Ideas and principles to keep in mind when selecting gas-pressure control devices for oxy-fuel cutting, laser cutting, or MIG/TIG welding applications.
Gas regulators and switchover systems used in fabrication shops typically deliver gas molecules to an oxy-fuel cutting application, laser cutting system, or a MIG/TIG welding application at a pre-determined pressure or flow.
A single-stage regulator, when attached to a standard high-pressure cylinder of gas product at an outlet pressure set at 100 psi, will deliver that pressure until the cylinder is emptied.
But the delivery pressure will have slipped from the original setting of 100-psi delivery pressure. At process end, the operator will notice the delivery pressure is now somewhere between 120-150 psi, a phenomenon referred to as the decaying inlet pressure characteristic.
When using a good two stage-regulator in this same application, the delivery pressure set at the beginning will be the same pressure you return to when the high-pressure cylinder is almost empty. If the cylinder pressure at the source is less than 1000 psi, or if the gas being used is a liquid-going-to-vapor type, as for example CO2, there is little or no benefit to be gained by using a two-stage regulator.
Often an application will require a two-stage regulator that will deliver 400 or 500 psi, which is not an easy piece of equipment to find. The solution is to install a single-stage regulator, 0-500 psi at the gas source, and a line regulator 0-500 psi at the end-use point, providing two stages of regulation to the application.
Manifolds and switchovers
Using gas manifolds or switchover systems is a way to supply uninterrupted gas product to an application without running the process out of gas. Because basic manifolds, however, typically are set manually, there is still the possibility of a gas run out.
Pressure-differential or fully automatic switchover systems can keep the operation running without exhausting the gas supply, providing that someone is assigned to monitor the equipment. An alternative is to incorporate telemetry devices to the system.
The key factor for measuring or comparing performance with gas-handling equipment is knowing how long the manufacturer warrants its products in the field. A 90-day or 6-month factory warranty is a tipoff that the equipment represented is probably not the best value. A one-year warranty is better, while a 5-year warranty is best.
Considering that a cylinder regulator may be flowing inert, toxic, corrosive, flammable, oxidizing, or high-pressure gas, safety should always be the first consideration. A leak of any of these gases can cause serious injury to anyone in the immediate vicinity of where these gases are handled or dispensed.
Visual and periodic bubble-leak testing of these systems is recommended by the National Fire Protection Association (NFPA) and endorsed by most safety programs as a way to minimize potential incidents.
The reality of manufacturing dictates that high-leak integrity and quality cannot be supplied at the lowest price. The true cost of inexpensive gas handling equipment is realized in the loss of the margin of safety provided by a high-quality product or design.
Most gas handling equipment is fabricated in brass or stainless steel to withstand the forces exerted by the pressure it handles. When the equipment is being used for toxic or corrosive gases, the optimal choice of material for the body and diaphragm is stainless steel. Material considerations are normally dependent on the gas being handled, the application it will be supporting, or the environment in which the application is located.
Design improvements on gas handling equipment may sometimes be nothing more than packaging or physical dimensional changes, as opposed to real design improvements.
One novel and real improvement is the practice of using a pre-filter before the gas enters the regulator. But, purchasing an industrial grade of gas to save money and hooking it up to one of these pre-filtered regulators and expecting to get a Grade 4 or 5 (99.99% to 99.999% purity) gas product out on the other end will not work. Doing so is about as practical as putting a pre-filter on a regulator.
If the gas in the cylinder is not clean enough for the application to begin with, it probably won’t be clean enough after it has passed through the pre-filter either. Purchasing the correct grade of gas is the only real answer.
Many equipment manufacturers now have encapsulated seat assemblies in their regulators and switchovers. By assembling multiple components into one capsule assembly, the process of building a regulator becomes more efficient and the end result more becomes reliable. Inlet filtration also can be incorporated at this level, which can help maintain gas purity in the system. This step also makes repairing or rebuilding the regulator easier and less complicated.
Materials of composition
The service life of a regulator or switchover system depends on the materials composition of the diaphragm. A regulator or a system with a neoprene diaphragm has a life span of approximately 40,000 cycles, whereas a regulator or system with a stainless steel diaphragm has a life span of about 20,000 cycles. A cycle is complete when the regulator is hooked up to a cylinder of gas, the gas is turned on, the regulator is adjusted into a given pressure or flow, and then the process reversed.
Keeping in minde these key practical pointers about gas delivery systems and equipment can make it possible for equipment distributors to deliver quality products to their customers each day, every day.
John Karpus is an applications specialist for CONCOA, a manufacturer of gas-control devices and systems for analytical, scientific, aerospace, environmental, pharmaceutical, electronic, medical, petrochemical, and metalworking applications. Contact him at firstname.lastname@example.org.