Despite the fact that the vast majority of the world’s petrochemical process plants and pipelines are either welded or bolted together, there is often a perceived difference between the two methods.
Whereas the welded joint has mandated procedures and competency requirements to ensure its quality and fitness for purpose, the bolted flange joint has historically had few such requirements, despite the fact that it serves the same purpose of pressurised process containment.
However, times are changing. There is a growing case for bolting in petrochemical process plants with the development of bolting techniques, standards and training.
The objective: preventing leaks
Welded and bolted joints are a common feature in petrochemical plants today and both fulfil the same purpose in containing a pressurised liquid or gas and preventing leaks. Whereas welded joints are permanent connections, bolted joints are a ‘temporary’ connection and tend to be required where assembly or maintenance dictates. Moreover, problems with a welded joint tend to be more disruptive than a bolted joint which can be readily examined and replaced.
Figure 1. Comparison between compliance requirements of a bolted and welded joints.
When it comes to preventing leaks, the stakes are indeed high with any leaks, typically carrying heavy financial penalties – due to rigorous environmental emissions targets – and severe corporate reputational damage. For petrochemical projects in construction or turnaround, leaks can also severely disrupt schedules, causing delays, cost overruns, and uncertainty when production finally begins. One offshore multi-asset operator calculated the average cost of their UK leaks to be in the region of US$100 000 per leak in repair time and materials alone.
In addition, commissioning companies estimate that up to 20% of integrity test budgets can be consumed by reworking bolted flange joints that leaked during a system test. In worst case scenarios, leaks can lead to a full shutdown, mobilisation of emergency services, evacuation and casualties. It is against this backdrop that there is a need for the highest industry standards for both welding and bolting.
When dealing with welded joints, engineers must comply with the American Society of Mechanical Engineers (ASME) B31.3 for process piping welding standards. This stipulates strong material control, and documented and approved procedures with only formally coded welders able to perform the welding. In addition, the standard requires that welds are tested using non-destructive techniques and verified through hydro and/or gas-testing methods. All these processes are recorded with full traceability.
Yet, given their same purpose, why have the mandatory standards for welded joints not been applied to bolted connections? Things are changing in this respect too as industry guidance has emerged to raise standards for bolting. A key reason for this is the increased acceptance of specific bolting techniques when tightening bolted joints.
Increased control to bolt tightening
Tightening a bolted joint is not as simple as it may first appear. Variables, such as the design standard, flange size, the choice of flange/gasket/bolt material, choice of lubricant and the method of assembly, all work together to determine the optimum values and procedures to use.
When it comes to actually tightening the joint, there have historically been two schools of thought: controlled and uncontrolled tightening. Whereas uncontrolled tightening is essentially a ‘best guess’ methodology where a hammer is used to hit a manual wrench and turn the nut, controlled tightening uses trained and competent personnel who follow prescribed procedures and are supported by calibrated and measurable equipment.
There are two main controlled tightening techniques: firstly torque tightening using a hand or hydraulic wrench to turn the nut down the length of the bolt; and secondly bolt tensioning, which works by stretching the bolt axially so that the nut can be turned down by hand. Both have the same goal - to use the elastic properties of the bolt to compress the joint and seal the gasket.
Controlled bolt tightening ensures known, controllable and accurate bolt loads. It also provides greater uniformity of bolt loading – especially important on gasketed joints as an even and consistent compression is required for the gasket to be effective.
Importantly, controlled bolt tightening also imposes a ‘safe operation’ approach using prescribed procedures, eliminating the dangerous activities of manual uncontrolled tightening and requires that the operators be skilled and follow procedures. Controlled tooling also removes many of the uncertainties surrounding in-service joint failures and ensures that the correct assembly and tightening of the joint are carried out first time round.
There is little doubt that controlled bolt tightening has increased industry acceptance of bolting.
Enhancing standards for bolting
While bolting technicians had not been held to the same competence standards as welders in the past, this has now changed. Industry guidance has gradually emerged over the last few years to raise standards for bolting.
Figure 2. Technician assembling a MorSafe flange clamp.
