Because August is vacation time for many, it s time to think about major projects you normally put off, er, don t have time for. Like moving, remodeling a shop or starting a new business.
Investing a little foresight into planning the neon-workshop layout and gas/air installation can save considerable labor and frustration in the long run. Excluding hands and eyes, the neon bender s primary work tools are the fires. If they re not constant or easily adjustable, even the best bender can t perform well with them. In many shops I ve visited, the gas and air plumbing caused inconsistent flame settings.
Don t toy with gas plumbing. A plumber who s licensed by the local gas company should set it up. Most residential gas explosions result from unprofessional homework on gas systems.
Natural gas vs. propane
In most commercial and residential areas, natural gas is piped into build-ings. If natural gas isn t available, use a tank of propane in an out-door shed. Some municipality regulations require special tank-storage cabinets.
Propane flames, which are hotter than natural-gas flames, require a different gas/air ratio in the mixer (see ST , July 2000, page 36). Because propane is stored under high pressure in the tank, in liquid form, you need a reducer (regulator) that normally produces a pressure of 0.8-1 psi. Special, high-pressure installations run up to 20 psi. When selecting a regulator, make sure you can crank up all fires and still have some capacity to spare.Advertisement
Normally, a gas company guarantees a roughly 0.3- to 0.5-psi pressure (which appears directly behind the gas meter). If your shop is in an industrial zone in which other, large, gas consumers are frequently switched on or off, or if you re located so far from the gas works that the pressure is very low, or your neon area is 100 ft. or more from the main gas meter, consider installing a gas booster.
In the winter, I saw a glass-melting furnace stop due to low-pressure conditions in the gas line — in Brooklyn! Thus, to regulate gas pressure in your area, ask a licensed plumber if it s advisable to install a gas booster, which is merely a gas-tight roots blower (a positive displacement pump with two lobed impellers, shaped roughly in a figure-8). The regulator must be a closed-cycle model that feeds surplus gas back into the booster s intake.
You can buy a used gas booster, but its packings and valves must be tight so they won t spit oil into your system. Normally, gas boosters are adjusted to an approximate 2-psi, natural-gas pressure.
The pressure throughout a branched, gas-distribution system remains equal as long as no gas flows. At the other extreme, when you completely open the end of the line, there s no pressure at the end, but full pressure at the meter. Thus, when gas flows, the pressure at all the line s points depends on the throughput — and various geometric line parameters.Advertisement
To fathom how complicated pressure-loss calculation is, conduct an Internet search using the keywords gas-flow dynamics, Hagen-Poiseulle s Law, Reynolds n numbers or Poisson s equations. Typically, a 3-in. black pipe for each of two or more bending tables can accommodate low-pressure natural gas. Avoid galvanized gas pipe, because the metal coating may peel off and clog your valves, mixers and burner heads.
Gas-service applications ask for the shop s maximum total consumption (in cu. ft./hr. or BTU/hr.) so the gas company can size the incoming pipe and meter accordingly. For a quick reference, use Table 1, but add at least one complete workstation load for possible expansion. Don t forget to add such gas utilities as your heat, stove, etc. You can never have too much service, and too little hurts.
After having determined the main service-line size for the neon area, apply this diameter to each bending table. I prefer a tree structure, which prevents the pressure changes with fluctuating loads, to a linear line structure. At each bending table, every fire must have a side T (as in your vacuum manifold) with its own shut-off ball valve.
The fires should follow a certain sequence as they T-off from the line to minimize pressure changes and avoid a possible Venturi effect (when fluid speeds up through a constriction, the pressure drops, produces a partial vacuum and thereby changes the other fire settings). Follow this simple rule: Side-T the largest consumer first, and the smallest consumer last. I don t recommend reducing the pipe size after having removed the large consumers from the line. All those hardware fittings are less of a hassle if they re the same size. After installation, apply a coat of paint to prevent corrosion and color-code the pipe (gas, air, water, oxygen, etc.).
Here s a worst-case scenario: Air enters the gas line. Now the lines contain an explosive mix. This may happen simply by kinking a hand-torch hose. When air pressure exceeds gas pressure, air is forced back into the gas line. When this explosive mix reaches an ignition source (another torch), it s too late to escape the inferno.
The big bang can be prevented by two simple devices, and at least one should be employed in each fire s gas and air lines. A check valve (or non-return valve) permits gas to flow in only one direction. I d recommend using it only in the air line, where there s normally enough pressure. Opening the valve requires a minimum pressure level, so the valve reduces the available pressure.
Contrastingly, a flashback arrester reduces throughput, but not the static pressure. Arresters comprise fine, metal (often copper) wire gauze in the line. If a flame (an explosion is just a fast-traveling flame front) hits the gauze, the metal cools the flame gases so that no further ignition can take place, and the flame normally will be extinguished – and never travel further into the line. In 1815, Sir Humphry Davy invented this method to block a flame for a safe mining lamp.
Another way to easily prevent a gas backflow into the air line (but not air into the gas line) is to use an electromagnetic, solenoid, main gas valve (plus a manual shutoff, of course), wired together with the blower, so the gas can flow only when air pressure is present. Don t scrimp on safety measures. In the long run, you never save by buying inadequate, unprotected, but less expensive, materials.
Valves and Venturi mixers
Every neon fire must be equipped with a gas (and air) shutoff ball valve, plus a needle/cone valve to adjust the proper flame. To achieve complete combustion, gas must be mixed with the proper amount of oxygen (in the air).
Most mixers rely on the Venturi principle. Here, the higher-pressure component (normally air from the blower at 4 to 5 psi) line is reduced in diameter, which causes the flow speed to increase. An increased hydrodynamic speed creates lower pressure (that s why airplanes fly). At the orifice, low pressure creates a partial suction where the second component (gas) enters. The common line diameter is larger to again reduce the speed.
A proper Venturi mixer is selected after you ve determined:
1. Air pressure and consumption;
2. Gas type, pressure and consumption; and
3. Fire head and hose backpressure at the given flow rate.
You can make your own Venturi from hardware plumbing parts. Remember the assembly sequence. The inner assembly must be brazed so it won t disassemble when soft-soldering the T s outer parts. Some shops use simple T s as a mixer, but they rarely provide stable mixes in different burner-setting conditions.
For any crossfire, ribbon fire or bench torch, use only safety gas hoses (like those used in welding). Also, use these for the gas supply to the handtorch mixers.
However, when extended from the bench-mounted mixer to the handheld (tipping) torch, the safety gas hose is simply too stiff. Instead, use medium-wall, pure-silicone hose. Silicone rubber doesn t burn if it accidentally contacts the flame, but it s easily cut by broken glass. A medium wall prevents the hose from being kinked or squeezed too easily.
In the glass shop, the operator must pay attention to cuts. Inspect the hose twice a day.
Again, gas installs are nothing to toy with. Rely on a licensed installer to avoid possible liability issues. Keep the fire where you need to work, but only there!
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