Flare System Piping and Header Design: Protecting Plant Safety

II JAY SHRI KRISHNA II

A Flare System is a critical safety element in process industries like oil and gas, petrochemical and chemical plants. It is designed to safely dispose of excess or undesirable gases by controlled ignition. These gases create from various sources, such as relief valves, blowdown valves and emergency vents.

Flare System Piping and Header Design: Protecting Plant Safety

The primary function of Flare System Piping is to safely and efficiently transport potentially hazardous relieved gases from their source to a designated flare stack for controlled combustion. These gases are typically released, due to overpressure conditions caused by; process upsets, equipment failures or emergency shutdowns.

Flare System Flow Diagram

Types of Gas Disposal Systems:

There are commonly three types of Gas Disposal Methods to ensuring the Plant Safety:

1. Atmospheric Discharge: Releasing gases directly to the atmosphere under specific conditions.

2. Pressure Relief to Vessel: Transferring gases to a lower pressure vessel for further processing or storage.

3. Flare System: Controlled combustion of gases in a flare stack.

Criteria for Atmospheric Discharge:

Atmospheric discharge is permitted only if:

• Gas is in vapor phase.

• Gas temperature, is below auto ignition point.

Gas meets at least one of these criteria:

• Molecular weight (MW) less than 2/3 of air (MW 28.83) and flammable.

• Molecular weight between 28.83 and 80, flammable, and discharged at minimum velocity of 152 m/s without creating hazards.

• Non-toxic, non-flammable, non-hazardous, and non-condensable, regardless of molecular weight.

Note: Strict adherence to environmental regulations is crucial for atmospheric discharges.

Now, this post will delve into the critical role of Flare Systems and their associated piping in managing gas disposal within industrial plants.

What is a Flare?

Flare is a purging device which is capable of,

• Rapidly leading the toxic & hazardous gases to a safe distance from the operating plant and burn before allowing it out to the atmosphere, and

• At the same time, taking care that heat intensities do not exceed tolerance limit of human beings working around it.

Types of Flare:

1. Elevated Flare:

• Tall vertical stack, which is either imitated with a supporting structure or self-supported.

• Combustion is at higher elevation, hence does not need large unobstructed area thus less area required.

• Main disadvantages: high initial cost, difficulty in maintenance & flame visibility.

2. Ground Flare:

• No supporting structure is required, as the burning is carried out at ground level.

• Required long space as it has to be well isolated from process plant, minimum unobstructed area is about 75-100m.

• Main disadvantage: High area requirement.

Potential Causes of Overpressure:

Overpressure can arise from various operational disturbances, including:

• Equipment failures: Breakdown of critical equipment like pumps (cooling water, reflux, and feed), fans, compressors, or heat exchangers.

• Utility disruptions: Loss of power, cooling water, or instrument air supply.

• Process upsets: Blockages, valve errors, or uncontrolled reactions.

• External factors: Fires or extreme weather conditions leading to increased heat input or equipment damage.

These factors can disrupt normal process conditions, which leading to pressure build-up and necessitating the use of Flare Systems.

Importance of Flare System Piping:

• Safety: Stops hazardous gas release, protecting personnel & equipment in plant.

• Environmental Protection: Controls emissions by, directing gases to controlled combustion.

• Process Consistency: Make sure safe operation, during upsets or emergencies.

• Efficiency: Maximizes gas transportation, to the flare stack.

• Asset Integrity: Protects equipment, from overpressure damage.

• Compliance: Adherence to safety & environmental regulations.

• Design Codes: Design based on ANSI B31.3 (Process Piping) and API RP 521 (Pressure Relief).

Factors Affecting Flare System Selection are:

The selection of a Flare System is influenced by few critical factors which are:

• Gas properties: Composition, flow rate, and pressure.

• Environmental rules: Emissions, and safety standards.

• Plant needs: Capacity, emergency relief, and cost.

• Site conditions: Space, terrain and climate.

Considerations for Capacity Calculations in Plant Fire Case

Fire Scenario Assumptions:

• Equipment Inventory: Assume all equipment is isolated and contains normal liquid levels plus liquid from tower trays.

• Fire Exposure Area: Consider equipment and piping within 7.6m (25ft) above grade or fire-supporting platforms for fire exposure.

• Fire Circle: Assume all equipment within a 21.4m (76ft) diameter circle contributes to relief load, with only one fire considered per area.

• Hot Insulation Credit: Allow for hot insulation systems that can withstand flames.

Piping Requirements:

• Condensation Prevention: To prevent liquid accumulation and increased PSV set pressure, all relief lines must slope towards a knockout pot (KO pot) located upstream of the flare header. Discharge lines should slope at a 45-degree angle towards the flare header.

• Self-Draining Design: Suction and discharge lines must be designed to prevent liquid traps & ensure proper drainage.

Rationale:

Proper piping design is crucial to prevent liquid accumulation in relief lines, which can hinder PSV performance and potentially lead to equipment failure. By following to these guidelines, the integrity of the safety system is well maintained.

