Sealing performance assessment of in‑service flanges is an important part of piping integrity management. Unlike quality control during manufacturing, in‑service inspection faces special challenges such as limited space, inability to disassemble, and unknown operating conditions. A progressive screening methodology from macro to micro enables effective identification of potential risks in flange connections without affecting operation.
Level 1: Macroscopic visual inspection. This is the most basic and rapid screening method. Inspection items include: visible leakage traces at the flange connection (rust, stains, frost, or ice), obvious bolt loosening or missing bolts, signs of gasket extrusion or blow‑out, and visible flange deformation or misalignment. Although limited in precision, macroscopic inspection can quickly identify “suspect objects” requiring further attention.
Level 2: Indirect assessment of bolt preload condition. When direct measurement of bolt preload is not possible, various indirect methods can be used for judgement. The hammer tap method is the most traditional—striking the bolt with a hammer and judging whether it is loose by the crispness of the sound. More precise methods include using ultrasonic bolt stress meters, which infer remaining preload from changes in acoustic transit time through the bolt. The goal of this screening level is to identify bolts whose preload has significantly decayed—these are the objects requiring priority attention.
Level 3: Near‑field assessment of sealing face condition. If conditions permit (e.g., the flange is accessible and the system can be briefly isolated), a borescope or fibrescope can be used for near‑field observation of the sealing face. Key inspection points include: corrosion, pitting, or cracks on the sealing face; signs of gasket aging or hardening; and traces of medium seepage at the sealing face gap. This level of assessment can provide first‑hand information on sealing face condition but requires system isolation and appropriate preparation.
Level 4: Indirect leakage monitoring based on operating parameters. For flanges that cannot be isolated, changes in system operating parameters can be monitored to indirectly judge sealing condition. For example, if the flow balance downstream of the flange becomes abnormal, this may suggest a small leak at the flange. Temperature monitoring can reveal local temperature anomalies caused by evaporation or condensation of leaking medium. This method requires establishing baseline data, and its sensitivity to changes depends on the precision of the monitoring system.
Level 5: Specialised non‑destructive testing. For critical flanges, when opportunities for shutdown maintenance arise, ultrasonic, radiographic, and other NDT methods can be used to inspect the flange body and welds. Ultrasonic testing can detect crack or inclusion propagation within the flange; radiographic testing can reveal defects near the sealing face. Although these methods require shutdown coordination, they provide the most in‑depth quality information.
In‑service flange inspection strategies should be differentiated based on risk assessment. High‑risk flanges (toxic media, high temperature and pressure, critical paths) should receive more frequent and in‑depth inspection; low‑risk flanges may have appropriately simplified inspection procedures. The value of progressive screening lies in quickly filtering out “healthy” flanges at the lowest cost and concentrating inspection resources on the objects that truly require attention.
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