Abstract

Flare tips are essential for safety. Maintenance is difficult and costly. Flare tips are subjected to high combustion temperatures, thermal cycling, oxidation and marine corrosion. Following a number of flare tip failures an in depth study by Imperial College was carried out into the failure of a flare tip from a UK platform, looking for service life improvement. Materials selection and design solutions were considered. The study considered alternative materials and concluded that materials selection was the smaller part of the answer; design changes can double service life. This study used failure investigation, high temperature experimental and thermo-mechanical modelling analysis.

The modelling process simulated two common flaring conditions and correctly predicted the observed failure of initiation and crack propagation from holes used to bolt on flame stabilizing plates to the top of the flare. The calculated thermal stress and strains enabled the low cycle fatigue life and minimum creep life to be predicted. It was concluded that service life could be improved by replacing Incoloy alloy 800HT (UNS N08800) with Inconel alloy 625 (UNS N06625), an alloy with attractive mechanical properties and improved high temperature corrosion resistance. Repositioning or eliminating bolt holes can double service life.

Introduction to flare tips

Flare tips are an essential part of a hydrocarbon producing platform's safety system. Flare systems allow the safe disposal of inventory. Flare tips are subjected to high combustion temperatures, thermal cycling, oxidation and marine corrosion. The full cost of a flare tip failure and planned replacement was £9 million in 2004; improving flare tip life would increase both safety and cost performance.

There are a number of common flare tip designs; two of the most common are the vertical chimney pot and the inclined tulip. The chimney is more common and is the focus of this study. Failure of the chimney design is progressive and in extreme cases, vertical cracks and distortions split the flare tip causing the tip to appear 'y-shaped'.

Flare tip life is highly variable and failures frequently occur early in the platform life when well tests are frequent, although, some tips last many years. Figure 1 shows a typical distribution of flare tip life (including failed and currently operating). The solid boxes show recent failures (mean service life 5.4 years) and the broken boxes show the life of the current flare tips (which increases the mean service life to 6.3 years).

As safety devices, flare tips are subjected to extremely varied and unpredictable service conditions. The USA Environmental Protection Agency[EPA, 1997] has suggested that gas flow rates can range from 50 kg/hr for relief valve gas leakage rates to 750 T/hr under emergency situations. Outlet gas temperatures were estimated to range from -50°C to room temperature (from process modeling). Under conditions of high gas flow rates, flame temperatures can rise to 1100°C or more, while at low gas flow rates the flame temperature is about 600°C. Flare tips are designed to operate at maximum gas flow rates producing a vertical flame. However, such conditions seldom apply and with low gas flow rates and a high wind velocity, external burning with the flame impinging on the outer surface of the flare tip is a common feature (Figure 2). Under these conditions the flame is in contact with the metal, which even for short exposures would heat it to a high temperature. However, verifiable information of actual in service metal temperatures is not available.

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