Calcady™

Acetylene Withdrawal Limit

Calculate maximum safe and recommended continuous acetylene withdrawal rates to prevent explosive acetone depletion.
Trades & Construction
Standard: OSHA 1926.350 / CGA G-1

Cylinder & Torch Parameters

CFH
OSHA 1926.350 & CGA G-1 Core Safety Principle:Acetylene is dissolved in liquid acetone within a porous cylinder mass. Withdrawing faster than 1/7th of tank volume per hour draws liquid acetone out with the gas, degrading hoses and creating severe cylinder destabilization.
Calculated result for Max Intermittent Limit (1/7th):

Max Intermittent Limit (1/7th)

20.71 CFH
Short bursts (< 15 min duration)
Calculated result for Max Continuous Limit (1/10th):

Max Continuous Limit (1/10th)

14.5 CFH
Sustained continuous preheating
Calculated result for Manifolded Cylinders Required:

Manifolded Cylinders Required

2 Cylinders (Intermittent) / 3 (Continuous)
Parallel manifold configuration for safe gas delivery

Live Acetylene Withdrawal Safety Determination

Verification of safe gas withdrawal limits for 30 CFH demand from a 145 CF cylinder:

Mathematical Solution
1Step 1: Calculate Maximum Intermittent Limit (1/7th Rule)

OSHA 1926.350 / CGA G-1 maximum allowable gas withdrawal rate for intermittent heating cycles.

Max_{CFH} = \frac{Cylinder\,Capacity}{7} = \frac{145}{7}
20.71 CFH
2Step 2: Calculate Continuous Duty Safe Limit (1/10th Rule)

Recommended continuous withdrawal threshold to ensure acetone vapor remains dissolved in liquid state.

Continuous_{CFH} = \frac{Cylinder\,Capacity}{10} = \frac{145}{10}
14.5 CFH
3Step 3: Evaluate Tip Flow Demand Against Thresholds

Demand exceeds allowable single-cylinder rate; acetone extraction will occur without manifolding.

\text{Ratio} = \frac{30\text{ CFH}}{20.71\text{ CFH}} \times 100\%
144.8% (DANGER_ACETONE_DEPLETION)
4Step 4: Determine Parallel Manifold Requirement

Number of identical cylinders manifolded in parallel to maintain individual withdrawal below the 1/7th threshold.

N_{cylinders} = \lceil 30 \div 20.71 \rceil
2 Cylinders
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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
MaxCFH=CylinderCapacity7Max_{CFH} = \frac{Cylinder_{Capacity}}{7}
CylinderCapacityCylinder_{Capacity}= Measured in Cubic Feet (CF)
Safe Continuous Withdrawal
ContinuousCFH=CylinderCapacity10Continuous_{CFH} = \frac{Cylinder_{Capacity}}{10}

Laws & Principles

  • The OSHA/CGA 1/7th Withdrawal Limit: OSHA 29 CFR 1926.350 and CGA G-1 establish that acetylene should not 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 hazardous consequences: (1) liquid acetone degrades Grade R rubber hoses on contact (Grade T hose is required for acetone resistance), (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 an external ignition source.
  • The 15 PSI Working Pressure Ceiling: Free gaseous acetylene becomes shock-sensitive above 15 PSI and can decompose from mechanical impact, friction, or heat. This is why standard acetylene regulators are factory-limited to 15 PSI maximum delivery pressure, and why acetylene is not permitted 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. 1. Calculate max intermittent withdrawal for Size 3: 145 CF / 7 = 20.7 CFH.
  2. 2. Calculate safe continuous withdrawal: 145 CF / 10 = 14.5 CFH.
  3. 3. Compare to tip demand: 40 CFH >> 20.7 CFH (max intermittent) — a single cylinder is DANGEROUSLY undersized.
  4. 4. Calculate cylinders needed for intermittent use: ceil(40 / 20.7) = 2 cylinders minimum.
  5. 5. Verify: 2 x 20.7 = 41.4 CFH combined limit > 40 CFH demand — PASSES for intermittent.
  6. 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 establishes that acetylene withdrawal should not exceed 1/7th of total cylinder 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 dangerous cylinder destabilization.

