Gas pipe sizing

Introduction

Pipework in the home is used to deliver gas from the gas meter to each of the your appliances. The manufacturer of each appliance will have specify the maximum rate of gas consumption for the appliance. The pipework must be large enough able to supply this rate of consumption to all the appliances at the same time, without excess pressure drop, for the appliance to work correctly and safely.

This article explain how you can calculate the sizes required, and for example, assess if the existing pipework will be adequate for a new or upgraded appliance.

Design Parameters

Your gas meter and the main gas rate governor, should be set to deliver a gas at dynamic pressure of 21 mBar (note the dynamic pressure is the pressure measured when gas is flowing - the static pressure may be a few mBar higher).

Gas appliances in the UK are typically designed to expect an input pressure of 20 mBar. So the pipework can drop no more than 1 mBar between the supply and the point of use.

Flow Resistance

The pressure drop of a fluid flowing in a pipe will be dictated by:

• Bore of the pipe (usually specified in mm diameter)
• The flow rate of the gas in cubic meters / hour
• The effective length of the pipe

Effective Length

The flow resistance of a pipe will be higher at the point it changes direction. The easiest way to make an assessment of the higher pressure drops caused by direction changes, is to adjust the effective length of the pipe. The effective length is the actual length of the pipe, with and additional 0.5m added for each 90 degree elbow, and 0.3m added for each bend or swept elbow.

Total Rate

When checking a pipe design, you will need to assess the total gas rate that needs to be delivered though each section of pipe. Some sections of pipe will likely branch to more than one appliance, so the section of the pipe before the branch will potentially carry the sum total maximum gas rates of all the appliances that follow the branch.

Design Data

This table (taken from BS 6891, "Installation of low pressure gas pipework of up to 35 mm (R1¼) in domestic premises (2nd family gas) — Specification"), contains a typical gas discharge rates for various pipe types and sizes:

Discharge in a straight horizontal pipe with a 1.0 mBar differential pressure between the ends for the gas of relative density 0.6 (air = 1)

Nominal Pipe size				Length of Pipe
Steel (medium)	Copper	Corrugated Stainless Steel	PE	Discharge (cubic meters/hour)
BS 1387	BS EN 1057	BS 7838	BS 7281	3	6	9	12	15	20	25	30
6	8	—	—	0.29	0.14	0.09	0.07	0.05	—	—	—
8	—	—	—	0.8	0.53	0.49	0.36	0.29	0.22	0.17	0.14
—	10	10	—	0.86	0.57	0.50	0.37	0.30	0.22	0.18	0.15
10	—	—	—	2.1	1.4	1.1	0.93	0.81	0.70	0.69	0.57
—	12	12	—	1.5	1.0	0.85	0.82	0.69	0.52	0.41	0.34
—	15	15	—	2.9	1.9	1.5	1.3	1.1	0.95	0.92	0.88
15	—	—	—	4.3	2.9	2.3	2.0	1.7	1.5	1.4	1.3
—	—	—	20	4.0	2.7	2.1	1.8	1.6	1.3	1.2	1.05
20	—	—	25	9.7	6.6	5.3	4.5	3.9	3.3	2.9	2.6
—	22	22	—	8.7	5.8	4.6	3.9	3.4	2.9	2.5	2.3
25	—	—	32	18	12	10	8.5	7.5	6.3	5.6	5.0
—	28	28	—	18	12	9.4	8.0	7.0	5.9	5.2	4.7
—	—	32	—	29	20	15	13	12	10	8.5	7.6
32	35	—	—	32	22	17	15	13	11	9.5	8.5

Examples

The table can be used to work out what pipe size is required for a given gas rate. So for example a 15mm copper pipe of 6m effective length can supply a maximum rate of 1.9 cu m/h at a 1 mBar drop in pressure. Stepping up to a 22mm pipe, and it can supply that rate (and more - 2.3) at 30m in total effective length.

Worked Example

If we take the following example layout:

Calculate the gas rates in each pipe section

For each section of pipe, we need add up the total gas rate that may need to be supplied at that point. Its often easier to work back from the end points of the installation. We can include an educated guess of the pipe size likely to be required at this stage

Total gas rate at each point, and initial estimate of pipe size

Pipe Section	Supplies Sections	Total Gas Rate (m³/h)	Effective Length of Section (m)	Estimated copper pipe size (mm)	Pressure drop
G	none	0.6	2	15	TBD
F	none	1.0	2	15	TBD
E	F, G	1.6	1	15	TBD
D	none	0.5	1	15	TBD
C	D, E	2.1	7	22	TBD
B	none	2.8	5	22	TBD
A	B, C	4.9	3	22	TBD

In this application we can assume that the pressure drop in a pipe is proportional to its length, all other things being equal. So if we take pipe section G, and look at Table 1, we can see that a 3m pipe can discharge 2.9 m³/h using the full 1 mBar pressure drop available. So we can work out the actual pressure drop by scaling that 1 mBar by the actual gas rate and pipe length. To do this divide 1 by the length and discharge rate from the table to give us a value per m of pipe, and per m³/h of gas rate, then multiply by the actual values. so we get:

${\frac {1mbar}{3m\cdot 2.9m^{3}/h}}\times 2m\times 0.6m^{3}/h=~0.14mbar$

So using this approach, we can now work out the pressure drop in each pipe section, and also check the total cumulative drop for each section (its normally easiest to do this in a spreadsheet):

First Iteration

Pipe Section	Effective Pipe Length (m)	Pipe Diameter (mm)	Actual Gas Rate (m³/h)	Pipe Length Table 1 (m)	Discharge Rate Table 1 (m³/h)	Actual Pressure Drop (mBar)	Path to Appliance	Total Pressure Drop
G	2	15	0.6	3	2.9	0.14	Oven: G, E, C, A	1.24
F	2	15	1.0	3	2.9	0.23	Hob: F, E, C, A	1.33
E	1	15	1.6	3	2.9	0.18
D	1	15	0.5	3	2.9	0.06	Fire: D, C, A	0.98
C	7	22	2.1	9	4.6	0.36
B	5	22	2.8	6	5.8	0.40	Boiler: B, A	0.97
A	3	22	4.9	3	8.7	0.56

We can see a problem here, While the feeds to the Boiler and Gas fire are just inside the 1 mBar limits, the Oven and Hob will be out of spec. So we need to change pipe size(s) somewhere to compensate. Sections of pipe A, and C both feed other sections, and also introduce a fairly large drop due to the high flow rate. If we change those sections to 28mm pipe, and pull the new figures from the table we get: