Zero leaks and 100% reliability in bolted joints
16 September 2009
Since the early eighties, the gospel of tension control has been preached as an assured method of bolted joint integrity. There are three factors that will assure bolted joint integrity – joint design, quality of components and correct tension across the bolted joint. Get all three right and you have maintenance free, reliable bolted joints.
Zero leaks and 100% reliability in bolted joints
Tightening effort, management procedures, operator competence and training are frequently highlighted as key factors in achieving leak free joints and yet research into bolted joint failures consistently reports that around 90% can be attributed to one factor - insufficient bolt tension at installation.
Traditional tightening methodology measures the tightening force being applied to the bolt - it does not measure the tension being achieved across the bolted joint. Torque does not equal tension! At the end of the tightening procedure the tension across the joint is unmeasured and unknown, often with disastrous consequences.
Another major factor can be the engrained "tightening culture." Promoting prevention can be a long and difficult path when there is a mindset of tightening and then fixing leaks. Add to this an almost in-built reluctance to accept solutions that appear too simple, and the status quo can be easily perpetuated.
However, the good news is that growing numbers of enlightened operators are enjoying the benefits of effective tension control ...
Dow Corning, global supplier of silicone based materials, is using over 4,000 tension control fasteners on high integrity pipe work, vessels and large body flanges at its new plant currently under construction in Jiangsu province, China.
The fasteners have been designed-in because of their successful track record in Dow Corning’s flange management control systems at its plant in Barry, South Wales, where they have been used for a number of years. "We are using RotaBolt fasteners more extensively at the new plant," commented the Senior Mechanical Engineering Specialist at Dow Corning, who is involved with the company’s new projects, "they have had a good record in South Wales and we are using them more, particularly on large body joints, to help assure joint integrity."
The fasteners have been selected by Dow Corning based on two main criteria. Firstly, they them to ensure that a minimum bolt tension has been achieved and to monitor loss of the minimum bolt tension throughout the life of the plant. Secondly, Dow Corning’s requirements on the control of materials were more than met by RotaBolt’s own assurance procedures which see every fastener being 100% load test calibrated.
At South Wales, Dow Corning came to these fasteners as an important "hardware component" in their management of critical joints. Over 10 years ago the company began to investigate all the measures that could be taken to increase the reliability of bolted joints as part of their commitment to high integrity systems.
This resulted in tension control fasteners being fitted on a number of pressure vessels and heat exchangers. They were also used on some of the high integrity pipeline flanges. The company found that another major advantage was the tactile tension monitoring system which meant that an instant "finger test" of the cap on each fastener was all that was required to let the engineer know that the correct load was being maintained.
The application of a technology driven approach to bolted joint integrity recently achieved a total leak-free start at Antwerp’s Nuclear Generation plant following its annual shut down. For the first time in seven years, a 24 inch steam valve remained leak free using tension control.
The 1500lbs steam valve had leaked on every start-up since it had been installed. A technical analysis of the valve and its joints indicated that a complete re-design was necessary if joint integrity was to be achieved.
RotaBolt’s technical team recommended the use of RotaBolt 2 to achieve the necessary installed design tension and for the ongoing monitoring in service. RotaBolt 2 provides an even greater range of tension control on installation, tightening and in-service checking. It offers two tension settings in a single sensor and features a dual load indicator cap – the outer cap for high tension setting and inner cap for low tension – giving the choice of an operational tension range for overload or maintenance control.
The design of the new joint incorporated the use of a Metaflex gasket, the fitting of a silver plated C-ring and new housing dimensions of the male/female flange. Twenty, 3 inch RotaBolt 2’s were used, with an operational tension range set at 125 and 150 tonnes. Test pressure for the valve was set at 130 bar at room temperature, with start-up at 160 bar at increasing temperature. The process pressure was set at 60 bar at 260° C.
The steam valve with its newly designed joints was fitted during the shutdown and remained totally leak-free when the plant was re-started. The Company is now working with Antwerp’s plant engineers to see how tension control can achieve joint integrity in other areas of the plant and its processes.
Just over three years ago, the concept of applying tension control technology to achieve bolted joint integrity was unheard of in Poland. Today, tension control systems are being successfully used in power plants, refineries, chemical processing plants, steel production and in general industry.
The technology has made significant inroads in situations where pressure, vibration and fatigue have been causing bolted joint failure over many years.
"Our breakthrough came when a major refinery owned by Poland’s largest petrol company became the first customer, using tension control fasteners to cure an issue on a heat exchanger," commented Tomasz Rogowski, the company’s representative in Poland, "we had begun promoting the benefits of tension control in 2005, and we found that wherever we went there was a universal openness and willingness to try the technology." The same refinery – PKN Orlen - is now using tension control on seven heat exchangers, with more planned this year.
One of the country’s largest producers of nitrogen fertilisers and plastics was the next to benefit from tension control, fitting fasteners to a high capacity heat exchanger which had caused many shutdowns at its melamine plant. The plant has been running at full capacity, without shutdown, since the technology was applied.
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