LTCS or SS: Material selection in Flare system design.

Just recently, someone asked me: what’s the difference between a HP and LP Flare?

It was a simple yet interesting question that made me pause and reflect.

Well, in oil and gas operations, flare systems are essential for safely releasing excess gases during emergency depressurization or routine maintenance.

LP Flare System collects gases from equipment operating at pressures below 10 barg. This system primarily handles wet gas and design temperatures ranging from -46°C till 210 °C. Piping typically made from low-temperature carbon steel (LTCS), which is suitable for these conditions.

On the other hand, HP Flare System manages gases from equipment with pressures exceeding 10 or 15 barg.

Why do you think stainless steel is preferred for HP flare piping?

The Joule-Thomson effect is a key phenomenon in HP flares. When high-pressure gas expands rapidly through a valve or nozzle, it undergoes adiabatic cooling. This can cause temperatures dropping to almost -200°C, especially during emergency blowdowns.

In addition, HP flares are designed to manage large gas volumes during emergency depressurization. For example, HP flare may need to handle relief loads exceeding 800,000 kg/h, compared to the LP flare’s maximum relief load of 100,000 kg/h. This significant difference arises from the higher operating pressures and volumes associated with HP systems.

The impact of elevated temperatures to material selection:

While most scenarios focus on low temperatures caused by the Joule-Thomson effect, flare systems must also account for elevated temperatures during fire cases ??

In such situations, the system is exposed to extremely high temperatures, and materials must be selected to maintain structural integrity under both extremes. This dual requirement—handling cryogenic cooling and fire heating—makes material selection even more critical.

So the design temperature for HP Flare might be selected as - 200 °C / +200 °C.

Typically, studies and flow simulations is carried out using depressurisation tool in HYSYS V.10, and provided by EPC contractors. These studies help gain critical insights into the adiabatic cold depressurization process, calculate the minimum temperatures that can be reached during depressurization, and ensure that the selected materials meet the required Minimum Design Metal Temperature (MDMT).

If the simulation results indicate that the calculated minimum temperature falls below the capabilities of the initially selected materials (like LTCS) , basically equipment metallurgy cannot withstand, then the study may suggest switching to more suitable materials, such as stainless steel (SS316), to ensure the system’s safety and compliance with the required MDMT.

In addition in simulation you can create "Pressure-Time" profile for Pipeline Depressurisation, similar as in below:

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