Process safety - with leak-free sealing technology

With process safety high on any operator's priority list, new technologies are now available to ensure leak-free connections: a review by Ian Robinson of Vector International

Avoiding leakage is one of the more significant elements contributing to process safety at any refinery, petrochemical, chemical, pharmaceutical or other processing plant.

And 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 of plant components.

Where once this was because there was no real alternative, this is no longer the case.

The latest pressure connection and sealing technologies promise leak-free performance.

The conventional ANSI flange, on the other hand, while it has provided the industry with a sealing solution for decades, has not been significantly developed or improved in that time.

The time has come to recognise the failings of this traditional technology and consider alternative, more reliable options, which are now available.

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, ultimately compromising seal integrity.

Moreover, the pressure within the line is in effect working to force the two faces apart, so the conventional ANSI flange is essentially an inefficient seal, requiring 50 per cent of the bolt load to compress the gasket, leaving only 50 per cent to hold the pressure.

Compact flange and clamp connector technologies, 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 connection and sealing technologies provide the best of both worlds - combining the integrity of a welded joint with the versatility of a mechanical one.

The SPO Compact Flange, for example, features a triple seal design for maximum joint integrity.

The gasket of a conventional flange is replaced in the compact flange by an internal seal-ring 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.

The radial compression as the flange is made up is, importantly, combined in the compact flange with a wedging action.

This is brought about by 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.

When assembled, the flange faces first make contact at the seal-ring, 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.

Only a small proportion of the bolt pre-load is used to compress the seal; the majority is transferred through the mating flange faces, with 90 per cent of the loading on the heel area and a minor compressive force transferred through the outer wedge, creating a static, face-to-face connection which is unaffected by internal pressure or external loads acting on the flange.

Importantly, the SPO Compact Flange design creates a dual inner seal, in addition to the environmental seal at the outer edge.

When the joint is made up the metal-to-metal seal created at the inner diameter of the flange faces is a fully qualified axial seal, while the compressed seal-ring (which is pre-stressed diametrically and is independent of pre-stress after the flanges have been mated) provides a second, radial, 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 seal-ring intensifies the seal action.

Once pressure-tight, these compact flanges will remain pressure-tight, providing a leak-free joint.

Neither dynamic nor static loading will degrade seal performance or adversely affect the corresponding pipe or interfacing component.

The strength of clamp connector technology such as Techlok, 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 seal-ring 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 - in fact, being transversely mounted, the bolts are almost completely isolated from the operating loads.

The resulting joint can withstand high tensile loads, severe bending loads, and can be used in extreme temperature applications and subject to transient thermal conditions, while still retaining full joint integrity, with the ability to withstand pressures in excess of 30,000 psi.

Where zero-leak performance is critical - as is the case in many process industry sectors - these compact connections offer significant advantages.

In one typical example, compact flanges were selected for a modification to introduce an adiabatic loop to improve the efficiency of a cracker at a vinyl chloride monomer (VCM) plant, where the volatile and hazardous pipe content involved in the VCM process meant leakage was unacceptable.

The compact flanges meet these high integrity requirements, while offering the flexibility of a mechanical joint (an important advantage when it comes to, for example, the decoking process).

Moreover, 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 in the case of the compact flange, for example, dynamic loading of the joint is not transferred to the bolts, so the bolts will not fail due to fatigue.

Further, the internal seal-ring is protected from corrosion (unlike the gasket of a conventional flange), and the external barrier or environmental seal created by the outer wedge when the joint is made up keeps the seal-ring, flange faces and bolts out of contact with the external medium, protecting them from corrosion and in turn significantly reducing maintenance demands.

At one refinery these minimised maintenance requirements were a significant driver in the selection of compact flanges for the re-tubing of a furnace convection bank and replacement of the outlet transfer lines to the radiant bank (being upgraded from carbon steel to stainless steel).

Given the flange locations at 15 metres above ground, making them inaccessible during normal operation without scaffold erection, the ability to install the flanges with the confidence that they would not leak or need regular attention was of high value.

In addition to the significant safety advantages offered by these reliable, high integrity 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 these latest compact connections means they are typically 70 to 80 per cent smaller and lighter than conventional flanges for the equivalent pressure rating, making them particularly suitable for application in confined spaces or congested plant where clearance can be a problem, or where added weight could introduce additional load bearing considerations.

Where a conventional six inch Class 2500 flange, for example, will weigh some 172kg, the equivalent compact flange can weigh as little as 32 kg; an 82 per cent weight saving.

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 process industries to stick to conventional ANSI flanges if only because they have been the traditional method, it is time to take a fresh look.

Vector International is a specialist in compact connection and sealing techniques, and can advise on these now proven alternatives that are better suited to meeting the processing industry needs of today.

The author, Ian Robinson, is vice president for marketing and sales at Vector International.

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