OIL & GAS EQUIPMENT | Updated August 2026 | 8 min read

Sour gas flares handle waste gas streams containing hydrogen sulfide (H₂S), and the design requirements differ materially from sweet gas applications. Carbon steel components, standard elastomers, and pilots designed for methane-rich gas all fail prematurely in H₂S-laden service unless the system is specified for the actual gas composition.

Hero Process Solutions, founded in 2011 and headquartered in Kellyville, Oklahoma with operations in Midland, Texas, manufactures flare systems and combustion equipment for the full range of upstream and midstream sour and sweet gas applications. This guide covers the engineering decisions that matter when specifying a sour gas flare for production, gathering, or processing service.

1. What Makes Sour Gas Flare Design Different from Sweet Gas Applications

The defining characteristic of a sour gas flare application is the presence of hydrogen sulfide in the waste gas stream. H₂S changes the engineering picture in four ways.

Corrosion attack on carbon steel. H₂S causes sulfide stress cracking (SSC) in high-strength carbon steel at stress levels well below the material’s yield point. Flare headers, knockout drums, and tip components in sour service must be specified per NACE MR0175/ISO 15156, which sets hardness limits and approved alloy designations for wetted metallic components.

Combustion chemistry. H₂S burns to produce sulfur dioxide (SO₂) rather than carbon dioxide and water. Incomplete combustion produces sulfur compounds including COS and CS₂. Combustion temperature and residence time requirements for a sour gas flare are set to achieve complete oxidation of H₂S, which means pilot and assist medium design cannot simply be copied from a sweet gas application.

Low BTU and variable composition. Sour gas streams, particularly in amine treating unit tail gas and sour water stripper overhead service, often have low heating values and variable H₂S concentrations. Maintaining stable combustion across wide BTU and composition swings requires pilot sizing and assist medium control that account for the actual heat content range.

SO₂ dispersion and air quality permitting. When H₂S combusts, every mole of H₂S produces one mole of SO₂. State and federal air quality agencies require SO₂ dispersion modeling to establish a ground-level SO₂ impact before a sour gas flare permit is issued. Stack height, assist medium selection, and site meteorology all affect the modeling outcome.

2. Metallurgy Selection for Sour Service Flare Components

NACE MR0175/ISO 15156 divides sour service materials into three categories based on H₂S partial pressure and pH of the aqueous phase. For above-ground combustion equipment, the relevant requirements focus on the flare header and knockout drum where liquid condensate may contact the gas stream.

At the flare tip itself, the operating environment is primarily high-temperature combustion gas, which reduces SSC risk but introduces oxidation and thermal cycling attack. Alloy 625 (UNS N06625) and 316L stainless steel are commonly specified for sour service flare tips because they combine sulfidation resistance at elevated temperature with NACE-acceptable corrosion behavior at ambient conditions during outages.

3. Combustion Temperature and Efficiency Requirements for H₂S Destruction

Destroying H₂S in a sour gas flare requires combustion conditions that ensure complete oxidation. The minimum temperature to achieve reliable H₂S-to-SO₂ conversion in a flare flame is generally recognized as 1,400°F (760°C) with adequate residence time. Below this threshold, partial combustion produces reduced sulfur compounds, which are more toxic than SO₂ at the same concentration.

For EPA 40 CFR 60 Subpart OOOOb-affected facilities, the 98% combustion efficiency requirement applies to the total organic compound fraction of the waste gas, not specifically to H₂S. However, state air permits in Texas (TCEQ) and Oklahoma (OEPA) often impose supplemental H₂S combustion efficiency requirements specific to the sour gas flare, depending on H₂S concentration and proximity to populated areas.

Air-assist flare designs are frequently specified for sour gas service because the turbulent mixing zone created by the assist air improves combustion completeness across variable-BTU sour streams. The air-assist flares product line at Hero Process Solutions is available in sour service metallurgy configurations. For low-flow sour streams such as amine unit vent gas, vapor combustors offer an enclosed combustion alternative with tighter temperature control than an open flare.