Pressure Drop Criteria and Flare System Components

Pressure Drop Criteria for Safety Valves (API 521)

To ensure optimal PSV performance, pressure drop limitations apply:

• Suction Line: Pressure drop from the protected equipment to the PSV inlet should not exceed 3% of the set pressure.

• Discharge Line: The combined pressure drop through the discharge line, sub-headers, and main header must not exceed 10% of the individual PSV set pressure.

Calculation Methods:

• Node Concept: Pressure is calculated, at each connection point in the system.

• Finite Element Method: A more detailed analysis for compressible gases.

Flare System Components and Terminology:

• Flare System: The overall system planned, to safely dispose of waste gases.

• Flare Header: The pipeline collecting relief valve discharges, vents, etc., and directing them to the flare stack.

• Knockout drum: Takes out liquids from the gas stream, before it enters the flare header.

• Water seal drum: Prevents flashback of flames, into the system.

• Quench Drum: Cools gas before flare stack, preventing soot. Uses water spray to reduce temperature & remove impurities.

• Flare Stack: The tall vertical structure that routes gases to the atmosphere and ignites them using a flare tip.

Design Aspects: For Flare Header Sizing & Adequacy Check

Flare Header Sizing:

The procedure for Flare Header sizing includes, the following steps:

• Identifying the Maximum Flare Loading: Determine the worst-case scenario for gas discharge.

• Estimating Relief Load: Estimate, the discharge capacity for each safety relief valve.

• Developing Pressure Profile: Create a pressure distribution map for the flare header.

• Checking Header Adequacy: Ensure the header can handle the calculated flow without exceeding allowable back pressure.

Adequacy Check for Flare Header

The suitability of a Flare Header is determined by:

• Back Pressure Limits: Define maximum allowable back pressure for conventional, balanced bellows, and pilot operated safety valve types.

• Economic Evaluation: Compare the costs of using, conventional valves with higher-sized piping versus opting for balanced bellows valves.

• Noise Control: Restrict flare header Mach number to 0.7 for noise reduction.

• Flare Header Segmentation: Consider dividing the Flare Header into sections, based on safety valve set pressures for potential cost savings.

Inlet Piping:

• Pipe Sizing: Match the inlet pipe diameter to the safety valve inlet for optimal flow.

• Pressure Drop: Maintain inlet pressure drop below 3% of set pressure to prevent valve chattering.

• Block Valves: Eliminate block valves, upstream of safety valve. If required, use a full-bore valve locked open.

• Corrosion Protection: Install a rupture disk upstream of the safety valve for corrosive fluids.

Layout Considerations:

• Drainage: It design for the system, from the safety valve to the flare knockout drum for self-drainage.

• Valve Elevation: Position safety valves higher than the flare header to prevent condensate accumulation.

• Manual Drains: Include manual drain valves at safety valve outlets if self-draining is impractical.

• Top Connection: Connect safety valve outlet lines to the flare header from above to avoid condensate backup.

• Liquid Reliefs: Direct liquid reliefs to a nearby drain funnel or discharge within 150mm of grade.

• Atmospheric Vents: For high-elevation atmospheric vents, terminate the outlet pipe at least 3m above surrounding platforms.

• Accessibility: Maintain platform accessibility, for safety valve maintenance.

• Weep Holes: Install a 1/4" weep hole at low points in the outlet piping to prevent condensate buildup.

Flare Header Materials, Supports, and Insulation

Materials:

• Carbon Steel: Often used due to its cost-effectiveness. Suitable for most hydrocarbon services.

• Stainless Steel: Working for highly corrosive or high-temperature services.

• Alloy Steel: Used in specific cases where extreme conditions necessitate specialized materials.

Supports:

• Pipe Supports: Designed to withstand the weight of the flare header and its contents, including thermal expansion forces.

• Structural Steel: Provides additional support for heavy headers or complex configurations.

• Seismic Restraints: Essential in regions prone to earthquakes to prevent damage.

Insulation:

• Thermal Insulation: Protects personnel from burns, prevents equipment damage from thermal stress, and minimizes heat loss.

• Acoustic Insulation: Reduces noise levels generated by high-velocity gas flow.

• Insulation Material: Commonly used materials such as; mineral wool, fiberglass, and ceramic fiber.

Simulator Use for Flare Header Sizing

• INPLANT: Utilize the INPLANT simulator for analyzing flare line and header networks.

• Flow Rate Input: Define flow rates from individual safety valves.

• Back Pressure Calculation: Calculate back pressures based on flow rates and pipe sizes.

• Pipe Sizing: Determine optimal pipe sizes to achieve desired back pressures.

• Safety Valve Sizing: Obtain safety valve sizes based on calculated parameters.

By following these guidelines, engineers can design efficient and safe Flare Header Systems.

Conclusion:

Flare Systems are vital for safely managing extra gases in process plants. With Proper piping design, including material selection, sizing, and layout, it is crucial for system efficiency & plant safety, by adhering to design codes and seeing factors like pressure drop, noise and drainage ensures optimal performance.

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