They may be small, but they are mighty. Flame or flashback arrestors used in oxy-acetylene equipment setups are a key component of safeguarding the wellbeing of personnel undertaking hot work onboard vessels and lowering the risk of damage to assets. Here we explain everything you need to know about flashbacks and help to prevent them.
(Note: In this article, the term flashback arrestors also denote flame arrestors)
Oxy-acetylene torches have been used in the maritime industry for many decades for cutting, welding, brazing and heating of steels during onboard maintenance. When purchasing gas welding and cutting equipment, one often overlooked aspect is ensuring that the equipment meets technical specifications for safety and compliance. By investing in goodquality equipment that adheres to strict safety standard requirements, you can significantly reduce the risk of fire and explosions onboard.
Despite close attention paid to gas welding and cutting safety, there are still too many cases of personnel being injured or even killed
due to the use of poor or substandard quality equipment. There have been incidents where flashback into acetylene cylinders has triggered decomposition of the gas causing the cylinder to explode. In one case, a cylinder exploded only three minutes after a flashback with devastating results (see Figure 1). Fortunately, no one was injured because the operator realized that the cylinder was in a dangerous condition, raised the alarm and the location was evacuated (Reference: AIGA Safety Bulletin 01/05). Protecting against flashbacks during hot work is in line with the International Safety Management (ISM) Code and the Maritime Labour Convention (MLC) 2006, under which ship owners and operators are obliged to do their utmost to safeguard the occupational health and safety of personnel and prevent accidents both at sea and in port. Awareness is everything in ensuring the right behaviour. In a recent Welding Safety Inspection (WSI), for instance, our inspector discovered that flashback arrestors were missing from the gas welding and cutting equipment, where the consequences in the event of a flashback accident could be severe and disastrous. A strong safety culture and the right mindset among crew members are essential for reducing safety risks
Figure 1: Oxy-acetylenegas cylinders that caughtfire and exploded ina flashback arrestorincident where the firetravelledthrough thehose and directly into thegas cylinders. Source:https://safety.army.mil/ON-DUTY/Workplace/Oxygen-Acetylene-Torch
Flashbacks explained
Under correct conditions, where there is a balance between the gas mix exit velocity (i.e., speed of the gas out of the nozzle) and the combustion velocity (i.e., burning speed of the gas), the gas will burn correctly in a stable manner. A phenomenon known as blow-off
occurs if the gas mix exit velocity is higher than the combustion velocity, i.e., the flame burns at a distance away from the nozzle tip. This can be corrected by manually decreasing the pressure and thereby the velocity. Conversely, a flashback will occur if the combustion velocity exceeds the gas mix exit velocity, with the flame burning into the equipment. Flashbacks typically occur at the cutting nozzle or welding attachment tip (orifice) of the torch setup. There are three distinct types of flashback phenomena:
Backfire happens when the flame burns back into the welding torch with a sharp bang. Either the flame is extinguished or reignites at the nozzle opening. A backfire is harmless in itself but can be a sign of a fault in either the equipment or the gas supply.
Sustained backfire happens when the flame burns back into the torch mixing point (where oxygen and acetylene is mixed). A sustained backfire is characterized by an initial bang (backfire) followed by a whistling sound. If it is not swiftly interrupted, the torch will melt and the escaping combustion products can cause injuries. To stop a sustained backfire, rapidly close the oxygen valve followed by the acetylene valve on the shank.
Flashback happens when the flame burns back at very high speed through the torch, hoses and regulator. The flashback consists of a pressure front moving at twice the speed of sound, followed by a flame front and the gas back-feeding. If the flashback feeds back into an acetylene cylinder that lacks the necessary safety equipment, it may enter the cylinder and cause it to explode Typical causes of flashbacks can be attributed to either human factors or equipment / environment factors as follow
Human Factors
- Wrong gas sequence during start-up. Insufficient purging to clean hoses before use.
- Wrong gas pressures. Where practicable, gauges should be within sight of the operator.
- Flame held too close to work surface. Inappropriate selection or use of equipment for the job.
Equipment / Environment Factors - Blocked, worn, undersized oroverheated blowpipe tip.
- Blocked section in the blowpipe, cutting attachment or heating attachment.
- Leaked, kinked, restricted or incorrect diameter hoses that create pressure loss or insufficient supply in either one of the gas hoses.
- Old gas hoses that have become stiff and brittle.
- Hot metal falling on and burning through hose or hose damaged by sharp-edged object such as hose clamps etc
How to prevent flashbacks
(In reference to Code of Safe Working Practices for Merchant Seafarers (COSWP, UK) – British Compressed Gases
Association (BCGA) Code of Practice No. 7 (CP7) guidelines)
At Procurement and Assembly
- ‘Fit for purpose’ gas equipment purchased and fitted in conformance with international standards such as EN ISO 5175.
- Ensure gas regulators for both acetylene and oxygen are fitted with flashback arrestors and that the welding shank is fitted with
- either a nonreturn valve or a shank-mounted flashback arrestor (on long length of gas hose) to prevent gas back-feeding. Use equal-pressure torch instead of injector (low-pressure) torch for gas
welding and cutting equipment. - Prevent oil, grease and contamination contact with gas equipment.
- Check condition of threads and sealing surfaces. Ensure no signs of PTFE
white tapes. - Leak test all joints at working pressure
Figure 2: Gas regulator outlet with no flashback arrestors installed on either the acetylene and oxygen gas regulator connections, thus increasing risk of flashback fire or explosion.Photo: WSS
Before Use
- Ensure all crew are aware of the use of flashback arrestors to promote optimal safety, and that all personnel using the oxy-acetylene equipment are properly trained and certified.
- Double check all equipment before proceeding with the work task. Check the state of the cutting nozzle and blowpipe and keep these items in good condition. Make sure they are not blocked by dirt or spatter.
- Replace damaged nozzles if necessary. Poorly maintained cutting nozzles and blowpipe cause turbulent gas flow disturbing the gas mix exit velocity, which increases the risk of flashback.
- Leak test all joints at working pressure. If the equipment is already under pressure, remember to purge the hoses by opening the gas supply to each hose for a few seconds before lighting the torch. This will flush out any flammable mixture of gases in the hoses. If there is a flammable mixture of acetylene and oxygen present in the hoses when the torch is lit, the flame will ignite the mixture and travel backwards from the torch along the hoses, through the regulator and into the cylinder.
Check your Flashback Arrestor before use:
Appropriate for the gas in use
The body is free from damage, contamination & correctly labelled
Appropriate Maximum Working Pressure of the FBA
Leak test all joints at working pressure
Reset button or lever check for gas flow in the operating position.
If there is no gas flow, check for signs of damage caused by previous
flashback on the FBA or any attached hoses & regulators
During Operation - Use the correct nozzle size/blowpipe for the job and ensure the pressure setting is correct (consult the pressure setting chart detailing the correct setting for the particular item). Compensate with a 30% increase if the hoses used are more than 10 metres long and/or gas consumption is more than 1,800 litres/hour.
- Ensure lighting-up and shutting-down procedures are followed correctly. For example, if using UNITORTM gas welding and cutting torch, open and light the acetylene then introduce the oxygen. Not doing this may cause oxygen to drift back into the torch and hoses, creating a flammable gas mixture.
- Do not bump or hold the torch nozzle too close to the work piece during welding or cutting operations. The white inner core of the flame should be held 1mm from the surface of the work piece. Bumping the tip will disturb the gas mix velocity and increase the risk of flashback
After Use
- Check for any damage, contamination, defects, or faults and replace as necessary.
