A comprehensive look at the options
July 13, 2004
When laser gases are used in significant volumes or in an around-the-clock operation, a centralized gas delivery system is a practical necessity. A well-conceived delivery system reduces operating costs, increases productivity, and enhances safety.
|Photo courtesy of Weldstar, Aurora, Ill.|
A centralized system allows all of the cylinders to be stored in one location. Having all cylinders in one place streamlines inventory control and simplifies and improves cylinder handling. Gases can be separated by type to enhance safety.
Maintaining proper gas purity also is simplified with a centralized system in which all materials are consistent. For laser-purity-grade gases, regulators with stainless steel diaphragms, brass barstock-constructed internal inlet filtration, and diaphragm packless valves should be used.
A centralized system reduces the frequency of cylinder changeout. By connecting multiple cylinders to manifolds in banks, one bank can be vented, replenished, and purged safely, while a second bank provides continuous gas service. Such a manifold system can supply gas to multiple lasers in the entire shop, eliminating the need for separate cylinders and regulators for each laser.
Since cylinder switchover is accomplished automatically by the manifold, cylinders in a bank are exhausted uniformly, which results in improved gas use, higher gas purity, and lower costs. Also, the delivery system is more reliable when cylinder changeouts are performed in a controlled environment.
The manifolds should be equipped with check valves (nonreturn) to prevent gas backflow and to purge assemblies, eliminating contaminants during changeout and ensuring system and gas purity.
Gas manifold systems can be supplied in various designs depending on the application. Following is a brief description of available systems.
Single-stage Systems. For some applications, laser gas is required only in small amounts each day. Such an application may not appear to need an automatic changeover manifold, but the delivery system still must be designed to protect against gas contamination and to minimize cylinder changeout costs.
A single-station manifold with a bracket is an ideal solution for low-flow applications. It provides a safe and cost-effective way to connect and change out cylinders by eliminating the need to struggle with the regulator. The gas system should incorporate a purge assembly in the manifold, which allows the regulator to be purged with an inert gas (usually nitrogen) to protect it from contamination.
The single-station manifold also can be equipped with a second pigtail and isolation diaphragm valve. This arrangement allows an additional cylinder to be connected and held in reserve. The gas switchover must be performed manually. Manual switchover may require resetting or shutting down the laser and is not desirable in 24/7 operations.
Multiple-cylinder Systems. Some laser applications require a higher flow rate than can be supplied by a single-station manifold. In this situation, a header manifold generally is the first choice.
The header manifold is a cost-effective means of connecting two or more cylinders to the same line for continuous gas supply. Each cylinder connection point, or station, is fitted with a valve to permit individual cylinders to be isolated for changeout. To preserve system purity, these valves should be the diaphragm packless type, which prevents oxygen, hydrocarbons, water vapor, and other airborne contaminants from entering the gas stream.
Header manifolds are used in both single-row and double-row configurations, allowing almost any number of cylinders to be connected to the delivery system. Header manifolds also are used with switchover systems to connect more than one cylinder to each bank.
Many fabrication shops require a constant, uninterrupted gas supply. For these shops, any pause in the gas supply can result in lost productivity, ruined workpieces, and even downtime for an entire shop operation.
Systems that can switch from a primary to a reserve bank without interrupting the gas supply can minimize or eliminate such costly downtime. Selecting the correct system for a given application depends on a number of factors.
One of the most important considerations in designing a laser-gas delivery system is the level of gas purity required at the end-use point. Laser gases must be supplied at a certain purity level, and the materials used to construct the supply system must maintain the purity requirements.
Acceptable construction materials include regulators with brass barstock bodies and stainless steel diaphragms.
The manifolds at this level must be designed to minimize trace component adsorption. Acceptable materials include 316-series stainless steel convoluted pigtails assemblies. All tubing must be 316, cleaned and passivated. Flow shutoff valves must be of the diaphragm packless type.
Several methods can be used to change over cylinder banks. These methods vary substantially in their sophistication levels. As might be expected, cost usually increases with sophistication. Selecting the correct method depends on the application. The additional features in the more sophisticated versions can justify their expense in critical applications.
Differential Manifolds. The simplest manifolds are designed to change over when pressure drop is sensed in one cylinder bank relative to the other. Such a manifold is called a differential type.
As an example, the regulator on bank 1 is set for a delivery pressure of 200 pounds per square inch gauge (PSIG). The regulator on bank 2 is set at 180 PSIG.
As long as there is sufficient gas in bank 1 to maintain the 200-PSIG delivery pressure, bank 2's regulator stays closed. As the gas in bank 1 is exhausted, delivery pressure drops. When bank 1's pressure drops below 180 PSIG, bank 2's regulator opens at about 180 PSIG. The regulator pressure gauges must be visually monitored to determine when changeover has occurred.
