PHASED ARRAY NDT
Raise Face Flange Face Inspection
Utilizing this phased array technologies allows for inspection of complex geometries, which are generally too difficult to performed with conventional ultrasonic techniques. Phashed array opens the door for inspections that are thought to be impossible or impractical. One of these ‘impossible’ examinations is the inspection of Raised Face Flanges.
When raised face flanges are used in very corrosive environments such as hydrofluoric acid (HF) or C02 service, the flange face is susceptible to corrosion which in turn can lead to subsequent failure. With our phased array technique and specially designed probes & wedges, the flange face can be inspected with detection capabilities as low as 0.020” from the flange face.
With a properly calibrated probe and with depth beam focusing at the flange face, then examining the flange from between the bolt holes, a sectorial display will depict a cross sectional view of the flange (as shown above).
While using the phased array instrument’s flaw cursor feature, place them on the ultrasonic geometric responses of both the flange face (Horizontal) and the flange throut (Vertical) thus creating a cross-hair at which this cross-hair should be directly over the corner trap of the flange, creating a flange face profile (see below). No ultrasonic responses should deviate from this artificial cursor profile and the profile should be perfect 90 degrees; if this is the case, then it would be safe to say no corrosion is present. If any ultrasonic responses create a profile deviating upward form the artificial cursor profile, typically at an angle from the flange face, or you see a corner trap response further to the left of the vertical cursor position (throut profile) then this is a good sign corrosion is present. By using the same flaw cursors, this corrosion can be measured in terms of thru-wall height and width. Flange face corrosion is typically 360 but has been found in isolated areas.
Types of Flange Face Corrosion
This method is an excellent screening technique for flange face corrosion, however it must be noted that it is not perfect and does not have 100% detectability; an example of this is HF corrosion. HF corrosion can cause a slight taper to the flange face which can be difficult to detect at times and it has been know that corrosion with a slight slope of 0.050″ can cause failure. With this said, phased array is still a good technique to screen flanges for face corrosion and can be done successfully when performed correctly. The only other alternative would be to break open all flanges for visual inspection during plant shut-down and this can become very costly.