Pressure loss and flow

Gas only flows because there's a pressure difference pushing it. Along the way, friction in the pipe and the turns in fittings use up some of that pressure. The more gas that flows, and the longer and narrower the route, the more pressure is lost before the gas reaches the burner.

What makes the loss bigger

BS 6891's sizing method shows what the pressure loss depends on:

• Flow rate: the more gas flowing, the bigger the loss per metre.
• Pipe size: a smaller bore gives a much bigger loss for the same flow.
• Length: the loss adds up metre by metre.
• Fittings: each elbow, bend and tee adds loss, counted as an extra length of pipe.

So the same pipe can be fine for a hob and far too small for a combi boiler. Pipe sizing is covered in 5.2.

The limits

With the meter outlet at no less than 18.5 mbar under high load, and no more than 1 mbar lost in the pipework, each appliance should get enough pressure at its inlet. Appliance manufacturers state the inlet pressure they need, and you check it at commissioning (see 3.3).

When there's too much loss

BS 6891 doesn't require you to measure the actual loss between two points, because typical on-site test methods aren't accurate enough. Instead, if you suspect too much loss:

1. Check the meter outlet pressure with everything at high load. If it's below 18.5 mbar, the problem is upstream of the pipework: report it (see 3.3).
2. If the meter outlet is fine, check each appliance reaches its heat input (see 3.5), then all of them running together.
3. Check the pipe sizes against the appliances they feed.

Undersized pipework that's shown to affect an appliance's safe operation, for example by causing incomplete combustion, is classed as Immediately Dangerous in the Unsafe Situations Procedure.

Sources: BS 6891:2015 5.3.2, Annex A1 mbar, sizing factors BS 6891:2015 10.4checking loss IGEM/G/13 Table 1 note 14 mbar meter IGEM/G/11 Ed. 2 Table 1situation 3.4 CCN1 Issue 16 §10 K&U 1–3assessed

▸Tutor notes

• Run the artefact and ask learners to predict where the biggest loss will appear as each appliance switches on.
• CCN1 §10 knowledge items 1–3 cover the reasons for and effects of excessive pressure loss, and meter pressure absorption at full load. Ask learners to list possible causes from this lesson and from BS 6891 (undersized pipe, too many fittings, a long run, a bore restricted by kinks, burrs or debris).
• Ask why measuring the drop between the meter and the appliance isn't required, and what is checked instead.