FLASHBACK ARRESTOR

By David Pryor
September/October 2008

Oxy-fuel welding can be safe if common safety procedures and equipment are employed. 

Welding has been done virtually every day for years in a variety of settings. Despite this wealth of experience, one cannot take safety for granted but must constantly keep safety in mind—and that goes for the oxy-fuel process also. Numerous safety issues are taken into consideration during the design and manufacturing of today’s oxy-fuel equipment, but it is also the responsibility of the end user to employ proper practices to prevent accidents and injury. Considerations from engineers, manufacturers and marketers of these products, along with the proper awareness from end-users can help make oxy-fuel welding and cutting an even safer profession. 

Safety Features in the Equipment

Some of the most common incidents in oxy-fuel welding and cutting involve backfire and flashback. There are two types of backfire: backfire itself, and sustained backfire. When oxygen is added to the fuel gas it accelerates the burning rate of the gas. For example, acetylene and oxygen burn at about 26 feet per second and alternate fuel gases (propane, natural gas, MAPP, or propylene) together with oxygen burn somewhere in the 14 to 16 feet per second range. A backfire occurs when the velocity of fuel gas, burning at the tip end with the support of oxygen, is reduced to a point where it is less than the burning rate and therefore backfires into the tip or torch, usually to that location where the fuel gas and oxygen are mixed. This is often very audible especially when using large multi-flames, since noise is created by the flame backfiring into the mixer. There are times when the backfire continues to burn in the mixer area supported by the flow of fuel gas and oxygen. This is generally accompanied by a loud whistle noise and is termed “sustained backfire”. 

Flashback, on the other hand, is the ignition of mixed gases that develop in either the fuel gas or the oxygen passages. This occurs when one or more of several situations develop while using a torch, which can cause the reversing of one gas into the other side of the system. Reverse flow can typically occur when systems are over-pressurized for the size of tip being used, the tip is plugged or dirty, the oxygen or fuel gas supply depleted or due to operator procedural error. When fuel gas backs into the oxygen line or oxygen into the fuel gas line, the mixture can travel through the torch, into the hose, through the regulator and possibly into the supply cylinder or system. This condition represents a very dangerous situation if ignited. Reverse flow check valves at the torch help reduce the possibility of the reverse flow of gases. (See Figure 1). However, they are mechanical devices and may malfunction if not properly maintained.

Flashback arrestors are not mechanical by design. Flashback arrestors depend on a fine sintered filter that prevents transmission of a flame through the filter itself. The flashback arrestors will stop the ignition of the mixed gases even if the reverse flow check valve is not operable. The result is that the ignition cannot propagate into the hose, regulator supply cylinder, or system. Ignition does take place in the torch as it does with a backfire. As a general rule, the torch design will withstand the ignition. (See Figure 2).

Therefore, it is recommended that both reverse flow check valves and flashback arrestors be used at the rear of the torch. As a result, manufacturers are beginning to build these components into most industrial equipment with the ability to include them with larger, heavy-duty equipment at the request of the buyer.

Safety With Fuel and Gas

There are many issues with the storage and use of gas and fuel that can lead to incidents as well. For starters, the storage of all cylinders is extremely important. Cylinders should always be stored in an upright position with the protective valve cap installed. Cylinders should also be securely chained on a suitable cart or to a fixed object such as a post or wall to prevent accidentally tipping.

When using acetylene, handling and storage becomes even more critical. Acetone in the tanks is used to help stabilize the acetylene. This is why it is important to always use acetylene cylinders in the upright position. It is also important to pay attention to the rate at which the acetylene is taken from the cylinder. As a general rule, no more than 1/7th of the tank contents should be removed per hour. It is also important to only open an acetylene cylinder valve ¾ of a turn and no more than 1-1/2 turns. All other cylinder valves should be opened fully. 

Safety With the End User

Often, the end user may not be aware of safety concerns or become complacent about them. Most notably, the safe practice of purging the hoses and checking the system for leaks before use is often neglected. It is common to see operators using gas equipment that is in need of repair. The oxyfuel system should always be checked for leaks before using. Any leaks found during this check should be remedied before the equipment is used.

Also, using oxy-fuel equipment in tight spaces can be dangerous. There should be ample air flow to eliminate the possibility of creating an environment of asphyxiation. Heating, melting or burning certain materials can produce toxic fumes. This condition is dependant on the materials being heated and the amount of free flowing air in the area. The proper ventilation and protective respiration should be used. In addition, any small leak that causes excess oxygen or acetylene to be released into a small environment can become the catalyst for explosion.

The End Result 

The oxy-fuel industry is one that has made great strides in the last decade, and in order to continue that progress it will take the help of manufacturers and end users both. Remaining cognizant of current safety issues on all levels will help to create better equipment and practices in the future. We can never be too careful. Nothing can take the place of the using the proper operating procedures of the equipment as recommended by the manufacturer.

source : Gasses & Instrumentation Free Magazine