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Tip of the Month

History of MIG Shielding Gas Flow Control
Avoid Pitfalls In Device Selection

Download Tech Paper PDF : History of MIG Shielding Gas Flow Control

We are often asked questions, including from engineers, that request more knowledge about MIG (and TIG) shielding gas flow control. This “history” will help. When the first viable MIG system patent stated one of Claims 8, 9, 11 and 12 the shielding must be, “ nonturbulent to exclude air from the arc."

CHOKED FLOW.
The two main companies developing MIG welding were Airco and Linde the and there R&D engineers used “choked flow designs.”  Understanding how choked flow works will provide knowledge of what controls MIG flow rates.  It will help avoid the pitfalls of using devices that elimCHOCKEDinate this essential feature when trying to control gas uses and avoid gas waste.
A unique situation occurs when the velocity in a small orifice or needle valve flow control reaches the speed of sound it cannot flow any faster.  Once that velocity is reached, the downstream pressure has NO INFLUENCE on gas flow rate.  It’s all controlled only by the upstream pressure. 
To have choked flow control and maintain the preset flow required the upstream pressure to be over 25 psi.  It’s no coincidence that quality flow control regulators for MIG and TIG welding all exceed 25 psi!  


HOW SHIELDING GAS FLOW IS SET AND MEASURED

Cylinder Supply
There are two types of flow controls used for cylinder supply.  Both have a regulator that attaches to the cylinder.  The first and a very common type is called a Regulator/Flowgauge.  It has two gauges.  One is a cylinder contents gauge that displays the pressure in the cylinder. 
The other gauge is calibrated in CFH.  There is a very small hole or orifice often about 0.025 in diameter at the exit of the regulator.  It operates in a choked flow mode. Typical operating pressures for the typical MIG flow rates of 25 to 50 CFH the pressure may vary from 40 to 60 psi.  The exact pressure is dependent on the specific model design.
The other common device is called a Regulator/Flowmeter and uses a fixed pressure regulator with a flow tube and float or ball.  That type was previously discussed and most common are set at 25 psi, 50 psi and some for CO2 service 80 psi.  It consists of a very accurately tapered tube, with a small round ball.  The ball is pushed up by the drag force of the shielding gas flow and pulled down by gravity.  A higher volumetric flow rate through a given area increases flow speed and drag force, so the float will be pushed upwards. 

Pipeline Supply

Controlling shielding gas flow and setting the desired flow rate differs on a pipeline gas supply.  At each welder gas flow control drop from a pipeline a regulator is not needed  Typically, that pressure is set at 50 psi.
So, a flowmeter with variable area tube can be attached directly to the 50-psi outlet and the desired volume set with a needle valve adjustment.  It can be the same flowmeter that is on the output of a cylinder regular.  It’s important that the flowmeter be calibrated at the pipeline pressure.  We have found a majority are NOT!  We see many installations with flowmeters calibrated at 25 psi, which will read about 28% low!  So, when reading 35 CFH they are actually flowing 45 CFH!  A source of waste.  Our Portable Flowmeter (PFM) can be used, and a mark made on the flowmeter tube with a Sharpie!
For lower volume needs, some distributors are manifolding a number of cylinders on a fabricated pallet arrangement. The cylinders are piped together like the 12 shown left.  The output is then connected to the fabricators pipeline often with a standby 2nd cluster of cylinders.  This is often called Mini-Bulk supply, and some have automatic switching to the back-up manifolded cylinders so the empty can be replaced.
A simple flowmeter can be used as the output of the pipeline drop as the cluster output is controlled by a regulator.

BUT HIGH PRESSURE CAUSES PROBLEMS
Pressures needed to assure “choked flow” or automatic flow compensation regardless of inevitable downstream restrictions cause a problem at weld starts.  Gas continues to flow in the needle valve or orifice TURBULANCEuntil the pressure in the gas delivery hose quickly equals the higher pressure. That extra gas stored at increased pressure “blasts out” of the MIG gun nozzle at every start.  We (and others) have measured peak gas flow of over 200 CFH.
Osborne Reynolds work published in 1890 defined even a worse problem than the audible gas surge. He showed that once Turbulent flow occurred, it took time for it to become smooth Laminar flow even after the flow velocity lowered to the Laminar level.  Therefore, after each weld start, the high peak flow velocity pulls moisture laden air into the shielding gas stream causing excess spatter and other quality problems.

WHAT FLOW RATE CAUSES TURBULENCE?

Although little is published about the flow rates causing turbulence in MIG (and TIG) welding one technical paper used interesting methods to directly define turbulence in shielding gas flow.  One was to measure the oxygen near the weld to define what flow created air mixing with the shielding gas stream.  Other approaches were also utilized.


Need for Some Extra Gas to Purge Air Defined by Stauffer in 1982

StaufferIn a 1982 Patent Stauffer defined the need for extra start gas in his shielding gas waste control device. He added a rather large starting gas storage volume element (item 112 in patent figure right) because his device also used relatively low pressure, less than the 25-psi needed to achieve choked flow.  The patent states:    "... air leaks back into the MIG gun 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."

OUR PATENTED SOLUTION TO GAS WASTE IS INEXPENSIVE, MAINTAINS AUTOMATIC FLOW COMENSATION AND IMPROVES WELD START QUALITY
Our patented Gas Saving System solution (GSS) has no moving parts and is inexpensive.  Most important, welders love the improved starts. 
Yep, some fabricators understand and calculate how much shielding gas they are wasting.  But a major precept of “Lean Manufacturing” states most waste is invisible.  Must work to quantify. Sure fits shielding gas!

Our patented solution is straight forward and simple.  It employs a custom extruded small ID, large OD gas delivery hose from flow control at the gas source cylinder of pipeline.  That reduces the amount of shielding gas stored when welding stops.
To keep the peak flow from becoming excessively turbulent at each weld start it incorporates a “peak flow limiting” orifice.  NOTE, it does NOT control the steady state flow only the peak flow.  If welders are foolishly setting very high flows, it will limit flow to under ~90 CFH.  But in that situation welders should be trained that anything over about 55 CFH is just pulling air into the shielding gas stream and making weld quality worse.  The combination small ID hose and peak flow control orifice typically cut total gas use in half!  That assumes leaks are monitored and fixed.
It’s simple to install.  Just replace the existing gas hose from flow control at gas source to wire feeder or welder.  That’s it!  Set the proper flow and start welding. 
EXAMPLE:
aswWe have hundreds of users and many who have documented their savings results with careful measurement of up to 63%.  However, this early customer experience defines not only the 40+% savings they found BUT most important, improved weld start quality.
This was a pipe weld repair application and as soon as the welding engineer and I installed the GSS and he was going to measure gas savings the welder said, “This is much better!”  NOT in gas savings but weld start!
He was having many rejected repair welds after they were ultrasonically checked.  He knew the cause was the high gas surge at the start.  He would cut the wire close to the tip and keep the MIG gun high to give time for the audible gas blast to reduce.  As can be seen he was facing a 3+ second problem where the flow was turbulent pulling in moisture laden air.  After 6 months of use he said he was experiencing very few defects! The start quality sayings were at least as important to reduced gas waste!

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What do you think of the Tip Of the Month?

Received this positive feedback regarding a Tip of the Month discussing the effects of leaks in a gas delivery system:

"I have been in Metal Manufacturing for over 25 years. Recently I have been assigned to a department manufacturing centrifugal compressor impellers where a cover is TIG and MIG welded to the top of blades. Your Tip of the Month is the first plausible explanation (for occasional defects) I have come across that can explain this phenomenon and why it may be more prevalent in the springtime and in high humidity periods."