Corrosion management program : X-Ray : Radiography
An effective corrosion management program predicts the remaining life of equipment such as piping systems, pressure vessels, and storage tanks. A radiographic or ultrasonic measurement of a wall thickness by itself only reflects the current condition. For an effective program, the rate of wall thickness metal loss must be established. The best way to establish the rate of wall loss is to have a complete initial survey (baseline survey) followed at a later point in time by another survey (re-inspection). The change in wall thickness divided by the time between surveys gives the real rate of wall loss.
Radiography is the use of x-rays and gamma rays to produce a radiographic type image (radiograph) of an object on a sheet of x-ray film. A sheet of film is placed behind the object (pipe), and a focused source of high energy x-rays is projected through the object to the film (sidewall shot). Thicker areas of steel will block more of the x-ray from reaching the film than will thinner areas. If the object is a pipe, the wall thickness can be measured on the film. Radiography is recommended for confirming suspect areas revealed by ultrasonic inspection. It is used to detect the deterioration of welds and the adjacent heat affected zones revealing wall loss.
Ultrasonic (UT) technology is used as a non-destructive procedure for measuring wall thickness in steel piping systems, vessels, and tanks. An instrument is used to generate an ultrasonic frequency signal on one side of a wall. When the signal reaches the opposite side of the wall, it is reflected back to the instrument. The instrument determines the wall thickness by measuring how long it took for the signal to return. Ultrasonic examination is recommended as the primary means of inspection.
Re-inspection is generally a follow-up survey to establish corrosion (wall loss) rates and to verify that a system is not having problems. Re-inspection of specific pieces of equipment may be needed to verify suspect data, or to scan an area for thickness averaging in accordance with API 510 guidelines. A piece of equipment may be inspected with an alternate technology (such as ultrasonic vs. radiographic) if a problem is suspected and needs to be verified.
BRINELL hardness testing is an accepted procedure for determining the approximate tensile strength of shell and head materials used to build pressure vessels. In the absence of other information, the values can be use for MAOP calculations. A BRINELL hardness tester will damage the protective coating of the vessel.