Check Valve Installation in Natural Gas Premixed Combustion Systems

Abstract: Natural gas premixed combustion technology is widely used in industrial and residential applications because of its high thermal efficiency and clean combustion characteristics. However, it also presents safety risks, including flashback and reverse flow of the air-fuel mixture. China's national standard GB 55009-2021, Code for Gas Engineering Projects, requires check valves or equivalent protective devices to be installed upstream of gas-consuming equipment in forced-draft premixed combustion systems. This requirement has sparked considerable debate within the industry regarding its scope, technical necessity, and implementation. This paper reviews the operating principles of premixed combustion and the associated safety risks, examining the protective functions and limitations of check valves. By clarifying the distinction between gas-consuming equipment and gas-fired appliances and considering the multiple safety interlocks incorporated into modern combustion systems, the paper argues that check valves should serve as supplementary safeguards rather than the primary means of protection. It also proposes a risk-based safety management strategy and practical recommendations to support standardized engineering practices and improve the inherent safety of gas systems.

As a clean and efficient energy source, natural gas plays an important role in China's energy transition and its “Dual Carbon” strategy. Premixed combustion technology, in which natural gas is mixed with air before combustion, improves combustion efficiency and reduces pollutant emissions. It is therefore widely used in industrial boilers, commercial kitchen equipment, and residential gas appliances. However, premixed combustion also presents safety risks, particularly flashback, in which the flame propagates upstream, and backflow of the gas-air mixture. These phenomena can cause pipeline deflagration, equipment damage, and even casualties.

To address these risks, Article 6.2.6 of China's national standard GB 55009-2021, Code for Gas Engineering Projects (hereinafter referred to as GB 55009), requires check valves or equivalent protective measures to be installed upstream of gas-consuming equipment that uses blower-assisted premixed combustion. Since the standard took effect, this requirement has sparked debate over its scope, technical necessity, and enforcement. Consequently, implementation has been inconsistent, with some cases involving blanket requirements for check valve installation and others involving the omission of protective devices where they are required. Clarifying the intent and scope of this provision is therefore essential to resolving these differences in interpretation, promoting consistent implementation across the industry, and improving the safety of gas systems.

Research on premixed combustion safety has primarily focused on flame arrester optimization, burner flow-field design, and pressure control. Existing studies generally indicate that increasing flow velocity and optimizing burner structure can help suppress flashback. By comparison, research on check valves has mainly addressed valve design and sealing performance, while their specific application in natural gas premixed combustion systems has received relatively little attention. At the standards level, differences exist among China's current technical requirements. GB 50028-2006, Code for Design of City Gas Engineering (hereinafter referred to as GB 50028), requires a check valve or pressure relief device to be installed downstream of the gas meter. However, product and equipment safety standards, including GB/T 19839-2005, General Specifications for Industrial Oil and Gas Burners (hereinafter referred to as GB/T 19839), and TSG 11-2020, Safety Technical Regulation for Boilers (hereinafter referred to as TSG 11), do not explicitly mandate the installation of an external check valve in the gas supply pipeline. These differences in standard requirements have contributed to ongoing industry debate and uncertainty regarding implementation.

This paper investigates the installation of check valves in natural gas premixed combustion systems. It compares relevant standards and evaluates their applicability, examines the combustion characteristics of premixed systems and the operating principles of check valves, and explores risk-based strategies for optimizing their application. Using a framework of theoretical analysis, standards interpretation, and countermeasure development, the study draws on a review of the literature and relevant regulations to develop clear, technically sound, and practical recommendations for engineering applications.

(1) GB 55009

Article 6.2.6(3) states: “Where a blower is used for premixed combustion, measures shall be taken to prevent the gas-air mixture or flame from entering the gas pipeline, such as installing a check valve in the gas pipeline upstream of the gas-consuming equipment.”

