Facilities should have a written program to ensure flanges are properly made up. Proper makeup of every flange in a piping system is important for reliability. Proper makeup includes:
- the use of the proper gasket and fastener material,
- type, and size, proper positioning of the gasket, and
- proper torquing of the fasteners
The quality assurance program should include procedures for gasket and fastener selection and for fastener torquing. ASME PCC-1 offers sound guidance on the proper makeup of bolted flange joints. The program can incorporate varying degrees of sampling, visual inspection, field testing, and destructive testing of components.
Gasket selection can usually be confirmed by visually examining the gasket’s color and markings on the OD surface. Spiral-wound gaskets should be marked and color-coded in accordance with ASME B16.20. Fasteners can be visually examined for proper stampings or markings, and PMI tested in accordance with API 578.
Proper gasket positioning and torquing depend on the training and craftsmanship of the pipe fitters making up the flanges. Gasket positioning can be checked visually. Proper torquing is difficult to check, but flange deformation can indicate improperly torqued fasteners.
Flanged joints should be visually inspected for cracks and metal loss caused by corrosion and erosion when they are opened.
Flange joints can be inspected while in service by applying single-element or phased array UTs to the external surfaces to measure flange face corrosion and to detect ring groove cracking. Flange bolts should be inspected for stretching and corrosion. Where excessive bolt loading is indicated or where flanges are deformed, a nut can be rotated along the entire length of the stud. If the stud is stretched, the thread pitch will be changed and the nut will not turn freely. Inspection involves checking to determine whether bolts of the proper specification have been used, and it may involve chemical analysis or physical tests to determine the yield point and the ultimate strength of the material. If flanges are bolted too tightly, they can bend until the outer edges of the flanges are in contact. When this occurs, the pressure on the gasket can be insufficient to ensure a tight joint. Visual inspection of the gasket will reveal this condition. Permanently deformed flanges should be replaced or refaced.