February 26, 2004
The introduction of gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) in the 1940s changed the way metals were joined. The automation of these processes meant the gas supply system also had to be automated to achieve optimal productivity.
In any welding process, the gas supply system must address basic equipment requirements for the gases at the point of supply and at the point of use.
The type of shielding gas required determines the point-of-supply equipment needed, the mode in which the gas can be supplied, and the volume of gases to be used. The point-of-use equipment depends on the method of supply.
For GMAW processes, a variety of shielding gases may be used—usually in a mixture of two or more gases. Shielding gases for GTAW are pure. Normally, an argon-rich mixture with either oxygen or carbon dioxide as the minor gas component is used for welding carbon steels, while a helium and argon mixture is used to weld aluminum, stainless steel, and other nonferrous metals.
How each gas is supplied determines the most efficient point-of-supply method. All gases can be supplied in high-pressure compressed-gas cylinders. Unfortunately, this method is the most expensive in terms of unit cost. It also diminishes productivity because of increased cylinder handling.
Argon, oxygen, carbon dioxide, and certain mixtures of argon/oxygen can be supplied in cryogenic liquid form. Although helium also can be supplied in this way, it is not cost-efficient for welding applications. Gases supplied in cryogenic liquid form cost less per unit than gases supplied in compressed-gas cylinders. Although unit cost for the gas is a major determinant in a gas supply system, it is not the only factor to be considered. Shielding gases may have to be supplied in compressed-gas cylinders when the requisite volumes do not justify the use of cryogenic liquid supply, or if the required gas is a mixture. See Figure 1.
When the situation demands that the shielding gas be supplied in high-pressure compressed-gas cylinders, a centralized supply minimizes cylinder handling and provides a continuous, uninterrupted gas supply.
Manifolds. A centralized supply requires manifolding the cylinders, plus piping to the workstations. A manifold system that consists of a primary in-use supply plus a reserve supply offers the best solution. A manifold system equipped with an automatic switchover, plus an alarm to indicate low supply, eliminates downtime caused by supply gas unavailability.
In most cases, the cost of the manifold plus the piping system is easily justified when compared to the potential loss of productivity associated with downtime.
Gas Blenders. The gas blender is another cost-effective point-of-supply system for high-pressure compressed-gas cylinders. When the shielding gas is a mixture, the gas distributor normally passes on any blending cost to the buyer.
Another point to consider is that in a gas blend, one gas usually is more expensive than the other. Argon can be four times or more expensive per unit than either the oxygen or carbon dioxide used in the blend. The final mixture is priced the same as the most expensive gas in the mixture.
Additional cost savings can be achieved by using cryogenic liquid cylinders for shielding gases, although they require a use of a gas blender to achieve the proper mixture in most cases.
Bulk Tanks. If the volumes required are high, installing bulk tanks may be the most cost-effective gas supply method.
Each supply option has a unit cost for the gas, plus a rental fee or facility fee for cylinder use. Sometimes optimum cost-effectiveness and efficiency can be obtained by using a combination of different gas supply methods. See Figure 2for comparative gas and supply method costs.
Although much effort can and should be given to securing the lowest-cost gases and point-of-supply method, point-of-use inefficiencies can undermine any cost savings. Each welding process requires a select flow rate based on a variety of parameters, such as wire diameter, arc voltage, and material type. When the desired flow rate is not achieved, faulty welds or wasted shielding gas can occur. Delivering the correct flow rate requires a delicate combination of pressure control and flow control; too little or too much shielding gas can be costly to the welding process.
A traditional flowmeter regulator delivers a preset pressure to an adjustable flowmeter. A flowgauge regulator is different in that it delivers an adjustable pressure to a set orifice for the desired flow. These devices normally are connected directly to the compressed-gas cylinders and must be removed and exchanged when the cylinder is depleted, causing downtime. Another cost issue with these regulators is that a positive pressure creates a surge of gas flow each time a weld is started. Using a gas surge device to reduce shielding gas waste during the weld start-up is recommended.
When a pipeline gas system is installed, a combination of devices can be used. Each combination must incorporate the proper pressure and flow devices for the desired flow.
In pipeline applications, the pressure from the point of supply already has been reduced before reaching the flow control device. The control device can be either an adjustable flowmeter or an orifice if the flow rate is constant for all applications. Regardless of the device used, the correct pressure must be delivered. Incorrect pressure will result in incorrect flow. Figure 3is a corrected flow factor chart.
Many factors are involved in selecting the best shielding gas delivery system. A local welding and gas distributor can evaluate current supply systems and suggest possible improvements.