The key implications of this provision are as follows:

Scope of application: The requirement applies specifically to premixed combustion systems that use a blower. It does not explicitly cover premixing methods that do not rely on a blower, such as atmospheric aspirating systems based on the Venturi effect.

Terminology: The provision explicitly refers to “gas-consuming equipment” rather than “gas-burning appliances.” This distinction is important when determining which types of equipment fall within the scope of the requirement.

Protective measures: The wording “such as installing a check valve” indicates that a check valve is an example of a protective measure rather than the only permissible solution. Other suitable measures, such as flame arresters or pressure relief devices, may also be considered, provided they meet the applicable safety requirements.

GB 55009 distinguishes between “gas-consuming equipment” and “gas-burning appliances.” The former refers to larger-scale equipment that uses gas as a fuel for heating or mechanical power generation, including gas boilers, gas-fired absorption units, gas heat pumps, gas internal combustion engines, and gas turbines. The latter is a general term for gas-fueled combustion devices, including gas water heaters, gas-fired heating boilers, gas cooking appliances, gas baking appliances, and gas space heaters. The two categories differ in scale, structural complexity, and potential risk, making the distinction relevant to interpreting the scope of the check valve installation requirement.

(2) GB 50028

Clause 10.3.3, “Installation of Gas Meter Protection Devices,” Item 2, states: “Where oxygen-enriched burners are used or a blower is used to supply combustion air to a burner, a check valve or pressure relief device shall be installed downstream of the gas meter.”

Clause 10.6.7, in Section 10.6, “Gas Usage in Industrial Enterprises,” specifies that where pressurized air and oxygen are used for gas combustion, measures shall be taken to prevent air and oxygen from flowing back into the gas pipeline and to prevent flashback. The clause further requires a back-pressure regulator on the gas pipeline, pressure relief valves on the air and oxygen pipelines, and a flame arrester between the burner and the pipeline conveying the gas-air-oxygen mixture. The pressure in the mixture pipeline must not exceed 0.07 MPa.

Compared with GB 55009, GB 50028 has the following characteristics:

Scope of application: It does not explicitly distinguish between “gas-consuming equipment” and “gas-burning appliances,” potentially resulting in a broader scope of application.

Installation location: It explicitly specifies that the check valve or pressure relief device must be installed downstream of the gas meter.

Alternative protective measures: It expressly permits a pressure relief device as an alternative to a check valve in the circumstances specified in Clause 10.3.3, reflecting an approach that allows different protective measures to be selected according to the applicable safety requirements.

(3) Relevant Requirements in Equipment Standards

Equipment standards such as GB/T 19839 and TSG 11 do not explicitly require check valves to be installed in gas supply pipelines. Instead, they focus on burner safety functions, including flame-failure protection and pressure interlocks. For example, GB/T 16508-2022, Shell Boilers, requires the fuel supply to be shut off automatically when the gas pressure falls below a specified limit. It also requires two automatic safety shut-off valves to be installed in series in the main gas control valve assembly, providing redundant protection against fuel leakage.

One possible explanation for the absence of an explicit check valve requirement in these equipment standards is that large-scale gas-consuming equipment may already be equipped with comprehensive pressure-monitoring and safety interlock systems. Under normal operating conditions, these systems can help prevent unsafe operating conditions and reduce the likelihood of gas backflow, potentially limiting the additional protection provided by a check valve.

Current standards contain conflicting requirements for check valve installation, giving rise to ongoing debate within the industry. The main points of contention concern the scope of application and the effectiveness of check valves in preventing flashback.

(1) Dispute over the Scope of Application

GB 55009 limits check valve requirements to “gas-consuming equipment,” whereas some local standards and regulatory authorities extend these requirements to gas-burning appliances, including large-capacity commercial stoves. For example, DB 4201/T 692-2023, Code for Installation and Acceptance of Commercial Gas-Burning Appliances and Facilities, requires check valves to be installed on the branch pipes serving steam generators connected in parallel. The dispute centers on whether large-capacity commercial stoves should be subject to the same check valve requirements as gas-consuming equipment, given the following considerations:

Combustion method: These stoves use forced-draft premixing.

