How to calculate pipe size in domestic installations

Whenever pipework is to be installed, or appliances replaced, gas operatives should ensure that the design allows adequate flow of gas to each appliance. Mike Heads looks at how this is achieved by calculating the pipe size in domestic installations using BS 6891: 2005.

Low pressure natural gas installations should be designed to provide an adequate flow rate and have a maximum pressure loss of 1 mbar between the meter outlet and the appliance inlet. LPG installations should be designed to have a maximum pressure loss of 2.5mbar.

Non-domestic pipe sizing tends to be calculated using computer programs or pipe sizing calculators. One of the problems is the increase in equivalent length for fittings as the pipe size increases.

It is very tempting when doing a boiler change from a storage heat-only boiler to a combi boiler to use the existing pipework, especially with the massive increase in the price of copper tube. Unfortunately, the increase in gas rate will usually necessitate an increase in pipe size.

Understanding the principles

The factors that affect pressure loss are length, quantity of gas flowing, diameter of the pipe, relative density and friction.

Although calculating pipe size has been carried out for many years, one of the key developments in low pressure installations has been Pole’s formula (see panel). This provides us with the relationship between all the factors that determine flow rate in low pressure installations.

A variation in this formula is provided in IGEM/UP/2.

When designing any gas pipework installation, all calculations should be based on the maximum gas rate of each appliance. Consideration may also be required for any extensions to existing supply.

BS 6891 Installation of low pressure gas pipework up to 35mm (R1¼) in domestic premises (2nd family gas) provides a method for calculating the required pipe size by using the proportionality of pressure loss and length.

The method used in BS 6891:2005 is to divide the installation into sections. Each section would then have a maximum pressure loss depending on the number of sections. For example, if there were two sections, each section can have a maximum loss of 0.5mbar, if there were three sections, each section would have a maximum loss of 0.33mbar, etc. Each tee is normally used to separate the sections.

A line diagram should be produced, if required, indicating the route of the pipework from the meter outlet to the connection point of each gas appliance together with a table showing your calculations.

It is recommended to keep this after the installation is completed in case the gas user questions your calculations at a later date. This process should provide a degree of margin, as pipe that is cut with a pipe slice will have a small internal burr and, if this is not removed, this will have an effect on the frictional loss.

Worked example

An installation with just a central heating boiler and a cooker could be broken into two sections. Each section would be made up of pipe length and fittings, and the fittings would have an equivalent length which is added to the length to provide a total length. Doubling this total length and then referring to the table, the maximum loss will be less than 0.5 mbar for each section.

Fitting

Equivalent length (m)

Elbow

0.5

Tee

0.5

90° bend

0.3

Table 1. Equivalent length of fittings up to and including 28 mm

An allowance has to be made for fittings – elbows, tees and bends – for copper pipe to BS EN 1057. Up to 28mm is shown in Table 1, and for 35mm the equivalent length for elbows and tees would be increased to 1m.

This information is not provided in BS 6891 but is provided in IGEM/UP/2. Table 2 provides discharge rates for copper pipe to BS EN 1057 and for corrugated stainless steel pipe (10–28mm) to BS 7383.

Tube size (mm)

Discharge rate (m3/h)

3m

6m

9m

12m

15m

20m

25m

30m

8

0.29

0.14

0.07

0.05

10

0.86

0.57

0.5

0.37

0.3

0.22

0.18

0.15

12

1.5

1

0.85

0.82

0.69

0.52

0.41

0.34

15

2.9

1.9

1,5

1.3

1.1

0.95

0.92

0.88

22

8.7

5,8

4.6

3.9

3.4

2.9

2.5

2.3

28

18

12

9.4

8

7

5.9

5.2

4.7

35

32

22

17

15

13

11

9.5

8.5

Table 2. Discharge rates for copper tube to BS EN 1057

Limiting the use of elbows – or using 90° bends instead – will reduce the equivalent length of the pipe and help with pipe sizing.

Worked example

See Figure 1:

• Length A to B is 3m plus two elbows and a tee
• Length B to C is 3.5m plus four elbows
• Length B to D is 2m plus three elbows and a bend Section A to B has a length of 3m and there are 2 elbows and a tee. Each elbow and tee has an equivalent length of 0.5m, giving a total for the fittings of 1.5m plus the original length of 3m – a total effective length of 4.5m (see Table 3).

Section

Length of actual pipework (m)

Allowance

Total equivalent length of fittings (m)

Total length pipework plus fittings (m)

Number of sections

Maximum length (m)

Gas rate (m3/h)

Pipe diameter (mm)

Elbows

Tees

Bends

A– B

3

2

1

1.5

4.5

x

2

=

9

5

28

B– C

3.5

4

2

5.5

x

2

=

11

3.6

22

B–D

2

3

1

1.8

3.8

x

2

=

7.6

1.4

15

Table 3. Pipe sizing calculation

If the installation is split into two sections, we multiply the 4.5m by 2, which gives an answer of 9m. The gas rate of the combined appliances of cooker and boiler is 5m3/hr.

Using Table 2, the maximum length of 9m is cross-referenced against a gas rate is 5m3/hr.

22mm tube delivers a gas rate of 4.6m3/hr, which is too small. Therefore, it is necessary to upgrade to the larger size of 28mm, providing a gas rate of 9.4m3/hr.

Section B to C has a length of 3.5m and there are four elbows.

Each elbow has an equivalent length of 0.5m, giving a total for the fittings of 2m. Adding this to the actual length of 3.5m produces a calculated length of 5.5m. The installation is split into two sections, so 5.5m is multiplied by two to produce a total length of 11m. The gas rate for the boiler is 3.6m3/hr.

Referring to Table 2, the maximum length of 12m must be used and the nearest gas rate is 3.9m3/hr, providing a pipe size of 22mm.

Section B to D has a length of 2m with three elbows and a bend.

The elbows have an equivalent length of 0.5m and the bend has one of 0.3m, giving 1.8m for the fittings.

Adding the actual length of 2m produces a calculated length of 3.8m. The installation is split into two sections, therefore we multiply the 3.8 3 2 = 7.6m.

Table 2 shows that with a maximum length of 7.6, 9m needs to be used, and this is crossreferenced with a gas rate of more than 1.4m3/hr. This provides us with a pipe size of 15mm.

Final pipe sizes for the example

• A to B = 28mm
• B to C = 22mm
• B to D = 1 5mm

To Sum Up

The maximum pressure loss on a low pressure natural gas installation between the meter and the appliance
inlet is 1 mbar (for LPG installations it is 2.5 mbar).

Pole’s formula can be used as the basis for the relationship of flow rate pipe size, pressure loss and length, on low pressure installations.

BS 6891: 2005 uses the concept that length is directly proportional to pressure loss, double the length double the pressure loss.

By producing a line diagram and the tables from BS6891: 2005, the pipe sizing can be quickly determined.

About the author

Mike Heads has been in the gas industry for nearly 40 years. He began his career as an apprentice Service Engineer with British Gas. He completed various gas utilisation qualifications then progressed to become a trainer for 14 years. Mike joined Ideal Boilers as a trainer and centre manager, completed a BSc (Open) and then a PGCE. Became a college lecturer teaching plumbing and gas, and now works for NGST as a trainer/assessor. He is a member of various organisations including MIGEM Eng Tech, MIDHEE, past president of the Yorkshire Gas Association. Presented various papers on combi boilers and condensing boilers, written a central heating fault-finding book for CORGI. Writes much of the training material used by NGST.