Setting Up a Welding Shop by the Numbers
Chapter 4 of 10 · 10 min read
The gas supply
Estimate a new shop's yearly use of shielding, fuel and cutting gases from its arc hours, decide how many cylinders to hold, store them safely, and measure what each job really uses.
A welding shop runs on gas as much as on electricity. Its wire welders need shielding gas for every metre of weld, its tungsten arc work needs argon, and its heating and some cutting need oxygen and a fuel gas. All of it arrives in cylinders, on rental, from a supplier who must be chosen, and all of it must be stored where it cannot fall, leak into a closed room or feed a fire. A new owner who guesses the gas use either ties up money in cylinders sitting idle on rent, or runs out on a Friday afternoon with a job half welded.
This chapter estimates the gas Mara’s shop will use from its expected arc hours, decides how many cylinders of each gas to hold, sets out the storage rules, and shows how the early manuals’ advice on measuring gas use per job still applies. Industrial Gases by the Numbers covers the gases themselves, their hazards, cylinder and bulk supply and leaks in detail; this chapter applies it to a new shop.
Which gases
From the work mix in Chapter 1, Mara’s shop needs:
- an argon and carbon dioxide mixture for solid-wire and gas-shielded flux-cored welding of steel, the bulk of its welding;
- pure argon for tungsten arc welding of stainless steel and aluminium, a small share of the work;
- oxygen and acetylene for heating, bending, freeing seized parts and occasional cutting where the plasma cutter cannot reach, mostly on repairs and site work.
The plasma cutter uses compressed air from the shop compressor, not bottled gas. For site work where shielding gas is awkward, Mara planned to use self-shielded flux-cored wire and manual metal arc electrodes, which need no gas.
Shielding gas from arc hours
Shielding gas use follows arc time, and Chapter 3 estimated about 585 hours of arc time a year for the shop. This is an illustration. If about 80% of that arc time is gas-shielded wire welding at a flow of 15 L/min, the arc needs about 0.8 × 585 × 15 × 60 ≈ 421,000 L, or about 421 m³ a year.
- Arc time a year
- about 585 hours
- Share with gas-shielded wire
- 80%
- Flow at the gun
- 15 L/min
- 585 × 0.8 × 15 × 60 ≈ 421,000 L ≈ 421 m³
- with a 15% allowance for purges, surges and small leaks: about 485 m³
The 15% allowance is a deliberate choice. Industrial Gases by the Numbers shows, in its chapter on shielding and purging gases, how one workshop found that about a third of its shielding gas was lost to high flowmeter settings, surges and leaks. Mara set out to hold the waste to about 15% from the start, by setting flowmeters to the procedure, fitting surge-limiting controls and leak testing every connection.
The other gases
Pure argon for tungsten arc work, about 5% of arc time at about 10 L/min, comes to only about 18 m³ a year. Oxygen and acetylene for heating depend on how much heating the repair and site work need. Mara estimated about 60 hours a year of heating with a tip using roughly 0.8 m³ of oxygen and 0.67 m³ of acetylene an hour, as an illustration, giving about 48 m³ of oxygen and 40 m³ of acetylene.
| Estimated use | Illustrative gas price | Gas cost | |
|---|---|---|---|
| Argon and carbon dioxide mixture | about 485 m³ | A$9.00/m³ | about A$4,365 |
| Pure argon | about 25 m³ with waste | A$11.00/m³ | about A$275 |
| Oxygen | about 48 m³ | A$6.00/m³ | about A$290 |
| Acetylene | about 40 m³ | A$14.00/m³ | about A$560 |
| Total gas | — | — | about A$5,490 |
The mixture is about four-fifths of the gas cost, which tells Mara where to concentrate: on the flowmeters, surges and leaks of the wire welders. The oxygen and acetylene, by contrast, are small enough that their cost matters less than their safety: the fuel gas and oxygen on the cutting trolley carry most of the shop’s gas hazards for a small part of its gas bill, and they get the most care in storage and handling.
How many cylinders to hold
Cylinder rental is charged on every cylinder held, full or empty, so the number held matters. The shop needs enough of each gas to keep working between deliveries, plus a spare so that a cylinder running out does not stop the job.
This is an illustration. The mixture cylinders hold about 10 m³ each, so the shop uses about 485 ÷ 10 ≈ 49 a year, about one a week. With weekly deliveries, Mara decided to hold three: one on each welder and one full spare. For argon, used at two or three cylinders a year, she held one. For oxygen and acetylene, one of each on the cutting trolley, with the oxygen swapped every two or three months and the acetylene less often. At an illustrative A$160 a year each, six cylinders on rent cost about A$960 a year, about a sixth of the gas cost.
Gas on site
Site work needs gas too, and the site vehicle carries its own small set of cylinders: oxygen and acetylene for heating and cutting, and sometimes a small mixture cylinder for wire welding where conditions allow. Cylinders carried in a vehicle bring their own rules. They are secured upright, in a ventilated space, never in a closed cab or a closed van body, with their valves closed and regulators removed or protected for travel, and the vehicle carries the signage and documents the transport rules require. Mara fitted a ventilated, lockable cylinder rack to the tray of the site ute, and removed the cylinders to the shop store at the end of each site day rather than leave them in the vehicle overnight. Small cylinders cost more per cubic metre of gas than full-size ones, so she kept site gas use low by using self-shielded wire and electrodes for most site welding.
