N E W S   A R T I C L E   article reproduced by kind permission of
International Power Generation

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February 2005 
Managing joint integrity for critical connections     

Tony Nicholls, managing director of Furmanite, looks at the factors causing leaking joints, and the actions required as part of an outage management programme to ensure joint integrity.

The need to reduce leaks and fugitive emissions is perhaps greater today than ever before, driven by both environmental and economic demands. Recognising the constant loss in terms of heat and condensate experienced with steam systems through areas such as flange leaks, gland leaks and passing steam traps, plant operators are under increasing environmental and financial pressure to minimise these losses. Inevitably, joint integrity comes under the spotlight.

Action pays dividends
Certainly avoiding leaking joints and associated emissions will pay dividends. Environmentally-speaking, reducing energy use and increasing system efficiency will minimise greenhouse gas emissions as legislated via the Climate Change Levy, EU Emissions Trading and other climate change instruments, for instance. Equally, reduced energy and water usage affords a direct cost reduction, along with the indirect cost benefit from reduced water treatment. On top of these come factors such as downtime and repair costs, and safety issues. Indeed, where critical joints are concerned – that is, where leakage would cause plant shutdown, the process to be affected, or danger to personnel or equipment – failure can be costly in many senses, and integrity is crucial.

Ideally, effective maintenance programmes should be sufficient to ensure leak-free joints, but too often planned maintenance outages are followed by leaking joints on start-up.

By contrast, steps to eliminate leaking joints will help to drive down costs and, in the case of safety-critical joints, remove unacceptable risk. Furthermore, achieving a leak-free start-up after a scheduled shutdown will avoid delays, reduce equipment and testing costs, avoid re-work, and enable earlier demobilisation of labour – representing additional cost-saving benefits.

Factors for integrity
While there is growing acceptance of the need to manage joint integrity as a key element of good practice and an asset integrity strategy, what is not always recognised is the level of engineering and management required to ensure leak-free performance. Achieving a reliable leak-free joint is about far more than installing a gasket and tightening the bolts.

The level of management required will depend on a number of criteria, from the physical size of the joint and the operating pressure and temperature, to factors such as any fluctuations in temperature or pressure it may be subjected to. As a broad rule, flange distortion, sealing surface damage, inappropriate gasket selection, incorrect bolt loads, and uncontrolled tightening methods are typically among the primary causes of leaking flanged joints. Moreover, the importance of appropriately trained and skilled technicians, and of keeping effective records of work undertaken, loads applied and other relevant data cannot be over-emphasised, particularly on pressurised safety-critical systems.

It is with these factors in mind that Furmanite, the engineering company geared to maximising asset uptime, has launched a Pressurized Systems Integrity (PSI) Management service that can be applied to critical joints from pipework flanges to heat exchangers, pressure vessels, valves and compressors, reactors and more. The programme involves a number of elements, reflecting the fact that effective joint integrity management requires every stage to be managed and documented, from initial engineering analysis of the joint, through all the necessary work to closure and bolting.

The first requirement is to identify the critical flanges, including a risk assessment to allocate a criticality rating. Factors such as whether a flange is operating at high or fluctuating pressures or temperatures, has a history of leaking, is particularly large, inaccessible, or non-standard, will all be considered.

Save time at outage
Engineering analysis of the identified critical joints is then undertaken. For example, the flange will be reviewed against the relevant design standard, determining the target load based on the optimum bolt load for sealing the flange. Sufficient load to overcome all forces acting to part the flange is required, but if too high it can place undue stresses on the flange. Gasket design is also evaluated, to assess whether an alternative (particularly if the joint is old and still using the original gasket type) may be more suitable.

Flange and bolt materials and the thermal co-efficient are reviewed, since differential thermal expansion (often the root cause of a joint leaking on start up or coming off line, but sealing when up to temperature) can be solved by measures such as using a different bolt material or altering the bolt’s grip length. Stress relaxation behaviour of bolt materials over a range of temperatures is also examined, as a bolt material with reduced relaxation may be advantageous where a flange leaks some time after plant start-up.

Specific documented recommendations and work requirements, including all relevant data and the selected tightening method, are produced for every critical flange as a result of this analysis stage, which is carried out in advance of shutdown, and helps to minimise time demands and workscope pressures and avoid delays during the outage.

Outage status overview
When it comes to shutdown, Furmanite implements a flange-tagging system in line with the identified work requirements for each joint, providing immediate status recognition using a series of colour-coded tags, updated as work progresses.

This information is simultaneously recorded electronically into the innovative PSI Management system; a key component of the service. This bespoke-developed software system offers real-time reporting on the current status of each joint, and is accessible not just to the Furmanite site manager, but also to the customer as user-friendly HTML pages via the web, with a passcode entry system. The software system (held and managed by Furmanite and requiring no purchase from the client) uses the same colour coding process and carries all the relevant mechanical and work status data for every joint. Accessible at any time during shutdown without having to be on-site, the system provides the customer with a unique clarity and overview of the outage workscope status and progress.

Work carried out during the shutdown will typically include ensuring an appropriate surface finish, flatness and condition of the existing gasket face, including any re-machining as required. The rougher the surface finish the higher the bolt loads required to obtain a seal, for example, while any marks or defects greater than 30 per cent of the flange sealing face width, or flatness that is outside the maximum tolerance, will be difficult to seal so should be re-machined. If a new gasket is required this is inspected and installed, followed by flange alignment (significant misalignment of the flange holes can require an additional load) and controlled bolt tightening.

Controlled bolting
Bolt tensioning is generally accepted as the most accurate method, using advanced hydraulic technology to induce accurate bolt stresses, without creating torsional or bending stress. The specialist equipment grips the bolt and stretches it axially to a pre-determined load using hydraulic pressure. Since the stud is axially loaded no bending or torsional stress is induced, and as friction is an insignificant factor in the technique, repeatable and accurate residual bolt loads to specific requirements are obtained, and can be repeatedly produced. The residual stud tension can be confirmed by ultrasonic or mechanical stress measuring equipment.

A large number of tensioners can be used simultaneously to keep time to a minimum, and can be readily applied even in areas of difficult access, meeting the most stringent requirements. Even the largest bolt sizes can be tightened to specific design requirements.

Where tensioning is not required, torque tightening (turning the nut to stretch the bolt) offers a simple and safe method of ensuring controlled tightening and loosening of bolts. Again, Furmanite uses a wide range of light, compact, safe and user-friendly hydraulic torque tools, and a complete range of wrenches are available to a torque load of 80,000 ft lb or 108 Nm. Various measurement techniques enable the bolt loading to be verified.

Full traceability
Importantly, a full and detailed history of each critical joint is built up as work proceeds, providing a unique comprehensive record that is easily accessed, incorporating all relevant information from mechanical data to work history for full traceability. Moreover, post-outage the data can be accessed for future maintenance planning, helping to eliminate unscheduled downtime or disruption to operation, and enabling the next scheduled shutdown to be handled with maximum efficiency.

Perhaps not surprisingly, the service is attracting significant attention. Moreover, Furmanite offers a guarantee covering that all-important start-up period when the vast majority of leaks will appear – testament to its confidence in the service’s ability to eliminate leaking joints. Given the ever-more challenging environmental, legislative and economic demands facing power plant operators, while always striving to ensure maximum operational efficiency, effective management of critical pressure-containing joints can reap rewards in terms of cost-savings and operational efficiency, not to mention removing risk. Ultimately it is worth remembering that the true cost of a leak is far greater than the cost of avoiding it.