DesignIn a conventional pipe joint, there are two raised-face flanges bolted together with a gasket inserted in between. There is no contact between the flanges at the bolt circle. When the bolts are tightened, a toroidal moment is induced, causing the flanges to rotate about a primary centre located about one-third of the width of the gasket from the outside. In service, however, under load generated by internal fluid pressure, axial or lateral forces and/or bending moments, the flanges rotate about two secondary centres.
The problem with the gasketed pipe-joint is that the loads acting on it are balanced by the rotational resistance in the cross-section of the flange. This means that the degree of warping, or angle of flange rotation, varies according to axial/lateral forces. As a result, gasket wear or failure induced by fatigue in the bolts are likely.
Worse still, concentrated bending occurs in the fillet under the bolt head, already at a load equivalent to about 20% of the bolt yield strength. So, although some design codes claim that their figures on allowable bolt stress are conservative, in reality they are somewhat unsafe, which explains why incidents of conventional pipe joint leaking are so common.
When gross leakage does occur offshore, a shutdown of operations becomes mandatory. This entails shutting off valves, cooling down equipment, purging lines with inert gas and other stringent procedures. This in turn leads to lost production and unexpected costs.
A solution to these problems is a safer, alternative pipe-joint method, namely the Verax Compact Flange (VCF) system. This provides flanges in full-face, metal-to-metal contact with no gasket in between. Consequently, a perfectly static mode operates, regardless of fluid pressure, bending moments or axial/lateral forces.
Normally no seal is incorporated, although in special cases an elastomeric O-ring or a metallic seal may be installed in a groove. Through use of high strength, high quality bolts and nuts and by applying modern principles of bolted joint engineering, the weight of such a modern, non-gasketed pipe-joint is reduced to less than one fifth of an equivalent standard gasketed type. In general, any pipe-joint according to the VCF system can be made at least as strong as the associated pipe itself.
The system's package covers not only flanges, but also bolts, nuts and washers, a set of proper tools for the pre-loading of bolts, plus education, training and instructions to the mechanics assigned to the installation of VCF-joints, and complete documentation. Pumps and valves can be equipped with flanged connections, complete and ready for installation by welding into the piping system. Through the application of such a simple and logical procedure, the alignment of joint faces is no problem, while at the same time, they are protected against dirt and damage.
No special surface finish of the flange faces is needed, merely one produced by an NC-lathe in good condition. And there are no real limits regarding size, maximum operating pressure, temperature or medium contained in the pipe.
Over the years, several experiments have been performed to verify the theories and principles of VCF-joint engineering. Among these, a small vessel with pipe of 2-in. OD and 0.43-in. wall thickness was constructed, with a VCF-joint in the middle, and tested for tightness using helium as a tracer-gas and a mass spectrometer. The surface seal was made of pure gold.
Results showed that at least 500 years would pass before 1 gram of helium would escape. The unit was then tested for strength by raising the pressure. The joint was originally designed for 693 bar maximum working pressure and 1,314 bar minimum guaranteed pressure. At 1,683 bar, the hydraulic system could go no higher, but the VCF-joint remained intact.
In another experiment, three vessels of 2-in. OD and 0.14-in. wall thickness, each with a VCF-joint in the middle, were charged with a rocket propellant and blown to pieces at the Bofors proving grounds in Sweden. Although the pipe walls yielded/burst, the VCF-joints stayed intact.
Based on the results from numerous experiments and from years of difficult applications, the VCF-joint can be considered safe as there is no gasket to blow and no dynamic load to cause bolts to fail from fatigue. Nor are there tensile stresses in the flanges as high as those in the pipe itself.
Authors Note: This is an edited version of a paper presented at the Institute of Marine Engineers conference, Innovative Technology for Challenging Environments, held in London, March 1997.
Verax flange supported
Professor Thomas Gray is head of the Department of Mechanical Engineering at Strathclyde University, Glasgow. The Department is well known for its expertise in pressure vessel engineering, and was recently contracted by an international oil company to evaluate conventional versus compact flanges.He has had ten years of contact with Verax, having been invited by Jan Webjorn to test to destruction a small pressure vessel with a Verax joint. This test was "highly successful", he concluded. According to Professor Gray, "my colleagues and I are well aware of the shortcomings of conventional flange design (leakage, inability to cope with severe thermal cycling, fatigue failure). The structural principle of the Verax joint is sound and offers many fundamental advantages over the conventional type of joint, apart from reduced size and weight.
"The statements and claims made by Verax with regard to comparisons between compact joints and conventional joints are upheld and are in no sense exaggerated. In particular, the use of properly specified bolting materials with quality controlled tightening procedures will give much higher structural integrity than the conventional design, where much lower bolt stresses have to be imposed."
Copyright 1997 Oil & Gas Journal. All Rights Reserved.
