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Why is Extra Gas Needed for MIG Weld Starts?

We are often asked why certain devices such as restriction orifices mounted at the feeder to control flow do not work or why welders complain about their use.  The following are some answers:

Question: Why is some extra gas needed at the weld start?

Answer: Some devices designed to reduce gas waste supply  insufficient extra gas at the weld start to counteract the air entering the torch nozzle, body and cable when welding is stopped or in the weld start area.  Some of these devices control flow at the wire feeder.    Stauffer in a patent published in 1982 ( Patent Number 4,341,237) discusses this issue.  He clearly understood the problem and designed around it using a "surge storage tank"; he states in the patent teaching, "... air leaks back into the torch and lines when welding is stopped.  The air must be quickly purged and replaced with inert gas to produce high quality welds. Also, it is critical to displace the air at the weld zone of the work piece upon initiating the weld.

Question:  What are the devices that cause lack of sufficient extra gas at the weld start?

Answer:  Any device that attempts to control flow at the wire feeder will not have sufficient "stored gas" that can be quickly delivered to the welding torch cable, torch nozzle or weld start area.  This can be a:
Flowmeter,
Orifice that is controlling gas flow,
Needle valve or
Regulator/ flowgauge (low or high pressure.)  Note, low pressure devices have added problems. (See Q&A About Automatic Flow Compensation)

Question:  We're using a flowmeter at the gas source (cylinder or pipeline.)  Do we have a lack of start gas problem?

Answer:  No you probably have much more than enough excess gas but the initial flow surge velocity is so high your pulling moisture laden air into the weld start area and creating similar problems. You can hear the gas surge when you pull the torch nozzle - (CAUTION be sure the wire feeder pressure roll is disconnected when you listen and do not touch an electrically energized part of the torch to your body!! You Need Our Gas Saver System (GSS), Check It Out.

Question: What happens if we don't quickly have sufficient extra gas at the weld start?

Answer:  You're essentially starting the weld in moisture laden air until the MIG gun cable, gun nozzle and weld start area get fully displaced by the shielding gas.  About 5% air is sufficient to create Nitrogen porosity (See Reference.)  Welders often see this problem and try to counter with higher steady state flow rate settings.  That is only partially successful and the higher steady state flow rate wastes gas! They may also increase the rate so high they create turbulence in the shielding gas stream  This only pulls in moisture laden air and is counterproductive.

Question: Is that why when we have used orifices mounted at the wire  feeder to control shielding gas flow welders complained and sometimes drilled out the orifice?

Answer:  We have found that to be the case.  Welders may drill out the orifice believing they need higher gas flow (which they may - but just at the start.)  However the smallest drill commonly available in a drill bit set is 1/16 inches.  Sounds like a small hole but on a 50 psi shielding gas pipeline that will provide in excess of 150 CFH flow!  This not only wastes gas but also pulls in moisture laden air creating internal if not visible weld porosity.

Question:  Are there specific production instances where a switch to some extra gas at the start helped weld performance?

Answer:  Yes, the following two examples validate the need for some extra gas at the weld start:

First Case:  A bar joist manufacturer was using flow control orifices mounted at the wire feeders.  Argon/CO2 shielding gas is supplied in a pipeline through about 15 feet of gas delivery hose.  The flow control orifice established the flow at 45 CFH.  However the welders wanted higher flow rates with some even drilling out the orifice!  The welding engineer wanted to avoid wasting shielding gas. With this flow setting arrangement where control is mounted at the feeder next to the gas solenoid  there is insufficient extra gas provided at the weld start.  This lack of extra gas prevents  to purging the weld start  area of moisture laden air.  

