4.2 Pipe Sizing Calculations & The 1 mbar Maximum Pressure Drop Rule
Key Takeaways
- The maximum allowable pressure drop between the outlet of the primary gas meter regulator and the inlet of any connected gas appliance is 1 mbar under maximum connected load.
- Nominal standing pressure at the meter outlet is 21 mbar (acceptable working pressure range 19 mbar to 23 mbar), ensuring appliances receive a minimum working pressure of 20 mbar (or 18 mbar if meter outlet operates at 19 mbar).
- Volumetric gas flow rate in cubic metres per hour (m³/h) is calculated by dividing total appliance gross heat input in kilowatts (kW) by the gross calorific value factor of 10.5 kWh/m³ for Natural Gas (G20).
- Equivalent length calculations account for frictional resistance from pipe fittings by adding specified meter lengths (e.g., 0.5 m for 15 mm elbow, 0.7 m for 22 mm elbow, 1.0 m for 28 mm elbow) to physical pipe length.
- Pipe sizing tables in BS 6891 must be used to select pipe diameters that ensure total cumulative pressure loss from meter to appliance does not exceed 1 mbar.
4.2 Pipe Sizing Calculations & The 1 mbar Maximum Pressure Drop Rule
Quick Answer: The fundamental design requirement under BS 6891 is that the maximum allowable pressure drop between the outlet of the primary gas meter regulator and the inlet connection of any gas appliance must not exceed 1 mbar under total maximum connected load.
1. The 1 mbar Maximum Allowable Pressure Drop Rule
Installation pipework must be sized correctly to ensure that all connected appliances operate safely and efficiently at their full rated heat input. If pipework is undersized, gas flow is restricted, leading to an excessive pressure drop across the pipe run. This pressure starvation can cause flame instability, incomplete combustion, dangerous carbon monoxide production, or appliance burner extinction.
Under BS 6891, installation pipework must be engineered such that under full peak demand—when every connected appliance operates simultaneously at maximum heat input—the total friction-induced pressure loss from the outlet of the primary gas meter regulator to the inlet connection of any appliance does not exceed 1 mbar (100 Pascals).
2. Operating Pressure Baselines & Appliance Inlet Requirements
To evaluate pressure compliance on site:
- Standing Pressure: The nominal standing pressure at the primary meter regulator outlet for Natural Gas (G20) is 21 mbar (with no gas flowing).
- Working Pressure Range: Under flow conditions, the acceptable working pressure at the meter outlet is 21 mbar (+/- 2 mbar), meaning a range of 19 mbar to 23 mbar is legally compliant at the meter outlet.
- Appliance Inlet Working Pressure: Because the maximum allowable pipework pressure loss is 1 mbar, an appliance supplied by a meter working at 21 mbar must receive a working pressure of at least 20 mbar. If the meter outlet working pressure is at its lower statutory limit of 19 mbar, the appliance inlet working pressure must not fall below 18 mbar.
3. Gas Volumetric Flow Rate Calculations (kW to m³/h)
Before sizing pipe diameters, the engineer must determine the total volumetric gas flow rate passing through each pipe section. Appliance ratings are supplied in kilowatts (kW) gross heat input.
For Natural Gas (G20) with a standard Gross Calorific Value (CV) factor of approximately 37.78 MJ/m³ (equivalent to 10.5 kWh/m³), the required gas flow rate in cubic metres per hour (m³/h) is calculated using the formula:
Gas Flow Rate (m³/h) = Total Gross Heat Input (kW) / 10.5
Practical Calculation Example:
If a property contains a 28 kW combination boiler, a 7.5 kW gas hob, and a 4.5 kW gas fire:
- Total Connected Heat Input: 28 kW + 7.5 kW + 4.5 kW = 40.0 kW
- Total Gas Flow Rate at Meter Outlet: 40.0 kW / 10.5 kWh/m³ = 3.81 m³/h
4. Pipe Resistance & Fitting Equivalent Length Allowances
As gas flows through pipes and fittings, friction creates resistance. To account for the additional pressure drop caused by fittings (such as 90-degree elbows, tees, and bends), BS 6891 assigns an equivalent length of straight pipe to each fitting type.
Equivalent Length Table for Copper Pipe Fittings (BS 6891)
| Pipe Diameter (mm) | 90° Elbow Equivalent Length (m) | Tee (Flow Through Branch) (m) | 45° Bend Equivalent Length (m) |
|---|---|---|---|
| 15 mm | 0.5 m | 0.5 m | 0.25 m |
| 22 mm | 0.7 m | 0.7 m | 0.35 m |
| 28 mm | 1.0 m | 1.0 m | 0.50 m |
| 35 mm | 1.2 m | 1.2 m | 0.60 m |
To calculate the Total Equivalent Length of a pipe run:
Total Equivalent Length = Physical Measured Pipe Length + Sum of Fitting Equivalent Lengths
5. Worked Sizing Scenarios to BS 6891
Scenario A: Single Appliance Branch Run
A 24 kW system boiler is located 12 metres away from the meter outlet. The pipe run contains four 22 mm 90° elbows.
- Gas Flow Rate: 24 kW / 10.5 = 2.29 m³/h
- Physical Length: 12.0 metres
- Fitting Allowances: 4 elbows x 0.7 m = 2.8 m
- Total Equivalent Length: 12.0 m + 2.8 m = 14.8 metres
- Pipe Selection: Consulting BS 6891 discharge tables for copper tube:
- 15 mm copper pipe over 15 m equivalent length has a capacity of only 0.87 m³/h (undersized).
- 22 mm copper pipe over 15 m equivalent length has a capacity of 2.70 m³/h (compliant, pressure drop <= 1 mbar).
- Conclusion: Install 22 mm copper pipework.
Scenario B: Multi-Appliance System (Main Stem Sizing)
A property has a 31.5 kW combi boiler and a 6.3 kW gas cooker connected to a main stem.
- Main Stem Flow Rate: (31.5 + 6.3) kW / 10.5 = 37.8 / 10.5 = 3.60 m³/h
- Over an equivalent length of 10 metres, 22 mm copper pipe has a capacity of 3.40 m³/h (undersized).
- Conclusion: The main stem must be sized in 28 mm copper pipe (capacity 7.8 m³/h over 10 m) to keep cumulative pressure drop under 1 mbar.
BS 6891 Copper Pipe Discharge Capacity Matrix (m³/h for 1 mbar drop)
| Pipe Outside Diameter | 3 m Equivalent Length | 6 m Equivalent Length | 9 m Equivalent Length | 12 m Equivalent Length | 15 m Equivalent Length |
|---|---|---|---|---|---|
| 15 mm | 2.10 m³/h | 1.40 m³/h | 1.10 m³/h | 0.95 m³/h | 0.87 m³/h |
| 22 mm | 6.50 m³/h | 4.40 m³/h | 3.50 m³/h | 3.00 m³/h | 2.70 m³/h |
| 28 mm | 18.0 m³/h | 12.0 m³/h | 9.50 m³/h | 8.10 m³/h | 7.20 m³/h |
| 35 mm | 35.0 m³/h | 24.0 m³/h | 19.0 m³/h | 16.0 m³/h | 14.5 m³/h |
What is the maximum permissible pressure drop from the outlet of a primary gas meter regulator to the inlet connection of any domestic gas appliance under BS 6891?
A domestic gas boiler has a gross heat input rating of 31.5 kW. What is its volumetric gas flow rate when burning Natural Gas (G20) with a gross calorific value factor of 10.5 kWh/m³?
When calculating the total equivalent length of a 22 mm copper gas pipe run under BS 6891, what allowance is added for each standard 90-degree elbow fitting?