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Acetylene Withdrawal Limit

Calculate maximum safe and recommended continuous acetylene withdrawal rates to prevent explosive acetone depletion.

Cylinder Capacity

Target Torch/Tip Flow Rate

CFH

The Acetylene Bomb 💣

Acetylene is highly unstable above 15 PSI. To store it safely at 250 PSI, cylinders are packed with a porous monolithic mass soaked in liquid Acetone. The acetylene gas dissolves into the acetone like carbon dioxide in soda.

If you draw gas out of the cylinder too fast, the liquid acetone is pulled out with it. Not only does this ruin your hoses and mix, but it depletes the stabilizing agent in the bottle. This causes the remaining acetylene inside the cylinder to become unstable and explosively decompose. The iron-clad rule in welding is NEVER exceed 1/7th of the cylinder capacity per hour. Constant duty withdrawal should be kept to 1/10th.

Max Safe Withdrawal Limit

20.7 CFH

Cubic Feet per Hour (1/7 rule)

Recommended Continuous

14.5 CFH

For continuous duty (1/10 rule)

DANGER

Tip requires more flow than the cylinder can safely provide. Acetone withdrawal imminent.

Manifold multiple cylinders together.

Draw Status

For estimation purposes only. Always consult a licensed professional before beginning work. Full Trade Safety Notice →

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What is Acetylene Cylinder Thermodynamics: Acetone Dissolution, the 1/7th OSHA Rule & Manifold Sizing?

Acetylene (C2H2) is the only commercially used fuel gas that is inherently unstable — it can explosively decompose at pressures above 15 PSI (103 kPa) without any ignition source. To safely store it at cylinder pressures of 250 PSI, the cylinder is packed with a porous monolithic mass (calcium silicate or charcoal) saturated with liquid acetone. Acetylene dissolves into the acetone under pressure following Henry's Law — similar to CO2 dissolved in a sealed soda can. When the valve opens, reducing pressure, acetylene comes out of solution and is delivered at working pressure. The 1/7th rule governs how fast this release can safely occur.

Mathematical Foundation

Maximum Intermittent Withdrawal

Max_{CFH} = \frac{Cylinder_{Capacity}}{7}

Cylinder_{Capacity}

= Measured in Cubic Feet (CF)

Safe Continuous Withdrawal

Continuous_{CFH} = \frac{Cylinder_{Capacity}}{10}

Laws & Principles

The OSHA/CGA 1/7th Withdrawal Limit: OSHA 29 CFR 1926.350 and CGA G-1 mandate that acetylene must never be withdrawn faster than 1/7th of cylinder capacity per hour (intermittent use) or 1/10th for sustained continuous use. Drawing faster causes the acetone solvent to boil out alongside the gas. This has three catastrophic consequences: (1) liquid acetone destroys Grade R rubber hoses on contact (acetone is an aggressive solvent), (2) the flame turns orange/sooty as acetone burns, and (3) the remaining acetylene loses its stabilizing medium, dramatically increasing the risk of spontaneous explosive decomposition inside the cylinder — even without a flame or spark.
The 15 PSI Absolute Pressure Ceiling: Free gaseous acetylene becomes shock-sensitive above 15 PSI and can detonate from mechanical impact, friction, or even heat. This is why all acetylene regulators are factory-limited to 15 PSI maximum delivery pressure, and why acetylene must never be used for pressure testing, pneumatic tools, or any application requiring pressures above 15 PSI. Inside the cylinder, the 250 PSI storage pressure is safe only because the gas is dissolved in acetone within the porous mass — it is not free gas at that pressure.

Step-by-Step Example Walkthrough

" A welder needs to use a #8 rosebud heating tip that demands 40 CFH of acetylene for sustained preheating of a 2-inch thick carbon steel plate. Available cylinders are Size 3 (145 CF). Determine if a single cylinder is safe, and if not, calculate the manifold requirement. "

1. Calculate max intermittent withdrawal for Size 3: 145 CF / 7 = 20.7 CFH.
2. Calculate safe continuous withdrawal: 145 CF / 10 = 14.5 CFH.
3. Compare to tip demand: 40 CFH >> 20.7 CFH (max intermittent) — a single cylinder is DANGEROUSLY undersized.
4. Calculate cylinders needed for intermittent use: ceil(40 / 20.7) = 2 cylinders minimum.
5. Verify: 2 x 20.7 = 41.4 CFH combined limit > 40 CFH demand — PASSES for intermittent.
6. For sustained continuous preheating: ceil(40 / 14.5) = 3 cylinders needed (3 x 14.5 = 43.5 CFH).