- When shutting down the equipment, first close the acetylene supply followed by the oxygen. Close cylinder top valves, open the valves on the shank and drain the gas out of the system. Unscrew regulator knobs on regulators, then close the shank valves. Check that gauges return to zero during the venting process. Never keep the equipment under pressure when not in use.
Annually
- Conduct a full visual inspection.
- Check manufacturing dates for expiry.
- Conduct functional tests to ensure continued ‘fit-for-purpose’ such as leak test on the system, creep test for gas regulator integrity, flow restrictions and reverse flow to ensure correct operation of non-return valves of flashback arrestors. Bend the gas hose in a tight radius to ensure reinforcement is not visible and there are no signs of abnormalities or distortion especially at length 50 centimetres (cm) from torch/shank, etc.
Five Year Replacement/Renewal
• For maritime use, especially for ships following OCIMF SIRE 2.0 and/ or RIGHTSHIP RISQ 2.0/3.0 requirements, gas regulators and flashback arrestors shall be replaced five years from date of manufacture
• For the Gas Distribution System (GDS) i.e., Cylinder Central Storage to Outlet Station, conduct pressure and leak testing, and obtain a new certificate from an approved third-party service provider.
• Ensure safety through purchasing and standardizing your welding and cutting equipment or spares and replacements from reputable suppliers instead of a “mix and match” approach.
Inspect your Flashback Arrestors:
Check the manufacturer’s date stamp or coding
If the code is not present, ascertain from purchase records or management whether the FBA is under 5 years old
Replace Flashback Arrestors every 5 years
If in doubt, replace the Flashback Arrestor
What to do in the unlikely event of a flashback
Acetylene is extremely flammable, unstable and under certain conditions can decompose explosively into its constituent elements, carbon and hydrogen. A flashback can trigger decomposition in the acetylene cylinder.
Therefore, it is crucial to keep the cylinder on the uppermost continuous deck and ensure direct access to the open deck. This allows for easy removal in case of an emergency. Never keep cylinders inside the hull. Note that UNITORTM acetylene cylinders are designed in line with international standards, incorporating features that allow for safe filling and storage of acetylene gas, and retard any acetylene decomposition should it occur, providing enough time for emergency actions to be taken.
• If a flashback occurs and it is safe to do so, • Carefully check the cylinder and any other
immediately close both acetylene and oxygen cylinder valves. The flame should go out when the acetylene is shut off. If the flame cannot be extinguished at once, evacuate the area. It is important to keep mittens or gloves in a convenient location near to the gas cylinder. The mittens or gloves should be made from flame-retardant material and can be used on either hand to close the top valve of an acetylene cylinder that is on fire
acetylene cylinders that may have been affected by fire or flames. If the cylinder becomes more than hand warm (60-70°C) or starts to vibrate, evacuate the location immediately. Do not attempt to move an unstable cylinder; moving it could restart or accelerate the gas decomposition. It could be several days before it can be moved. Use the fire hose from a distance of at least 20m and hose down the cylinder body with a direct water spray. Cooling the cylinder down is the only way to stop decomposition.
• Depending on the situation, throw the cylinder overboard.
Types of flashback arrestors
Gas regulator outlet flashback arrestors
A flashback consists of a pressure front, a flame front and back-feeding of the gas, hence the flashback arrestor must be constructed accordingly (see Figure 3). The pressure-sensitive shut-off valve (PV) will be triggered if there is a sudden change in pressure of approximately 1.2 bar. This will also trigger the non-return valve (NV) and cut off further gas supply from the cylinders. The UNITORTM flashback arrestor models W66S and S55 are resettable, however this should not be done before the cause of the flashback has been identified. For UNITORTM flashback arrestor model 85-10 used at cylinder central station (unmanned compartment), it is a high flow and non-resettable type, which needs to be replaced in an event of flashback for safety.
