Driven by environmental and financial pressures to reduce leaks and minimize losses, power plant operators worldwide are focussing on joint integrity in an attempt to eliminate critical joint failure.
There is no question that avoiding leaking joints and glands and associated emissions will pay dividends, in the ongoing quest for optimum plant performance and maximum efficiency. So it is not surprising that joint integrity is taking on an increasingly high profile within power plant maintenance programmes. Leaking joints are both costly and potentially damaging or dangerous, with safety and environmental consequences, not to mention the negative impact on corporate image.
The constant losses of heat and condensate experienced with steam systems, for example, through flange and gland leaks and passing steam traps, can be considerable. At one UK power station a study undertaken to quantify the magnitude of these losses identified the potential for savings of over à‚£1 million ($1.8 million). Water and energy loss was monitored for the six months leading up to a major plant shutdown and again after the necessary remedial work had been undertaken during the shutdown. Not only did the reduced steam loss result directly in increased plant output, the make-up water usage also reduced significantly, leading to lower direct water costs and a reduction in associated water treatment costs. These savings were estimated at around à‚£1.5 million annually, and their value was considerably greater than the cost of the work. Environmentally-speaking, reducing energy use and increasing system efficiency will minimise greenhouse gas emissions, as legislated via the Climate Change Levy, EU Emissions Trading Scheme (see article, p 17) and other climate change instruments.
Figure 1. Many factors are applied to calculate the optimum bolt load and select the tightening method of either torquing or tensioning.
Clearly, steps taken to eliminate leaking joints will help to drive down costs, and in the case of safety critical joints will remove unacceptable risk. Moreover, achieving a leak-free on-time startup after a scheduled outage will avoid delays, reduce equipment and testing costs, avoid re-work, and enable earlier demobilization – each in themselves representing additional cost benefits.
Where critical joints are concerned – in other words, where leakage would cause plant shutdown, would affect the process under way, or would pose danger to personnel or equipment – failure can be costly in every sense, and integrity is crucial. Effective maintenance programmes should be sufficient to ensure leak free joints. Yet it remains a worrying fact that planned maintenance shutdowns are all too often followed by leaking joints on startup.
As drives to reduce leakage and emissions start to bite, there is growing acceptance of the need to manage joint integrity as a key element of good maintenance practice. It is in this climate that Furmanite International has introduced a pressurized systems integrity (PSI) management service for critical joints, from pipework flanges to heat exchangers, pressure vessels, pumps and compressors, and reactors.
Furmanite managing director Tony Nicholls points out that while the need for joint integrity is widely acknowledged, the level of engineering and management required to achieve it is not always recognized. “Maintenance and assembly of a truly reliable leak-free joint is about far more than simply installing a gasket and tightening the bolts,” he said.
The level of management required will depend on a number of criteria, including the operating pressures and temperatures and physical size of the joint, as well as factors such as temperature or pressure fluctuations to which it may be subjected. Typically numbering among the primary causes of leaking joints are flange distortion, unsuitable gasket selection, sealing surface damage, incorrect bolt loads and uncontrolled tightening methods. The importance of appropriately trained and skilled technicians, using suitable tools and equipment and with detailed procedures to work to is vital, as is keeping effective records of work undertaken, loads applied and other relevant data. Both these elements are too often overlooked, but can negatively affect joint integrity if not properly addressed.
For effective management of critical joint integrity the first stage is to identify the critical flanges and undertake a risk assessment to allocate a criticality rating. Whether a flange is operating at high or fluctuating temperatures or pressures, is particularly large, has a history of leaking, is inaccessible, or is non-standard, are all factors to be considered in the rating allocation.
The next stage is an engineering analysis of the critical joints. Undertaken well in advance of the shutdown, it is this stage that enables the work requirements to be identified and documented for each joint (and, as importantly, allows those joints not requiring attention to be identified as not needing to be worked on) to facilitate maximum speed and efficiency during shutdown and remove workscope pressures, helping to ensure that start-up is not only leak-free, but on time.
During this engineering analysis stage, the flange is reviewed against the appropriate design standard, to determine the optimum and therefore target load to seal it. The target load must be sufficient to overcome all forces acting to part the flange, but not so high as to place unduly high stresses on the flange.
Gasket design is also reviewed, to assess whether an alternative design (especially, 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 any problems resulting from differential thermal expansion (which can be the cause, for example, behind a joint that leaks on start up or coming off-line, but seals when up to temperature) can be overcome by measures such as using a different bolt material or altering the bolt’s grip length.
Figure 2. The PSI Management software is available to both the Furmanite site manager and the customer, who can access the html pages via the internet
Stress relaxation behaviour of bolt materials over a range of temperatures is also examined, since high relaxation can be a contributory factor where a flange leaks some time after plant start-up, so a bolt material with reduced relaxation can be advantageous.
These factors are all applied to calculate the optimum bolt load and select the tightening method (torquing or tensioning), which in itself has a potential impact on the long-term sealing of the joint by affecting the accuracy of the bolt loading. From this engineering analysis the specific documented recommendations and work requirements can be specified for each critical joint requiring attention during the outage.
Furmanite’s PSI Management programme then uses a flange tagging system in line with the identified work requirements for each joint to provide immediate status recognition, using a series of colour-coded tags which are updated as work progresses. The information is also recorded electronically into the PSI Management system – a key component of the service.
“This bespoke-developed software offers real-time reporting with the current status of each joint automatically recorded into the system,” Nicholls explains. The software is not only available to the Furmanite site manager, but also to the customer, who can access the html pages via the internet, using a secure passcode entry system. The Windows-based software system uses the same colour coding process and carries all the relevant mechanical and work status data for every joint.
While it is managed by Furmanite and requires no purchase by the customer, the customer has access to it at any time during and after shutdown without having to be on site, giving a clear overview of the outage workscope status and progress.
Work on the joints carried out during shutdown is logged via the software system and will typically include re-machining the gasket face to ensure an appropriate finish (the rougher the finish the higher the bolt loads required), condition (any defects over 30 per cent of the sealing face width will be difficult to seal), and flatness (if out of tolerance), or inspecting and installing a new gasket.
This is followed by flange alignment, because significant misalignment of the flange holes can require an additional load to overcome the misalignment, and controlled bolt tightening to the determined load. Bolt tightening may use hydraulic tensioning, which induces accurate bolt stresses without creating torsional or bending stress by gripping the bolt and stretching it axially or torquing, where the nut is turned to stretch the bolt, and is verified using ultrasonic or mechanical stress measuring equipment.
In line with the importance of clear procedures and appropriate record-keeping highlighted by bodies such as the HSE, a full and detailed history for each critical joint should be retained.
Nicholls says the PSI Management system provides that history, built up as work proceeds. “This provides a comprehensive, computerised record that is easily accessed incorporating all relevant information from mechanical data to work history for full traceability,” he said. “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.”
Adequately trained and competent personnel are important when it comes to bolting associated with flanged joints on pressurised systems, to ensure correct assembly tightening and inspection. The PSI Management service is carried out by certified, externally accredited Furmanite technicians.
Nicholls concludes, “This is an issue that deserves to move up the operation and maintenance agenda.”