Risk-based inspection and assessment at gas plant

Citation

(1999), "Risk-based inspection and assessment at gas plant", Anti-Corrosion Methods and Materials, Vol. 46 No. 6. https://doi.org/10.1108/acmm.1999.12846fab.001

Risk-based inspection and assessment at gas plant

Risk-based inspection and assessment at gas plant

Keywords: Risk assessment, Inspection, Gas, Tubing

CATS International Limited, of Southport, has recently completed a comprehensive damage and integrity assessment survey, based on a risk-based inspection approach, at the Hannibal plant of British Gas Tunisia Limited (BGTL) to establish the cause of premature failure of heater tubes within an amine re-heater. The results of the survey have been validated by ERA Technology Ltd, of Leatherhead, using replication, metallurgical analysis and fracture mechanics techniques, to provide BGTL with in-depth information on the probable cause of tube failure and recommendations for future operation of the unit.

The re-heater is one of four at the Hannibal plant and comprises an electro-resistance welded (ERW), thermally insulated, steel-clad module containing four banks of thin wall (6.7mm), 316 stainless steel, welded tubes, which are connected to form four heat exchange coils. These coils carry dilute amine solution, whose temperature is raised by around 22úF, by means of convection from four fan-assisted, gas burners, equi-spaced along the length of the floor of the unit.

When the re-heater failed in service and was taken out of commission, the source of the problem was visually identified as leakage from a through-wall circumferential crack in one of the tubes. This defect was further examined, radiographically and by dye penetrant testing, both before and after weld repair and was then successfully hydro-tested to 150psig.

Bearing in mind the criticality of the component and the fact that the BGTL, Hannibal plant is responsible for 80 per cent of the natural gas supply of Tunisia, it was decided to commission CATS to carry out a detailed assessment of the complete re-heater unit before putting it back in service.

Using a variety of inspection techniques

The CATS on-site team, which had been mobilised at very short notice, first carried out a visual and dimensional assessment of the mechanical damage to the tube. This revealed previously undetected distortion in the tube which had failed and indicated similar distortion in the equivalent tube in the coil immediately behind. It also revealed the presence of a externally tarnished band, which roughly corresponded to the liquid/vapour interface in the process pipe work.

CATS then undertook an ultrasonic examination, using a number of sophisticated "imaging" techniques, both to obtain an initial assessment of the damaged area and to establish the optimum ultrasonic approach to quantify the nature and full extent of the problem.

Inspection techniques using the imaged responses from zero degree compression wave, creep wave, 70 degree shear wave and Lamb wave probes were evaluated. The responses from the Lamb wave probes proved to be totally ineffective at resolving the damage, due to the structural "noise" from the material. However, the compression scans confirmed that there was no wall thickness reduction and the shear wave scans highlighted far-surface breaking reflectors, indicative of micro-cracking. Finally, although below the usual thickness limits of applicability, the decision was taken to carry out further ultrasonic examination using time of flight diffraction (TOFD). This technique monitors signals diffracted from the tips of defects within the structure of the material itself and, although it is not normally effective on thin section material, because of the existence of a "lateral" wave, the resultant scans confirmed the presence of through-wall cracks. As a result, the TOFD approach was fine-tuned and was shown to be the most defective defect detection and sizing technique, allowing proportioned, through-wall imaging, which permitted the extent of the damage to be established.

Conclusive results

The results of the position-encoded ultrasonic examination were presented, online, in graphical form, using the latest digital data recording, processing and imaging equipment that also allowed offline processing.

The collected data, in its "imaged" format, demonstrated that there had been a significant change in the structure of the material itself, such as that attributable to micro-fissuring or stress-related, corrosion cracking. To determine the precise cause of the failure, BGTL, on the recommendation of CATS, asked ERA Technology Ltd to assess the surface structural condition of the pipe using replication techniques. This assessment established that the cracking was typical of high cycle fatigue and that its transgranular nature was characteristic of chemically-assisted stress corrosion cracking (SCC).

Remedial action

A report of the comprehensive visual, mechanical, ultrasonic and metallurgical assessment was submitted to BGTL within two weeks of the initial intervention. BGTL engineers then used the results of this report, combined with their detailed knowledge of the plant operation, to decide on a course of remedial action. As a first step, it was decided to replace the damaged heater tubes at the earliest opportunity and to stress relieve the butt welds to reduce the risk of future SSC. The checking of burners for flame shape and appearance is now a standard procedure and, in future, periodic examination will be carried out on any further tubes which exhibit any signs of malfunction. Material conformity, process parameters, operating procedures and product chemistry are also under review and it is anticipated that optimisation of these parameters will further mitigate future problems.

Furthermore, as Bill Browne, managing director of CATS explains, "All of the techniques used by CATS were subsequently assessed by BG Technology at Loughborough by performance demonstration on cut-out samples from the repair areas. This exercise conclusively confirmed the validity of the approach and its effectiveness at detecting and through-wall sizing of the integrity- and production-critical defects".

CATS International is structured and resourced to provide the expertise experience and technology to help industrial and process organisations manage expensive, and often potentially hazardous, assets in a safer, more cost-effective manner. Its asset management and inspection services are offered world-wide, in conjunction with affiliates of the Cooperheat Group, who operate from 50 offices in key market locations in industrialised regions.

Further details are available from CATS International Ltd. Tel: +44 (0) 1704 213311; Fax: +44 (0) 1704 25800.