Testing, Purging, and Leak Detection Procedures
Chapter Introduction
This chapter covers one of the most critical areas of the gasfitter Class A trade: leak testing, purging of piping systems, and leak detection. These procedures are governed by the Canadian Electrical Code, Part I (CE Code) and the CSA B149.1 Natural Gas and Propane Installation Code, specifically the rules in Section 5 (pressure testing) and Section 6 (purging). An error in these steps can result in an explosion, carbon monoxide poisoning, or fire. The Red Seal exam tests not only your ability to perform these procedures but also your understanding of the underlying physical principles and precise regulatory requirements.
1. Fundamental Principles of Pressure Testing
1.1 Purpose of Testing
A pressure test is designed to confirm that a piping system is gas-tight before it is put into service. It is not about testing the mechanical strength of the pipe (that is an engineering calculation), but rather about detecting any leak, even microscopic, that could allow gas to escape into an occupied space.
1.2 Authorized Test Fluids
According to CSA B149.1, Rule 5.4, only certain fluids may be used for leak testing:
| Fluid | Use | Limitations |
|---|
| Compressed air | Standard test | Maximum pressure limited by component strength |
| Nitrogen (N₂) | High-pressure test | Inert, no ignition risk |
| Inert gas (helium, argon) | Very fine leak detection | High cost, specialized equipment |
| Water | Hydrostatic test | **Prohibited** for gas piping (risk of internal corrosion) |
| Combustible gas | Test with service gas | **Prohibited** for initial test (except specific exceptions, see below) |
> Key Rule: The initial test must be performed with air or an inert gas. Combustible gas may only be introduced after purging, and only for commissioning.
1.3 Minimum Test Pressures
The minimum test pressure is defined in Rule 5.5 of CSA B149.1:
Low-pressure systems (service pressure ≤ 7 kPa or 14 in water column): test at 140 kPa (20 psi) for at least 15 minutes.
Medium-pressure systems (7 kPa to 400 kPa): test at 1.5 times the maximum service pressure, with a minimum of 350 kPa (50 psi).
High-pressure systems ( > 400 kPa): test at 1.5 times the service pressure, with a minimum of 700 kPa (100 psi).
Summary table of test pressures (CSA B149.1, Table 5.5):
| System Type | Service Pressure (kPa) | Minimum Test Pressure (kPa) | Minimum Duration |
|---|
| Low pressure | ≤ 7 | 140 | 15 min |
| Medium pressure | 7 – 400 | 1.5 × P_service (min. 350) | 15 min |
| High pressure | > 400 | 1.5 × P_service (min. 700) | 15 min |
Important Note: The 15-minute duration is a minimum. If the pressure drops during this period, the test fails, even if the drop is due to a change in ambient temperature. You must wait for thermal stabilization before starting the timing.
1.4 Step-by-Step Testing Procedure
21.Isolation: Close all shut-off valves and cap open ends with threaded plugs or blind flanges.
22.Pressurization: Connect the compressed air or nitrogen source to a test point (usually a test valve or purge tee).
23.Pressure build-up: Increase the pressure gradually (never suddenly) to the test value.
24.Stabilization: Wait 5 to 10 minutes for the temperature to stabilize (air compression generates heat, which increases pressure; you must allow the system to cool).
25.Timing: Close the supply valve, record the exact pressure, then time 15 minutes.
26.Final reading: At the end of the timing period, record the pressure. Any pressure drop indicates a leak.
27.Leak detection: If the pressure has dropped, apply a foaming solution (soapy water) to all joints, fittings, and welds to locate the leak.
28.Repair and retest: Repair the leak, then restart the complete test. It is prohibited to "repair" a leak by tightening a fitting under pressure.
1.5 Factors Affecting Pressure Readings
Temperature: A temperature drop of 1 °C can cause a pressure drop of approximately 0.35% (Gay-Lussac's law: P₁/T₁ = P₂/T₂). For a test at 140 kPa, a 5 °C drop can cause the pressure to fall by 2.5 kPa, which can be mistaken for a leak.
System volume: The larger the volume, the longer a small leak will take to cause a detectable pressure drop.
Humidity: Condensation can distort readings on dial gauges.
> Exam Tip: If the pressure drops slightly but the temperature has also dropped, the test may still be valid. You must calculate the temperature-corrected pressure. The formula is: P₂ = P₁ × (T₂ / T₁) where T is in kelvins (K = °C + 273.15).
