The need to reduce leaks and fugitive emissions is perhaps greater today than ever before, driven by both environmental and economic demands. Inevitably joint integrity comes increasingly under the spotlight -- whether on or offshore -- particularly given the costly implications of leaking joints, from lost product and inefficient plant operation, downtime and repair costs, to matters of safety and environmental issues and fines, not to mention the negative impact on corporate image.

Ideally, effective maintenance programmes should be sufficient to ensure leak-free joints, but too often planned maintenance shutdowns are followed by leaking joints on start-up. Indeed, figures from the UK Offshore Operators Association (UKOOA) indicate that some 25percent of critical joints leak on start-up, and research shows that 10percent of hydrocarbon leaks offshore are from leaking flanged and bolted joints. Initiatives such as UKOOA's Hydrocarbon Release Reduction programme, following the Health & Safety Executive's drive to improve performance regarding hydrocarbon releases substantially, highlight the need to build strategies to cut leaks. Where critical joints are concerned -- in other words 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 particularly crucial.
By contrast, steps to eliminate leaking joints will help directly to drive down costs, and in the case of safety-critical joints will remove unacceptable risk.

Factors for integrity

As drives to reduce leakage and emissions hit their mark, 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. Maintenance and assembly of 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. Further, bodies such as the Health & Safety Executive stress the importance of appropriately trained and skilled technicians, using appropriate tools and equipment, and of detailed procedures to work to, with effective supervision and inspection. Moreover, the importance of keeping effective records of work undertaken, loads applied and other relevant data is also stressed by the HSE, particularly on pressurised safety-critical systems.

The way forward

The need to manage joint integrity effectively to remove the causes of failure is apparent if leak-free results are imperative, and it is with this in mind that Furmanite, the engineering company geared to maximising asset uptime, has launched a Pressurised Systems Integrity (PSI) Management service that can be applied to critical joints from pipework flanges to heat exchangers, pressure vessels, pumps and compressors, reactors and more.
Given that every stage must be managed, from initial engineering analysis of the joint, through all the necessary work to closure and bolting -- all with full documentation -- the PSI Management programme involves a number of elements.
The first stage 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 is non-standard, will all be considered in the rating allocation.
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. Crucially, while there must be sufficient load to overcome all forces acting to part the flange, if it is too high it can place unduly high stresses on the flange. Gasket design is also reviewed, to assess whether an alternative design (particularly, for instance, if the joint is old and still using the original gasket type) may be better suited to the application.
Flange and bolt materials are considered and the thermal co-efficient reviewed, since differential thermal expansion (which can be the cause, for example, 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. Additionally, stress relaxation behaviour of bolt materials over a range of temperatures is examined, since high relaxation can be a factor where a flange leaks some time after plant start-up, and selection of a bolt material with reduced relaxation can be advantageous.
These factors are all applied to calculate the optimum bolt load, and the tightening method -- torquing or tensioning -- can then be selected. Importantly, since the chosen method will affect the accuracy of the loading, this in itself has a potential impact on the long-term sealing of the joint. The net result of the engineering analysis is specific documented recommendations and work requirements, including all relevant data, for every critical flange. Further, only those joints requiring attention need then be worked on at shutdown, saving valuable time.
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 which are 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 with the current status of each joint automatically recorded into the system, 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 (which is held and managed by Furmanite and requires 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.
Typically work carried out during the shutdown will 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 marks or defects greater than 30percent of the flange sealing face width will be difficult to seal so should be re-machined. Re-machining should also be considered if the face flatness is outside the maximum tolerance.
Bolt tensioning is generally accepted as the most accurate method of cold-tightening threaded fasteners. The technique makes use of 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. Importantly, 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 produced again and again. The residual stud tension can be confirmed by ultrasonic or mechanical stress measuring equipment.
Where tensioning is not required, or hydraulic tensioning equipment cannot be used, 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 80000ftlb or 108Nm.
In line with the importance of clear instructions and procedures and appropriate record-keeping stressed by the UK's Health & Safety Executive, particularly for pressurised systems, a full and detailed history of each critical joint is built up as work proceeds, to provide a record that is easily accessed, incorporating all relevant information from mechanical data to work history for full traceability. Moreover, post-shutdown 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.
Given the ever-more challenging environmental, legislative and economic demands facing plant operators, while always striving to ensure maximum operational efficiency, joint integrity must be seen as a crucial element of any maintenance programme.

Tony Nicholls is managing director of Furmanite International, Kendal, Cumbria, UK. www.furmanite.com