Things to Consider When You Select a System that Delivers Assist Gases to Lasers

You may recall that in the last issue of SGR, focus articles dealt with several aspects of the laser for material processing systems. One article addressed the fact that gases are commonly an afterthought to the purchaser of a laser, and it also discussed the practical applications of gas handling equipment in getting the lasing gases to the resonator without compromising purity.

In the other article, discussions centered on considerations of pressure and flow requirements that offer distributors the opportunity to pick the right gas mode or supply container to deliver the assist gas, and to use their knowledge of the gas industry as a “profit added service.”

In this issue, we asked a number of equipment suppliers to help complete the discussion on lasers by addressing the “considerations for delivery of the assist gases” to lasers.

For reference, a quick and simple recap of the laser cutting process is in order here. The lasing gases are mixtures that are delivered into the resonator cavity. There the laser beam is generated. That laser beam is then directed through a complex series of highly polished mirrors to the laser head. At the laser head the beam is focused on the material to be processed; i.e., it is used to melt a thin sliver of the material. The force of a gas being blown onto the area then removes the melted material. This gas is called the assist or process gas.

I know this is an over simplification of the process, but the bottom line is that if the melted material is not removed properly, some of the value of the laser cutting instrument is lost. This is a critical consideration, and one worthy of the following inputs from the companies who responded to SGR’ s request for discussion on the subject.

Here is what they said:

What needs to be considered when delivering assist gases to lasers?

The two primary elements to consider are flow and pressure. The primary purpose of the assist gas is to remove the metal that has been heated by the laser beam. There is no more critical time in making a cut than when it is initiated. Whether it is a long cut or simply a pierce, this is the time that having the wrong type of pressure control makes a poor cut. Most pressure regulators are spring controlled.

The spring in the bonnet provides a force above the diaphragm or piston that generates the outlet pressure from the regulator. When a high flow is required, the regulator seat opens further than at low flow causing the spring to stretch a little more. If you look at the flow performance curve of any spring loaded regulator, you will see that as flow increases there is a natural drop in outlet pressure, this is due to the change in the spring rate. The proper style of regulator to control assist gases is a dome loaded regulator.

With a dome loaded regulator, pressure control is provided by a cushion of gas above the diaphragm or piston. As the flow to the laser increases, the pressure above the diaphragm is replenished rapidly and therefore there is virtually no fade in the outlet pressure at high flows. 

If you look at the flow performance curve of a dome loaded regulator you will see that the curve is relatively flat throughout the flow range of the regulator. Another characteristic of a dome loaded regulator is response time. When a cut is initiated, flow at pressure is required immediately, providing a high quality cut from start to finish. The reaction time of a dome loaded regulator is unparalleled allowing the laser to live up to its capabilities. Whether cutting thin material or thick, continuous cuts or piercing, the laser will get the flow and pressure required right there right now.

Dave Durkin is a Senior Products Manager for Victor Equipment. He can be reached at Dave_Durkin@thermadyne.com

High-Pressure Cradles, Cryogenic Manifolding of Liquid Cylinders, and Micro-Bulk Considered

It is not uncommon each week to receive a call asking, “What is the best way to deliver nitrogen at 450 psi?” My response is always, “What material/thickness will be welded and how often?” Typically, the material is one-quarter to halfinch stainless that the job shop needs to cut every two to three weeks. This requires peak flow of 1,347 to 4,042 cfh. 

Solutions range from high-pressure cradles, cryogenic manifold of liquid cylinders and micro-bulk. If the job only requires two to three hours of production, then cradles are a reasonable alternative. However, this is a nightmare logistically for the gas distributor. With minimal capital investment in a vaporizer and a cryogenic manifold as illustrated in Figure 1, the job shop is capable of four to 15 hours of continuous operation.

If the stainless jobs are more frequent, then a 1,000-liter or 1,500-liter micro-bulk cylinder becomes feasible. The micro-bulk option usually requires a contract and a facility fee.

