Heat-exchanger sealing plug wastage detection
In March 2018, a high-pressure heat exchanger of the Fin Fan-type cooling arrangement reported leaking in service, the consequence of this being that the plant required an inspection to assess the blanking plugs and subsequent investigation. The failure involved a number of leaking plugs on the tube-plate face.
Following identification of the leakage, the process plant could not readily be shut down to remove the blanking plugs. This critical piece of infrastructure had to be operational at short notice, so an accelerated solution was needed to ensure the safe resumption of operations. The aim of the inspection was to identify any signs of SCC and thread wastage, as well as providing sufficient detail for the threads to be measured.
The hexagon-headed blanking plugs in question were located on the front face of the header box, with some 177 blanking plugs requiring inspection on each face. The header box operated at a temperature of between 70 and 100°C, which provided an additional complication.
Various methods of inspecting the blanking plugs were evaluated. The most appropriate solution came in the form of ultrasonic immersion testing using a special housing arrangement carrying the acoustic couplant and focussed transducer.
Although UT inspection was the only feasible means to inspect this line, doing so required a delivery device and special probe arrangement to be put in place. Applus+ RTD was awarded this project, having the skills and experience to manage the design, manufacture, set-up and delivery of the inspection.
To inspect the heat exchanger, Applus+ RTD prepared a complete project plan to help with design, development and manufacture. The inspection included procedure preparation, production of a calibration sample with reference targets and a client demonstration.
A UT inspection method was chosen as this would facilitate delivery to the point of interest, the mounting of special focussed transducers and housing that provided a near-perfect fit within the tubes.
The transducers were mounted facing forwards and rearwards, allowing inspection of both butt and fillet welds in the tubes.
The first stage involved internal cleaning and preparation so as to provide a good, clean surface for acoustic coupling.
Due to the importance of the heat exchanger and its around 500 hexagon plugs, the inspection device had to be designed to accommodate a relatively quick inspection. It was tailored to couple with the header box face, coupling between the bolts’ hexagon heads. It also needed to accommodate high-temperature coupling and provide an encoded image of the thread form of each blanking plug.
The first run was completed using an Applus+ RTD ultrasonic-inspection tool to detect the threads through the head of the bolt. Embossing could have been an issue due to mode conversion, but the design prevented problems.
Scanning easily provided a 360-degree image of the thread form and, from this, we were able to evaluate the image with the echo dynamics from the thread pitch providing a means of assessing fitness for purpose. The inspection-tool design could accommodate variations in the hexagon-head dimensions and fit between rows on the header box.
This combined inspection was able to identify any wastage and thread cracking. Since this heat exchanger had been in service for some years, this was an ideal opportunity to take a detailed baseline survey using the highest-resolution direct-measurement inspection tool available on the market.
This novel NDT application has proved to be a useful tool for the client and removed the burden of having to shut down the plant for plug removal. It also provides a means to quickly assess fitness for purpose.
A team of two ultrasonic technicians was deployed, and each inspection image recorded against a master report sheet.
Work completed ahead of schedule and the heat exchanger was handed back to the client for remedial work to exchange the faulty plugs at the next shutdown.
A complex heat exchanger with complex geometric and access features requires the correct technology to ensure success. In this case, this involved a combination of specialised ultrasonic inspections. Applus+ RTD took full responsibility for the project, seeing it through from beginning to end.
The inspection method had previously been field-tested on faulty plugs, as well as on reference bolts with various features. The scan images provide details of pitch, flattening and SCC.
This heat exchanger is a critical asset, so Applus+ RTD provided express reports within 24 hours of each run and full reports within the agreed timeline. Express reporting was instrumental in returning the plant to normal operation as soon as possible.