Hi
Today, I want to share critical insights about flare system design - a fundamental aspect of process safety that every engineer should master. This is because the flare is the final component of the relief system that handles the whole relief load from the plant, whether from pressure relief valves or from depressuring/blowdown scenarios as we have gone through before. If it failed, it shall paralyze the whole plant or even lead to catastrophic accidents.
Why Flare System Knowledge Matters
The flare system is more than just a pipe with a flame at the end. The specific service conditions dictate the type of flare you need, and getting this choice wrong can have major consequences for safety, the environment, and the project budget.
In this email, we'll discuss 5 flare types that may be considered depending on the fluid service, operating conditions, and plant layout.
✅ 1. Typical Elevated Flares
These are the most common flares you'll encounter in refineries and petrochemical plants. Used for general hydrocarbon service, its design is governed by strict parameters like velocity limits (0.2 Mach for normal operation, up to 0.5 Mach for short-term events). Heights typically range from 30-200 meters, determined by thermal radiation limits and ground-level pollutant concentration requirements.
✅ 2. High-Pressure Flares
Operating at pressures of 2-6 barg, these flares can achieve sonic velocity at the tip. They're commonly used as a relief destination from sources at a very high pressure as it will accept much higher backpressure.
This provides 3 advantages over a normal elevated flare:
- High backpressure will allow for smaller flare header and subheader sizes
- High velocity leads to excellent air mixing, leading to better combustion and dispersion.
- Requires less height than a conventional low-pressure flare for the same service due to better dispersion.
✅ 3. Acid Flares
These are specifically designed for H₂S-rich streams and other corrosive gases. The entire system, from piping to the tip, is typically constructed from stainless steel to resist corrosion. Segregating acid gas streams into a dedicated flare is a common strategy to avoid the massive cost of building a single, large flare system out of expensive alloys. However, auxiliary fuel gas may be needed if relief gas is of endothermic nature.
✅ 4. Cold Flares
For cryogenic services (e.g., ethylene, LNG) with temperatures below -29°C, a cold flare is essential. Materials like Low-Temperature Carbon Steel (LTCS) or stainless steel are used to prevent brittle fracture.
When there's significant normal-temperature load at normal temperature, vs the cryogenic load, cold flares should be segregated to protect the main carbon steel flare header from dangerously low temperatures while avoiding a large flare system with a higher grade material same as acid flare.
✅ 5. Offshore Flares
Platform limitations drive unique solutions. They're often installed on angled booms that use horizontal distance to minimize heat radiation on the platform, and extensive radiation shielding is common to protect personnel and equipment.
Since steam is rarely available offshore, they use air-assisted or water-assisted tips for smokeless flaring.
Practical Application Tips:
- Study well the applicable relief scenarios and relief fluid nature and conditions for a proper flare system design
- Consider segregation strategies early in design if various flare types are needed
- Document your design basis thoroughly, including expected flare segregations and their conditions
- Always verify flare tip diameter and pressure drop with vendors to avoid excessive backpressure
That's it for today! If you'd like to understand more about the main criteria for relief system sizing, you can check out the below video:
I hope you enjoy these emails. If you have suggestions related to the content to be presented in emails, feel free to reply to this email.
Have a great weekend!
Mohamad
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