2. Purging of Piping Systems
2.1 Definition and Purpose
Purging is the operation of replacing the air or gas present in a piping system with the service gas (or the reverse when taking a system out of service). The goal is to prevent the formation of an explosive air-gas mixture in the piping.
2.2 Why is Purging Dangerous?
Natural gas has a lower explosive limit (LEL) of approximately 4% and an upper explosive limit (UEL) of approximately 15% in air. Propane has an LEL of 2.1% and a UEL of 9.5%. Any mixture between these two limits is explosive. Purging must therefore be designed to quickly pass through this dangerous zone without an ignition source.
2.3 Purging Methods
2.3.1 Direct Purging (or Displacement Purging)
This method involves introducing the service gas at one end of the piping and venting the air from the other end. It is used for small-diameter piping (≤ 50 mm or 2 in) and short lengths (≤ 30 m).
Required conditions:
The piping must be horizontal or slightly inclined.
The vent point must be at the highest point.
The gas velocity must be sufficient to prevent mixing (≥ 1.5 m/s).
2.3.2 Sweep Purging (or Dilution Purging)
For large-diameter or long-length piping, direct purging is dangerous because the gas and air can mix. In this case, a sweep purge with an inert gas (nitrogen) is used:
49.Introduce nitrogen into the piping to displace the air.
50.Verify that the oxygen content is less than 1% at the purge vent.
51.Then introduce the service gas to displace the nitrogen.
52.Verify that the gas content is greater than 80% of the LEL (or that oxygen is still < 1%) before putting the system into service.
2.3.3 Vacuum Purging
This method is used for underground or inaccessible piping. A partial vacuum is created in the piping, then the gas is introduced. It is rarely used in practice due to the risk of implosion on thin-walled piping.
2.4 Purge Points
Purge points must be installed:
At each end of a piping run.
At high and low points of the system (for purging and drainage).
Every 30 m maximum on long straight piping runs.
2.5 Purge Gas Venting
Gas vented during purging must never be released inside a building. It must be vented outdoors, at a minimum distance of 3 m from any opening (window, door, air intake) and 1.5 m above ground level. A purge vent pipe must be securely fastened to prevent movement due to the force of the discharge.
2.6 End-of-Purge Detection
To confirm that purging is complete, use a gas detector (combustible gas indicator) or an oxygen analyzer:
| Parameter Measured | Target Value for Commissioning | Target Value for Decommissioning |
|---|
| Oxygen (O₂) | < 1% | > 20.9% (ambient air) |
| Combustible gas | > 80% of LEL | < 10% of LEL |
> Exam Trap: A piping system is considered purged when the oxygen content is below 1%, not when the gas content is 100%. A mixture of 99% gas and 1% oxygen is still potentially explosive.
3. Leak Detection
3.1 Detection Methods
3.1.1 Foaming Solution (Soapy Water)
The most common and reliable method for locating a leak on a fitting or joint. It involves applying a solution of water and detergent to the suspected surface and observing bubble formation.
Precautions:
Never use a flame to detect a leak.
The solution must be concentrated enough to form stable bubbles (approximately 1 part soap to 10 parts water).
Apply to threads, welds, flanges, fittings, and valve bodies.
3.1.2 Electronic Gas Detectors
Electronic detectors (combustible gas indicators, semiconductor detectors) are used for very fine leaks or in hard-to-reach areas. They must be calibrated according to the manufacturer's recommendations, usually with a reference gas (methane or propane).
Limitations:
Catalytic bead detectors (pellistors) do not work in oxygen-depleted atmospheres (< 10%).
Semiconductor detectors can give false positives in the presence of other volatile organic compounds.
3.1.3 Manometer Test (Pressure Drop Test)
For very slow leaks, a liquid manometer (water column) or a high-precision digital manometer can be used. The system is pressurized, isolated, and the pressure drop is observed over an extended period (1 hour or more).
3.2 Mandatory Check Zones
According to CSA B149.1, Rule 5.8, the following areas must be checked for leaks during any test:
All threaded and welded joints.
All mechanical fittings (compression, push-fit).
Valve bodies and packing glands.
Appliance connections (flexible hoses, connectors).
Expansion joints.
Test points and purge tees.
3.3 Leak Search Procedure on a System in Service
91.Visual inspection: Look for traces of oil, rust, or discoloration on fittings (indications of old leaks).
92.Olfactory test: Natural gas is odorized with tetrahydrothiophene (THT) or mercaptan. A gas odor indicates a leak, but the human sense of smell must never be the only detection method.
93.Application of foaming solution: On all suspected joints.
94.Appliance check: Inspect appliance connections, flexible hoses, and burners.
