PetroStreet

What are the major inspection & replacement guidelines for in-operation flare tips?
What are the fundamental regulations, code & standards which are applied - US or other regulators.
Need to have an outlined understanding on the subject please.

I would strongly recommend you to read EI guidelines for the Integrity Management of Flare Tips. This is widely considered the most comprehensive framework for managing flare health, especially in the North Sea and Middle East. Unlike API 537, which focuses heavily on design, the EI guidelines focus on the lifecycle and risk-based inspection (RBI) of the tip.

The EI guidelines categorize inspections into three distinct levels of detail:
Level 1: Routine Ground/Deck Observation
Frequency: Daily or Weekly.
Method: Binoculars or high-zoom ground-based cameras.
Focus: Checking for "Flame Lick" (flame hugging the leeward side of the tip) and ensuring all pilots are visible.
Indicator: Excessive smoke or unusual noise (whistling or roaring), which suggests internal damage to the steam or air injection nozzles.

Level 2: Remote Aerial Survey (UAV/Drone)
Frequency: Annually, or after a major "Emergency Blowdown" event.
Method: 4K Video and Thermal IR.
Critical Checks:
Tip Distortion: Checking for "ovality" or warping of the top circumference.
Pilots & Ignitors: Inspecting the mechanical bracketry of the pilots. EI notes that pilot failure is often mechanical (vibration fatigue) rather than just "blowing out."
Refractory Health: Looking for cracks or "spalling" in the internal lining.

Level 3: Close-Out / Turnaround Inspection
Frequency: During scheduled plant shutdowns (Typically every 5–8 years).
Method: Physical "hands-on" inspection and Non-Destructive Testing (NDT).
Critical Checks:
Dye Penetrant (PT): On welds to find thermal fatigue cracks.
Ultrasonic Testing (UT): Measuring wall thickness of the tip shell, especially near the top 1 meter where heat is most intense.

Would suggest to have a baseline: Perform a UAV survey immediately after a new tip is installed to have a "Day 1" reference.
Better than do a trend: Compare annual UAV footage side-by-side to track the rate of metal loss or deformation.
Take an action: If a "Level 2", as mentioned by novice123, survey shows probable metal loss or significant cracking, move the replacement to the next available window.

I think the operating side monitoring helps as well before we try to look for actual damages:
Some three parameters which come to my mind:
Purge Gas Flow: Monitoring the flow rate to ensure it stays above the minimum required to prevent oxygen ingress and flame lick.
Thermocouple Data: Continuous monitoring of pilot temperatures. A sudden drop signals a flame-out; a sudden rise might indicate burn-back.
Acoustic Monitoring: Used in some advanced systems to detect internal combustion or flow irregularities by listening to the tip.

Most in-operation failures are due to Thermal Fatigue and High-Temperature Corrosion (Sulfidation).
Also need to consider, creep & stress rupture: Because tips operate at temperatures where the metal softens, it is recommended to check for elongation or sagging of the tip structure.
And the major sigma phase embrittlement: In certain stainless steels, long-term heat causes the metal to become brittle like glass.
novice123 has already covered everything on inspection part of identifying these damages.