Gathering and Analysis of Heat Exchanger Tube Inspection Thickness Data

Introduction

The tubes of heat exchangers (HX), whether for a shell and tube bundle or an airfin, are typically subject to some form of nondestructive examination (NDE) to try and quantify the remaining wall thicknesses and corrosion rates to help a plant to determine remaining life or the need for intervention via re-tubing or replacement of these thin wall components. Often, this NDE only provides semi-quantitative results and often on only a sampling of tubes. This is typically due to the ability to obtain sufficient cleanliness of the tubes and the cost and time available to perform the inspections. Questions then arise on how one can use this data to best estimate the remaining life of the exchanger bundle when the first tube may reach a target minimum thickness or perforation. The principles of sound practices in quality data gathering still apply, as discussed in previous articles published in Inspectioneering Journal from 2012-2014^{1 2 3 4 5 6}. The remaining life analysis using the full internal inspection data can be significantly different than that of small sample size piping data. It is the author’s experience that, when reviewing plant inspection data for heat exchanger bundles in preparation for risk based inspection assessments, the quality of thickness data and corrosion rates for heat exchanger bundles are not as critically assessed as that for piping or vessels. This is a concern, particularly for airfin tubes which will leak into the atmosphere and can be critical in shell and tube bundles where the consequence of a bundle leak may be significant either due to safety (i.e. high pressure hydrocarbon, acids leaks to cooling towers) or business consequences (i.e. product quality). This article will outline some specific techniques to sample, inspect, and analyze the thickness data generated during a heat exchanger tube inspection.

It is important to keep in mind that this article focuses only on thinning, or corrosion, which in many plants, is the most prevalent form of degradation. However, numerous other mechanisms can lead to heat exchanger failures, such as fatigue, dealloying, environmentally assisted cracking (e.g. chloride SCC), etc. Effective inspection strategies should be developed to detect the particular types of anticipated or potential damage as appropriate.

Heat Exchanger Tube NDE – Decision

The decision to perform a tubular inspection is the first step. This decision should be based on both safety and business risk assessments. Simple life cycle cost analysis would help the decision in whether to inspect, proactively replace, or even upgrade the bundle. So, for example, an inexpensive (i.e. carbon steel) high potential margin loss bundle would probably trigger a proactive replacement equipment plan. Inspections would probably be triggered for expensive alloy bundles or high consequence leak driven exchanger such as airfins. Alloy upgrading for high risk bundles whether for safety or business risks to avoid the degradation exposure and reliance on inspections should also be considered.