Leak-free sealing technology

Avoiding leakage is one of the more significant elements contributing to process safety at any petrochemical or chemical processing plant. Yet a technology that is widely accepted as prone to leakage "” the traditional ANSI flange "” is still the most commonly used connection method for piping, pressure vessels, heat exchangers and other interfacing plant components.

The sealing principle of the conventional ANSI flange is to bolt two sealing faces together to compress a gasket and create a seal, with the loading on the bolts and joint integrity largely dependent on bolt torque. As a dynamic connection, however, factors such as thermal expansion and fluctuating cyclical loads can cause movement between flange faces, putting significant stress variations on the bolts and affecting the functionality of the gasket, compromising seal integrity. The pressure within the line is also working to force the two faces apart, making the conventional ANSI flange inefficient, requiring 50% of the bolt load to compress the gasket, leaving only 50% to hold the pressure.

Technologies such as compact flanges and clamp connectors, on the other hand, deploy alternative connection and sealing engineering, incorporating an internal seal ring which, when set up correctly, will provide a leak-free seal. Where leaks are unacceptable, but a welded connection (the ultimate leak-free solution) is not practicable due to its permanency, or because welding is not desirable, these compact flanges and connectors provide the best of both worlds "” combining the integrity of a girth weld with the versatility of a mechanical joint.

How compact flanges work

Compact flanges use radial compression like a conventional flange, but see the gasket of a conventional flange replaced by the internal sealring which sits in a groove in the flange face and works with the seal faces, not against them, to ensure face-to-face mating, so that only low seating forces are required to compress the seal faces. One such example is the SPO Compact Flange, which features a triple seal design for maximum joint integrity.

As the joint is made up, compression is combined with a wedging action thanks to the flange face geometry, which features a slightly convex bevel with the highest point, the heel, adjacent to the bore and a small outer wedge around the outer diameter of the flange face.

As the flange is made up by tightening or tensioning the bolting to pull the two halves together, axial forces are exerted on the taper of the metal sealring and translated into a radial sealing force. The wedging action created as the bevel is closed means the flange faces first make contact at the sealring, then at the heel. As the pre-tension increases, the pressure at the heel increases until full face-to-face contact is achieved, with an environmental seal created at the outer edge, and a dual inner seal. A fully qualified axial seal is created by application of the seal seating stress at the flange heel or inner diameter, while the compressed sealring (which is pre-stressed diametrically, and is independent of pre-stress after the flanges have been mated) provides a second seal. The provision of two metal seals in series based on different sealing methods provides maximum protection and high reliability. Should any accidental leakage occur at the heel, the internal pressure acting on the sealring intensifies the seal action, providing a leak-free joint.

In contrast to traditional flanges, only a small proportion of the bolt pre-load is used to compress the compact flange seal; the majority is transferred through the mating flange faces, with 90% of the loading on the heel area and a minor compressive force transferred through the outer wedge. This creates a static, face-to-face connection which is unaffected by internal pressure or external loads acting on the flange. Neither dynamic nor static loading will degrade seal performance of these compact flange connections or adversely affect the corresponding pipe or interfacing component.

Clamp connectors

The strength of clamp connector technology, on the other hand, lies in its pressure-energised metal-to-metal seal which, unlike a conventional flange, does not rely on bolting to maintain seal integrity. In installation, the hubs are drawn together as the bolts are tightened by the wedging action of the two clamp segments, and compress the sealring to make the first stage self-energised seal. Once in service, the internal pressure reinforces this seal, to the extent that its strength usually exceeds the burst pressure of the pipe.

Clamp connectors do not rely on gaskets and correctly tensioned bolting to maintain joint integrity "” indeed, being transversely mounted the bolts are in fact almost completely isolated from the operating loads. The Techlok clamp connector is one example, providing a joint that can withstand high tensile loads, severe bending loads, and can be used in extreme temperature applications and still retain full joint integrity, with the ability to withstand pressures in excess of 30,000 psi.

Reduced maintenance

Unlike conventional flanges, which require regular maintenance to maintain the seal, modern compact connections require only minimal maintenance. There is no need for periodic bolt tightening, and dynamic loading of the joint is not transferred to the bolts, so the bolts will not fail due to fatigue. Also, unlike a conventional flange where the gasket is vulnerable to the elements, the internal sealring of the latest connection designs is protected from corrosive elements, so maintenance demands are significantly reduced. In the case of the SPO Compact Flange, the external barrier or environmental seal created by the outer wedge when the joint is made up keeps the internal sealring, flange faces and bolts out of contact with the external medium, protecting them from corrosion.

Space and weight savings

In addition to the significant safety advantages offered by these reliable, leak-free pressure connections, clamp connectors and compact flanges provide further benefits in terms of the substantial space and weight savings they offer over conventional ANSI flanges, which tend to be large, heavy and cumbersome, particularly at high pressures. In a conventional flange, the substantial bolt force required to compress the gasket and retain the line pressure requires large bolts and nuts, and a wide diameter bolt-circle to allow for tightening, which in turn requires thicker flange sections to avoid susceptibility to bending. By contrast, the sealing design of the latest compact connections means they are typically 70 to 80% smaller and lighter than conventional flanges for the equivalent pressure rating, making them particularly suitable for applications in confined spaces or congested plants where clearance can be a problem, or where added weight could introduce additional load bearing considerations. Where a conventional 6" Class 2500 flange, for example, will weigh some 172 kg, the equivalent compact flange can weigh as little as 32 kg "” an 82% weight saving.

Conclusion

Given the important role of minimising or avoiding leakage in optimising process safety, connections that offer reliable, long-term zero-leak joint integrity have a valuable role to play.

While there has been a tendency within the industry to stick to conventional ANSI flanges, there are now alternatives better suited to meeting today's petrochemical and chemical processing industry needs.