When bank 1 has been replenished, the regulator settings should be reversed so that bank 1 is at 180 PSIG and bank 2 is at 200 PSIG. If this is not done, replenishing bank 1 will cause the bank 2 regulator to close. Bank 2 then will gradually deplete each time bank 1 is replaced, until there is not enough gas in bank 2 to effect changeover. Resetting the regulators alternates which bank is primary and which is reserve to prevent this possibility.
Differential manifolds require regulator monitoring and resetting and generally are selected for applications in which cylinder bank changeover is relatively infrequent, and when a drop in delivery pressure at changeover will not cause a problem.
Automatic Changeover. A change or drop in delivery pressure can, in some instances, result in an adverse effect on laser performance. To avoid this problem, an automatic manifold system may be selected. This type of system also is based on differential pressure, but delivery pressure is held almost constant during cylinder bank changeover.
The automatic manifold regulates pressure in two (or three) stages to keep delivery pressure steady, even during changeover. After bank 1 has been replenished, the bank selector must be rotated so that bank 2 is primary and bank 1 is reserve.
Thus, the flow of gas to the process is uninterrupted and is maintained at a constant delivery pressure.
When used in conjunction with a pressure switch and enunciator to provide remote indication of changeover, the automatic system manifold need not be monitored. Since resetting regulators is not required, the potential for operator error and reserve bank draining is minimized. Automatic manifolds are used for applications in which changeover is relatively frequent and variations in delivery pressure cannot be tolerated.
Logic-controlled Manifolds.In some the critical 24/7 manufacturing laser materials processes, an uninterrupted gas supply is an absolute necessity. Gas supply failure in these cases could result in loss of an entire job or even production line shutdown. The potential cost of either of these eventualities is so high that the installation of a gas delivery system designed to provide an uninterrupted gas supply clearly is justified. A logic-controlled manifold generally is selected for these applications.
A logic-controlled manifold is designed to sense cylinder bank pressures and to switch automatically to a full bank of cylinders with no disruption in delivery pressure or flow rate when one cylinder bank is depleted. This is accomplished electronically using pressure switches. In addition, these system manifolds are designed to interface with telemetry outputs.
By using pressure transducers, the logic-controlled manifold allows the user to monitor the gas supply pressure from a remote location. The system also can track and indicate which side of the manifold currently is in use. Telemetry-compatible systems are beneficial to both the laser shops and the gas supplier. When using Web-based telemetry, any event notification, such as cylinder or bank supply depletion, can be sent to either or both the supplier and the user via e-mail and text messaging to cell phones.
A logic-controlled telemetry system differs from an automatic switchover system in that no valves need to be switched manually. When the supply reaches a preset pressure, the manifold automatically switches to the reserve side, which continues to deliver gas without interruption. This changeover is accomplished by energizing a solenoid valve, which sends an air signal to open a valve on the reserve cylinder bank. At the same time another solenoid de-energizes to close a valve on the supply side, shutting it off. The outlet of both valves is connected to the inlet of a final line regulator, which maintains constant delivery pressure to the laser.
As the system switches, an alarm condition is indicated on the remote display, alerting the user that a cylinder bank needs to be replenished. The manifold continues to switch from a depleted side to a full side as required, each time notifying the user when the switch occurs.
With a logic-controlled manifold, the danger of inadvertently draining the reserve bank is eliminated, because no reliance on the operator is needed to switch a changeover valve or reset regulators. Fail-safe operation is ensured by using normally closed valves on each cylinder bank. In the event that either bank's electrical power should fail, these valves automatically default to one supply bank to avoid shutting down the entire system. When full electrical power is restored, the system can be reset.
Some shops have multiple lasers that use the same laser gases but may require different mixtures, delivery pressures, flow rates, or even purity levels. Unfortunately, even when a centralized gas distribution system is in place, these varying requirements often are dealt with by a maze of tubing, line regulators, and traps, which commonly are found scattered on benches behind laboratory and process equipment.
This mazelike arrangement can result in a number of serious problems. First, since regulators and tubing can be bunched together, it is possible to connect the wrong gas to the laser, which results in lost or degraded experiments or even laser damage. Second, safety can be compromised, because tubing, regulators, and traps often are not protected or marked adequately. Third, operating and maintenance costs increase as the difficulty of identifying and correcting problems increases.
A more practical arrangement to eliminate or minimize these problems is to install point-of-use regulator systems designed for dedicated laser gas service. A typical system provides a way to control both delivery pressure and flow rate at the point of use. Contaminant traps can be included in the system.
Carefully assessing your applications and considering your options can help you determine the most efficient, cost-effective, safe laser-gas delivery system for your operation.