Gas Properties

The Table below lists some of the more useful properties in building services for

natural gas and liquid petroleum gas (LPG).

Properties Natural Gas Liquid Petroleum Gas

(LPG)

Propane

Composition & Chemical symbol 92.6% Methane CH4

3.6% Ethane C2H6 0.8% Propane C3H8 0.2% Butane C4H10

100% C3H8

Gross Calorific Value (GCV)

(MJ/m3)(kJ/litre)

41.6 95

as gas

Gross Calorific Value (GCV)

(MJ/litre)

0.0416 25.5

as liquid

Gross Calorific Value (GCV) (MJ/kg) 53.3 50

Net Calorific Value (NCV)

(MJ/m3)

38.7 86

Density of gas at 15oC

(kg/m3)

0.78 1.85

Density of liquid under pressure

(kg/litre) or (kg/m3)

- 0.512 kg/litre or

512 kg/m3

Ignition temperature (oC) 650 510

Operating pressure (millibars) 20 37

Volume of gas per volume of liquid

(m3/m

3) or (m

3/litre)

- 274 m3gas /m

3 liquid or

0.274 m3 gas / litre liquid

Volume of gas per kg mass of liquid

(m3/kg)

- 0.54

LPG (Gas) Pipe Sizing Notes

Allowable pressure drops

Medium pressure lines - 450 Pa per m run ( 2 P.S.I. per 100 foot )

Low pressure lines - 4.1 Pa per m run ( ½” w.g. /100 ft)

- Maximum 1300 Pa total pressure drop in any run.

Gas Pressures

First stage regulation at tank pressure is reduced from 6 bar to 0.7 bar.

Second stage regulation at building pressure is reduced to 50 mbar (0.05 bar)

At equipment and appliances - 37 millibar (0.037 bar)

Natural Gas Pipe Sizing

Allowable pressure drops

The allowable pressure drop between the outlet of the meter and appliances must

not exceed 1 mbar or 0.001 bar or 100 Pa.

Pressure loss is generally limited to between 75 Pa and 125 Pa from the meter to

the point of use.

Gas Pressures

At equipment and appliances - 20 millibar (0.020 bar)

SIMPLE METHOD OF NATURAL GAS PIPE SIZING

A simple method of sizing Natural Gas pipes is to use the tables below.

The three tables are for Steel, thin wall copper and thick wall copper.

An allowance may be made for fittings.

This allowance is to add to the measured pipe lengths, 0.3 metres for 90o bends and

0.6 metres for an elbow or tee.

Procedure

1. Measure the length of pipe in each section.

2. Add equivalent lengths for fittings.

3. Determine heat output of appliance or heat to be carried in pipe in MW

4. Divide this by Gross Calorific Value (GCV) of gas i.e. 38.7 MJ/m3 to

give gas flow rate in m3/s.

5. Determine pipe size from relevant Table.

Example 1

Determine the steel pipe size for a gas boiler if the boiler rating is 28 kW and the

measured pipe run is 10.2 metres with 3 elbows.

Procedure

1. Measure the length of pipe in each section – answer 10.2 m

2. Add equivalent lengths for fittings – answer 3 x 0.6 = 1.8 m. Add to 10.2 m

gives 12.0 metres.

3. Determine heat output of appliance or heat to be carried in pipe in MW. –

answer 28 / 1000 = 0.028 MW

4. Divide this by Gross Calorific Value (GCV) of gas i.e. 38.7 MJ/m3 to give

gas flow rate in m3/s – answer 0.025 / 38.7 = 0.0007235 m

3/s

5. Determine pipe size from relevant Table – answer from Table 1 gives 20mm

pipe because the flow rate lies between two values on the table and the

larger pipe size is chosen.

Table 1 below is used for Mild Steel pipe, BS 1387: medium grade.

Table 1 – Gas Pipe Sizing for Steel Pipe

Nominal

Pipe size

(mm)

Pipe length (m)

3.0 6.0 9.0 12.0 15.0 18.0 21.0 24.0

Gas Flow Rate (m3/s)

15 0.00104 0.00072 0.00057 0.00050 0.00044 0.00039 0.00036 0.00034

20 0.00219 0.00149 0.00120 0.00102 0.00090 0.00083 0.00076 0.00071

25 0.00409 0.00275 0.00224 0.00193 0.00169 0.00153 0.00142 0.00134

32 0.00826 0.00574 0.00464 0.00393 0.00346 0.00315 0.00291 0.00275

40 0.01258 0.00865 0.00700 0.00598 0.00527 0.00480 0.00440 0.00417

Table 2 below is used for Copper Tube Table ‘X’ or EN 1057 thin wall, half hard

temper.

Table 2 – Gas Pipe Sizing for Copper Tube Table ‘X’

Nominal

Pipe size

(mm)

Pipe length (m)

3.0 6.0 9.0 12.0 15.0 20.0 25.0 30.0

Gas Flow Rate (m3/s)

15 0.00079 0.00054 0.00043 0.00035 0.00032 0.00027 0.00025 0.00024

22 0.00244 0.00165 0.00126 0.00102 0.00110 0.00079 0.00070 0.00063

28 0.00495 0.00330 0.00260 0.00220 0.00197 0.00165 0.00142 0.00134

Table 3 below is used for Copper Tube Table ‘Y’ or EN 1057 thick wall, half hard

temper.