Equipment classification: They are classified as gas-burning appliances.

Applicability of requirements: It remains unclear whether the check valve requirements for gas-consuming equipment should also apply to these appliances.

(2) Controversy over Protective Effectiveness

Some industry experts question whether check valves provide effective protection against flashback, arguing that their primary function is to prevent reverse flow of the gas mixture rather than to stop flashback itself. They also raise concerns about the potential impact on combustion performance:

Pressure loss: Check valves typically introduce a pressure drop of 400–500 Pa, which may impair combustion performance.

Insufficient heat output: With a rated gas supply pressure of approximately 2,000 Pa, large-capacity commercial stoves may experience reduced heat output and degraded operating performance if a check valve is installed.

Potential valve failure: Delayed closure and seal failure may further compromise the valve's ability to prevent reverse flow and provide the intended protection.

These concerns have led to ongoing debate over whether check valves can provide adequate protection without adversely affecting the combustion performance of large-capacity commercial stoves.

Premixed combustion involves mixing fuel gas and air before the mixture enters the combustion zone. Based on the mixing method and system configuration, premixed combustion systems can be classified into three main types:

Ejector-type premixing: A high-velocity gas jet entrains air to form a combustible mixture. This method features a simple structure and typically operates at relatively low mixture pressures and flow rates. It is commonly used in household and conventional commercial cooking appliances.

Forced-draft premixing: A blower supplies air for mixing with fuel gas, allowing more precise control of the air-fuel ratio and flow rate. These systems can handle high mixture flow rates and often involve relatively long pipelines and large mixture volumes. Typical applications include industrial boilers and high-capacity commercial combustion equipment, which are relevant to the check valve requirements addressed in GB 55009.

Centralized premixing and premixed-gas distribution: In these systems, fuel gas and air are mixed centrally, and the premixed gas is then distributed through a pipeline network to multiple combustion points. Because the network contains a combustible gas-air mixture, flashback may propagate through the pipelines under certain operating conditions, potentially resulting in serious

Risk of mixture backflow: This occurs when the air pressure exceeds the fuel gas pressure, forcing air or an unburned air–fuel mixture back into the fuel gas pipeline. This can dilute the fuel gas, create an explosive mixture, or damage equipment such as gas meters and pressure regulators. Potential causes include sudden drops in fuel gas pressure, abnormal increases in blower discharge pressure, and improper valve operation.

Risk of flashback: Flashback occurs when the flame propagation speed exceeds the flow velocity of the combustible mixture, allowing the flame to propagate upstream into the pipeline. This can cause a deflagration within the pipeline, resulting in severe damage. The risk depends primarily on the mixture ratio, flow velocity, burner design, and operating stability. It is particularly high during transient operations, such as startup and shutdown.

Modern gas boilers and burners that comply with applicable standards incorporate multiple, redundant active safety systems that serve as the primary line of defense for safe operation. These typically include:

• Dual or multiple safety shut-off valves installed in series.
• Gas pressure monitoring with interlocks for low- and high-pressure cutoff.
• Reliable flame failure protection.
• Programmed startup and shutdown sequences, including pre-purge and post-purge cycles.

In addition, a pressure regulator installed at the main gas inlet to the boiler stabilizes the gas supply pressure and may incorporate an overpressure shut-off function that automatically interrupts the gas supply when the outlet pressure exceeds a preset limit. However, the regulator's ability to prevent reverse flow depends on its design and configuration. If the upstream pipeline is depressurized or ruptures, the resulting pressure imbalance may allow reverse flow under certain conditions. The associated safety risks must therefore be assessed based on the specific system configuration and failure scenario.

