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Gas Propert ies 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(kg/litre) or (kg/m3)
- 0.512 kg/litre or512 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
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 0.00079 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 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
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 16 0.000413 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 4 0.000103 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
1,2,520.31 + 38.18 + 17.60
76.09
1,2,320.31 + 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 41.6L.P.G. ( Commercial propane)
96
Gas Pipe Sizing Table
1Heat
Outputin section
kW
2Flow rate
m3/s
3Pipe Size
mm dia
4Length of
pipe
m
5Total Equivalent length of
Fittings
m
6Total Pipe
LengthCol. 4+5
m
7Pressure drop per
metrePa/m
8TOTAL
PRESSURE DROP
Col. 6 x 7Pa
9Pressure at start of
sectionPa