2.2 Let-By Testing Procedures & Electronic / U-Tube Manometer Setup

2.2 Let-By Testing Procedures & Electronic / U-Tube Manometer Setup

Before undertaking any tightness test on a Natural Gas installation, gas engineers must verify that the Emergency Control Valve (ECV) or Meter Isolation Valve (MIV) seals completely when in the closed position. This verification procedure is known as the Let-By Test. Conducting a let-by test ensures that upstream gas under network distribution pressure cannot seep past the valve seat into the downstream installation during subsequent tightness testing, which would otherwise mask downstream gas leaks and yield false pass results.

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Purpose and Importance of Let-By Testing

The primary safety objective of the let-by test is to verify the mechanical seat integrity of the primary shut-off valve. If an ECV is "letting by" (passing gas while nominally closed):

• The downstream pipework will continue to receive gas even when the property owner believes the supply is safely isolated.
• During a tightness test at 20 mbar, gas leaking past the ECV seat will repressurize the pipework, causing the pressure gauge reading to rise or remain stable despite the presence of actual open ends or pipework leaks.
• Gas operatives working on pipework downstream could unknowingly cut into live gas lines under pressure, creating severe fire and explosion risks.

Under IGEM/UP/1B, performing a let-by test is a mandatory prerequisite that must be successfully passed before any tightness test is commenced.

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Manometer Selection, Setup, and Zeroing Procedures

Accurate pressure testing relies entirely on correctly set up and calibrated test gauges. Gas operatives typically use either traditional fluid-filled U-tube manometers or modern electronic digital manometers.

U-Tube Water Gauge Setup

A traditional U-tube manometer measures pressure differential by displacing a column of liquid (water or SG 1.0 indicator fluid) inside a clear glass or plastic U-shaped tube:

1. Visual Inspection: Check the manometer casing, rubber connecting hose, and brass push-on connector for cracks, splits, or blockages. Verify fluid level is adequate.
2. Vertical Alignment: Mount or hold the U-tube manometer perfectly vertical. Tilting the gauge introduces parallax errors and alters hydrostatic column height balance.
3. Zeroing Procedure: With both limbs of the U-tube open to atmospheric pressure (hose disconnected), slide the adjustable central millimeter scale until the zero mark aligns precisely with the bottom of the liquid meniscus in both columns.
4. Fluid Density Check: Ensure only approved gauge fluid (density 1.0 g/cm³) is used. Using heavier fluids will result in incorrect pressure readings.

Electronic Digital Manometer Setup

Electronic manometers use piezo-resistive pressure sensors to display digital pressure readings in millibars (mbar):

1. Pre-Check: Verify battery power, screen display integrity, and pressure range capability (must measure low pressures accurately to 0.1 mbar resolution).
2. Zeroing / Auto-Calibration: Switch on the instrument in fresh, clean ambient air at local room temperature with the test hose disconnected. Press the auto-zero button to establish a true atmospheric zero datum.
3. Temperature Stabilisation: If moving the instrument from a cold vehicle into a warm building, allow 5 to 10 minutes for the electronic sensor temperature to stabilize before zeroing. Sensor temperature drift during testing causes erroneous pressure readings.

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Step-by-Step Let-By Test Procedure (IGEM/UP/1B)

The let-by test must be carried out at a test pressure lower than normal operating pressure to maximize gauge sensitivity to seat leakage.

+-----------------------------------------------------------------------+
|                   LET-BY TESTING WORKFLOW                             |
|                                                                       |
|  1. Turn off all appliances & isolation valves.                        |
|  2. Connect calibrated manometer to meter test point.                  |
|  3. Crack open ECV to pressurise gauge to 7.0 - 10.0 mbar.            |
|  4. Close ECV completely.                                             |
|  5. Bleed excess pressure down to 7.0 - 10.0 mbar if necessary.        |
|  6. Start timer and monitor gauge for EXACTLY 1 MINUTE.               |
|                                                                       |
|  RESULT EVALUATION:                                                   |
|  - Pressure RISES  ==> FAIL (ECV letting by - Call Gas Emergency)      |
|  - Pressure FALLS  ==> Downstream leak (Investigate gross open end)   |
|  - Pressure STABLE ==> PASS (Proceed immediately to Tightness Test)   |
+-----------------------------------------------------------------------+

Detailed Testing Steps:

1. Appliance Check: Ensure all domestic gas appliances are turned off, pilot lights are extinguished, and individual Appliance Isolation Valves (AIVs) remain open (so that installation pipework up to appliance controls is included in the test).
2. Hose Attachment: Connect the manometer hose firmly onto the meter test point (nipple). Ensure the rubber hose fits tightly over the test point barb without air leaks.
3. System Pressurisation: Turn on the ECV very slowly to allow gas to enter the system until the pressure on the gauge reaches between 7.0 mbar and 10.0 mbar (typically targeting 7.0 to 10.0 mbar).
4. Isolate Supply: Turn off the ECV completely.
5. Bleed Pressure (if necessary): If the pressure rose above 10.0 mbar when opening the ECV, carefully bleed excess pressure off through a bleed valve or by loosening the hose connection slightly until the gauge reads between 7.0 mbar and 10.0 mbar.
6. Timing the Test: Start a stopwatch and observe the gauge reading continuously for exactly 1 minute (60 seconds).

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Interpreting Test Results: Pass, Let-By Failure, and Downstream Leakage

During the 1-minute test duration, the gas operative must closely monitor pressure movement:

Scenario A: Pressure Rises (Let-By Failure)

• Diagnostic: Any pressure RISE during the 1-minute let-by period indicates that higher-pressure gas from the mains network is leaking past the internal seating of the closed ECV.
• Action Required:
  • DO NOT proceed with the tightness test.
  • Keep the ECV shut off and attach a warning tag.
  • Immediately notify the National Gas Emergency Service (0800 111999) or relevant Gas Transporter to repair or replace the faulty ECV.
  • Inform the customer that the installation cannot be tested or re-commissioned until the ECV is replaced.

Scenario B: Pressure Drops Significantly

• Diagnostic: A noticeable pressure drop during the 1-minute let-by test indicates a downstream leak or an open pipe end on the customer's installation.
• Action Required:
  • Inspect the pipework for open ends, uncapped test points, or fully open appliance taps before attempting pressurisation to tightness test levels.
  • Rectify any open ends, cap disconnected pipework, and re-start the let-by procedure.

Scenario C: Pressure Remains Stable (PASS)

• Diagnostic: If the pressure reading remains completely stable (no rise) over the 1-minute period, the ECV seat is verified gas-tight.
• Action Required: The let-by test is marked as a PASS. The operative may immediately proceed to the 20 mbar tightness test.