Understanding liquid cylinder vaporization characteristics and laser gas requirements is key for optimizing gas withdrawal from cylinders
Allen County Fabrication in Lima, Ohio, is a fabrication job shop that uses a TRUMPF L3050 5kW CO2 laser for cutting a variety of materials. Typically it may have jobs for cutting carbon steel up to 1-inch thick, stainless steel up to ¾ inch, and aluminum up to ½ inch.
Jeff Thompson, Allen County Fabrication’s laser technician, called in Bob Montgomery, CONCOA’s district manager, for a consultation regarding the gas installations. The company wanted a gas installation package that would feed its laser. The lasing gas requirements would be supplied from high-pressure cylinders. The assist gases would initially be supplied from 500-PSI liquid cylinders, but would ideally be convertible to bulk tanks as the business grows without the need for replacing the regulation and the manifold equipment.
Additionally Allen County Fabrication wanted a system that would eliminate the persistent shut-down problems experienced when cutting 3/8- to ½-inch or thicker stainless steel or aluminum using a 2.3-mm nozzle. Under the high-flow nitrogen assist gas conditions required for these applications, the company was experiencing pressure drops in the line that would cause the laser protection circuits to shut down. When cutting these materials, the company was averaging shut down three times daily with an approximate 15-minute recovery time per episode.
The lasing or resonator gas requirements were fairly simple to supply. CO2, nitrogen, and helium are being supplied from 2200-PSIG high-pressure cylinders. This is a convenient and cost-effective gas source because of the low consumption rates of the resonator gases. These pressure requirements into the laser are 80 PSIG for each gas with flows ranging from 0.005 scfh to 0.70 scfh.
TRUMPF requires that the purity levels for its lasing gases be the following: Helium -Grade 4.6 or 99.996% purity; CO2-Grade 4.5 or 99.995% purity; Nitrogen-Grade 5.0 or 99.999% purity.
In actuality, there are three major contaminants that need to be minimized by specifying these gas purity levels: hydrocarbons, humidity, and particulate matter. Hydrocarbons should be limited to less than 1 part per million, moisture to less than 5 parts per million, and particles to less than 10 microns. These contaminants can cause significant loss of beam power. They also will cause deposits on or pitting of the resonator mirrors that will deteriorate their efficiencies and reduce their wear lives.
For the lasing gases, Allen County Fabrication installed a switch-over for each of the three gases, with one cylinder as the primary source and one in reserve. When the primary cylinder starts to empty, the reserve cylinder automatically kicks in to avoid laser shut down when a primary cylinder depletes. A point-of-use panel with three line regulators fine-tunes the inlet pressure at the inlet of the laser. Helium leak integrity of 1 x 10-8 scc/s for the regulation equipment (translating to 1 cubic centimeter of helium-leak every 3.3 years), stainless steel tubing, and tube compression fittings are used to maintain the high purity of the gases. The switchover includes built-in tee purge assemblies to flush out any contaminants that may enter the line during initial construction, when cylinders are replaced, or if any leaks should develop in the line. At the entrance to the laser, a 2-micron filter and a high-flow relief valve provide final protection against particulate contamination or over-pressurization.
The assist gas requirements at Allen are much more stringent. Oxygen is used as the assist gas for cutting carbon steel. Nozzle sizes with oxygen can vary from 1.0 mm to 2.3 mm with maximum nozzle pressures up to 50 PSIG and flows up to 250 scfh. Because there may be pressure drops of more than 100 PSIG in the laser and in the line, the pressures at the regulators will most probably need to be set significantly higher to ensure that the nozzle pressure requirement is met. TRUMPF recommends a minimum oxygen purity of 99.95% or Grade 3.5, although higher cutting speeds can be attained with higher gas purities.
Nitrogen can be used for cutting carbon steel, stainless steel, or aluminum. Carbon steel cutting speeds will be lower with nitrogen as the assist gas than with oxygen. However, the use of nitrogen avoids oxide deposits on the surface cut. Nozzle sizes with nitrogen vary from 1.0 mm to 2.3 mm, with maximum pressure requirements of 265 PSIG at the nozzle and flows up to 1,800 scfh. TRUMPF recommends minimum nitrogen purity of 99.996% or Grade 4.6. Here again, higher cutting speeds and cleaner cuts can be achieved with higher purities. All equipment for the assist gas side must also be of a design that will maintain these high gas purities.
