Gas Safety Valves: How They Work and Which One to Choose
A flashback in the gas supply circuit can travel all the way back to the cylinder in a matter of moments, with potentially serious consequences. Gas safety valves—also known as flashback arrestors—are an economical device that prevents this from happening by blocking the flow of gas or extinguishing the flame before it reaches the source.
They are used with oxygen, acetylene, propane, methane, hydrogen, and other combustible gases in all metal welding and cutting applications.
Knowing the types, positioning in the circuit, and maintenance requirements is essential for choosing the right valves and protecting the system and operators.
What are gas safety valves for?
A gas safety valve acts as a barrier between the point of use (cutter, torch) and the gas source (the cylinder), blocking the reverse flow or extinguishing the flame inside the device, protecting the system from the risk of serious damage or, worse, explosions.
It is generally installed at three points in the circuit—on the pressure regulator, along the connecting pipes, or on the torch handle—depending on the type and application. In all cases, the function remains the same: stopping the return flow before it reaches the gas source.
Gas safety valve: the three types
Non-return valves are divided into three categories, based on the level of protection they offer and the type of risk they are designed to manage.
Gas non-return valves
They prevent gas from flowing backward: they allow flow in only one direction, toward the point of use. However, they do not intervene in the event of a flame: if the return flow is already triggered, this type of valve does not stop it.
Dual-function flame-arrestor valves
They block the flame thanks to an internal filter, usually made of sintered steel microspheres: the gas continues to pass through the filter, but the flame is extinguished upon contact.
Three-Function Valves
Three-function valves add a thermal cut-off: a shutter held open by a thin filament of tin (or other low-melting-temperature material) that closes automatically when the temperature exceeds 95°C, interrupting the flow of gas before the flame even reaches the device.
Which one to choose
In most welding and cutting applications, two-function valves represent a sufficient safety standard, as they cover both gas and flame return.
Three-function valves are preferred in systems exposed to thermal stress, where an additional cut-off in the event of overheating provides an additional safety margin. Oxyturbo offers gas safety valves in all three configurations, to match the specific application.
Technical characteristics and mandatory markings for gas safety valves
Every safety valve must clearly display certain mandatory information: the model, the manufacturer's name or abbreviation, the reference to the product standard (EN ISO 5175-1), and the abbreviations for the functions it performs—FA for flame arrester, NV for gas backflow preventer—as well as the type of gas for which it was designed.
These markings allow you to quickly verify the valve's compatibility with the system, identify whether it is a gas-only, flame-only, or multifunction valve, and facilitate inspection and maintenance.
The flow rate varies from model to model, depending on the installation location and the gas used: this information should always be checked on the specific product's technical data sheet to avoid choosing a valve that is undersized for the system's needs.
Where to install them in the gas system and how many are needed
To be effective, gas safety valves must be installed so as to stop the return flow as close as possible to its point of origin.
Ideally, therefore, the valves should be installed on the torch handle. When this is not possible, the alternative solution is to place them along the pipes, about one meter from the handle. Generally, these are the locations:
● on the pressure regulator:
A first pair of valves should be mounted directly at the outlet of the pressure regulators, on oxygen and fuel gas. In this position, they protect the cylinders and regulators from any return flow that may travel up the pipes.
● on the torch handle:
The second pair of valves should be installed on the torch handle, so as to intercept the return flow as close as possible to the ignition point. This is the preferred solution because it blocks the gas and flame immediately "upstream" of the work area, reducing the length of the circuit potentially affected.
● along the connecting pipes:
when this is not possible they can be installed along the pipes, approximately one meter from the torch. In systems with particularly long pipes (over 15-20 meters), however, it is not necessary to insert additional valves every few meters: an excessive number of devices along the line risks reducing flow rate and forcing the operator to increase operating pressures beyond the correct values. A third pair midway through the circuit can still be useful as intermediate protection in the event of a pipe break.
How many gas safety valves are really needed?
The minimum recommended configuration therefore includes at least two pairs of safety valves: one on the regulators (oxygen/gas), one on the torch handle, or along the pipes near the point of use. An intermediate pair along very long pipes can provide additional protection, but it never replaces the role of the valves mounted on the regulators, which remain the last safety feature before the cylinders.
Maintenance and Replacement: What the Standard Says
Safety valves are protective devices and, as such, have a limited lifespan: they cannot be considered permanent components of the system.
To maintain their effectiveness over time, it is necessary to follow both the manufacturer's instructions and the requirements of applicable technical standards, such as UNI 11627 for manual oxygas equipment.
Checks Before Each Use
Before each use, it is good practice to check the tightness of safety valve joints to rule out external leaks. This check includes a visual inspection of the valve body and connections and, where applicable, a leak test with a leak detector or equivalent method to ensure there are no leaks of combustible or oxidizing gas.
