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Unit 5 — Pressure Testing, Tubing, and Piping
Section 2 — Air-Fuel

1.2 — Air-Fuel Operation

Correct operating procedures prevent gas leaks, uncontrolled flame events, and residual stored energy. This lesson covers fuel leak checking, normal and emergency shutdown sequences, and full system depressurization.

Leak Check Shutdown Depressurize 313A / 313D

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2.2.1 — Methods for Checking Fuel Leaks

Leak checking verifies system integrity before ignition. Leaking fuel gas can accumulate and ignite, especially in enclosed or poorly ventilated spaces. A leak check must be performed every time the outfit is assembled or a connection is disturbed, and before the outfit is left unattended.

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Never Use a Flame to Check for Leaks

Testing connections with an open flame is prohibited and has caused fatal incidents. Use an approved leak detection solution (soapy water or commercial leak detector) on all connections. A flame test cannot be controlled and can ignite an accumulation of fuel gas that a visual inspection missed.

Leak Check Procedure

  1. Pressurize the system to working pressure with the cylinder valve open and the torch valve closed. The hose and all connections are now under pressure.
  2. Apply leak detection solution generously to every connection point: regulator-to-cylinder, hose-to-regulator outlet, hose-to-torch body, and tip-to-torch body. Use a dedicated small brush or squeeze bottle.
  3. Watch for bubbles forming at any connection. Slow bubbles indicate a small leak; rapid frothing indicates a significant one. Do not attempt to tighten a leaking connection while under pressure — depressurize first.
  4. If a leak is found: close the cylinder valve, depressurize the system completely (see Section 1.4.3), tighten or re-seat the leaking connection, and repeat the leak check from step 1 before lighting.
  5. If no leaks are found: wipe the connections dry and proceed with the lighting procedure. Record that a leak check was performed in the job documentation.
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Approved Leak Detection Methods

  • Commercial leak detection solution — purpose-formulated, produces large stable bubbles, safe for all metals and rubbers
  • Soapy water — acceptable when no commercial solution is available; use dish soap diluted in water applied with a brush
  • Electronic gas detector — appropriate for checking in enclosed spaces where visual bubble detection is difficult; calibrate before use

Never use bare-hand feel alone to check for leaks — small leaks may not be detectable by touch and a hand placed near a leaking joint near an ignition source creates a burn hazard.

2.2.2 — Normal and Emergency Torch Shut-Down Procedures

Normal shutdown is the planned sequence that ends heat safely and leaves no stored energy in the system. Emergency shutdown prioritizes stopping gas flow as quickly as possible when abnormal conditions occur — such as a suspected hose leak, uncontrolled flame behavior, or a nearby fire hazard.

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Normal Shutdown Sequence

  1. Close the torch valve to extinguish the flame.
  2. Close the cylinder valve completely by turning clockwise until snug (do not over-torque).
  3. Reopen the torch valve to bleed residual gas from the hose; hold the torch away from your body and point it in a safe direction — do not re-ignite. Allow gas to bleed until both the high-pressure and low-pressure regulator gauges read zero.
  4. Close the torch valve.
  5. Back out the regulator adjusting screw counterclockwise until it turns freely, relieving spring tension. This prevents regulator creep during storage.
  6. Allow the tip to cool before placing the outfit in storage or in a carrying case.
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Emergency Shutdown Sequence

  1. Close the cylinder valve immediately — this is always the first action. Stopping the fuel supply at the source starves any flame or leak.
  2. Close the torch valve to ensure the tip flame is extinguished.
  3. Move the outfit away from any heat source, ignition source, or area of concern if it is safe to do so without additional risk.
  4. Ventilate the work area by opening windows and doors; do not operate electrical switches in the space until it is confirmed clear of gas.
  5. Do not relight until the cause of the emergency has been identified and corrected, and a fresh leak check has been performed on all connections.
  6. If a hose or regulator is on fire and cannot be controlled by closing the cylinder valve, evacuate, call emergency services (911), and activate any site emergency response procedure. Do not attempt to fight a gas-fed fire by extinguisher alone.
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Backfire vs. Flashback

A backfire is a momentary extinction of the flame at the tip, often with a pop. It is usually caused by touching the tip to the work, too low a gas flow, or a blocked tip orifice. Re-light after the tip cools slightly. A flashback is combustion travelling back through the hose toward the cylinder — indicated by a hissing or squealing sound and a flame receding into the torch body. This is a serious emergency: close both valves immediately, drop the torch, and treat as an emergency shutdown.

2.2.3 — Sequence for Depressurizing the Fuel Gas System

Depressurizing removes stored energy from the hoses and regulators so the equipment can be disconnected or stored without leaving pressurized fuel in the system. A pressurized hose left overnight or transported in a vehicle is an unnecessary fire and explosion risk. Full depressurization is required after every use before disconnecting any component.

  1. Extinguish the flame and allow the tip to cool for 30–60 seconds before beginning the depressurization sequence.
  2. Close the cylinder valve completely. Confirm it is fully closed by checking that the high-pressure gauge begins to fall as any remaining trapped gas flows out through the bleed sequence below.
  3. Open the torch valve in a safe direction (away from ignition sources and people) to bleed fuel from the hose through the tip. Continue until both gauges read zero (0 kPa / 0 psi).
  4. Close the torch valve once both gauges confirm zero pressure.
  5. Back the regulator adjusting screw counterclockwise to the fully relieved position. This releases spring tension on the diaphragm and is critical for long regulator service life.
  6. Replace the cylinder valve cap before transporting, storing, or leaving the cylinder unattended.
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Why Full Depressurization Matters

A hose that retains working pressure can release fuel gas if a fitting vibrates loose, a hose is pinched or abraded during transport, or a valve is accidentally opened. In an enclosed space — a van, a mechanical room, or a confined space — even a small fuel gas release can accumulate to an explosive concentration before it is detected. Full depressurization before disconnecting is a non-negotiable safe work requirement.

If the high-pressure gauge does not drop to zero after closing the cylinder valve and opening the torch valve, the cylinder valve may not be fully closed, or there may be a blockage in the hose. Investigate before disconnecting any fitting.

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