Question:
Are there production examples where low pressure devices have
caused problems?
Answer:
We have encountered a number of
examples where problems have
been observed with low pressure devices and they were removed and discarded.
The following are four specific reported problems:
First
Case.
A
Welding Engineer at a large automotive parts supplier installed low pressure
devices that mounted at the feeder and included a pressure calibrated flow
gauge (the same "Gas Guard" device used in the tests mentioned above.) Here are
his quotes
about the experience:
“After purchasing and using 32 low pressure gas saving
devices (the same product used in our tests mentioned above) that mounted at the wire feeders we decided to
discard all of them! There were two major problems:
1) Lack of sufficient extra gas at the start made inferior starts and
2) Large flow variations from preset levels were evident when flow was
checked at the MIG gun. In fact as he stated; "Even if the flow was blocked,
the flow calibrated pressure gauge supplied with these devices had the same
preset reading!"
Second Case.
A
Welding Engineer at a major Midwest heavy construction equipment
manufacturer
was observing porosity in the same weldment in one plant and not in another.
He performed a very careful, systematic analysis of the problem.
A fishbone troubleshooting diagram was developed with over 30 items
considered in attempt to solving the problem. It appeared to be a nitrogen
porosity problem so he looked at all the possible causes. He checked for
pipeline, solenoid, feeder plumbing leaks, tested a cylinder gas supply to validate the quality of
shielding gas and many others items. He even tried a different type and
manufacturer of the solid wire they were using.
After all this testing and
elimination of differences he found one major cause was the plant with the
porosity problem was using low pressure surge reducing devices on their
welders! The same "Gas Guard" device shown in the comparison test mentioned above. These were removed and the
problems went away! Variability in flow was probably allowing
nitrogen to enter the shielding gas stream. If shielding gas flow rate
is too high or too low this can occur.
We find fabricators often
use excessive flow rates. Flow settings above approximately 50 CFH with a typical 5/8 inch ID
gun
nozzle are just pulling air with it's 70% nitrogen and moisture into the
weld due to turbulence in the shielding stream.
Third Case:
When evaluating the fittings needed for adding
GSS's to 45 MIG welders at a bar joist manufacturer, it was observed
that one installation had
a low pressure regulator/flow control device mounted at the feeder (the same
"Gas Guard" device
used in case 1 and case 2). It had an output pressure gauge calibrated in CFH. The maintenance manager indicated these had been installed on all welders
several years before at a
cost much higher than the
GSS and had to be removed due to inconsistent results! The one
remaining was adjusted at a very high flow rate according to the flow gauge
reading. The maintenance manager indicated it would not be reinstalled and
was surprised it was still there!
Fourth Case:
A fabricator making Catalytic Converters had 70 new MIG Robots installed.
The systems integrator used a "Gas Guard" low pressure device mounted at
the pipeline drop (this is a model of the device used in the above cases
that mounts at the pipeline.) Experiencing flow variations and seeing
the information on our web site they were removed and replaced with
conventional flowmeters. They used flowmeters designed to read accurately at 50 psi
so flows could be read accurately directly, without the need for a
conversion. Flows at the MIG gun measured the same as the flowmeter
reading even when spatter and other restrictions occurred in production.
Bottom
Line:
Low pressure at first
appears to offer a solution to MIG weld start gas surge, as do flow restriction orifices mounted
at the feeder. But both approaches create other larger, but less obvious problems!
You'll note
the comments
about lack of sufficient extra gas making inferior starts in one of the
reported problem cases. That will
occur with any device that controls gas flow at the feeder, low pressure
devices, flowmeters or simple orifices. We have seen a number of
problems caused by the use of this flow control location.
A
recent survey of a shop with 100 MIG welders having flowmeters mounted at
the feeder showed all set with excess flow. The shop uses 0.045
diameter solid wire. No flowmeter was set below 50 CFH, about 25% were
set at the highest reading on the flowmeter (65 to 70 CFH) and approximately
25% had the flow indicator ball pinned to the top of the flow tube!
Our tests show shielding gas flow can be as high as 125 to 150 CFH when the
flowmeter needle valve is fully opened.
THE
WELDERS WERE NO DOUBT TRYING TO COMPENSATE FOR THE LACK OF SUFFICIENT EXTRA
GAS NEEDED TO PURGE THE WELD START AREA.
flowmeter at the gun nozzle (see photo
right.) This is an accurate and very repeatable measurement device. 
reading flow but pressure above an orifice - which didn't change! In
this instance the pressure was only 9 psi well below the 25 psi needed to
achieve "choked flow." Therefore the actual shielding gas flow
changes when the inevitable flow restrictions occur while welding, such as
spatter build-up in the nozzle and gas diffuser.
Sidebar: 
photo right (yellow arrow) attaches to a standard
flowmeter and sets flow by setting pressure. Where the low pressure
device test mentioned used above changed 4 CFH with a 1 psi increase in flow
resistance; in tests of this device the equivalent of a 1 psi increase in
resistance caused the flow to reduce 8 CFH! 