Piping systems in petrochemical refineries and oil and gas installations rely on hundreds of bolted joints that are exposed to corrosive conditions.
To ensure safe operation at these plants, it is vital to carry out regular inspections to assure joint integrity is maintained. Until recently, the normal procedure to do this was to physically disassemble each flange joint to check on its condition. Using this method is costly, as it requires shutdown and in many cases is entirely unnecessary. Before we look at the alternatives, let’s first take a look at the different types of flange corrosion.
Crevice corrosion is the “bugbear” of the oil and gas industry and is caused by a buildup of concentrated corrosive substances in the small confined area between two adjoining flanges. Because of this concentration, the rate of corrosion is much faster than elsewhere.
Environments containing seawater, acid or H2S are especially problematic, putting the flange face and the joint’s integrity at risk for potentially serious consequences.
In addition to crevice corrosion, flange face corrosion is also widely recognized as an integrity issue, due to the potential leak path created by the break-in line. This can be in the form of small pits, often appearing in clusters on the flange’s sealing face.
Flange face inspection, repair
As mentioned above, to disassemble a flange joint requires considerable cost in terms of both man-hours and shutdown time, but technology now exists to enable noninvasive checks to be carried out while the pipe joint is still in service.
Phased array technology is now used by specialist inspection companies to identify the condition of the flange face gasket seating surface condition.
This uses ultrasound technology, but with multi-element probes to modify beam properties such as angle, standoff distance and focal properties. Phased array inspection is now growing in popularity thanks to its flexibility and the compact size of the units now commercially available.
When inspecting the raised flange face area, encoded magnetic scanners direct sound from the outer diameter of the flange between the bolt holes. The ultrasound angles can be adjusted to fit the specific flange geometry and the multiple ultrasound angles used to optimize data displays of corrosion and/or erosion.
When a flange face is damaged, it is no longer sealable by a gasket and should be either replaced or repaired.
In general, there are four basic types of repair to consider: 1. Removing the corroded flange and welding a new one, 2. On-site machining of the sealing face or ring groove within the flange tolerance, 3. Adding material (weld buttering runs) and site machining the sealing face or ring groove, or 4. Using polymer composite repair materials to rebuild the flange face.
Portable on-site machines available to help
Flange face repair — To machine the flange face sealing surface, a flange facing machine is the ideal product to get the job done quickly and accurately. A wide range of machine options are available, including models that can be mounted internally or externally (and, in some cases, both). Smaller machines can be used for facing diameters as small as 2 inches, with the largest capable of facing up to a 120-inch diameter. In addition to machining a conventional flat sealing face, many machines can be used with various attachments or conversion kits for machining compact flange joints and grooves.
Flange removal — When repair of the flange is not possible, the remaining option is to remove the flange entirely. This can be carried out using a split frame/clamshell pipe cutter. As its name suggests, this machine comprises two sections that are fixed together and clamped around the pipe diameter.
Bolt removal — This flange bolt cutter (or bolt slitting saw) is designed for removing seized flange bolts and offers a spark-free cutting solution. It works by cutting each bolt through the gap between the two adjoined flanges and can be used in either a conventional or upside-down orientation.
For more information, visit www.miragemachines.com or call (281) 859-1234.