95.Static pressure test: Close the main shut-off valve and observe the manometer for 5 minutes. A pressure drop indicates a leak upstream of the valve.
3.4 Leaks on Buried Piping
For underground piping, leak detection is more complex:
Probe bar: Drive a steel rod into the ground along the piping to detect gas pockets.
Ground gas detector: Use a flame ionization or semiconductor detector by taking surface readings.
Extended pressure test: Pressurize the piping and observe the drop over 24 hours, accounting for soil temperature variations.
> Important Rule: If a leak is detected on buried piping, it is prohibited to dig with machinery without first precisely locating the leak and obtaining the necessary permits (locating other utilities).
4. Commissioning and Decommissioning
4.1 Commissioning a New System
105.Leak test: Perform the pressure test with air or nitrogen (Section 1 of this chapter).
106.Purge: Purge the piping with the service gas (Section 2).
107.Purge verification: Confirm that the oxygen content is < 1%.
108.Appliance lighting: Light each appliance one by one, checking the supply pressure at the burner.
109.Operational test: Verify that the flame is blue and stable, that combustion is complete (no carbon monoxide > 100 ppm in the combustion products).
110.Documentation: Complete the test report and provide it to the owner.
4.2 Decommissioning an Existing System
112.Close the main shut-off valve.
113.Purge residual gas: Vent the gas outdoors through a purge point.
114.Verification: Confirm that the gas content is < 10% of the LEL before dismantling.
115.Capping: Plug all open ends with threaded plugs.
116.Labeling: Affix a "OUT OF SERVICE" tag on the shut-off valve.
4.3 Recommissioning an Out-of-Service System
If a system has been out of service for more than 6 months, it must undergo a complete leak test before being recommissioned. If the period is less than 6 months, a reduced pressure test may be accepted, but a complete purge remains mandatory.
5. Test and Measurement Equipment
5.1 Manometers
| Type | Typical Range | Accuracy | Use |
|---|
| Dial gauge (Bourdon) | 0 – 700 kPa | ± 2% | Standard tests |
| Liquid manometer (water column) | 0 – 7 kPa | ± 0.1% | Low-pressure measurements |
| Digital manometer | 0 – 1000 kPa | ± 0.5% | Precise tests, recording |
| Electronic pressure sensor | Variable | ± 0.1% | Automated tests |
Requirements: The manometer must have a graduated scale such that the test pressure falls within the middle third of the reading range. A 0 to 700 kPa manometer must not be used to measure a pressure of 140 kPa (the reading would be in the first quarter of the scale, with insufficient accuracy).
5.2 Gas Detectors
Combustible gas indicator (LEL detector): Measures the concentration of combustible gas as a % of the LEL. Must be calibrated for the specific gas (methane or propane).
Oxygen analyzer: Measures the O₂ concentration in the atmosphere. Used to verify the end of purging.
Carbon monoxide (CO) detector: Used to verify appliance combustion (max. 100 ppm in undiluted combustion products).
Semiconductor combustible gas detector: Detects very fine leaks, but is sensitive to interference.
5.3 Test Valves and Purge Tees
Purge tee: A T-fitting with a purge valve, installed at high and low points of the system.
Test valve: A small valve (quarter-turn) installed on the piping to connect the manometer.
Test plug: A threaded plug with a central orifice for pressure measurement.
6. Safety Rules and Regulations
6.1 Applicable CSA B149.1 Rules
| Rule | Content |
|---|
| 5.4 | Authorized test fluids |
| 5.5 | Minimum test pressures |
| 5.6 | Test duration |
| 5.7 | Testing of buried piping |
| 5.8 | Leak verification |
| 6.1 | Purging of piping |
| 6.2 | Purge gas venting |
| 6.3 | Purging of appliances |
| 7.1 | Commissioning |
| 7.2 | Decommissioning |
6.2 Absolute Prohibitions
Prohibition on using a flame to detect a leak (CSA B149.1, Rule 5.8.2).
Prohibition on purging a piping system inside a building.
Prohibition on using combustible gas for the initial leak test.
Prohibition on tightening a fitting under pressure to stop a leak.
Prohibition on recommissioning a system without having performed the leak test and purge.
6.3 Gasfitter Responsibilities
The gasfitter Class A is responsible for:
Performing tests in accordance with the code.
Documenting the results (test pressure, duration, results).
Signing and dating the test report.
Providing the report to the owner or system operator.
Keeping a copy of the report for their records.