A test was made to check weld performance and potential shielding gas savings using two cylinders of shielding gas on two adjacent welders instead of their pipeline gas supply.  One was set with their standard flow control orifice system and a regulator providing a pressure that matched their pipeline, 50 psi.  The other with a regulator/flowmeter (also of a 50 psi design) using a 15  foot GSS without their flow control orifice.  Both steady state flows were set at 45 CFH.  Since welders stand side by side, it was easy to observe the weld start quality!   Instantly the welder using the GSS noticed  improved  starting.  After about an hour with observably better results the  welding engineer suggested we lower the shielding gas flow on the  welder with the GSS to 35 CFH!  The same improved weld start quality was observed and the welder was "happy."  In fact even though we lowered the steady-state flow to 35 CFH there was still about the same controlled amount of extra gas available at the start (that stored in the GSS hose when welding stopped.) The higher start gas flow rate established by the surge flow orifice in the GSS maintained the higher flow at the start.   This higher start flow rate quickly flooded and purged the weld start area of moisture laden air.  It was this air that was casing excess spatter and lack of shielding on all their other welders! After about 4 hours of observation it was obvious the spatter at the weld start was less with the GSS.  We also measured a reduced use of shielding gas of 25%.   After several months of testing to check this one system during windy days etc, this shop now has GSS's installed on all 50 welders!   

Bottom Line - - "Some extra gas flow at the start is very beneficial."  In addition, after about a year of use their gas supplier called to see if their business had turned down since they were using about 30% less gas- it had not!

Second Case:  A review of the shielding gas flow rates in a shop with 100 welders revealed the amount of excess gas flow on each welder.

DETAILS: A shop with ~100 MIG welders tried to reduce gas waste by installing flowmeters with flow controsl at the wire feeders. These were connected by a gas hose to a 50 psi shielding gas pipeline. Most of the flowmeters were model L-32 (shown at right)  which is designed to read accurately at 50 psi inlet pressure, so the flow readings are direct and accurate. 

As when mounting a flow control orifice at the wire feeder, this approach eliminates the start surge flow but there is insufficient extra gas available to purge air from the weld start area and MIG gun nozzle.  It is if you're starting in air!  As expected, the welders tried to compensate by increasing the steady state flow! However increasing the steady state flow can only partially help as a substitute for quickly needing extra gas at the weld start! it also wasted much more gas than moving the flowmeter to the wirefeeder reduced!

The following was observed:

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~50% of the welders were set at ~50 to 55 CFH.  None were found lower than that flow level.

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~25% of the welders were set near the top of the flowgauge, which for this model is 70 CFH.

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The remaining ~25% had the flow ball pinned to the top of the flow tube.  In our Lab we have measured flows with this model of 150 CFH when the needle valve is fully opened and the float ball pinned to the top of the flow tube!

Conclusion: Trying to compensate for the lack of sufficient extra start gas, the welders set the steady state flow far higher than needed.  Any flow over 50 to 60 CFH is also pulling air into the gas stream and is counter productive! Therefore increasing the steady state flow to the average 60 to 70 CFH observed, increased gas usage far more than the observed decrease in initial gas surge!! 

Question: But we use a flow control orifice just to limit start surge flow and still control the steady flow with a flowmeter at the gas source.  Is that a problem?

Answer:  That will eliminate the problem of excess gas surge but depending on the orifice size may not supply sufficient extra gas.  We have found most commercially available peak flow control orifices are actually not large enough to optimize start surge flow rate.  In addition you may be doing that since you perceive your saving gas.  You can hear the reduction that occurs in initial gas surge, right?  Yes the surge flow is reduced and you can hear the difference.  However if you put a pressure gauge in the gas delivery hose, you will see it still reaches the pipeline pressure (or with cylinder gas supply the regulator pressure) when welding stops!  The pressure will reduce to what is needed to flow the required amount of gas when welding starts (usually about 4 or 5 psi.)   Where does that extra gas go?  It is wasted almost as if you did not have the orifice in place!  It just takes longer to be expelled at the rate set by the orifice!  You may save some gas, may 10% of the surge waste.  Our GSS saves about 80% of the gas waste!  It pays for itself in a matter of weeks and includes, as part of its patented design, a surge flow limiting orifice at the feeder/welder end.  Check it out

Want More Details About Extra Gas Needed at Weld Start?

 

See Other Questions and Answers; Click Link Below:

Q&A About Ideal Gas Delivery Systems

Q&A About Automatic Flow Compensation