The flame arrestor (FA) inside the unit is made of porous sintered stainless steel that allows the gas to pass through. The flame front will not be able to penetrate this barrier and will be extinguished. Should the unit overheat, the arrestor is also fitted with a temperature-sensitive valve (TV) that will melt at 105°C. If this happens, the arrestor cannot be reset and will have to be replaced.
Safety Elements
1. Flame arrestor
High quality stainless steel sintered element. A flashback is stopped and quenched here.
2. Non-return valve
Spring loaded non-return valve against slow and sudden reverse flow.
3. Pressure sensitive cut-off valve
Closing automatically with increasing reverse pressure caused by reverse flow or flashback.
4. Temperature sensitive cut-off valve
Automatically closing at predetermined internal temperature.
Figure 3: Safety elements of UNITORTM gas regulator outlet flashback arrestors
Shank flashback arrestors and non-return valves (NVs)
A NV mounted on the welding shank (torch handle) prevents the passage of gas in the opposite direction. This stops oxygen or acetylene back-feeding and mixing within one of the hoses. However, in mind that a NV will not respond fast enough to prevent a full flashback, thus requiring an extra level of safety.
Figure 4: Non-return/backflow check valve
The use of a basic flashback arrestor such as UNITORTM FR-20 shank flashback arrestor, which incorporates a NV and FA in a single unit on the shank (torch handle), improves safety further by preventing a flashback fire on the gas hoses and potential harm to the operator.
Figure 5: Flashback arrestor with built-in NV
Note that the FR-20 does not substitute the flashback arrestor on the regulator. For optimal safety we recommend installing flashback arrestors both on torches and regulators for both acetylene and oxygen (see Maritime safety regulations on the next page).
Maritime safety regulations regarding flashback arrestors
Taking the lead from Code of Practice No. 7
(CP7), developed by the British Compressed Gas
Association (BGCA), the Code of Safe Working Practices for Merchant Seafarers (COSWP, UK) as well as key maritime industry bodies such as the OCIMF Ship Inspection Report (SIRE) programme
SIRE 2.0 for tankers and offshore, and the
RightShip Inspection Ship Questionnaire (RISQ 2.0 / 3.0) for bulkers, specify the mandatory use of flashback arrestors for gas welding and cutting equipment setups and their five yearly renewals. The use of flashback arrestors is also mandated by Class Societies like DNV and Flag States including the UK, US, Norway, Singapore, Australia etc. Other maritime value chain stakeholders that recognize how dangerous it is to use oxyacetylene equipment without flashback arrestors include marine insurers. For example, the American Steamship Owners Mutual Protection and Indemnity Association (The American Club) recommends that “flashback arrestors must always be fitted to the bottles” in oxy-acetylene setups.
Despite welding equipment being a low investment priority versus, for example, fuel costs, it is important to embed the right behaviours among your personnel to decrease the likelihood of incidents. Safety is paramount. Replace expired products (in terms of life expectancy, arrestors should be replaced every five
What to look for when purchasing flashback arrestors?
years in line with OCIMF SIRE 2.0 or RightShip RISQ 2.0/3.0 requirements) and only purchase flashback arrestors manufactured to the ISO 5175-1 standard and that have BAM (Bundesanstalt für Materialforschung und
-prüfung or German Federal Institute for Materials Research and Testing) certification and under-surveillance approvals. The key selection criterium is to only buy flashback arrestors from a trusted source or reputable manufacturer
“check that flashback arrestors are in place on both the oxygen and acetylene lines at both the torch and bottle ends”.
These general regulations are very positive in alerting ship owners and operators to the risks of gas equipment that is in poor condition and/or expired. We are also seeing increasing enquiries from customers on what action to take for compliance, especially with the latest SIRE 2.0 vetting inspection or RISQ 2.0 / 3.0 guideline publications. This is timely as, according to a recent analysis based on our Welding Safety Inspection data on 150 vessels of all types, we found that over 60% were using gas welding and cutting equipment that was either wrong, out of date and/or in poor condition.
