Inspection in industry is always an on-going process, whether it be to meet industry regulations, or simply for personal and environmental safety, or peace of mind.
Tubular heat exchangers are one of the key pieces of equipment in many industrial processes, used to cool processes rapidly, to maintain reliable and effective operating temperatures. They are widely used in industries such as power, chemical, petrochemical, natural gas, sewage treatment and more. A common example of a heat exchanger you might be more familiar with is the process of running fluid through radiator coils in a combustion engine. The process for tubular exchangers in industry is similar, and uses the same principles to achieve a similar effect.
In this article we’re going to focus on tubular heat exchangers specifically, as opposed to any other kind of heat exchanger, because they represent a distinct branch represented in the Non Destructive Testing (NDT) industry, and these are the inspection methods we’re interested in.
There are several defects that can arise in ‘tubes’ that threaten the reliability and integrity of the structure, that can lead to failure, and loss of time or money, or worse, life to the owner. These are the problems we attempt to mitigate through the use of NDT.
Problems can be surface level or internal, produced in the manufacturing process, or more likely from external damage, or corrosion. Different defects are more suited to different types of inspection, and for that reason we have several tools in our toolkit, which are often combined to provide a thorough or more in depth analysis.
The most common methods of inspection include;
RFET (Remote Field Electromagnetic Testing) – RFET is often the first method of inspection, it’s not as detailed as other methods, but it’s fast, and can detect a wide range of flaws, and is good for ferrous materials. Typical defects include, general corrosion/erosion, pitting, and baffle cutting. It works by sending a probe down the tube, which then produces an electromagnetic field, to be picked up either by the probe again, or a receiver. The magnetic field is analysed for anomalies by a skilled technician, and potential flaws are registered. Due to the nature of RFET, it cannot distinguish between internal and external flaws.
ECT (Eddy Current Testing) – Eddy current testing works on similar principles to RFET. It’s primarily used to inspect non-ferrous materials, although RFET is capable, ECT is often preferred. Eddy current as it’s often referred, commonly detects flaws such as cracks, and corrosion/wall loss. Two of the important features of ECT compared to RFET, are that it requires little preparation, and doesn’t requires highly clean surfaces. Also, ECT can detect small defects such as cracks and distinguish between internal and external defects, which RFET cannot.
IRIS (Internal Rotary Inspection System) – IRIS is often used as a follow up, or if a more detailed inspection is required, and is used on ferrous materials. A probe is sent into the tube as with RFET, and an ultrasonic pulse is projected at the tube wall to measure thickness and detect defects. In order to use IRIS the tube must be flooded, and be free of almost all contaminants, this is due to its highly sensitive nature. For this reason IRIS is often used sparingly.
RVI (Remote Visual Inspection) – On top of the methods above, RVI can be used to inspect further, or provide more detailed information/additional information on potential defects, or the environment. RVI uses tools such as fiberscope cameras, borescopes and flexible narrow lights. RVI is supplemental to the above methods, and is usually used in conjunction.