4. Pilot System Design for Sour Gas Flare Service

Pilot reliability is non-negotiable on a sour gas flare. An unlit pilot with active H₂S gas flow creates an immediate toxicity hazard downwind of the flare tip. Pilot system design for sour service must address three issues that do not arise in sweet gas applications.

Pilot gas supply isolation. The pilot fuel supply on a sour gas flare must be segregated from the waste gas header. Using sour waste gas as pilot fuel introduces variable BTU content, potential for H₂S-induced flame instability, and corrosion of small-bore pilot gas piping. Clean, sweet pilot gas from a separate supply is the correct specification.

Pilot tip metallurgy. The pilot tip operates in the same sour combustion environment as the main flare tip. NACE-acceptable alloys must be specified for pilot tip hardware. Standard carbon steel pilot tips corrode rapidly in H₂S flue gas environments.

Ignition system for variable-BTU streams. Electronic ignition systems on sour gas flares must be designed for reliable ignition across the full heating value range of the expected waste gas. High-energy spark systems combined with retractable pilot designs for high-flow service provide the ignition reliability required. The ignition systems offered by Hero Process Solutions include configurations for sour and variable-composition gas service.

5. SO₂ Dispersion Modeling and Air Quality Permitting

Every state air quality agency that permits a sour gas flare at a facility above specified H₂S throughput thresholds requires SO₂ dispersion modeling as part of the permit application. The modeling input data includes:

• Waste gas H₂S concentration and mass flow rate
• Flare tip stack height
• SO₂ emission rate calculated from H₂S combustion stoichiometry
• Site meteorological data: wind speed, wind direction distribution, and atmospheric stability class
• Terrain data for complex terrain sites

The modeling output establishes predicted ground-level SO₂ concentrations at the site boundary and at nearest receptor locations. Results must demonstrate compliance with NAAQS SO₂ standards: 75 ppb 1-hour average at the primary standard. In areas designated non-attainment for SO₂, the analysis is more complex and may require offsets.

6. Hero Process Solutions Equipment for Sour Gas Flare Service

Hero Process Solutions manufactures sour service combustion equipment for upstream production, midstream gas processing, and amine treating unit service. The flares product hub covers the full product line available in sour service configurations.

For upstream sour production sites, air-assist portable configurations on trailer-mounted skids are available for short-duration well test and startup service. For permanent sour gas flare installations at production facilities or gas plants, elevated air-assist or gas-assist designs with NACE-rated headers and alloy tips are the standard specification.

Field services from Hero Process Solutions cover commissioning, pilot system verification, and OOOOb compliance testing for new sour gas flare installations. Contact sales@hero-ps.com or (918) 941-2166 to discuss a sour service application.

7. Common Mistakes in Sour Gas Flare Specification

Mistake	Why It Hurts Operations or Compliance	Fix
Using sweet gas flare spec in sour service	SSC failure of carbon steel header within months	Specify NACE MR0175/ISO 15156 materials for all wetted components
Using sour waste gas as pilot fuel	Pilot flame instability at low-BTU periods	Segregate clean pilot gas supply from waste gas header
Skipping SO₂ dispersion modeling before permit	Permit denial or costly stack height redesign	Run dispersion modeling in parallel with equipment specification
Standard elastomers in H₂S environment	Rapid seal and gasket failure	Specify H₂S-rated elastomers (Viton, PTFE-encapsulated) throughout
Undersizing pilot for variable-BTU sour streams	Unreliable ignition; unburned H₂S releases	Size pilot for minimum expected BTU content, not average
Ignoring thermal cycling on alloy tip during outages	Fatigue cracking at tip welds	Specify controlled cool-down procedures and alloy tip designs rated for thermal cycling

