Materials and jointing

BS 6891 covers domestic natural gas installation pipework up to 35 mm (R1¼): from the outlet of the meter installation up to the inlet of each appliance isolation valve or self-sealing connector. This lesson covers what that pipework may be made of, and how its joints must be made.

Materials

Pipework and fittings must conform to the product standards that BS 6891 lists for each material:

• Steel, including rigid stainless steel, with steel or malleable iron fittings.
• Copper tube, with capillary, compression or press-fit fittings.
• Pliable corrugated stainless-steel tubing (CSST), made to BS EN 15266. It bends by hand and has a cover fitted by the manufacturer.
• Polyethylene (PE), for external pipework only. Where PE enters a building, GSIUR requires the part inside to be in a metal sheath that stops escaping gas getting into the building if the pipe fails.

When choosing a material, think about strength, appearance and protection against corrosion. Never use pipe made for water, including plastic central heating and plumbing pipe (with or without a metal barrier), for gas.

Lead. New lead or lead alloy gas pipe isn't allowed. Old lead composition pipe is still found in older homes. If it's sound, well supported and big enough, you may connect to it, but only with a soldered cup joint onto copper or a suitable brass union. Compression fittings for lead water pipe must not be used.

Joints in general

Every joint must be mechanically strong and gas-tight, and made without putting the fitting under strain. Cut pipe ends square and deburr them, and look over every finished joint for defects. Test new pipework for tightness before you paint, wrap or bury it.

Capillary (soldered) joints

• Check that the solder has run all the way round the joint.
• Use a flux that's only active while it's being heated, and wipe off all flux residue once the joint has cooled.
• Keep flux and solder off CSST completely, inside and out. Finish any soldering on nearby copper, and clean it, before connecting the CSST.
• Soft-soldered copper joints shouldn't be where they could get hotter than 100 °C.

Mechanical and press-fit joints

• Mechanical joints (compression and unions) must be somewhere you can reach to tighten them. Unions must be ground-faced or compression type.
• Press-fit fittings must be assembled exactly as the manufacturer says. Their yellow bodies should stay visible, and you must tell the customer not to paint them with oil- or solvent-based paint unless the manufacturer allows it.

Screwed joints

• Clean the threads before applying any jointing material.
• Use jointing compounds and PTFE tape to the BS EN 751 series, as the manufacturer says. Put compound on the male thread only, and wipe off any excess.
• Don't use compound and PTFE tape together.
• Don't use hemp or other fibres, except with compound on existing long-screw back-nut joints.
• Remove any cutting oil from inside and outside the pipe.

CSST

Gas Safe Register's Technical Bulletin 120 adds:

• Assemble CSST and its fittings to the manufacturer's instructions, and only use the tubing and fittings of the same manufacturer together.
• BS EN 15266 fittings are mechanical fittings. They must not be hidden in the building fabric, put inside a sleeve or buried. Put them somewhere accessible and ventilated, never in a fire-protected area such as a protected stairway or corridor.
• The factory cover is not a sleeve (see 5.4).

LPG: BS 6891 also covers LPG, which has some different rules (for example spring-loaded plug valves, and pipe entering buildings above ground only). This course covers natural gas.

Sources: BS 6891:2015 1, 6scope, materials BS 6891:2015 7jointing BS 6891:2015 8.15PE, water pipe BS 6891:2015 8.1100 °C Gas Safe Register TB 120 (2021)CSST CCN1 Issue 16 §5assessed

▸Tutor notes

• Lay out a range of fittings (compression, capillary, press-fit, CSST, screwed with tape and with compound) and ask learners what each needs to make a compliant joint.
• Ask why flux must be kept off CSST, and why mixing CSST makes is forbidden.
• CCN1 §5 knowledge item 26 covers protecting stainless-steel semi-rigid meter connections from flux; link it to the CSST flux rule here.