Final Result: A single Size 3 cylinder can safely deliver only 20.7 CFH — roughly half the rosebud's 40 CFH demand. Using it alone would cause rapid acetone depletion within minutes. For intermittent heating (short bursts with pauses), manifold 2 cylinders in parallel through a CGA-rated manifold with check valves (combined limit: 41.4 CFH). For sustained continuous preheating (the typical use case), manifold 3 cylinders (combined limit: 43.5 CFH) to maintain the 1/10th continuous safety margin.

Quick Answer: What is the acetylene 1/7th withdrawal rule?

The OSHA/CGA acetylene 1/7th rule states that you must never draw acetylene from a cylinder faster than 1/7th of its total capacity per hour — for example, a standard Size 4 (300 CF) cylinder has a maximum withdrawal rate of 300 / 7 = 42.9 CFH. For sustained continuous operation, the safe rate drops to 1/10th of capacity per hour. Exceeding these limits draws liquid acetone out of the cylinder alongside the gas — risking hose destruction, flashback, and catastrophic tank failure.

Acetylene Withdrawal Rate Formulas

Maximum Intermittent Withdrawal (OSHA/CGA 1/7th Rule)

Max CFH = Cylinder Capacity (CF) ÷ 7

Safe Continuous Withdrawal (Sustained Operation)

Continuous CFH = Cylinder Capacity (CF) ÷ 10

Cylinders Required for a Specific Flow Demand

Cylinders Needed = âŒˆ Required CFH ÷ Max CFH per cylinder âŒ‰

Cylinder CF— Total acetylene capacity of the cylinder in cubic feet (stamped on cylinder; also denoted by size: Size 3 = 145 CF, Size 4 = 300 CF)
Max CFH— Maximum cubic feet per hour that can be safely withdrawn intermittently (short bursts); governed by the 1/7th rule
Continuous CFH— Maximum CFH for sustained continuous use (heating applications, long welds); governed by the 1/10th rule
Why 7?— Acetone in the cylinder can release gas safely up to ~1/7th of total capacity per hour. Above this rate, liquid acetone boils out — contaminating hoses and destabilizing remaining acetylene

Real-World Withdrawal Rate Examples

✅ Safe — Welding Tip on Size 4 Cylinder

Tip demand: 15 CFH | Cylinder: Size 4 (300 CF)

Step 1: Max intermittent rate = 300 / 7 = 42.9 CFH
Step 2: Continuous rate = 300 / 10 = 30.0 CFH
Step 3: Tip demand (15 CFH) < continuous limit (30 CFH)

→ Safe on a single Size 4 cylinder — well within both limits

🚫 Dangerous — #8 Rosebud on Size 3 Cylinder

Rosebud demand: 40 CFH | Cylinder: Size 3 (145 CF)

Step 1: Max intermittent rate = 145 / 7 = 20.7 CFH
Step 2: Continuous rate = 145 / 10 = 14.5 CFH
Step 3: Tip demand (40 CFH) is 1.9× the max safe rate
Solution: Manifold 2 Size 3 cylinders → combined limit = 41.4 CFH ✅

→ DANGEROUS on single cylinder — manifold 2+ cylinders in parallel

Common Acetylene Cylinder Sizes & Safe Withdrawal Rates

Cylinder Size	Capacity (CF)	Max Intermittent (÷7)	Safe Continuous (÷10)
Size 1 (MC)	10 CF	1.4 CFH	1.0 CFH
Size 2	40 CF	5.7 CFH	4.0 CFH
Size 3 (B-Tank)	145 CF	20.7 CFH	14.5 CFH
Size 4	300 CF	42.9 CFH	30.0 CFH
💡 Source: CGA G-1 (Acetylene) and OSHA 29 CFR 1926.350. Rates shown are for a single cylinder. Manifolding cylinders in parallel adds their rates. Always keep cylinders upright — horizontal cylinders cannot safely release gas at labeled rates.

