Preparation and let-by testing

A tightness test is only as good as its preparation. Before you raise the test pressure you need the right gauge, an installation that's ready to test, and proof that the valve you're testing against is holding. That last check is the let-by test.

The gauge

Whatever you use, the gauge (or gas detector) must be suitably ranged, suitable for the gas and for the place it's used in, calibrated as the manufacturer requires, and zeroed at atmospheric pressure at the start of each test where that applies.

• Water gauge: must be readable to 0.5 mbar. It doesn't need calibrating, but it must be kept well maintained.
• Electronic gauge: must be readable to 0.1 mbar, and calibrated at least every 12 months (or as the manufacturer specifies), with a certificate available. Use it within the manufacturer's temperature range and let it settle to the room's temperature first, because electronic gauges can drift as the temperature changes. A finely divided display doesn't mean the gauge is that accurate.

Check the gauge and its tubing for integrity immediately before you take a reading.

Before you start

First establish the supply pressure. If it's above 75 mbar (a medium pressure supply), check that the parts upstream of the meter inlet valve were pre-assembled by the manufacturer and strength-tested to IGE/UP/1. Appendix 5 of the standard helps you recognise medium pressure meter installations.

If a section already contains gas, it's tested with gas. Air is used for new installations that aren't yet connected to a supply (see 6.3).

Then prepare the installation:

1. Inspect it. Every section to be tested is connected, every joint has been made correctly, and every open end is sealed with an appropriate fitting. Inspect newly made joints by eye before testing, and don't paint or wrap them until they've passed.
2. Check the appliances. Each appliance isolation valve is open. Every burner tap and any pilot supply is off. Any safety shut-off valve on the installation is open: on a cooker with one on the hob lid or grill door, that means lifting the lid fully or opening the door fully.
3. Turn off the supply control valve. This is the valve immediately upstream of the section you're testing: the ECV (or AECV) where the supply is no more than 75 mbar, or the meter inlet valve where it's above 75 mbar, with the ECV left open. The standard calls it the supply control valve.
4. Connect the gauge to a suitable test point.

On a medium pressure installation, if the work involved a new or replacement meter installation, a replacement regulator, or anything else that may have affected tightness between the ECV and the meter inlet valve, test every connection between those two valves with leak detection fluid as soon as the ECV is opened.

The let-by test

1. Adjust the pressure in the installation to between 7 and 10 mbar. On a medium pressure supply, make sure the safety device on the regulator before the meter inlet valve is open.
2. Close the supply control valve and note the reading.
3. Watch the gauge for 1 minute.

Any perceptible rise in that minute is a fail: gas is getting past the closed valve. No perceptible rise is a pass.

If you need to lower the pressure at any stage, release the gas to a safe area and take the same precautions as for a purge (see 6.6).

Reading what you see

• A rise, but the valve looks sound. The pressure or temperature in the installation may still be settling. Wait for a steady reading, then repeat the let-by test.
• A big fall. That isn't let-by. It probably means an escape on the installation, which must be put right before you start the test again.

If the valve is letting by

Investigate and put right the cause. To confirm let-by, disconnect the valve's outlet union and apply leak detection fluid to the valve's barrel or ball.

• ECV at the end of the service pipe: tell the Gas Emergency Service call centre straight away so they can arrange a repair.
• Meter inlet valve on a primary meter: tell the meter asset manager or the gas supplier straight away.
• AECV: IGEM/UP/1B doesn't say who to tell. The Unsafe Situations Procedure classes an AECV letting by (with no smell of gas) as At Risk: the AECV is maintained or replaced.

The valve must be repaired or replaced before you repeat the let-by test and carry on. If the repair can't be done, make the installation safe: disconnect it as appropriate, seal every exposed gas way with an appropriate fitting, and stop testing.

Sources: IGEM/UP/1B Ed. 4 4.1gauges, LDF IGEM/UP/1B Ed. 4 4.2.1no perceptible movement IGEM/UP/1B Ed. 4 5.1, 5.2.1supply pressure, medium IGEM/UP/1B Ed. 4 5.2.2–5.2.4gas or air, joints IGEM/UP/1B Ed. 4 5.3.2.1steps (a)–(d) IGEM/G/11 Ed. 2 Table 1situations 3.7, 3.8 CCN1 Issue 16 §6aassessed

▸Tutor notes

• Ask learners to say which valve is the supply control valve on three layouts: a standard low pressure meter, a meter remote from the flat with an AECV inside, and a medium pressure meter with a meter inlet valve.
• Use the Edition 4 change to open a discussion: what's gained by testing with the appliance valves open first, and isolating them only if there's a drop?
• CCN1 §6a knowledge item 6 (dealing with an ECV, AECV or MIV that is letting by) is covered here. IGEM/UP/1B names who to tell for an ECV and a meter inlet valve. For an AECV, the action comes from IGEM/G/11 Table 1 situation 3.8. An ECV letting by is situation 3.7 (At Risk, notify the Gas Emergency Contact Centre).