The higher flow requirements for the assist gases render liquid cylinders or dewars more cost-effective gas sources than high-pressure cylinders. The contents are cryogenically stored in liquid form with vaporized gases stored in the head space. Liquid cylinders are commonly available with 230, 350, or 500 PSI gaseous vent-relief valves. The 500 PSI models (a.k.a. laser cylinders) are normally the only ones suitable because of the high pressure requirements for laser assist applications. Contents from the cylinders can be withdrawn in either gaseous or liquid form. However, both the laser and the regulation equipment need to be supplied in gaseous form; if liquid gases are used, they must first be gasified with the use of an external vaporizer.
It is worth noting that gaseous withdrawal from a liquid cylinder can be complicated. The maximum gaseous withdrawal rate that can be expected from a single dewar is approximately 350 cubic feet per hour, and withdrawal rates decline in continual use and as the contents of the liquid cylinder decrease. Manifolding the gaseous outlets of multiple liquid cylinders does not always prove to be effective. Because the rate of head pressures in the different liquid cylinders will be unequal, there is a tendency for the higher-pressured ones to shut off flow from the lower-pressured ones. For each additional manifolded liquid cylinder, the additional flow capacity achieved is only approximately 20% of the initial dewar (that is, 70 cubic feet per hour). To improve the gaseous capacity of a liquid cylinder manifold, a vent manifold should also be installed. The vent manifold will equalize the pressures in the head spaces of all the liquid cylinders to withdraw the gas equally. Using a vent manifold will increase the flow capacity of each additional liquid cylinder to approximately 80% of that for the initial dewar (that is, 280 cubic feet per hour).
Allen County Fabrication preferred using liquid cylinders for both the oxygen and nitrogen assist gases. In the future, the company would like the flexibility to change the nitrogen mode to a bulk tank. Because the oxygen assist requirements are sporadic and rather moderate at 50 PSI and 250 scfh maximum, this can be supplied from two liquid cylinders manifolded to a dome-loaded, balanced-stem regulator. The balanced-stem design allows for very high flows exceeding 10,000 cubic feet per hour with moderate pressure drops of between 30 to 40 PSI. Traditional inverse seat-type regulators are not recommended for these applications because of the severe drooping characteristic of their flow curves. They exhibit drastic outlet pressure drops as the flow requirements for that regulator increase, causing the laser’s protection circuitry to shut down when minimum pressures to the laser cannot be maintained.
The dome-loaded feature of the regulator is one that bleeds a bit of the gas from the primary regulator to a second regulator that feeds it back to the dome side of that same primary regulator. Using this gas rather than a spring to press on the diaphragm opens the seat for downstream gas flow. This design is one that allows for outlet pressure ranges of between 0 to 100 up to 0 to 2000 PSI with constant delivery flow and pressure, regardless of supply source inlet pressure fluctuations.
The nitrogen supply was not practical from the gaseous outlet of the liquid cylinders. At peak conditions requiring 1800 scfh at 265 PSIG, it would require a manifold of eight liquid cylinders with vent manifolds to meet these needs. Instead, the liquid is being withdrawn from two liquid cans and fed to a 5000 scfh finned vaporizer. The nitrogen gas flowing from the vaporizer is then fed to a dome-loaded, balanced-stem regulator similar to that on the oxygen assist side.
To accelerate the withdrawal of the liquid and to maintain the head pressure of the two liquid cylinders, a third dewar is being used as a pusher. The nitrogen gas from this third liquid cylinder is fed through a regulator to the vents of the two primary liquid cylinders at around 450 PSIG.
Both the oxygen and nitrogen lines include a 40-micron tee filter and a high-flow relief valve at the inlet to the laser. These, similar to the resonator gas installations, are final protection against particulate contaminants and over-pressurization.
The new installations have led to improved efficiencies and cost savings to Allen County Fabrication. In the three months that they have been operating with the new gas systems, the laser has not shut down once, and the company is getting approximately 20 to 25% higher yields from liquid cylinders. Besides the savings in the gas itself, Allen has reduced a few of the change outs of liquid cans, which average approximately 30 minutes per change out. Estimated savings range from $700 to $1000 per month.
Dan Cruz ([email protected]) is director of sales at CONCOA, Virginia Beach, VA; www.concoa.com.