A Guide to Flare, Relief, and Blowdown Systems

1. Purpose and Principles

The primary purpose of a flare, relief, or vent system is to provide a safe and controlled method for disposing of flammable, toxic, or hazardous materials from a process. These systems are the most critical safety layer for protecting equipment from overpressure and for managing emergency events.

A poorly designed flare system can constrain plant operability, create a new hazard, or fail to operate correctly during an emergency, potentially leading to catastrophic equipment failure.

This guide outlines the best practices for the design, sizing, and selection of these critical safety systems, based on common industry standards (such as API 520, 521, and ASME B31.3 & ASME VIII).

2. Components of a Typical Flare System

A flare system is comprised of four main parts that work together to safely handle a release:

1. Flare System Sources: The individual devices that release fluid into the system. This includes Pressure Safety Valves (PSVs), rupture disks, automatic blowdown valves (BDVs), control valves (CVs), and drains.
2. Gathering System: The network of pipes (laterals and the main header) that collects the fluids from all sources and transports them to a central disposal point.
3. Knock-Out (KO) System: A vessel (or "KO Drum") that uses gravity to remove and collect any liquids from the vapor stream. This is critical, as flares are designed to burn vapor, and discharging burning liquids is a major hazard.
4. Flare and Vent System: The final disposal device, typically a flare stack or vent, which releases the vapors to the environment. Flares combust the vapors, while vents release them uncombusted.

3. Basic Steps for Flare System Design

A flare system design follows a logical, step-by-step process to ensure all scenarios are captured and the system is sized correctly.

1. Establish the Design Basis: Document all assumptions, standards, and governing philosophies.
2. Identify Relief Scenarios: Identify all credible overpressure scenarios (e.g., fire, blocked outlet, power failure) for each piece of equipment.
3. Calculate Relief Loads: Quantify the flow rate (kg/h) and properties for each individual relief case.
4. Identify Blowdown Systems: Determine which systems require emergency depressuring (blowdown) for fire survivability.
5. Calculate Blowdown Loads: Calculate the transient (time-based) flow rate for each blowdown case.
6. Determine Coincident Loads: This is the most critical step. Analyze plant-wide failures (e.g., total power loss, cooling water failure) to find the combination of events that results in the maximum simultaneous flow to the flare. This "global" case typically sizes the main header, KO drum, and flare stack.
7. Review System Segregation: Determine if any streams are incompatible and require their own dedicated flare system (see section 4.3).
8. Size the KO System: Size the knock-out drum to handle the liquid from the worst-case scenario, typically providing 20-30 minutes of holdup time.
9. Size Headers and Laterals:
   • Main Header: Sized by the governing "global" coincident load.
   • Laterals/Tailpipes: Sized by their own "individual" load, which is often a different, more severe case for that specific pipe.
10. Perform Hydraulic Analysis: Model the flare network to confirm backpressures, velocities, and acoustic/flow-induced vibration risks.
11. Perform Radiation & Dispersion Analysis: Set the flare stack height and location to ensure thermal radiation and gas dispersion (in a flame-out) are within safe limits for personnel and equipment.
12. Perform Low-Temperature Study: Check for low temperatures from Joule-Thomson (J-T) cooling, especially in blowdown lines, to select correct materials and prevent brittle fracture.
13. Document Everything: Create a comprehensive flare system design basis document that can be referenced for the life of the plant.

4. Defining the Design Loads

4.1 Pressure Relief (PSV) Loads

These are calculated based on specific overpressure scenarios as defined by API 521. Inlet and outlet lines for a PSV must be designed for the rated capacity of the valve, not just the calculated relief load.

4.2 Blowdown (BDV) Loads

Blowdown systems are designed to depressure equipment during a fire, reducing internal pressure as the vessel heats up to prevent rupture. The required rate is based on vessel survivability, with a common empirical guideline being to depressure the vessel to 50% of its design pressure within 15 minutes.

4.3 Coincident Loads (The Governing Case)

4.4 Segregation of Flare Streams

It is often unsafe or impractical to combine all streams into a single header. Separate flare systems (e.g., an HP flare and an LP flare) may be required. Segregation is necessary to prevent:

• Cryogenic and Wet Streams: Mixing a cryogenic stream (e.g., from a propane refrigerant PSV) with a water-rich stream can freeze the water and plug the flare header.
• Hot and Volatile Streams: Mixing a very hot gas (e.g., from a reactor) with a volatile liquid can cause rapid, explosive boiling in the header.
• High and Low-Pressure Sources: A high-pressure (HP) relief event will create high backpressure in the header, which may prevent a low-pressure (LP) source (like a storage tank) from being able to relieve at all.
• Liquid and Vapor Streams: A dedicated liquid disposal system is often safer than attempting to handle large liquid reliefs in a vapor flare system.
• Incompatible Chemicals: e.g., streams that could react or cause corrosion when mixed (like dry acid gas and wet gas).

