2.4 Permissible Pressure Drops, Calculations & Tracing Gas Leaks

2.4 Permissible Pressure Drops, Calculations & Tracing Gas Leaks

Once a 2-minute tightness test is completed under IGEM/UP/1B Edition 4, the gas operative must evaluate whether any recorded pressure movement is within acceptable safety thresholds. Determining whether a gas system passes or fails requires calculating the Installation Volume (IV), checking for the smell of gas, applying the Edition 4 perceptible-movement rules, and executing systematic diagnostic leak tracing if a leak is present.

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Calculating Total Installation Volume (IV)

The Installation Volume ($IV$) represents the combined internal volume of the gas meter and all downstream pipework up to appliance controls (plus an allowance for fittings).

$$\text{Total } IV = IV_m + IV_p (+ \text{fittings allowance})$$

Where:

• $IV_m$ = meter badged volume (see table below — meter type matters).
• $IV_p$ = $\sum (\text{Length of pipe in metres} \times \text{Internal volume per metre})$.
• Fittings: allow approximately 10% of $IV_p$ for elbows, tees, and couplings unless a more detailed volume schedule is used.

Meter Internal Volumes ($IV_m$)

Do not group U6, G4, and E6 as all 0.008 m³. E6 ultrasonic meters are 0.0024 m³.

Internal Pipe Volumes per Metre (NICEIC copper values)

Worked Calculation Example

Problem: A property contains a U6 meter ($IV_m = 0.008\text{ m}^3$), 5 metres of 28 mm copper pipe, 12 metres of 22 mm copper pipe, and 8 metres of 15 mm copper pipe. Calculate the total Installation Volume ($IV$), including a 10% fittings allowance on pipe volume, and confirm it falls within the IGEM/UP/1B scope limit ($IV \le 0.035\text{ m}^3$).

Solution:

1. $IV_m = 0.008\text{ m}^3$
2. $IV_{28} = 5\text{ m} \times 0.00054\text{ m}^3/\text{m} = 0.00270\text{ m}^3$
3. $IV_{22} = 12\text{ m} \times 0.00032\text{ m}^3/\text{m} = 0.00384\text{ m}^3$
4. $IV_{15} = 8\text{ m} \times 0.00014\text{ m}^3/\text{m} = 0.00112\text{ m}^3$
5. Straight pipe $IV_p = 0.00270 + 0.00384 + 0.00112 = 0.00766\text{ m}^3$
6. Fittings ≈ $10\% \times 0.00766 = 0.000766\text{ m}^3$
7. $\text{Total } IV = 0.008 + 0.00766 + 0.000766 = \mathbf{0.01643\text{ m}^3}$

Conclusion: Since $0.01643\text{ m}^3 \le 0.035\text{ m}^3$, the installation is within IGEM/UP/1B scope. Looking ahead to Table 3, this IV sits in the > 0.015 – ≤ 0.035 band (maximum 1 mbar drop if movement is perceptible and there is no smell of gas).

E6 contrast: Replacing the U6 with an E6 ($IV_m = 0.0024\text{ m}^3$) on the same pipework gives $IV \approx 0.0024 + 0.00843 = 0.01083\text{ m}^3$ — a different Table 3 band (> 0.010 – ≤ 0.015 → 2.5 mbar). Meter volume errors therefore change the permitted drop.

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IGEM/UP/1B Edition 4 Permissible Drops (Table 3 — Natural Gas)

Edition 4 keys maximum permissible pressure drop during the 2-minute test to Installation Volume, not to a simple pipe-diameter split.

Edition 4 Pass/Fail Sequence (Apply in Order)

1. No perceptible movement → PASS (gas tight). Perceptible movement ≈ 0.25 mbar on a water gauge; electronic gauges typically 0.25 mbar (0.2 mbar if one-decimal display).
2. Any perceptible movement and no smell of gas → calculate $IV$; compare measured drop with Table 3.
3. If within Table 3 limits with appliances connected → isolate all appliances and repeat a pipework-only test. Pipework-only must show no perceptible movement (any movement = fail).
4. New install / pipework-only / only new appliances → no perceptible movement required (do not use Table 3 relaxations).
5. Smell of gas → FAIL regardless of IV band or numerical drop.

Transition Note (Edition 3)

During the parallel-use period until 30 September 2026, Edition 3 historically presented meter/pipe tables that many operatives summarised as ≤ 28 mm → 4 mbar and > 28 mm → 2.5 mbar for existing installations with no smell of gas. From 1 October 2026, Edition 4 is mandatory: learn the IV Table 3 method above. Do not treat the old diameter-only matrix as the primary 2026+ exam rule.

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Diagnostic Procedures for Leak Location

When a tightness test fails (pressure drop exceeds the Table 3 limit, pipework-only shows movement, or gas smell is detected), the engineer must locate and repair the escape using systematic tracing methods.

+-----------------------------------------------------------------------+
|                   LEAK TRACING & DIAGNOSTIC STEPS                     |
|                                                                       |
|  1. VISUAL INSPECTION: Check joints, meter unions, compression nuts.  |
|  2. APPLIANCE ISOLATION METHOD: Close individual AIVs / all AIVs and  |
|     re-test — Edition 4 pipework-only requires no perceptible move.   |
|  3. LEAK DETECTION FLUID (LDF): Spray BS EN 14291 compliant fluid     |
|     on joints; observe active micro-bubbling.                         |
|  4. ELECTRONIC SNIFFER: Sweep sensor tip at 25mm/sec above pipe run   |
|     (Natural Gas is lighter than air and rises).                      |
+-----------------------------------------------------------------------+

Soap Solution / Leak Detection Fluid (BS EN 14291)

• Use only approved Leak Detection Fluid (LDF) complying with BS EN 14291.
• DO NOT use domestic washing-up liquid, as it contains chlorides and high salt content that induce stress corrosion cracking on copper pipework and brass fittings.
• Apply LDF onto compression joints, capillary solder elbows, threaded fittings, and test point nipples. Watch for growing foam bubbles indicating escaping gas.

Electronic Sniffers & Combustible Gas Detectors

• Electronic gas detectors utilize semi-conductor sensors tuned to detect methane hydrocarbons in parts per million (ppm).
• Buoyancy Rule: Natural Gas has a relative density of approximately 0.60 relative to air ($1.0$), making it lighter than air. Escaping gas rises upwards.
• Sniffing Technique: Move the detector probe along the upper side of horizontal pipe runs and above fittings at a slow rate of 25 mm per second.

Appliance Isolation Method

• Close all individual Appliance Isolation Valves (AIVs) for a pipework-only re-test.
• Under Edition 4, pipework-only must show no perceptible movement. Continued movement means the leak is on the fixed installation pipework (or meter), not inside an appliance.

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Unsafe Situations Procedure (GIUSP / IGEM/G/11)

If an uncorrected gas leak causes a tightness test failure and cannot be immediately rectified:

• Immediately Dangerous (ID): An unisolated leak or open end passing gas. The operative must isolate the ECV, disconnect/cap the supply, attach an ID label, and issue a Gas Safety Warning Notice (CP14).
• At Risk (AR): An installation fault where continued operation presents a safety hazard. Isolate with customer permission, attach an AR label, and issue a CP14 Warning Notice.