of relined upflow tubes at a pulp and paper mill

UltraAnalytix^® inspection results showed that the thickness and condition of the FRP in a pulp bleaching vessel were adequate to postpone scheduled relining

BACKGROUND

The Owner Company operated a moderately sized pulp and paper mill located in Ontario, Canada. Chlorine dioxide is used to bleach pulp. Chlorine dioxide is particularly corrosive and often results in loss of FRP thickness.

The mill has two bleaching upflow towers: one for softwood pulp and one for hardwood pulp. Both were fabricated in 1996, and have been relined internally due to visual identification of corrosion barrier damage by a previous Inspector. The details of the relining history were not available to UTComp.

Both upflow towers were scheduled for relining during the October 2017 plant shutdown. UTComp was hired to inspect the upflow towers in advance of the scheduled plant shutdown to determine whether the work could be postponed with minimal risk to the operation.

SOLUTION

UTComp, Inc visited the client’s site to conduct UltraAnalytix^® inspection of the towers, including External Inspection and UltraAnalytix data collection, and post-processing analysis. Internal Inspection is not required for UltraAnalytix inspection, so system shutdown and confined space entry was not required, and the inspection was conducted with the plant in operation. Key outputs of the UltraAnalytix analysis include:

FRP thickness
structural condition (measured as Percentage of Design Stiffness or PDS), and
corrosion barrier condition.

UltraAnalytix inspection results

The UltraAnalytix inspection revealed some corrosion barrier damage but no deficiency of thickness or PDS in either upflow tower.

For the hardwood upflow tower, some degradation of the structural FRP and corrosion barrier damage were found throughout all sections inspected. Despite the confirmation of corrosion barrier damage by UltraAnalytix readings, the UltraAnalytix data revealed that the FRP retained most of its original design stiffness, and the measured thickness at all sections met or exceeded the original thickness shown on the construction drawing. UTComp recommended leaving the tower in service without relining, and inspecting on an annual basis. UltraAnalytix results for the Hardwood tower are summarized below in Table 1.

For the softwood upflow tower, UltraAnalytix results revealed little to no corrosion barrier damage or degradation of the structural layers for most sections. The measured thickness at all sections met or exceeded the original thickness shown on the construction drawing. UTComp® recommended leaving the tower in service without relining, and inspecting on an annual basis. UltraAnalytix^® results for the Softwood Tower are summarized below in Table 2.