5. Advanced Flare Design Strategies

5.1 High-Integrity Protection Systems (HIPS)

A HIPS is an instrumented system (e.g., sensors, logic solver, and valves) designed to be reliable enough to prevent an overpressure event from happening in the first place. By using a certified HIPS, a specific relief scenario can sometimes be eliminated, reducing the required size of the flare system. This is an advanced, high-cost solution that requires rigorous lifetime testing and management.

5.2 Staggered Blowdowns

Blowdown loads are characterized by a very high initial flow that quickly tapers off. Instead of sizing the flare for the massive, instantaneous peak of all systems blowing down at once, an instrumented system can be used to "stagger" the sequence. For example:

• Time = 0 min: Compressor systems blow down.
• Time = 1 min: Separation systems blow down.

This reduces the peak flowrate and can significantly reduce the required flare header size. This approach is common in brownfield projects to add new equipment to an existing, constrained flare system.

6. Construction and Physical Design

6.1 Line Sizing and Inlet Pressure Drop (ΔP)

Proper line sizing is critical for PSV stability. If the pressure drop in the inlet piping to a PSV is too high, the valve will "chatter" (open and close rapidly), destroying itself and failing to protect the vessel.

6.2 Wall Thickness: Beyond Pressure

The wall thickness of flare piping is often governed by mechanical integrity, not just pressure containment. A thicker wall is required to resist vibration-induced fatigue.

• Acoustically Induced Vibration (AIV): Occurs downstream of pressure-reducing devices (PSVs, BDVs) operating at choked flow. The high-frequency acoustic energy can cause rapid fatigue failure.
• Flow Induced Vibration (FIV): Occurs in high-velocity lines, especially at bends and T-junctions.

6.3 Material Selection: Extreme Temperatures

Flare systems must be designed for both high and low-temperature excursions.

• High Temperature: Discharges from hot process sources can weaken the pipe material.
• Low Temperature: This is a more common and critical issue. During blowdown, the Joule-Thomson (J-T) cooling effect can cause gas temperatures to drop significantly, leading to brittle fracture of standard carbon steel. A low-temperature study is required to determine the Minimum Design Metal Temperature (MDMT) and specify low-temperature carbon steel or stainless steel where needed.

7. Relief Device Selection and Installation

7.1 Relief Valve (PSV) Selection

The choice between Conventional, Balanced, or Pilot-operated PSVs is critical and depends on the service and, most importantly, the backpressure.

PSV Selection by Backpressure

PSV Selection by Service

7.2 PSV Sparing Philosophy

The decision to install a spare PSV depends on the criticality of the equipment it protects.

• Installed Spare (100% spare):
  • Required for critical equipment that cannot be shut down without a full plant outage.
  • Required for any valve in a "dirty" or fouling process service.
• Future Spare (space and piping connections):
  • Acceptable for non-critical equipment in parallel trains (e.g., one of three pumps).
  • Acceptable for large, clean-service fire-case valves.
• No Spare:
  • Acceptable for equipment in intermittent service.
  • Acceptable for thermal relief valves (TRVs).
  • Acceptable for equipment that can be isolated without affecting production.

7.3 Rupture Disks (Bursting Disks)

Rupture disks are non-reclosing devices that burst at a specific pressure. They are generally not preferred because they empty the entire system inventory, but they are used in specific cases:

• Fast Response: For events like a heat exchanger tube rupture, where a PSV may not open fast enough.
• Corrosive/Fouling Service: A rupture disk can be installed *upstream* of a PSV to protect the valve's internals from the process fluid.

8. Disposal System: Flares and Vents

The final disposal system is chosen based on environmental, safety, and economic considerations.

8.1 Atmospheric Vents

Vents release uncombusted gas to the atmosphere. They are only used for low-flow, near-atmospheric pressure streams where the gas is non-toxic and environmental impact is minimal. They must be continuously purged to prevent air from entering the pipe, which could create an explosive mixture. Dispersion analysis is required to ensure gas does not reach grade or ignition sources.

8.2 Flare Types

Flares are used to combust waste gas, converting it to CO₂ and water. The type of flare is chosen based on capacity, cost, and public visibility.