Annual Periodic Checks
A more thorough inspection must be performed at least once a year: a leak test at maximum operating pressure to ensure there are no leaks, and a leak test of the gas non-return valve at both minimum and maximum operating pressure to ensure the device properly blocks reverse flow. These checks are part of the periodic activities required by UNI 11627 for manual gas equipment.
When to Replace Non-Return Valves
Non-return valves must be replaced whenever a backfire occurs: heat can cause invisible damage to internal parts and render the device ineffective in the event of subsequent incidents.
Regardless of incidents, UNI 11627 requires the scheduled replacement of safety devices within 5 years of commissioning, with intervals that may be reduced in the case of particularly heavy use or severe environmental conditions.
Role of the Manufacturer
These requirements supplement, but do not replace, the manufacturer's recommendations. In case of discrepancies, the more restrictive indication always prevails.
Gas safety valves: the most common errors
In our experience, errors lie not only in the choice of valve, but above all in how it is used and maintained over time.
1. Not replacing it after a backfire
After a backfire, the internal parts of the valve can be damaged even if the external body appears intact. Continuing to use it compromises its ability to stop any subsequent backfires and is contrary to the recommendations of technical standards and manufacturers, which require replacement after each such event.
2. Ignoring the 5-year expiration date
Treating backfire valves as components with no expiration date is a serious mistake: periodic maintenance and replacement within 5 years of commissioning are part of the required checks for manual gas equipment. Exceeding these intervals without replacement exposes oneself to an uncontrolled risk and possible non-compliance with the good practices required by the standards.
3. Positioning in the wrong place in the circuit
Mounting valves only on the regulators, neglecting the torch handle or the area near the point of use, reduces the effectiveness of the protection: the flame can still travel the entire pipe before being stopped. Correct installation always includes two levels of protection, with downstream devices (handle or line near the torch) and upstream devices (regulators).
4. Overloading the pipes with too many valves
Inserting safety valves every few meters on long pipes does not proportionally increase safety, but can cause pressure drops and reduce flow, forcing the operator to work with suboptimal settings. A third intermediate pair can be useful as additional protection on very long lines, but should not replace many unnecessarily duplicating devices already present on regulators and near the torch.
5. Neglecting leak tests and visual inspections
Simply "installing the valve and forgetting it" means sacrificing one of the main prevention tools: leak tests and periodic visual inspections allow for the early detection of damage, leaks, or anomalies that could compromise the safety function of the device.
Oxyturbo safety valves
Oxyturbo safety valves are specifically designed to protect the system and operator from gas and flame backfire in gas welding, cutting, and heating applications. They are built and approved according to the European standard EN ISO 5175-1, which defines design requirements, testing, and markings for safety devices for combustible gases and oxygen, including flashback arrestors.
Each valve is individually tested to verify tightness, proper backflow prevention, and flame arresting effectiveness, in line with the tests required by EN ISO 5175-1. The range includes single-, dual-, and triple-function versions, with configurations for mounting on regulators, torch handles, or along supply lines, to meet a variety of system requirements.
For more information on the features and available configurations, please visit the page dedicated to Oxyturbo safety valves.
Frequently Asked Questions
How often should a gas safety valve be replaced?
It must be replaced after every backflow and in any case no later than 5 years after commissioning, as required by UNI 11627, even if no faults are detected. In cases of particularly heavy use or harsh environments, it may be advisable to replace it earlier based on periodic checks.
What is the difference between a gas backflow prevention valve and a flame arrester valve?
A gas backflow prevention valve blocks reverse flow but does not stop a flame that has already ignited. The flame arrestor has an internal filter (usually sintered steel) that extinguishes the flame. For complete protection, dual-function valves are required, combining both mechanisms.
Where should safety valves be installed?
For effective protection of the gas system, flame arrestors should be installed at least on the pressure regulators (cylinder side) and near the point of use, on the torch handle, or along the pipes a short distance from the torch. This dual position allows the return flow to be stopped both near the ignition point and before it reaches the regulators and cylinders.
How many safety valves are needed on a system?
The minimum configuration includes two pairs: one on the pressure regulators (oxygen and fuel gas) and one on the torch handle, or along the pipes if the handle does not allow it.
If the pipes are very long, should they be placed at regular intervals?
Generally, no: on pipes longer than 15-20 meters, it's not necessary to add valves at regular intervals, because an excessive number of devices reduces the flow rate of the circuit. Valves on the regulators and near the torch are still necessary; an intermediate pair in the middle of the circuit is an option, not a requirement. Too many safety valves installed along the pipes can cause a drop in flow rate, which, in some cases, can even lead to a greater risk of backfire due to the "fuse effect."
What does it mean if a valve complies with EN ISO 5175-1?
Compliance with EN ISO 5175-1 indicates that the safety valve has been designed, tested, and approved according to the European standard that defines the requirements for flame arresters in gas welding and cutting systems. These tests include backfire resistance tests, leak tests, and performance checks and are an essential guideline for choosing reliable devices compatible with professional use.