In either case, it is important for the job shop to have command of its usage patterns in order to make a profitable decision.

(Richard Green is Manager of Business Development, CONCOA, manufacturers of gas flow control systems and equipment, headquartered in Virginia Beach, VA, 800-225-0473, richard. green@concoa.com, www.concoa.com.)

High Flow, High Pressure Regulator Ideal for Laser Gas Delivery

Piston-sensed regulators are ideal for pressure control of laser assist gases supplied from high pressure (up to 3750 psig) manifolds or tube trailer sources. High flow design with 1/2” NPT inlet and outlet porting allows flows in excess of 6,000 scfh. A large piston sensor gives excellent sensitivity while the balanced stem design minimizes the effect of changes in inlet pressure on delivery pressure settings. A large handknob provides fast low-torque pressure settings.

Glenn Haun is General Manager of Advanced Specialty Gas Equipment, Middlesex, N.J. He can be reached at ghaun@asgemail. com website is www.asge-online.com

Laser Considerations: Do The Math for Pressure and Source System

The major concerns we have at SGD when we are talking to a customer about a laser cutting system are not very much different than that of any specialty gas handling system. Simply, what are the delivery pressure requirements and what is the gas source going to be – high pressure cryogenic liquid containers, a bulk tank, multiple high pressure cylinders, etc. We need to know these parameters to properly size the pressure regulator and any filters that may be required.  But just as importantly, we need to be assured that the gas source is sufficient to meet the need. So often a user thinks that a single liquid container or high pressure cylinder has infinite capacity to deliver whatever he needs. For example, we just had a customer that wanted to deliver 25 SCFM oxygen from a single 300 cubic foot cylinder—do the math.

Frank Scornavacca is president of SGD Inc., a single-source, full-line wholesale specialty gas distributor with a unique support program. He can be reached at frank@sgd.com.

Major Consideration in Selecting Correct Equipment is the Application

Choosing the correct gas supply equipment for laser assist gases can be complex and, if not done correctly, can be quite problematic. The relatively high pressures and flow rates required must be carefully examined whenever choosing a gas source as well as the gas delivery equipment. 

For oxygen cutting, pressures of less than 100 psi and flow rates of less than 300 scfh are typical for laser cutting of carbon steel and can be supplied through liquid dewars or a small bank of high pressure cylinders.

For cutting with nitrogen, however, the pressures and flow rates required can exceed 400 psi and 2000 scfh respectively. A careful study of the requirements (and the costs) needs to be done before choosing a nitrogen delivery system. 

Large pressure drops and flow restrictions through inadequately sized regulators, manifold systems and/or piping can cause laser shutdown resulting in unnecessary downtime.

For most nitrogen assist gas applications, a high flow pressure regulator will be necessary. Servo-dome technology has been implemented into compressed gas regulator designs to eliminate outlet pressure fluctuations which cause performance issues with CO2 laser cutting.

These special application regulators are manufactured specifically for the laser fabrication industry. Typically, the units will come with 1/2-in NPT ports for high flow demands and are adaptable to either high pressure cylinder cradles or low pressure bulk and liquid systems. They easily handle the rigorous demands of high production/high capacity laser cutting using nitrogen as an assist gas.

For uninterrupted gas flow with no downtime, a high pressure, high flow automatic switchover system may be desirable. Like the stand-alone regulators, these systems can be connected to either high pressure cradles or low pressure bulk systems. Whenever gas supplies are depleted on the primary side, the system will automatically switch to the high pressure or mini-bulk/liquid reserve until the primary supply can be replenished. Switchover systems allow continuous supply of laser assist gases with no downtime or interruptions. Continuous flow switchover systems are invaluable in production environments where downtime can cost several hundred dollars per hour.