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 = ⌈ Required CFH ÷ Max CFH per cylinder ⌉

  • 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)

  1. Step 1: Max intermittent rate = 300 / 7 = 42.9 CFH
  2. Step 2: Continuous rate = 300 / 10 = 30.0 CFH
  3. 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)

  1. Step 1: Max intermittent rate = 145 / 7 = 20.7 CFH
  2. Step 2: Continuous rate = 145 / 10 = 14.5 CFH
  3. Step 3: Tip demand (40 CFH) is 1.9× the max safe rate
  4. 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)
Size 1 (MC) 10 CF 1.4 CFH
Size 2 (B-Tank) 40 CF 5.7 CFH
Size 3 145 CF 20.7 CFH
Size 4 300 CF 42.9 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. Keep cylinders upright during use — 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. Use a listed manifold with individual check valves to prevent backflow between cylinders.
  • ✓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

  • ✗Do not operate acetylene above 15 PSI working pressure. Free gaseous acetylene becomes shock-sensitive and can decompose at pressures above 15 PSI (103 kPa) — even without an external ignition source. 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 applications above 15 PSI is prohibited by OSHA and CGA standards.
  • ✗Don't use Grade R hose with acetylene — use Grade T hose rated for fuel gases and acetone resistance. Grade R hose is lined with SBR rubber, which acetone readily degrades and dissolves. Grade T hose features a polychloroprene (neoprene) synthetic inner tube that resists deterioration if liquid acetone is carried over. Inspect hoses regularly for softening, blistering, or 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 CO2 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 — 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). Use the maximum intermittent rate (1/7th) as the baseline limit, install a flashback arrestor at the torch and regulator, and ensure all cylinders are equally pressured before manifolding.

Related Calculators

Calculation Provenance & Validation Record

Method

Acetylene Safe Hourly Withdrawal Limits (1/7th and 1/10th Rules)

Formula
Qmax,intermittent=Vcylinder7,Qsafe,continuous=Vcylinder10Q_{max,intermittent} = \frac{V_{cylinder}}{7}, \quad Q_{safe,continuous} = \frac{V_{cylinder}}{10}
Assumptions
  • Cylinder volume corresponds to standard acetylene gas capacity measured in cubic feet at 70°F and 250 PSIG.
  • Calculations enforce OSHA 29 CFR 1926.350(a)(7) and CGA G-1 1/7th intermittent and 1/10th continuous rules.
  • Withdrawal limit assumes cylinder is secured in an upright vertical position with functional regulators limited to 15 PSIG working pressure.
  • Parallel cylinder manifolding assumes equalized pressures and listed flashback arrestors and check valves on each cylinder pigtail.
References
  • Acetylene Safe Handling & Withdrawal Limits (CGA G-1 / OSHA 1926.350) (2020 Edition) — 29 CFR 1926.350(a)(7) & CGA G-1 Section 5.3
Last substantive review:
Automated test status: 3 golden test vectors passing (ACW-01, ACW-02, ACW-03)
Method, assumptions & governing standards

Calculation Methodology

Trade estimation calculations derived from standard mechanical, electrical, and construction formulas.

Governing Standard 2020 Edition

Standard:OSHA 1926.350 / CGA G-1

Statutory building, electrical, and mechanical codes vary by jurisdiction. Confirm local municipality amendments before installation.

Key Assumptions & Constraints

  • Cylinder volume corresponds to standard acetylene gas capacity measured in cubic feet at 70°F and 250 PSIG.
  • Calculations enforce OSHA 29 CFR 1926.350(a)(7) and CGA G-1 1/7th intermittent and 1/10th continuous rules.
  • Withdrawal limit assumes cylinder is secured in an upright vertical position with functional regulators limited to 15 PSIG working pressure.
  • Parallel cylinder manifolding assumes equalized pressures and listed flashback arrestors and check valves on each cylinder pigtail.
Field Trade Notice: For trade planning and engineering estimates. Final installations must conform to project blueprints, authority having jurisdiction (AHJ) code approvals, and site-specific inspections.