Part I. Overview of compressed gases used in the metal fabrication industry
August 28, 2003
The metal fabrication industry has used compressed gases for more than one hundred years. Oxy-fuel cutting and welding have existed since the beginning of the 20th century. The more automatic welding processes, such as gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW), began as early as 1920.
|Photo courtesy of Harris Calorific Inc.|
GMAW and GTAW continued to grow in popularity with the introduction of lighter materials, such as aluminum and magnesium, in the late 1940s. Both of these processes use compressed gases to shield the weld from the atmosphere.
The introduction of plasma welding and cutting in the late 1980s required the use of compressed gases such as oxygen, nitrogen, air, and various shielding gases. Laser materials processing, specifically the use of carbon dioxide and Nd:YAG lasers, also rely on compressed gases.
The emergence of each of these processes, all of which are in use today, was driven by the desire for a more productive metal fabrication method. Achieving optimal productivity with these processes requires an uninterrupted supply of the appropriate compressed gases. Adequate uninterrupted supply alleviates downtime. Installation of the proper gas supply and point-of-use gas delivery systems can keep the processes running day and night.
When designing your gas supply and delivery system, you must do the following:
Each of the processes requires specific gases, and the gas supply system must be designed to handle the specific gases.
In oxy-fuel applications, acetylene has been the primary fuel gas selected for many years. While acetylene is still the most versatile fuel gas, alternate choices, such as propylene, propane, and natural gas, may offer advantages depending on the individual application. The alternative fuel gases are more cost-effective for brazing, cutting, and heating, but are not suitable for oxy-fuel welding. The biggest advantage alternative fuel gases have over acetylene is in the supply and storage. Acetylene must be supplied in individual cylinders with a maximum capacity of approximately 400 cubic feet per cylinder, while the alternative fuel gases can be supplied in either cylinders, bulk stations, and even from a pipeline. Acetylene cylinders can be stored in a manifold arrangement to increase the supply.
Oxygen can be supplied in a variety of containers, including individual high-pressure (3,000-PSI) cylinders, liquid cylinders, and stationary bulk stations. The selection of the supply method is determined by the proper identification of the four items in the design criteria. The appropriate solution may be simply to manifold the oxygen cylinders in a central supply area, or to change the method of supply from high pressure to either liquid cylinders or a bulk station.
Each method has advantages and disadvantages. Installing a manifold for the high-pressure cylinder removes the cylinders from the work area. Adding a switchover system to the manifold allows for a continuous gas supply. The change from high-pressure cylinders to liquid cylinders is quite common and referred to as modechange in the compressed gas industry.
Liquid cylinders, which contain cryogenic gas, have an advantage over high-pressure cylinders in increased supply. The disadvantages are limited flow capacity and maximum available working pressure.
Originally liquid cylinders were restricted to 235-250 PSI working pressure and flow rates of 400-600 SCFH. Installing a manifold could increase flow capacity. Some liquid cylinders systems today offer greater working pressure (400 PSI) and flow capacity (2,000 SCFH). Installing a permanent bulk station is the next step in the mode change equation. A bulk station normally offers the lowest unit cost for the oxygen, plus a large and continuous supply, but it involves more capital investment. Only large users of product (monthly demand of 100,000 SCF or more) should consider a bulk installation.
Various compressed gases are used as shielding for GTAW and GMAW processes. One or more gases or a mixture of gases are used depending on the application. GTAW processes normally require a pure gas, such as argon or helium. GMAW processes use pure or mixed gases. The mixture can be either two or three gases in a specific blend for a specific application. The gases can be a combination of argon, carbon dioxide, helium, and oxygen.
Using these gases involves different supply issues from those related to oxy-fuel applications. Although all the gases are available in compressed gas cylinders, either as the pure gas or the desired mixture, not all of the gases can be supplied in cryogenic form. Helium, for example, is best supplied in high-pressure cylinders, since the cryogenic (liquid) must be produced from the gaseous compressed helium. The use of mixtures also is limited to compressed gas cylinders since many of the required mixtures cannot be supplied in liquid form. Exceptions are some argon-oxygen mixtures that can be supplied in cryogenic cylinders.
Using either a manifold system for the cylinders and/or a blender to mix the gases onsite can be more cost effective than using single cylinders. Both the manifold and blender systems have advantages and disadvantages, but either will increase productivity by alleviating the need for shutting down to handle cylinders or replenish the supply.
Laser material processing has become increasingly popular with the metal fabrication industry for cutting, welding, and surface modification applications. The carbon dioxide (CO2) and Nd:YAG lasers are the main types of lasers used today. The Nd:YAG laser may have little or no use for compressed gases, requiring only assist (also referred to as process – cutting or welding) gases. The CO2 laser, on the other hand, requires compressed gases for three specific tasks:
1. Generating the laser beam – Referred to as resonator or lazing gases, gases for generating laser beams can be supplied either in a specific blended-gas mix or as pure gases to be blended by the laser. The lazing gases are helium, nitrogen, and carbon dioxide, and in some cases, other gases may be added. Gas consumption is very low, but the purity requirement is very high. Lazing gases must be protected from any contaminants, which can cause damage to the resonator optics. The presence of moisture (H2O) and hydrocarbons (THC) must not exceed the limits specified by the laser resonator manufacturer. Lazing gases almost always are supplied in compressed gas cylinders because of the purity requirements.
2. Purging the laser resonator cavity – The laser resonator cavity requires purging at regular maintenance intervals, and the laser beam delivery system may require compressed gases for protecting the laser beam as it travels to the work piece. Although compressed air can be used for purging and protecting, the use of nitrogen is becoming more popular. Nitrogen can be supplied in either compressed gas or liquid cylinders.
3. Assisting the process – Gases are used to assist the cutting, welding, and other surface modification processes. Oxygen, nitrogen, and compressed air typically are used for cutting, although argon and helium are used for the more exotic materials, such as titanium.
Cutting gases may require high working pressure of 300 PSI or more, plus a flow rate that exceeds 3,000 SCFH. The use of these gases supplied in cryogenic form is somewhat limited, since the required pressures and flows easily can exceed the capabilities of some cryogenic systems. Liquid gases normally are stored below 250 PSI, and flow rates are restricted by the rates at which the liquids can be converted to gaseous vapor.
The process gases used for welding are argon and helium, or a mixture of the two. These gases are used as shielding gases for protecting the weld area and may be material specific.
The design of the gas system for a CO2 laser demands that each requirement be considered individually. A laser material processing system must have an uninterrupted supply of gases for maximum efficiency.
Part II will discuss how you can maximum your profits with the correct oxy-fuel supply system.