Pro Tips & Critical Acetylene Safety Mistakes

Do This

✓Manifold cylinders in parallel when your tip demand exceeds a single cylinder's limit. Connect two or three cylinders through a common manifold header and check valve assembly. The combined withdrawal limit is additive: two Size 3 cylinders = 41.4 CFH max — enough for a #8 rosebud. Always use a listed manifold with individual check valves to prevent backflow between cylinders.
✓Always keep acetylene cylinders fully upright during use and storage. Acetylene cylinders store acetone as a liquid soaked into a porous mass. If the cylinder is tilted or laid on its side, the acetone can shift and pool near the valve — causing large quantities of liquid acetone to be discharged with the gas even at normal withdrawal rates.

Avoid This

✗Never use acetylene above 15 PSI working pressure. Free gaseous acetylene becomes shock-sensitive and can explosively decompose at pressures above 15 PSI (103 kPa) — even without a spark. This is why regulators must be set to 15 PSI maximum and why acetylene cylinders use dissolved storage in acetone at 250 PSI. Using acetylene for pressure testing, blow-off air, or anything above 15 PSI is prohibited by OSHA and the CGA.
✗Don't use Grade T (industrial) hose with acetylene — use Grade R or T welding hose rated for fuel gas. Acetone is an aggressive solvent. If acetone is drawn from the cylinder (due to excess withdrawal rate or tipped cylinder), standard rubber hose will swell, soften, and fail, potentially creating a fuel-gas leak at the torch. Inspect hoses quarterly for swelling and replace immediately if acetone contamination is suspected.

Frequently Asked Questions

What is the acetylene 1/7th withdrawal rule?

The 1/7th rule, codified by CGA G-1 and referenced in OSHA 29 CFR 1926.350, limits acetylene withdrawal to a maximum of 1/7th of the cylinder's total capacity per hour. For a 300 CF (Size 4) cylinder, that is 42.9 CFH. This limit exists because acetylene is stored dissolved in liquid acetone within the cylinder. Withdrawing gas faster than 1/7th causes the acetone to boil out with the gas, destroying hoses, contaminating the flame, and leaving behind destabilized acetylene that can detonate inside the cylinder.

Why is acetylene stored in acetone?

Acetylene gas is extremely unstable at pressures above 15 PSI — it can explosively decompose without a spark when compressed. To safely store enough acetylene in a portable cylinder, the cylinder is filled with a porous calcium silicate or charcoal mass soaked in liquid acetone. Acetylene dissolves into acetone at high pressure (up to 250 PSI in the cylinder), similar to COâ‚‚ dissolved in soda. The porous mass prevents any significant free-gas space where shock-initiated decomposition could propagate. When the valve is opened, acetylene comes out of solution and is delivered at working pressure — always 15 PSI or less.

What happens if I withdraw acetylene too fast?

Excessive withdrawal causes liquid acetone to be drawn out with the gas. This creates three serious hazards: (1) Acetone immediately destroys standard rubber welding hoses — hoses swell, soften, and may rupture, creating a fuel-gas fire or explosion risk. (2) The flame becomes orange and sooty as acetone burns alongside acetylene, creating poor weld quality and a carbon deposit on the tip. (3) As acetone depletes, the remaining acetylene in the cylinder loses its stabilizing medium and the risk of spontaneous explosive decomposition inside the cylinder increases dramatically. If you suspect acetone depletion, close the valve, tag the cylinder out of service, and contact your gas supplier immediately.

How do I safely use a large rosebud heating tip with acetylene?

Large rosebud heating tips (e.g., #5–#10) typically demand 30–100+ CFH of acetylene — far exceeding what a single standard cylinder can safely provide. The correct approach is to manifold multiple cylinders in parallel using a CGA-rated manifold and check valve assembly. For example, an 80 CFH (#8 rosebud) demand requires at least two Size 4 cylinders (2 × 42.9 CFH = 85.7 CFH combined limit). Always use the maximum intermittent rate (1/7th) as your limit, install a flashback arrestor at the torch and regulator, and ensure all cylinders are equally pressured before manifolding.