Gas Pipe Sizing - CIBSE Method - Page 1 2 3 4


 

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. = z 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. = z 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 z = 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. = z 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. = z 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 Pipe Sizing - CIBSE Method - Page 1 2 3 4