Gas as a share of the job
Shielding gas is a modest part of most jobs’ cost, but not a negligible one. This is an illustration. A trailer chassis that takes about 4 hours of arc time with gas-shielded wire uses about 4 × 60 × 15 = 3,600 L, or 3.6 m³, of mixture, about A$32 of gas plus a share of rental, against perhaps A$700 of labour and A$900 of steel. Gas is a few per cent of the job. That is why shops often lose track of it, and why a high flowmeter or a leak can run for months unnoticed: no single job shows the waste, but the year’s invoices do.
Choosing a supplier
Gas suppliers differ in price, rental terms, delivery frequency, cylinder sizes and service. Mara asked two suppliers for quotes covering the gas prices, the rental per cylinder, any delivery or service charges, the minimum contract term, what happened to rental if she held fewer cylinders in quiet months, and how quickly an urgent delivery could be made. She also asked each for the safety data sheets for every gas and for their advice on storage. The lowest gas price was not the cheapest overall once rental and delivery charges were added, and the supplier she chose offered weekly deliveries on a fixed day, which suited a shop that could not afford to run out.
Storing cylinders
Cylinders are stored to the requirements of AS 4332, the supplier’s instructions and the regulator’s requirements. Industrial Gases by the Numbers sets out the common rules in its chapter on storage: cylinders and bulk liquid, and the most important for a new shop are these: cylinders upright and chained; a store outdoors or ventilated, never a closed room; oxygen separated from fuel gases; full and empty cylinders segregated; and cylinders moved on a trolley. Mara’s landlord agreed to a caged, roofed cylinder store against the outside wall beside the roller door, out of the path of vehicles, which kept the cylinders out of the building except for those in use.
Measuring gas use per job
The early manuals took the measurement of gas use seriously, because the cost of gas was a large part of the price of a repair. The Imperial handbook of 1916 explained that the oxygen gauge could be used to measure the oxygen used on a job, because the cylinder pressure falls in proportion to the gas used, and that, because its torch used nearly equal volumes of the two gases, the acetylene used could be taken as equal to the oxygen. Dunham’s manual gave the same method in more detail and added a torch consumption test to find each torch’s actual ratio of acetylene to oxygen. Both warned that the acetylene gauge said nothing reliable about contents, because the gas is dissolved in acetone, and that acetylene used should be found by weighing the cylinder: Dunham gave about 14.5 cubic feet of acetylene to the pound, close to the modern figure of about 0.91 m³ per kilogram at 15 °C.
The modern version is simpler, because shielding gas use can be estimated from arc time and flow, but the principle is the same: know what each job uses. Mara’s job cards (Chapter 9) record arc time, or at least welding hours, and the gas used is estimated from them. For heating-intensive jobs, the oxygen gauge before and after gives a direct reading.
Checking deliveries
The Imperial handbook also advised owners to check, at intervals, that they were getting the gas they paid for, by weighing acetylene cylinders when received and when empty. A modern shop can do the same: weigh a full cylinder on delivery against its stated contents now and then, and keep a log of cylinders delivered and returned against the invoices. Cylinders returned with substantial contents, a common waste when a cylinder is changed early to avoid running out mid-job, are gas paid for and not used, as Industrial Gases by the Numbers explains.
Leak testing from day one
A new shop’s gas fittings are all new, and new fittings leak as readily as old ones if they are not tightened and tested. Mara and Sam leak tested every regulator, hose and fitting with a leak detection solution when the shop was set up, and do so at every cylinder change, and close the cylinder valves at the end of each day so that any leak stops overnight. Industrial Gases by the Numbers calculates in its chapter on piping, materials and leaks how a pinhole leak running day and night can cost thousands of dollars a year.
When to consider a manifold
A manifold, feeding the welding bays through fixed piping from cylinders in the outside store, removes cylinders from the bays and cuts handling, as Industrial Gases by the Numbers describes. For a two-welder shop using about one cylinder of mixture a week, the cost of the piping and manifold is hard to justify in the first year. The money is better spent on equipment that earns, such as the plasma cutter or the site welder. Mara noted it as a step to take if the shop grew to three or more welders, or if she reorganised the layout and wanted the cylinders out of the bays.
A failure: Friday afternoon
This is an illustration. In the shop’s second month, Sam was welding the last frame of a trailer due for collection on Saturday morning when the mixture cylinder ran out. The spare had been used earlier in the week and not yet replaced, because the delivery day was Tuesday and nobody had ordered an extra one. The nearest supplier’s depot was closed by the time Mara rang.
The frame was finished on Saturday morning with a borrowed cylinder from a neighbour, and the customer waited two hours. Mara’s fix was a simple rule: when the spare is connected, an extra cylinder is ordered the same day, and the cylinder count on the store door is checked every Thursday. The extra rental of occasionally holding a fourth cylinder is small beside a delayed customer, and a shop that promises Saturday collection cannot depend on a supplier’s weekday hours.
Exercise: the cost of a high flowmeter
If both of the shop’s wire welders had their flowmeters set at 22 L/min instead of 15, how much extra mixture would the shop use in a year, using the arc hours in this chapter, and what would it cost at A$9.00 per cubic metre?
The extra flow is 22 − 15 = 7 L/min over the gas-shielded arc time of 0.8 × 585 ≈ 468 hours, which is 7 × 60 × 468 ≈ 196,560 L, about 197 m³ a year. At A$9.00, that is about A$1,770, excluding GST, plus about 20 extra cylinder changes. It is more than the shop’s whole argon, oxygen and acetylene bill combined, and it is avoided by setting the flowmeters to the procedure and checking them at the start of each day.
