Pressurised systems in the oil and gas industry require secure and safe connections. Recent leaking joints have been the main cause of hydrocarbon releases on the UKCS offshore sites and a number of other facilities are earmarked as potential hazard areas too.
Following on from these incidences, the Energy Institute on behalf of Oil and Gas UK, came up with recommendations for best practice within the industry to eradicate future damage to the environment and reputation.
One aspect raised by the report was the usage of secure bolted flange joints. Here's 6 best-practice joints highlighted in the report that the industry can rely on including tips for better flange integrity management.
The most common type of joint is made up of 2 pipe flanges to ASME B16.5 design code, between which a gasket is compressed by the installed bolting.
The principle of a bolted joint is based on the bolting delivering sufficient joint compression and gasket sealing stress to withstand maximum service pressure and forces.
For integrity a minimum level of operational gasket seating stress must be maintained throughout joint service, therefore the design bolt load and compression target on installation should allow for creep, relaxation and tolerances of components.
Various types of compact flanges have been developed by specialist manufacturers. Some use gasket arrangements similar to the metallic ring joint whereas others use metal to metal. gasketless contact and the joint becomes a static entity with minimal flange rotation.
The design philosophy can vary from type to type so the manufacturer should always be consulted for advice on joint sealing, design bolt tension and installation procedures.
Clamped connectors use a split clamp to join the pipe. Hubs at the ends of the pipe have tapered shoulders sloping towards the joint and the clamps which form a wedging action to close two hubs together.
The hubs have internal sloping faces which bear on taper ring gaskets, causing them to be distorted elastically and form as seal.
Flanges and clamped connectors
Like pipes, flanges and clamped connectors operate differently under varying conditions of temperature and pressures. The most critical area on a flange or clamp is its sealing face, on which the gasket or seal ring sits to form a pressure retaining seal.
It is therefore paramount that the sealing face's surface finish complies with the design specification from the manufacturer.
On ASME B16.5 type flanges, nut sealing areas at the back of the flange must always be clean and have a smooth finish, reducing friction (unless stated by your manufacturer). All flanges and associated bolts and nuts will also carry some identification such as size, pressure, rating and flange material.
Gaskets and seal rings
Correct gasket or seal ring selection and installations are important and only those specified in the piping material should be installed. There are three main types of gasket:
- Made from elastomers, cork, compressed fibres, plate minerals and PTFE. They are generally used for low to moderate pressure and temperatures and used in many acid and alkaline applications.
- A combination of a non-mettalic filter for strength and flexibility, semi-metallic gaskets are adapted for high temperature and pressure applications. Spiral wound and Kammonprofile are also examples of semi-metallic gaskets.
- Made from a combination of metals in a variety of shapes and sizes, these are metal rings that fit into grooves that have been machined into flange faces. RX and BX rings are typical examples of these.
Correct bolting selection, procurement and installation are vital as only the one type of bolt will be sufficient for the flange to be fitted. Bolts are designed to carry pressure end load at the gasket as well as providing the load required to compress the gasket flange face together.
Bolts, nuts and washers used for joints should be clean, rust free and undamaged, However, fasteners can be considered for reuse after tracking their service history, original requirements and last environment. For any coated bolts and fasteners, these should also still have a 100% surface coverage.
Photo Credit: UL