Stabilisation and tightness testing

With the let-by test passed, you know the supply control valve is holding. Now you bring the installation up to test pressure, let it settle, and watch it for two minutes.

Raising the pressure

Open the supply control valve slowly to raise the pressure to between 20 and 21 mbar, then turn it off again. Avoid going higher, because a higher pressure can make the meter regulator lock up.

• Overshoot to no more than 23 mbar: bring it back to 20–21 mbar.
• Overshoot above 23 mbar: drop the pressure right back to 7–10 mbar, then raise it to 20–21 mbar again. This helps make sure the regulator isn't locked up during the test.

If you can't reach 20 mbar

Edition 4 allows the test to go ahead at a minimum of 18 mbar. If the test then passes, you must find out why the pressure was low by carrying out the operating pressure checks in IGEM/G/13 (see 4.4 Meter regulators). Below 18 mbar, don't test: isolate the installation and investigate the low pressure first.

Stabilisation

Wait 1 minute for the pressure and temperature inside the installation to settle. If the reading has moved by the end of the minute, bring it back to 20–21 mbar, turning the valve off again if you used it. Don't go on until the reading is steady: a slight rise or fall is normal while things settle, and you may need to allow more time.

The test

Watch the gauge for 2 minutes for any perceptible fall. Whether a drop is a pass or a fail depends on whether the installation is new or existing and whether appliances are connected; that's covered in 6.4. Whatever the gauge shows, a smell of gas is a fail.

If the installation fails, either trace and repair the escape and test again (see 6.5), or make it safe by disconnecting the appliance or section concerned and sealing every exposed gas way with an appropriate fitting.

Finishing off

1. Remove the gauge and reseal the test point.
2. Turn the gas on slowly.
3. Test the test point, the outlet connection of the ECV or AECV, the regulator connections and, where fitted, the meter inlet valve connections with leak detection fluid.
4. If the installation is new, or air may have got in, purge it (see 6.6).
5. Record the results on a suitable work record and, where appropriate, tell the responsible person.

Step 3 matters because the connections between the ECV and the regulator can run at a higher pressure than the test pressure. An escape there may only show once the full pressure is back on.

Testing a new installation with air

Where a new installation has no live gas supply yet, for example on a new housing estate, it can be tested with air first. Connect the gauge to one branch of a test tee and pump air in through the other, valved branch. For natural gas, the test pressure is again 20–21 mbar, with 1 minute to stabilise and 2 minutes with no perceptible movement. Once the gas supply is connected, the installation must be tested again with gas, by the procedure above, before it is purged.

Medium pressure without a meter inlet valve

Some older medium pressure installations (supply above 75 mbar and up to 2 bar) were fitted without a meter inlet valve. They're outside the scope of IGEM/UP/1B, but Appendix 4 gives guidance for testing most of them:

• the ECV is the supply control valve
• during the let-by test, operate the reset on the under-pressure shut-off or excess flow valve to balance the pressures across it, then let it shut again
• the test pressure is 18–19 mbar instead of 20–21 mbar, with the same 1-minute stabilisation and 2-minute test.

If you suspect the regulator is faulty, contact the Gas Emergency Service call centre. A faulty regulator on one of these installations means the whole meter installation is upgraded to BS 6400-2 or IGEM/GM/8, as appropriate, rather than the regulator simply being repaired or replaced.

ACS CCN1 Issue 16 assesses this in §6b, but still refers to Edition 3 Appendix 4. The equivalent in Edition 4 is also Appendix 4 (A4.3).

Sources: IGEM/UP/1B Ed. 4 5.3.2.1steps (e)–(k) IGEM/UP/1B Ed. 4 Appendix 3test with air IGEM/UP/1B Ed. 4 Appendix 4medium pressure, no MIV CCN1 Issue 16 §6a, §6bassessed

▸Tutor notes

• Before using the simulator, ask learners to write the sequence from memory: pressures, which valve is open or closed, and how long each stage lasts.
• Ask what an engineer should do if the gauge settles at 19 mbar, and at 17 mbar. Edition 3 had no 18 mbar allowance, so learners trained on it may not know the answer.
• Lock-up: the standard's LPG procedure (5.3.3.1, Table 4 note 4) explains why a test pressure that keeps the regulator open matters. Gas trapped at high pressure behind a locked-up regulator could hide an escape if that regulator is letting by. Ask learners how the same idea applies to the natural gas overshoot rule.