POWER PLANT PRODUCTS
How to protect your visible assets1 June 2007
Remote visual inspection techniques provide important information for power plant operators. But DIY is not a realistic option. The message is – get in the professionals
Remote visual inspection is playing an increasingly important role in preventative maintenance and troubleshooting at power generation plants. The technique uses remotely controlled video cameras, video probes or fibre-optic probes to obtain detailed visual information from inaccessible locations. In many cases, the inspection forms part of a wider investigative programme, which may also involve debris retrieval, corrosion assessment or routine repairs. Where applicable, video images can be used to support data from other techniques such as crack detection.
The photographs on these pages were taken during procedures carried out by Stockport UK, based AV Technology. Internal visual inspection is applicable to all types of plants, including oil, gas, coal, CHP, nuclear and hydro-electric. Areas suitable for routine inspection include:
• boiler headers
• attemperator nozzles
• pipe work and pipe linings
The trend towards increased outage intervals means that inspections, when they are carried out, must be very thorough. Although it is possible for operators to perform inspection work themselves, there are significant reasons why it makes sense to employ the services of a professional and experienced inspection company.
As AVT’s visual inspection expert Ken Lambert points out: “Although it may seem straightforward to hire the cameras and equipment, actually carrying out the work, effectively and safely, is far from simple. Without the experience, and the use of purpose built accessories for positioning cameras, companies that carry out their own inspections usually end up with low quality images which often do not provide sufficient useful information. In addition, interpreting and presenting the results so that further work based on the data can be carried out requires considerable expertise.”
Pipework and valves
Working in pipes typically involves routine inspections of welds, valve seat conditions, blockages and build-up of process product. Visual inspections can also be effective in assessing the effects of flow accelerated corrosion (FAC). Filming inside pipes and similar enclosed spaces requires a mixture of the right equipment, ingenuity and skill. Entrance apertures may be relatively small, and radial inspections require specially converted camera assemblies. For small diameter, short length inspections up to six metres, fibre optic endoscopes or video probes are used. For larger and longer inspections, a video camera mounted in a multi-legged centralising cradle allows pipe lengths up to 300 metres to be inspected. The cradles are usually propelled by flexible rods or by a remote controlled, motorised carriage. These remote crawlers provide greater versatility and are used in longer distance, more complex situations. In these applications, great operator skill is required to ensure that the crawlers do not become jammed or impeded.
During the past decade there has been an increasing number of incidences of failures or crack indications in utility boiler pressure components. This is primarily due to the ageing of the steam generating equipment base. Also, the condition of the components can be severely impacted by the continued trend towards cycling and load-swinging modes of boiler operation. Of particular concern are the ongoing occurrences of header and drum internal cracking.
Attemperators located in the superheater crossover piping are used to control the steam temperatures by introducing spray water to the steam flow. Cracking is typically due to thermal fatigue, a result of over-temperature conditions and cyclic or load swing operation.
An important associated part of remote inspection is debris retrieval. ‘Debris’ can include parts that have broken off or material left after construction or repair work. For example, a small, yet potentially problematic, broken thermocouple was recovered by the company’s inspection team at an HRSG plant in Ombilin, Indonesia. And it is not uncommon to find larger objects such as tools, nuts and bolts.
AVT are contracted to Alstom Power to carry out pre-commissioning inspections and removal of debris from all steam pipe work at new build gas power stations. To date AVT have carried out work at a variety of power plants, as the illustrations (p 70) indicate.
Waterproof cameras are available for underwater inspection and a wide range of accessories including lateral view heads, headlamps, and air-cooling jackets for high temperature applications increases the possibilities for remote inspection. As with all professional photography, the pre-requisite for clear and usable images is ensuring correct lighting and focus.
As Ken Lambert concludes: “It is clear that remote inspection provides a very powerful tool. Our experience in the power industry has shown that combining this technology with practical ideas and sound mechanical engineering methodology significantly extends its capabilities. We advise clients to discuss their application with specialists before they try to carry out any work themselves. We offer a highly professional service with all the relevant health and safety, QA and insurance certification.”
Working with Mitsui Babcock (now Doosan Babcock), AVT have carried out ligament cracking inspections on boiler headers at Aberthaw, Fawley and Tilbury power stations as well as header inspections on heat recovery steam generators (HRSG) and associated pipThe value of visuals in finding potentially damaging debris – this pin was found in a drain pot in attemperator pipework at Aberthaw power station The ability to carry out remote visual inspections to assess the condition of turbine blades forms a vital part of ongoing maintenance. The image shows turbine blades on a power generator at Wissington Sugar Factory following a survey carried out for Siem