So far, the maritime regulations do not explicitly mandate the use of a flashback arrestor on the oxygen side of the equipment installation. However, in the interests of optimal safety, we encourage customers not to take any chances with oxygen as it can fuel the combustion rate of acetylene. A flashback arrestor on the oxygen regulator can prevent oxygen from continuing to flow in case of fire.
with a good safety track record. Avoid buying second-hand units, incorrect type or flashback arrestors with no local standards.
Wilhelmsen Ships Service (WSS) has been supplying high-quality UNITORTM gas equipment for decades with an excellent safety track record in the maritime industry. All our UNITORTM flashback arrestors provide reliable protection against dangerous reverse gas flow and flashbacks according to EN ISO 5175-1. Our 85-10 series, S- 55 and W-66S are BAM-certified, while our basic FR-20 product conforms to EN ISO 5175-1 and are all manufactured by an industry leading manufacturer and subjected to rigorous testing.
UNITOR Flashback Arrestor Product Range and Application
FA = Flame arrestor, NV = Non-return valve, TV = Temperature sensitive cut-off (105oC), PV = Pressure sensitive cut-off;
Figure 6: UNITORTM Flashback arrestor product range and application(s)
Case Study: Flashback and fire in oxy-acetylene hose onboard
The International Marine Contractors Association (IMCA) details that a member reported an incident in which a sub-contractor’s oxy-acetylene cutting hose had burst and caught fire.. The incident occurred when the welder was ready to ignite the cutting torch head. The acetylene valve on the quad was slowly opened; the welder then turned on the oxygen and acetylene valves on the cutting head and ignited the tip. Instantly, there was a bang and flames were seen coming from a coil of oxygen and acetylene hoses on the deck. The burst created sufficient ignition and force to cause the hose to split in three separate places. The splits were located at the in-line hose connection points. The fire was immediately extinguished by the fire watcher who was able to fold over the two hoses thus preventing further gas flow. The gas supply was isolated at the cylinder and hoses were vented. There were no injuries.
Figures 7 & 8: Ruptured gas hose and flashback arrestors installed prevent major accident. Source: IMCA
Suggested cause of the incident: Possible blockage at the nozzle tip of the cutting torch due to the equipment being old and worn out, resulting in flashback and fire in the hose. This was borne out by the investigation, which identified brittle and perished sections along the whole hose, as well as the suspicion that the non- return valve on the cutting torch was inoperative, allowing the flashback to occur within the hoses. Flashback arrestors had fortunately been installed and prevented gas back-feeding into the cylinder and explosion.
In its Safety Flash identifying lessons from the incident, the IMCA writes that “the primary safety condition breached in this incident was: always use the correct tools safely to carry out the job”. Among the items related to inspection of welding and cutting equipment prior to use, it categorically advises checking that “flashback arrestors are fitted at both gas regulator’s outlets” and that “non-return valves or flashback arrestors should [also] be fitted at the torch ”.
Core preventive action: Ensure that tools are cleaned after use, replace worn-out components and keep equipment in good order over time.
Figure 9: Similar incident on another vessel whereby flashback arrestors installed prevent further damage. Source: WSS
Weld safely with us
It pays to be fully aware of all the safety hazards relating to hot work onboard vessels, ensuring that all your welding and cutting equipment is shipshape.
Independent inspections by a third party can verify that you are up to scratch. We see many customers returning to us having had negative experiences using, for example, lowpressure injection equipment, which we believe is inherently much less safe than our solution.
We also offer industry-leading Welding Safety Inspections where we will immediately highlight urgent issues including the absence or expired flashback arrestors. You can also take advantage of UNITORTM welding training courses at UNITORTM welding schools so your crew is fully up to speed on the latest cutting and welding techniques and safety parameters.
With WSS you are in the best hands – improving safety, optimising performance.
Contact your local WSS representative to find out more. For contact details, visit https://www.wilhelmsen.com/ contacts-and-locations/