7. Practical Calculations
7.1 Calculating Temperature-Corrected Pressure
Formula: P₂ = P₁ × (T₂ / T₁)
Where:
P₁ = initial pressure (kPa)
P₂ = corrected final pressure (kPa)
T₁ = initial temperature (K)
T₂ = final temperature (K)
Example: Test at 140 kPa at 20 °C. After 15 minutes, the temperature has dropped to 15 °C and the pressure reads 138 kPa. The corrected pressure is:
T₁ = 20 + 273.15 = 293.15 K
T₂ = 15 + 273.15 = 288.15 K
P₂ = 140 × (288.15 / 293.15) = 140 × 0.9829 = 137.6 kPa
The measured pressure (138 kPa) is higher than the corrected pressure (137.6 kPa), so the test is valid (no leak).
7.2 Calculating Piping Volume
Formula: V = π × (d/2)² × L
Where:
V = volume (m³)
d = inside diameter (m)
L = length (m)
π ≈ 3.1416
Example: Piping with an inside diameter of 50 mm (0.05 m) and a length of 30 m:
V = 3.1416 × (0.025)² × 30 = 3.1416 × 0.000625 × 30 = 0.0589 m³
7.3 Calculating Purge Time
Formula: t = (V × N) / Q
Where:
t = purge time (min)
V = piping volume (m³)
N = number of air changes (generally 3 to 5 for a direct purge)
Q = purge flow rate (m³/min)
Example: Piping of 0.0589 m³, purge flow rate of 0.02 m³/min, 3 air changes:
t = (0.0589 × 3) / 0.02 = 8.8 minutes
8. Pitfalls to Avoid
184.Confusing service pressure and test pressure: The test pressure is always higher than the service pressure (1.5× or 140 kPa minimum, whichever is greater).
185.Forgetting thermal stabilization: Starting the timing immediately after pressurization leads to false failures.
186.Using an unsuitable manometer: A manometer too large for the measured pressure gives inaccurate readings.
187.Purging indoors: Purging must always be done outdoors, 3 m from openings.
188.Using service gas for the initial test: This is prohibited, except in very specific cases (systems already in service).
189.Ignoring oxygen content: A purge is complete when O₂ < 1%, not when the gas smell is strong.
190.Tightening a fitting under pressure: This can damage the threads and worsen the leak.
191.Not documenting the test: The Red Seal exam tests your knowledge of documentation requirements.
192.Confusing LEL and UEL: The LEL of natural gas is 4%, the UEL is 15%. Propane: LEL 2.1%, UEL 9.5%.
193.Forgetting to check appliances: The leak test does not cover appliances; they must be checked separately.
9. Summary
The leak test is performed with air or an inert gas, never with the service gas.
The minimum test pressure is 140 kPa for low-pressure systems, 1.5 × P_service (min. 350 kPa) for medium pressure, and 1.5 × P_service (min. 700 kPa) for high pressure.
The minimum test duration is 15 minutes, after thermal stabilization.
The purpose of purging is to prevent the formation of an explosive mixture (between LEL and UEL).
A purge is complete when the oxygen content is < 1%.
Leak detection is done with foaming solution, electronic detectors, or pressure drop testing.
Manometers must be suitable for the measurement range (middle third of the scale).
Any commissioning or decommissioning must be documented.
Absolute prohibitions: flame for leak detection, indoor purging, tightening under pressure, testing with combustible gas.
10. Self-Assessment Questions
207.What is the minimum test pressure for a low-pressure system according to CSA B149.1?
208.Why is nitrogen used instead of air to purge a large-diameter piping system?
209.What is the maximum acceptable oxygen content at the end of a purge?
210.Can a 0 to 700 kPa manometer be used for a test at 140 kPa? Justify your answer.
211.What is the minimum distance between a purge vent point and a building opening?
212.What should you do if the pressure drops by 2 kPa during a test but the temperature has dropped by 3 °C?
213.What are the LEL and UEL values for propane?
214.What is the maximum period a system can remain out of service before requiring a complete test?
11. Regulatory References
CSA B149.1: Natural Gas and Propane Installation Code, Section 5 (Testing), Section 6 (Purging), Section 7 (Commissioning).
CSA B149.2: Propane Storage and Handling Code (applicable to propane systems).
CSA B149.3: Code for the Field Approval of Fuel-Related Components (appliance verification).
Canadian Electrical Code, Part I, Chapter V: Requirements for electrical installations in classified areas (applicable when installing gas detectors).
This chapter prepares you for exam questions on testing, purging, and leak detection. Master the numerical values, procedures, and prohibitions — these are the most frequently tested elements.