David W. Gailey is Manager, Specialty Products, The Harris Products Group, 2345 Murphy Blvd, Gainesville, GA 30504. Tel: (770) 538 6171; Fax: (800)841-3738; email:david_gailey@lincolnelectric.com

Purity is Paramount in Laser Gases.

Quality ranks highest among the demands made by laser manufacturers, and the way to ensure peak performance is to use high-purity laser gases, along with other good practices. Dust and impurities inhibit laser performance since impurities reduce output power and disturb the uniformity of the electrical discharge. Also, frequency of service and maintenance have a direct effect on the utilization cost.

The purity of the gas delivered is not always the culprit that leads to poor performance. Even when cylinders themselves are clean, particles can be introduced by the equipment. That means that the choice of proper equipment is also very crucial. But even this is not enough; the quality of the installation is also critical.

CO2 laser gases are commonly delivered unmixed. The mixture ultimately used is made in the laser itself. For excimer applications, the laser gases are delivered premixed in cylinders or bundles.

Equipment and installations: For many years it has been well known that water vapor and hydrocarbons are the most critical impurities found in lasers. Where these are present, it becomes necessary to purify these gases.

Impurities principally affect the optics by causing condensation to form, reducing the level of reflectivity and inducing a slow, but inevitable destruction if preventive actions are not taken.

Heavy and costly repairs, as well as lost production can be avoided when high purity gases are used, and a good level of high purity equipment is installed by a knowledgeable company.

Equipment: It is strongly recommended that high purity equipment be used even when the laser is dedicated to an industrial application, the objective being to maintain the purity level of the gas at its highest level, avoiding pollution that can be traced to the materials used for the production of the switch-over boards, supply boards, and regulators.

All regulators installed in a laser process should have a Hastelloy diaphragm to avoid diffusion, rather than the classical industrial rubber diaphragm.

The level of the specifications for the regulators also must be held to a higher standard. Generally speaking, the creeping coefficient of a standard industrial regulator is approximately 30 percent, according to some norms such as ISO 2503. In these cases, it is necessary to use regulators with a much lower coefficient and a better regularity and reproducibility.

This is the reason we recommend the use of a two-stage regulator when the laser is connected directly to a cylinder. A balanced regulator, when installed on a switch-over board or a supply board, maintains the quality of the supply gas to the laser when a second level of regulation is applied at the entry of the laser, just after a filter.

When a filter is installed, it should be a one micron filter with an inlet pressure rating of 290 psi.

A second stage of regulation should be used for oxygen laser gases used in systems for cutting and welding applications since the process is very sensitive. This second stage should be compulsory when the oxygen pressure must be reduced to a level less than 15psi.

For applications with other gases the demanded performances are much higher in terms of flow, even when the purity requirements are not very high; i.e., nitrogen. This is mainly due to the fact that the high flow helps to clean the regulator by itself. Then the use of a filter, located just before the point of use, is essential, regardless of the gas and the equipment used. The minimum outlet flow currently required for this application is from 50 to 70 m3/h (1750 to 2470 ft3/h) to up to 250 to 300 m3/h (8830 to 10595 ft3/h).

The use of hoses to connect the gas supply to the process is a common practice, and even if you have been very careful in your equipment selection, you must not forget that the less expensive components can also play a major role in maintaining the quality of the installation.

If in doubt regarding the quality of the hoses (material, internal flow restriction, dust traps, etc.), do not hesitate to adopt more rigid standards and use an all stainless pigtail in term of flow, and use a pig tail (See Figure 2) instead of a common hose.

We stress the importance of gas quality without impurities, and the utilization of high quality equipment that is connected to purge the system. This is mandatory to keep your process free from particles entering when replacing the gas supply.

This is the point at which your hoses or any other connections will come in direct contact with the ambient atmosphere, and the danger of polluting your installation increases if you do not use a purge when replacing the cylinders. This is the time when switch-over boards, with an integrated purge line (As shown in Figure 3), should be a requirement to ensure better purity without creating extra costs and ensuring better safety by reducing the leak risks.