These protective measures are designed to prevent hazardous conditions or rapidly shut off the gas supply when unsafe conditions arise. Accordingly, for combustion equipment that complies with applicable product standards, a check valve on the main gas supply line is not necessarily a universal requirement for safe operation. Its necessity should be determined by the applicable codes and standards, the system design, and the results of a specific risk assessment.

Check valves operate in response to the flow and pressure conditions of the medium. Their intended protective functions are twofold:

Physical isolation: Rapid closure can interrupt reverse flow and help limit the propagation of hazardous conditions along the pipeline.

Prevention of cross-contamination: They can restrict air or combustible mixtures from flowing back into the fuel gas supply line.

However, check valves have several important limitations:

Passive protection: They do not prevent flashback from initiating. Their protective effect depends on their ability to respond to reverse flow, and conventional check valves are not necessarily designed to arrest flame propagation.

Performance limitations: Effective protection depends on closure response, sealing performance, and suitability for the operating conditions. Conventional check valves may not close quickly enough to prevent the passage of a rapidly propagating flame, and leakage may compromise their protective function.

Effects on operating conditions: Check valves introduce additional flow resistance. The resulting pressure drop may reach 400–500 Pa, potentially affecting the thermal output and combustion efficiency of low-pressure combustion equipment.

Maintenance requirements: Check valves introduce additional components that may malfunction, wear, or become fouled, increasing maintenance requirements.

Therefore, check valves cannot replace active safety control systems. Instead, they should be regarded as supplementary protective devices within a multilayered safety system, where their necessity and effectiveness are determined by the specific operating conditions and applicable safety requirements.

The requirement for check valves in GB 55009 may reflect several safety considerations:

Defense in depth: Providing an additional mechanical safeguard against low-probability events involving the failure of automatic safety control systems.

Protection of public gas distribution networks: Limiting the risk of hazardous incidents propagating to upstream public gas facilities, consistent with the safety principles underlying GB 50028.

Applicability to aging and non-compliant equipment: Establishing safety requirements that account for a wide range of operating conditions, including those involving non-standard equipment, aging installations, and inadequate maintenance.

Therefore, check valves should be considered one component of an integrated safety system rather than a standalone or universally sufficient solution. Their role should be evaluated in the context of the overall system design, applicable regulatory requirements, and other protective measures.

Pressure and flow compatibility: The valve's rated pressure shall be no lower than the pipeline design pressure. The pressure drop across the valve should preferably not exceed 500 Pa under the specified operating conditions.

Closing response: The valve should provide sufficiently rapid closure to meet the system's safety requirements. A response time of no more than 0.1 s may be specified where justified by the risk assessment and verified by testing. Spring-loaded or gravity-assisted designs may be considered, while swing-type check valves should be avoided where their closing characteristics are unsuitable for the intended application.

Sealing performance and corrosion resistance: Leakage shall meet the applicable acceptance criteria of GB/T 13927-2022, Industrial Valves: Pressure Testing, where this standard applies to the selected valve. The valve body and sealing components shall be made of materials compatible with the gas composition and resistant to the expected corrosive environment.

Explosion and fire safety: Where the installation is located in a hazardous area, the valve's electrical or instrumentation components, if any, shall meet the applicable explosion-protection requirements. The valve and its associated components shall also satisfy the relevant fire-safety requirements where applicable. Pressure resistance shall be verified against the specified design and foreseeable operating conditions.

High-risk scenarios: Mandatory compliance with applicable national standards. These scenarios include large industrial gas boilers, direct-fired absorption units, and industrial furnaces that fall within the scope of the relevant requirements for gas-consuming equipment. They are typically characterized by high rated thermal loads, medium-pressure gas supplies, long premixed-gas pipelines with large internal volumes, and direct connections to public gas networks. In these applications, check valves with verified performance characteristics, such as rapid-closing designs, should be installed upstream of the equipment where required by applicable standards or justified by the risk assessment. Flame arresters and pressure relief devices may also be considered where appropriate to the identified hazards. These applications should receive priority in safety management.

