The long wait for a non-destructive testing system for PE piping systems may be over. Dr Ken Murphy explains the benefits of the Evisive Microwave inspection.
The increased use of PE piping systems in an ever-increasing range of services and applications has resulted in a call for the development of a reliable non-destructive testing (NDT) method of inspection for these materials to reduce cost, time and safety. Currently, the proven method for rehabilitating old pipelines and extending the service life of an existing pipeline by protecting the pipeline from corrosion and abrasion is swagelining. The method uses polyethylene (PE) pipe with an outside diameter that is slightly larger then the inside diameter of the pipe to be lined. Depending on the length of pipe to be lined, PE stalks need to be butt welded together before being pulled through a reducing die to allow it to be pulled through the original pipeline. The loads experienced by the PE pipe during this operation can be very high. Although PE butt welding is a very well understood and controlled welding method, joint failures can still occur, usually due to a lack of fusion or fine particulate contamination. Albeit infrequent, failure of butt welds especially during an operation such as swagelining has significant consequences with respect to cost, time and safety of the project. Unlike metallic welding, there is no reliable NDT technique for non-metallic pipe systems such as PE currently available. No codes or standards have been developed with respect to the NDT inspection of PE joints to date, while requirements for mechanically testing PE butt welds have only been adopted in the past couple of years through CSA Z662 and ASME B31.3. Extensive review Currently the main method of NDT inspection of PE butt welds is by visual inspection and the bead twist test, which requires removal of the external bead upon cooling of the weld and twisting it. If any splitting along the length of the bead joint is observed the joint is considered to be suspect. Inspection of PE butt welds has been attempted using conventional NDT methods. While these techniques are well established and understood they do not reliably detect issues associated with poor polyethylene joint performance such as lack of fusion (LOF) and fine particulate contamination. Advances in NDT technology have seen the development of improved inspection techniques some of which have shown promise in the inspection of non-metallic materials. The Polymers specialists at Exova have carried out an extensive review of the NDT techniques available, with a new state-of-the-art technique known as Evisive Microwave imaging has shown the most promise with regard to identifying the issues associated with poor PE butt weld performance. Fully portable, so ideal for use in the field, the testing system can be used to inspect new installations or pipelines already in service with no need to shut down or empty the pipeline and can take as little as five to ten minutes depending on the pipe diameter. Evisive Microwave inspection is a form of electromagnetic (EM) inspection. The probe used consists of a transmitter and two receivers. The transmitter emits a single coherent frequency of microwave energy. The energy is reflected at every interface between materials with differing dielectric constants. All the energy reflected from interfaces within the cone of transmitted energy is detected by the receivers. An advantage of the technique is that it is a volumetric method of inspection; which means there are no thickness limitations, and the resolution does not change with depth. Electromagnetic inspections by their nature create digital information, therefore mapping and reporting of the joint using the microwave imaging software allows various software manipulations and visualisations such as colour, gray scale, 2D, 3D and raw voltage data to be generated and recorded as JPEG files. Blind samples The basic equipment comprises the probe and a control box housed in a durable pelican case. All software is run from a standard laptop. During inspection the probe requires to be moved in a continuous fashion along the surface, either in contact or near to the surface. The system is field ready and has been successfully trialed in the field. It is portable, fast and intrinsically safe. Work was carried out at Exova to determine the effectiveness and functionality of the technique in the inspection of PE butt fused joints. Nine butt welds of PE100 250mm SDR11 pipe were manufactured by Plasflow – one best practice and eight with varying defects within the weld. Three of the welds were blind samples to help assess the interpretation of the technology. The results obtained highlight the significant potential that Evisive microwave imaging has as a valid NDT method for the inspection of PE butt welds and EF joints and its ability to detect issues associated with poor joint quality. The ability of the technique to distinguish between a best practice butt weld and a weld containing fine particulate contamination is a significant development in the NDT inspection of PE joints. This issue has not been reliably detected in the past and highlights the potential of the technique. It has also shown significant potential in identifying issues that affect the performance of EF joints. The system can be used in conjunction with current assessment methods for PE joints to further improve the quality assurance and reliability of PE joints in the field. It may also reduce the need to destructively test certain joints and therefore reduce the amount of time lost cutting out and replacing joints that have already been manufactured. Dr Ken Murphy is project manager at Exova’s Polymers division. T: 0161 787 3250
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