Purging

Purging drives the air out of an installation and replaces it with gas, so that the appliances light on gas and not on a gas–air mixture. Done carelessly, it releases unburnt gas into the property. Done properly, you pass a known volume of gas through every branch, in a controlled way, and finish with a steady flame at each appliance.

When to purge

Always tightness test immediately before a purge. Purge after a passed test when the installation is connected to a live supply and it's new, any gas way has been open to air, or the work may have let air in some other way. While purging, don't let the pressure exceed the maximum operating pressure of any pipework being purged.

Installation volume and purge volume

The purge volume (PV) is the total volume of gas that must pass through the installation to make the mixture in the pipe safe. It's worked out from the installation volume (IV):

The two figures are always different, and each has its own job. IV decides whether IGEM/UP/1B applies (no more than 0.035 m³), what the permitted drop is on an existing installation, and how you purge. PV tells you how much gas to pass.

Working out the volume

Survey the whole section to be tested, unless it's very simple, such as one straight length of pipe. Note the meter, every pipe run and size, the fittings (regulator, bends, tees) and any pipework you can't see. Where hidden pipework is of different sizes and unknown length, assume the largest diameter you can see, for the longest length it could be.

1. Meter (IVm). From Table 7 of the standard: 0.0024 m³ for an E6, 0.008 m³ for a U6 or G4, and 0.025 m³ for a U16 or G10. For any other meter, ask the manufacturer.
2. Pipe (IVp). Multiply each length by the volume of 1 m of that pipe from Table 8. For copper, 1 m holds 0.00014 m³ (15 mm), 0.00032 m³ (22 mm), 0.00054 m³ (28 mm) and 0.00084 m³ (35 mm).
3. Fittings (IVf). Add the volume of valves, fittings and any vessels. If you can't work it out, add 10% of IVp.
4. Total. IV = IVm + IVp + IVf. Then PV = 1.5 × IV.

To add up the pipework as you walk round, try the free installation volume tool.

Edition 4 uses the meter's installation volume. The standard notes that the cyclic volume of diaphragm meters was used to work out the purge volume in the past.

Example. A U6 meter with 2 m of 28 mm, 6 m of 22 mm and 3 m of 15 mm copper:

The IV is under 0.035 m³, so IGEM/UP/1B applies, and under 0.02 m³, so either purge method below may be used.

Before you start

Throughout the purge, in the area where you're purging:

• avoid any build-up of gas in confined spaces
• stop anyone operating electrical switches or appliances by accident
• put out every possible source of ignition, with no smoking or naked lights
• ventilate well, for example by opening doors, windows and passive stack vents
• tell the responsible person, and anyone else nearby, that you're about to purge and they may smell gas.

These precautions still apply when you're holding a flame at the burner, because unburnt gas–air mixture comes out until the mixture is right to burn.

The purge

1. Make sure a satisfactory tightness test has been done immediately before.
2. Make sure every appliance is off.
3. Turn the gas on slowly. Note the position of the test dial or drum on a diaphragm meter, or the reading on an electronic meter.
4. Purge from a suitable point, using the method that suits the IV (see below).
5. Go back to the meter and note how much gas has passed. Repeat until the full purge volume has passed. Unless burners are lit while you purge, don't go beyond the purge volume, so as to release as little unburnt gas as possible.
6. Purge every branch of the pipework the same way.
7. Get a stable flame picture at each appliance.

IV up to 0.02 m³. Either open the burner tap on an appliance with an open burner, or loosen the fitting that seals a gas way. Using a burner, you may hold a flame at the burner head or keep operating the appliance's igniter to try to light the purged mixture. That helps you tell when the purge is complete, and it limits the unburnt gas released. Confirm that gas is present (the burner lights, or you smell gas), then close the tap or tighten the fitting and test it with leak detection fluid as needed.

IV over 0.02 m³ (up to 0.035 m³). Purge only through an open burner, and light the mixture as soon as possible by holding a flame at the burner head or keeping the igniter going. Every installation with a U16 or G10 meter is over 0.02 m³, and occasionally one with an E6, G4 or U6 meter is too. If there are no appliances fitted, the appliances have flame supervision devices, or there are no open burners, you may need to connect a temporary burner. Confirm gas is present when the burner lights, then turn it off.

Finishing

If you find an appliance that hasn't been commissioned, either commission it, or disconnect it or seal it off with an appropriate fitting and label it as not commissioned.

The law then requires that pipework not going straight into use is sealed at every outlet; that a connection loosened to purge is retested once it's tightened; and that every seal fitted after the purge is tested.

Taking an installation out of use

When an installation is being decommissioned, for example before it's removed or the property is demolished, consider purging the gas out of it. Here the purge volume is the volume of air to pass through. In every case, seal any exposed gas way with an appropriate fitting and label any part that may still contain gas.

When a meter is removed for good, for example on a meter exchange or when the supply is ended, fit dust-proof caps to its open gas ways straight away. A meter of over 6 m³/h should be purged of gas first. Guidance on purging gas out of installations is in IGE/UP/1 and IGE/UP/1A.

Sources: GSIUR 1998 reg. 22(2)purge, seal, retest IGEM/UP/1B Ed. 4 6.1when, pressure IGEM/UP/1B Ed. 4 6.2PV = 1.5 × IV IGEM/UP/1B Ed. 4 6.3.2procedure IGEM/UP/1B Ed. 4 6.4.3–6.4.4decommissioning IGEM/UP/1B Ed. 4 Appendix 7Tables 7 and 8 CCN1 Issue 16 §6aassessed

▸Tutor notes

• CCN1 §6a knowledge item 11 is a calculation exercise: E6, U6 and G4 meters on 35 mm pipework, and U16 meters on any pipework up to 35 mm. Set learners a few of each and have them decide the purge method from the IV.
• Ask why the purge method changes at 0.02 m³, and why a U16 installation always falls on the "burn it" side.
• Ask how an engineer will know when the purge volume has passed, using the meter they have in front of them.