Northern NDT solves tricky inspection problem

Citation

(2002), "Northern NDT solves tricky inspection problem", Anti-Corrosion Methods and Materials, Vol. 49 No. 4. https://doi.org/10.1108/acmm.2002.12849dab.001

Northern NDT solves tricky inspection problem

Northern NDT solves tricky inspection problemKeywords: Corrosion, Mapping, Inspection, Technology

Northern NDT Services Ltd, with offices in Stockton and Ellesmere Port, UK, reports that it has solved a difficult corrosion-mapping problem by an innovative application of ultrasonic inspection technology.

A major chemical producer in the North East of England required accurate internal corrosion mapping of the riser and downcomer legs of a synthesis reactor vessel, both to locate any in-service corrosion and to monitor any slight changes in pre-existing variations. The construction consists of six pairs of vertical risers and downcomers, 15m in length and made from seven sections; 20in. diameter outer pipes enclose each of the internal 17in. diameter fluid-carrying pipes. Steel spiral strips act as spacers between the two carbon steel pipes through which cooling water is circulated; the complete jacketed assembly is externally lagged (Plate 1).

This complex structure precluded the use of external corrosion-mapping systems; the surface curvature and the pipes being in the vertical position further complicated the inspection requirement. The only access is afforded via a flange joint at the bottom of each pipe.

To solve this difficult problem Northern NDT proposed the use of their "P" scan ultrasonic corrosion-mapping system. This was linked to a special purpose internal pipe scanner-head, which was equipped with CCTV as an operator aid and to assist in the monitoring of surface cleanliness. The "P" scan is described as a versatile software-based system, which is capable of multiple collection of NDT and positional data to generate comprehensive C-scan images, which are stored for subsequent processing and analysis.

Plate 1 The internal pipe scanner head being used by Northern NDT Services for corrosion mapping of riser and downcomer legs of a synthesis reactor vessel

The corrosion mapping of a selection of legs was carried out over a four-day shutdown period by a two-man team operating from a mobile control facility, this being located close to the reactor vessel with an umbilical connection to the scanner-head.

The scanner-head was introduced into each leg through the bottom flange and driven vertically upwards via magnetic wheels to perform a 320mm wide raster scan of the 15m of pipework. The results indicated a very slight corrosion loss but were further able to demonstrate that the subtle changes in the pre-existing variations in the pipe sections, caused during pipe forming, had no detrimental effect on the system's structural integrity.

By careful consideration of the information required to satisfy the vessel inspection scheme, linked with the engineering restrictions, the client and Northern NDT were able to find a solution that could give long-term confidence. In this case, the assurance that the operator can achieve maximum life expectancy from his plant is associated with considerable cost savings.

Northern NDT Services has a long history in corrosion mapping, covering both individual and repeat inspections. It offers a range of conventional and specialist inspection techniques, from manual NDT carried out by experienced and qualified personnel, to specialist services such as the "P" scan system with a wide range of scanners to meet virtually any mapping specification.

The company's extensive range of services includes time of flight diffraction, boiler/heat exchanger tube inspection, remote visual inspection using miniature CCD camera systems, thermography, as well as long-range ultrasonic inspection and the RTD-Incotest system.

Details available from: Northern NDT Services Ltd. Tel: +44 (0)151 373 4800; Fax: +44 (0)151 373 4801;E-mail: martin.hockley@northern-ndt.co.uk