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Inspectioneering Journal

Inspection of Heat Exchanger, Condenser & Fin Fan Coolers Tubes - An Update

By Brian Beresford at TechCorr Inspection & Engineering. This article appears in the May/June 2007 issue of Inspectioneering Journal

Tube inspection is a vital tool for the refining and petrochemical industries. Heat exchangers and condensers are designed to sustain 100% separation between the products in the tube (tube side) and the products in the vessel (shell side). A leaking tube can not only cause a significant impact to production it can cause major environmental issues and the potential for loss of life.

Tube Inspection techniques have been available for decades. Historically the costs for inspections have been extraordinarily high due to probe manufacturing and instrumentation costs. Only nuclear facilities which are heavily regulated could afford these services. Over the past decade improvements in manufacturing capabilities have helped to decrease the cost for testing devices. The 1986 Process Safety Improvement Act also resulted in an increased demand for inspection services. These two factors have contributed to more cost effective probe design and decreased cost to perform inspections. Tube inspection services are much more cost effective for the oil and gas industry equipment operators than in the past.

Now exchangers and condensers are being inspected on a more regular basis. This has lead to improved bundle reliability for the oil and gas companies. The inspection allow the operators to improve preventative maintenance programs by identifying damaged tubes requiring immediate replacement during maintenance outages and the ability to more accurately determine remaining life so maintenance activities can be scheduled during future outages and finally the ability to manage “Risk” by reducing the number of unforeseen unplanned outages. These benefits have provided significant reliability improvements for refinery and petrochemical operators and will continue to grow as technology and implementation practices continue to improve over time.

Current Technology

Tube inspection techniques include Eddy Current, Remote Field Eddy Current, Magnetic Flux Leakage, IRIS (UT technique) and LOTIS (Laser) profilemetry. Although this article focuses on electromagnetic based techniques the ultrasonic and laser techniques require mentioning as the techniques are very complimentary and often used in parallel.

Tube inspection is typically broken down into two (2) categories; ferrous and nonferrous. Ferrous materials refer to materials with magnetic properties such as carbon steel and 400 series stainless steel. Nonferrous materials refer to materials with nonmagnetic properties such as copper, brass, Inconel and most stainless steels. The following table shows the techniques that are used for the different tubes materials.

Technique Flaw Detection Table for Nonferrous TubingTechnique Flaw Detection Table for Nonferrous Tubing

Technique Flaw Detection Table for Ferrous TubingTechnique Flaw Detection Table for Ferrous Tubing

Eddy Current

Eddy Current Testing (ET) is very sensitive to a great number of variables making it a powerful examination tool. The eddy current testing method is based on inducing electrical currents (eddy currents) in electrically conductive materials. For tube inspection, bobbin type probes are used containing coils as shown. In theory, any defects in the material such as cracks, pitting, wall loss or other discontinuities will disrupt the flow of the eddy currents and be detected by the instrumentation. Most heat exchanger bundles contain supports that are many times likely targets for service type damage such as fretting, galvanic or oxygen concentration cell corrosion. Multi-frequency channel systems are capable of suppressing or mixing out the unwanted signals responses from supports in order to closely interrogate the material under and near the supports. Mixing is also used for the detection of defects near, at, or within the tubesheet. Conventional eddy current testing is employed primarily on non-ferrous (nonmagnetic) materials due to permeability effects of ferrous materials. Many times the owner/users of the exchangers prefer eddy current testing to IRIS (internal rotary inspection system) inspection as the cleanliness of the tubes is less critical. Additionally, the productivity of eddy current testing can be as much as 3 – 4 times faster than this type of UT inspection.

Although the choice of technique used is primarily influenced by the type of “failure mechanism” needed to be detected many times the technique used is dictated by the tube cleanliness. For example, whereas the utilization of IRIS and Laser require a high degree of tube cleanliness, ET, RFT and MFL do not require the same level of cleanliness. There are several damage mechanisms and flaws that can occur on the outside or inside diameter surfaces (O.D. or I.D.) as well as volumetric damage. The following table shows the various flaws that can be detected with the various techniques for both nonferrous and ferrous tubing materials.

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