Piping Design

Understanding compact pipe connection technology benefits

How lighter-weight, smaller-diameter bolted flanges with internal sealing features offer advantages over conventional flange systems

 The most popular pipe connection method throughout the oil and gas industry is the ANSI flange. But, despite its popularity, it is a technology that has not been developed to meet today’s operational demands. An overall lack of knowledge regarding modern pipe connection alternatives is causing industry to fail to capitalize on the benefits that high-performance compact connectors offer. Described here is the basic problem/limitations of conventionally used flanges and the basic design/advantages of compact flanges developed by Vector International. 

  THE PROBLEM

 The ANSI flange has provided industry with a suitable sealing technology for decades, often to great success but, whereas industry’s demands have changed, the ANSI flange has not. Cost efficiency, environmental responsibility and optimum health and safety are three issues facing 21st century operators/contractors. 

 The ANSI flange sealing principle is straightforward – bolt two seal faces together to compress a gasket and create a seal. To maintain this seal, enormous bolt force must be maintained and, to achieve this, bolts are made larger. Bigger bolts means bigger nuts, which means a wider diameter bolt circle. The wider the diameter of the bolt circle, the more susceptible flanges become to bending; and the only recourse is to increase flange section thickness, Fig. 1. Combined, this means significant size and weight – a particular problem offshore. Further, the ANSI flange is essentially an inefficient seal, e.g., 50% of the bolt load is required to compress the gasket, leaving only 50% to hold the pressure. 

Fig. 1. Comparison of SPO Compact Flange and conventional ANSI type flange, each for 6-in. welding necks, Class 2500. Weights of the two flanges, respectively, are 32 kg and 172 kg, a weight saving of 82% for the compact flange.

 The primary fault of the design is that it is not guaranteed leak-free. The connection is dynamic, and factors such as thermal expansion or fluctuating cyclical loads can cause movement between flange faces. This affects gasket functionality and compromises the seal integrity, leading to leakage. 

  THE SOLUTION

 The compact flange (CF) provides everything that a standard flange does not – space saving, reduced weight and, perhaps most importantly, a guaranteed leak-free connection. 

 The CF’s reduced dimensions are achieved by reducing the seal diameter which, in turn, reduces cross-sectional area of the seal faces. Secondly, the gasket is replaced by a seal ring which 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. With lower pressures needed, both size and number of bolts can be reduced, ultimately providing a smaller, lighter-weight connection (70% to 80% smaller and lighter than ANSI flanges in the case of Vector’s products). 

 The key advantage of the CF over the ANSI flange is that it will not leak, providing cost savings by avoiding lost product and maintenance, as well as improving health and safety. In a CF, only a small amount of total bolt pre-load is used to compress the seal. The major part of the bolt pre-load is transferred through the mating flange faces, creating a static face-to-face connection unaffected by internal pressure or external loads. Once pressure-tight after assembly, a compact flange, being a static connection, will always remain pressure-tight. 

 Vector’s SPO Compact Flange (SPO CF) takes the compact sealing principle still further, essentially providing a double seal – face-to-face mating and internal seal ring. In more than 50,000 applications worldwide, it reportedly has never leaked in service. 

  UNIQUE DESIGN FEATURES

 Vector’s design is unique in that flange faces include a slightly convex bevel with the highest point (the heel) adjacent to the bore, and an outer wedge around the outside diameter of the flange, Fig. 2. As with the ANSI design, the flange is made up by tightening bolts, which pulls the two connector halves together. When assembled, the flange faces first make contact at the seal ring, then at the heel. As pre-tension increases, pressure at the heel increases, until full face-to-face contact is achieved, and the bevel is closed. An environmental seal is created at the outer wedge. The full face-to-face contact makes a completely static joint with no relative movement between seal faces. The seal created at the inner diameter of the flange faces is a fully qualified seal. 

Fig. 2. Cross section of the SPO Compact Flange illustrating the three sealing functions initiated as bolt clamping force is applied, including, successively, the seal ring, the heel and the wedge.

 Independent of the heel seal, a second seal is the seal ring, which is compressed inside grooves on the flange faces according to dimensions and their tolerances. Should any accidental leakage occur at the heel, the internal pressure acting on the seal ring inside the flange forces it toward the outer-diameter, intensifying the sealing action. 

 Further, the external wedge barrier prevents ingress of corrosive agents from damaging the bolts or the internal seal ring, making the compact flange ideal for use in harsh operating conditions, like those offshore, and when interfacing different metals with a potential for galvanic corrosion. This is an advantage over the ANSI flange, which is susceptible to corrosive element effects, with ramifications for seal integrity. 

  APPLICATIONS OFFSHORE NORWAY

 The no-leak capabilities of the SPO CF have been proven recently during modification work on Norsk Hydro’s Troll C Floating Production Unit (FPU) to tie it into the Fram West field on the Norwegian Continental Shelf. 

 Conventionally, the modification work would have required welding; but, with the significant man hours, preparatory work and procedures required to create a gas-tight habitat in which welding could commence (all of which inherently drive the cost of construction), Norsk Hydro looked to alternative options. 

 Typically, flanged spools are not accepted as a construction method for such piping modifications and welding is preferred, it being guaranteed not to leak. In this instance, however, Vector’s compact flange design was accepted as an alternative to a fully welded system, due to its static design which guarantees no leaks. 

 A further recognition of the flange’s strength is that it has been accepted by the Norwegian Petroleum Directorate as suitable for applications on risers above sea surface – where traditionally only welded joints are permitted – regarding its strength and integrity as equal to a girth weld. 

 The application on Troll C is just one example. SPO CFs are in operation on piping systems with design temperatures up to 720°C and down to -175°C, and pressures up to 15,000 psi, exceeding the ANSI pressure classes 150 to 2,500. 

  CONCLUSIONS 

 In medium- to high-pressure piping, and in situations where volatile substances are being transported and leakage is unacceptable, compact flanges have been proven to be the optimum connection method. Yet the “comfort factor” with the traditional flange design leads to a propensity toward the ANSI flange. 

 With issues of assured joint integrity and reduction in hydrocarbon emissions more important than ever, compact technology has a valuable contribution to make in reducing leaks, as well as providing weight and space savings. There certainly seems to be a trend developing. Compact flanges are now the connectors of choice in the North Sea and there are widespread applications for the technology in deepwater developments, making it reasonable to suggest its use could increase on such projects.

 By reducing topside weight of tension leg platforms required to access deepwater petroleum reserves, the Department of Commerce’s National Institute of Standards and Technology (NIST) estimates $250,0000 could be saved per meter of water depth – a significant amount. 

 Vector International’s SPO Compact Flange is just one example of a technology that has proven itself to be a reliable, safe and cost-effective connection method. And, although use of such products is increasing, all too often it is relegated to where leaks are a critical concern. It is time for the oil and gas industry to start recognizing the numerous potential benefits on offer.