GAS PIPE SIZING

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 95as gas

Gross Calorific Value (GCV)(MJ/litre)

0.0416 25.5as liquid

Gross Calorific Value (GCV) (MJ/kg) 53.3 50Net Calorific Value (NCV)(MJ/m3)

38.7 86

Density of gas at 15oC(kg/m3)

0.78 1.85

Density of liquid under pressure - 0.512 kg/litre     or

(kg/litre) or (kg/m3) 512 kg/m3

Ignition temperature (oC) 650 510Operating pressure (millibars) 20 37Volume of gas per volume of liquid (m3/m3) or (m3/litre)

- 274 m3gas /m3 liquid   or0.274 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 MW4.                 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 m2.     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 MW4.     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 m3/s5.     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

Pipe length (m)

3.0 6.0 9.0 12.0 15.0 20.0 25.0 30.0

(mm)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 82 43 134 55 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   

GASGROSS CALORIFIC

VALUEMJ/m3

Natural gas 38.7

L.P.G. ( Commercial propane) 96

 

Gas Pipe Sizing Table

Section

Ref.

1Heat

Outputin

section

kW

2Flow ratem3/s

3Pipe Size

mm dia

4Length of pipe

m

5Total Equivalent length of Fittings

m

6Total Pipe

LengthCol. 4+5m

7Pressure drop

per metre

Pa/m

8TOTAL

PRESSURE DROP

Col. 6 x 7Pa

9Pressure at start

of sectionPa

10Pressure at end

of sectionPa

1 41 0.00106 28 8

Zeta factor for Tee =0.5 + 1.0 + 0.25 = 1.75T.E.L. =     x   le          =  1.75  x  0.7          =   1. 23 m

8 + 1.23= 9.23

2.2 20.31

2 160.00041

3 15 4

Tee 0.5 + 1.0  = 1.5T.E.L. =     x   le           = 1.5 x  0.4           =  0.6 m

4 + 0.6= 4.6

8.3 38.18

3 40.00010

3 15 13

3 bends @ 1.0  = 3.01 plug valve     = 1.0      Total        =  4.0T.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  = 2.01 plug valve = 1.0T.E.L.      =     x   leT.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 = 1.0T.E.L.      =     x   leT.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: 

SectionsPressure 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.

GASGROSS CALORIFIC VALUE

(MJ/m3)Natural gas 41.6

L.P.G. ( Commercial propane)

96

 

Gas Pipe Sizing Table

Section

Ref.

1Heat Outp

utin

sectionkW

2Flow

ratem3/s

3Pipe Sizemm

dia

4Length of pipe

m

5Total

Equivalent

length of

Fittings

m

6Tota

l Pipe Leng

thCol. 4+5m

7Pressure

drop per

metrePa/m

8TOTALPRESSU

RE DROPCol. 6 x

7Pa

9Pressure at start

of sectio

nPa

10Pressure at endof

section

Pa