Article Summary

• A sour gas flare is designed specifically to combust waste gas streams containing H₂S, requiring engineering that differs from sweet gas applications in metallurgy, combustion design, pilot specification, and permitting.
• NACE MR0175/ISO 15156 governs materials selection for sour service flare headers, knockout drums, and tip hardware, with a maximum HRC 22 hardness limit for all carbon steel components in H₂S service.
• Complete H₂S combustion to SO₂ requires temperatures above 1,400°F (760°C) with adequate residence time. Below this threshold, partial combustion products including COS and CS₂ are generated.
• EPA 40 CFR 60 Subpart OOOOb’s 98% combustion efficiency requirement applies to sour gas flares at affected facilities, with supplemental H₂S combustion requirements commonly imposed by state permits.
• Air-assist flare designs are preferred for sour service because the assist air improves combustion completeness for variable-BTU, variable-composition H₂S streams.
• Pilot gas supply must be segregated from sour waste gas to prevent flame instability and pilot corrosion in H₂S service.
• SO₂ dispersion modeling is required in most jurisdictions before a sour gas flare permit is issued, and must begin in parallel with equipment specification.
• Thermal cycling and sulfidation attack at the flare tip require alloy specifications, including Alloy 625 or 316L SS, for sour service tip hardware.
• Hero Process Solutions manufactures sour service flare configurations from its Kellyville, Oklahoma facility, with field commissioning support available for new installations.

Frequently Asked Questions

What is a sour gas flare and when is one required?
A sour gas flare is a combustion device designed to safely destroy waste gas streams containing hydrogen sulfide (H₂S). Sour gas flares are required at production facilities, gas processing plants, and amine treating units where H₂S is present in vent gas, relief gas, or other waste gas streams that must be combusted. When H₂S concentration in the waste gas exceeds the threshold defined in applicable air quality permits, sour service equipment specifications and SO₂ dispersion modeling become mandatory.

How does H₂S concentration affect sour gas flare design?
Higher H₂S concentrations drive three design decisions: higher corrosion allowances and tighter NACE metallurgy requirements for headers and fittings, stricter minimum combustion temperature to ensure complete H₂S-to-SO₂ conversion, and higher SO₂ emission rates requiring more detailed dispersion modeling and potentially greater stack height to meet ground-level SO₂ standards.

What metallurgy is required for a sour gas flare?
NACE MR0175/ISO 15156 governs material selection for sour service flare components. Carbon and low-alloy steel is permitted with a maximum hardness of HRC 22 and post-weld heat treatment verification. Flare tips and pilot hardware operating in the combustion zone are typically specified in Alloy 625 or 316L stainless for combined sulfidation resistance at high temperature and H₂S corrosion resistance during outages.

Can a standard flare be used for sour gas service?
Standard sweet gas flare designs should not be used for sour gas service without material and design upgrades. Carbon steel headers without NACE qualification are susceptible to sulfide stress cracking. Standard elastomers fail in H₂S environments. Pilot designs sized for methane-rich gas may not reliably ignite variable-BTU sour streams. All three issues must be addressed before a flare is placed in sour service.

What SO₂ permitting is required for a sour gas flare?
Most state air quality agencies require SO₂ dispersion modeling demonstrating compliance with NAAQS SO₂ standards (75 ppb 1-hour average at the primary standard) before issuing a permit for a sour gas flare at a regulated facility. Texas TCEQ and Oklahoma OEPA each have specific permit pathways for sour gas flare installations. Begin the permitting process in parallel with equipment specification to avoid project delays.

Does Hero Process Solutions manufacture sour gas flare systems?
Yes. Hero Process Solutions manufactures combustion equipment for sour and sweet gas service from its Kellyville, Oklahoma facility. Sour service configurations include NACE-rated headers, alloy flare tips, and segregated pilot gas supply systems for H₂S-bearing waste gas streams. Contact sales@hero-ps.com or (918) 941-2166 for application engineering support.