Medium-risk scenarios: Selective installation based on risk assessment. These scenarios mainly involve high-capacity commercial forced-draft premix appliances, such as large steamers and commercial wok ranges, as well as modular boiler arrays. Although these appliances may be classified differently from industrial gas-consuming equipment under applicable regulations, their high thermal output can present additional safety risks. Priority should be given to verifying the completeness and effectiveness of built-in safety controls. Manufacturers should also be encouraged to incorporate supplementary protective devices, including check valves, where appropriate. If installing a pipeline-mounted check valve would significantly impair combustion performance, alternative measures may be considered following a documented risk assessment. These may include enhanced safety certification, more frequent inspections, and additional functional testing of the equipment.

Low-risk scenarios: Check valves are generally unnecessary unless specifically required. These scenarios mainly involve household and standard commercial appliances that use atmospheric, venturi-type (inspirating) premixing. Their relatively simple configurations and small mixing volumes may reduce certain risks associated with mixture backflow. However, the actual risk depends on the appliance design, gas supply arrangement, and operating conditions. Safety management should therefore focus on compliance with applicable product standards, required CCC certification where applicable, effective flame-failure protection, and installation in accordance with relevant codes and regulations. This approach helps avoid unnecessary technical measures and costs without compromising safety.

Scientific and standardized management of gas premix combustion systems requires coordinated efforts in several areas.

Standards development: Existing standards should be revised to clarify the distinction between gas-consuming equipment and gas combustion appliances. Specific classification criteria, such as rated thermal input and operating pressure, should be established, and representative equipment types should be listed to reduce ambiguity in interpretation and implementation.

Clarification of responsibilities and technological development: Relevant standards should clearly define the responsibilities for supplying, installing, and maintaining check valves. As a general principle, manufacturers should be responsible for check valves integrated into their equipment, while installers or construction contractors should be responsible for externally installed valves, subject to the applicable contractual and regulatory requirements. From a technological perspective, greater emphasis should be placed on integrated safety design, including the incorporation of check valves where appropriate, to enhance safety at the source. Developing low-resistance check valves specifically designed for low-pressure gas systems should also be encouraged.

Safety supervision and industry management: Regulatory authorities should move beyond checking merely whether a check valve is installed and instead assess the completeness and effectiveness of the equipment's internal safety controls, including flame-failure protection and pressure interlocks. A performance-based compliance assessment framework should be adopted, supported by enhanced technical training for industry personnel and a robust system of random product quality inspections.

(1) Article 6.2.6 of GB 55009 has a specific scope of application: it applies to "gas-consuming equipment." Extending this requirement indiscriminately to all gas-burning appliances may lead to misinterpretation of the provision, unnecessary technical measures, and inconsistent enforcement in engineering practice.

(2) In premixed combustion systems, check valves should be regarded as supplementary protective devices rather than primary safety mechanisms. They provide passive protection against reverse flow but cannot replace active safety systems incorporating automatic interlocks. Modern combustion equipment that complies with applicable standards typically incorporates multiple protective measures, including dual safety shut-off valves, gas pressure monitoring, and flame-failure protection.

(3) Safety management strategies should be differentiated according to risk level. For large industrial gas-fired equipment, protective devices such as check valves shall be installed where required by applicable national standards. For commercial combustion appliances, the need for such devices should be determined through an assessment of the specific operating conditions and associated risks. For conventional gas appliances using atmospheric, inspirating premixing, check valves are generally unnecessary unless specifically required by applicable standards or justified by the risk assessment. This differentiated approach avoids a one-size-fits-all solution while balancing safety and cost-effectiveness.

(4) Gas system safety ultimately depends on a comprehensive, multilayered protection strategy. Its essential elements include selecting compliant and reliable equipment, following standardized installation procedures, and conducting regular inspection and maintenance. A check valve is only one potential component of this system and should not be regarded in isolation as the sole means of ensuring safety.