Table 1 – Gas Pipe Sizing for Copper Tube Table ‘Y’

Nominal

Pipe size

(mm)

Pipe length (m)

3.0 6.0 9.0 12.0 15.0 20.0 25.0 30.0

Gas Flow Rate (m3/s)

15 0.00071 0.00047 0.00037 0.00032 0.00028 0.00027 0.00024 0.00020

22 0.00220 0.00149 0.00118 0.00094 0.00087 0.00072 0.00064 0.00057

28 0.00456 0.00307 0.00244 0.00205 0.00181 0.00149 0.00134 0.00118

It must be remembered that the above method of pipe sizing is not as accurate as

using the CIBSE guide method as detailed in page 3, but can be useful for

checking pipe sizes quickly.

CIBSE method of Gas Pipe Sizing

Use CIBSE guide Table C4.45 (steel) and C4.46 (copper) for natural and LPG pipe sizing.

If LPG is used then refer to correction factors on page C4.76 of the CIBSE guide.

Most domestic natural gas appliances operate at a pressure of 20 millibar (0.020 bar)

Domestic LPG appliances operate at a gas pressure of 37 mbar (0.037 bar).

For natural gas it is usual to limit the pressure loss to 75 Pa to 125 Pa from the gas meter to

the point of use.

A maximum pressure drop from meter to appliance of 1millibar (100 Pa) can be used.

Natural Gas Pipe Sizing Example

Size the gas supply pipe shown below. Copper pipework is used.

Appliance pressure is 20 mbar.

Use the maximum pressure drop given above of 100 Pa from meter to appliance.

Section Length

(metres)

1 8

2 4

3 13

4 5

5 4

Example 1 – Gas Pipe System

To calculate the gas flow rates use the following formula:

Q = H / GCV

Where;

Q = Gas flow rate in pipe section (m3/s)

H = Heat output in pipe section (convert to MW)

GCV = Gross Calorific Value of gas (MJ/m3) given in Pipe Sizing Table.

For example, in Pipe section 1 the Heat output to all appliances is 25 + 12 + 4 kW = 41

kW.

Flow rate Q = H / GCV

Q = 41 x 10-3

(MW) / 38.7 (MJ/m3) from Pipe Sizing Table.

Q = 1.05943 x 10-3

m3/s

Q = 0.00105943 m3/s

Q = approximately 0.00106 m3/s

GAS

GROSS CALORIFIC

VALUE

MJ/m3

Natural gas 38.7

L.P.G. ( Commercial propane) 96

Gas Pipe Sizing Table

Section

Ref.

1

Heat

Output

in

section

kW

2

Flow

rate

m3/s

3

Pipe

Size

mm

dia

4

Length

of pipe

m

5

Total Equivalent

length of Fittings

m

6

Total

Pipe

Length

Col.

4+5

m

7

Pressure

drop

per

metre

Pa/m

8

TOTAL

PRESSURE

DROP

Col. 6 x 7

Pa

9 Pressure

at start of

section Pa

10 Pressure at end

of section Pa

1 41 0.00106 28 8

Zeta factor for Tee =

0.5 + 1.0 + 0.25 = 1.75

T.E.L. = x le

= 1.75 x 0.7

= 1. 23 m

8 + 1.23

= 9.23

2.2 20.31

2 16 0.000413 15 4

Tee 0.5 + 1.0 = 1.5

T.E.L. = x le

= 1.5 x 0.4

= 0.6 m

4 + 0.6

= 4.6 8.3 38.18

3 4 0.000103 15 13

3 bends @ 1.0 = 3.0

1 plug valve = 1.0

Total = 4.0

T.E.L = 4.0 x 0.2

= 0.8 m

13 + 0.8

= 13.8

1.3 17.94

4 25 0.00065 22 5

2 bends @ 1.0 = 2.0

1 plug valve = 1.0

T.E.L. = x le

T.E.L = 3.0 x 0.5

= 1.5 m

5 + 1.5

= 6.5

3.0 19.5

5 12 0.00031 15 4

1 plug valve = 1.0

T.E.L. = x le

T.E.L = 1.0 x 0.4

= 0.4 m

4 + 0.4

= 4.4 4.0 17.6

Gas pipe sizes and pressure drops can now be put on the drawing.

The pressure drops as shown in the following Table:

Sections Pressure drop

(Pa) Pressure

drop (Pa)

1,4 20.31 + 19.50 39.81

1,2,5 20.31 + 38.18 +

17.60 76.09

1,2,3 20.31 + 38.18 +

17.94 76.43

The maximum pressure drop is in sections 1, 2 and 3 and amounts to 76.43 Pa.

This is less than the recommended maximum of 100 Pa; therefore the pipe sizes

are appropriate.

If the pressure drop in sections 1, 2 and 3 and was too high then the pipe size in

section 2 could have been increased to 22mm, and a second calculation of pressure

drop carried out.

GAS

GROSS

CALORIFIC

VALUE

(MJ/m3)

Natural gas 41.6

L.P.G. (

Commercial

propane)

96

Gas Pipe Sizing Table

Section

Ref.

1

Heat

Output

in section

kW

2

Flow rate

m3/s

3

Pipe

Size

mm dia

4

Length of

pipe

m

5

Total Equivalent length of

Fittings

m

6

Total

Pipe

Length

Col. 4+5

m

7

Pressure

drop per

metre

Pa/m

8

TOTAL

PRESSURE

DROP

Col. 6 x 7

Pa

9

Pressure

at start of

section

Pa

10

Pressure at

end

of section

Pa