Two major publications concerned with the assembly and management of bolted flange joints were released in 2013. CEN (the European committee for standardisation) published EN1591 Part 4 as a standard entitled ‘Flanges and their Joints – Part 4: Qualification of Personnel Competency in the Assembly of the Bolted Connections of Critical Service Pressurized Systems’. ASME then released an updated PCC-1-2013 Guidelines for Pressure Boundary Bolted Flange Joint Assembly to include Appendix A relevant to the ‘Training and Qualification of Bolted Joint Assembly Personnel.’
The growth in bolting training programmes
Since the early 1990s, supporters of bolting have been developing industry training programmes, facilitating a common approach and increasing the degree of control and competence needed to secure leak-free bolting.
In 1994, the Engineering Construction Industry Training Board (ECITB) certified a range of specialist bolting training programmes including the Mechanical Joint Integrity (MJI) programme available from 2011. Approved training courses derived from the ECITB Mechanical Joint Integrity technical training standards cover isolations, dismantling techniques, inspection of components, alignment techniques and clamp connectors, as well as assembly and tightening techniques in specialist critical bolting. Training and customer endorsement in this area has been further enhanced though ‘Step Change in Safety’, a member-led organisation committed to improving safety in the oil and gas industry in the UKCS.
It is important to add that experience and knowledge are also often confused with one another. Training alone does not guarantee competence and experience alone does not verify knowledge or understanding. The ‘Route to Competency’ is a combination of four essential elements which follows a documented and traceable path of evidence including: structured training, workplace experience, initial assessment, and ongoing assessment.
The competence of the personnel involved in the disassembly, inspection, repair, assembly and tightening of bolted connections – including subcontractors during a shutdown when resource is at its peak – is fundamental to the joint integrity assurance process. Developments in training have made this a reality.
The benefits of seeking a third party
Today the use of third party specialists and access to experienced, skilled site service personnel with the highest levels of training and competency have further increased control of the process of bolted flange joint assembly and reduced the risk of leaks.
For a petrochemical facility where there may be hundreds of thousands of bolted joints, for example, the complete outsourcing of all requirements for controlled flange management, and integrity assurance to specialist bolting contractors, is an attractive option for many companies looking for greater control and leak elimination.
Taking this approach also encourages the adoption of global standards in controlled bolting, flange management, and integrity assurance, ensuring that a ‘right first time’ approach is adopted, as opposed to a ‘leak and fix’ or ‘search and repair’ approach. The latter approach tends to be adopted by companies without the specialist knowledge or competencies required to assure bolted flange joint integrity.
A joint integrity management system
Managing a plant-wide bolting programme is aided by the adoption of a joint integrity management system including a flange management programme with an embedded bolt load calculation engine for common flange standards such as ASME B16.5, ASME B16.47, API 6A and API 17D. In this way, joint integrity programmes offer the same quality control and assurance requirements as a welded joint assembly.
Figure 3. Technicians being trained in hydraulic tensioning.
Recommended bolt loads for standard joints should be derived from independently verified calculation methods (such as ASME PCC-1) and traceable back to these standards. It should include common gasket materials and configurations, comprehensive flange and bolt materials, and a wide range of lubricants.
The use of such a system enables storage of the entire lifecycle data relating to every critical bolted joint on an installation. Moreover, it provides managers and technicians with essential information about joint history, the components used in assembly, and the tools and the torque or tension values to deliver a leak-free joint.
In addition, all previous history and experience of the joint is available whenever the joint is worked on, allowing any particular requirements of the joint to be taken into account proactively prior to joint assembly and tightening.
This can also be used to aid planning, ensuring joint integrity, and can reduce construction and maintenance schedules and costs. The correct information needed by technicians also removes the possibility of guesswork and/or incorrect procedures. It is also important that such a system should be implemented at the start of a project from design and through to installation and eventual decommissioning.
A compelling alternative to welding
Today, the technical demands of bolting are every bit as important as those of welded joints. Through the evolution of controlled bolt tightening, enhanced bolting standards, high quality training and competency, and joint integrity management systems, controlled bolting provides a compelling alternative to traditional welding in the petrochemical industries. The result is a project or facility where joints are properly installed and certified, leading to reduced leakage from assets, improved safety and environmental performance, more efficient operations and reduced costs.
Written by Mike Williams, Hydratight, UK.
Read the article online at: https://www.hydrocarbonengineering.com/special-reports/18082020/tightening-the-bolts/