# The speed of gas – how fast does gas travel in pipework?

Until several decades ago, gas used in British homes was called “town gas” and manufactured by heating coal or coke, commonly in local “gas works”. I can remember our local gas works and the massive operation to change to natural gas, accessed from natural underground reservoirs under the North Sea. The television advertising slogan of the era was “High Speed Gas”, presumably because cooking and heating times were reduced. Natural gas has a calorific value about double that of town or “coal” gas – burning a given volume produces about twice the energy (I guess “High Calorific Value Gas” was rejected by advertisers).

How speedy gas is when travelling through the pipes in our homes is easy to work out. The volume of a length of circular cross-section pipe is its cross-sectional area multiplied by its length. If we know the volume rate at which gas is flowing, we can obtain the speed, as follows:

The calorific value of natural gas is often taken to be 10.76kW per cubic metre per hour (though in practice it can take a range of values). In other words, if gas is being burned at the rate of 1 cubic metre per hour, 10.76kW of heat is produced (the “gross heat input”). To produce 1kW of heat, the gas rate is therefore The cross-sectional area of a 15mm diameter copper pipe, taking into account a wall thickness of 0.7mm is (from p.r2, where r is the radius ie half the diameter), 145.3 mm2 or 0.0001453m2 (more neatly expressed as 1.453 x 10-4 m2).  The volume of a 1 metre length of this pipe is therefore 0.0001453m3.

So, to get 1kW of heat, we need to burn the gas contained in of pipe in an hour, which is equivalent to gas passing through the 15mm pipe at a speed of 640 metres per hour – not very fast at all. For a 7kW gross input gas fire or hob supplied by a 15mm diameter pipe, the speed of gas is 640 x 7 = 4480 metres/hour or around 4.5km per hour, which is still walking speed.

Alternatively, it can be expressed as 4480/3600 metre/sec or 1.24m/sec. So if the hob or gas fire has been connected up for the first time using 12.4 metres of pipe, there’ll be a 10 second wait from turning on the gas to its emergence at the burners.

For a 22mm copper pipe, 0.9mm wall thickness, the corresponding speed is 290m/hour per kW.

Take the case of a relatively powerful domestic combi boiler with a net heat input of 35kW for domestic hot water. That’s a gross input of 35 x 1.11 or about 39kW. Burning gas at that rate, the speed of gas in a 22mm pipe is 290 x 39 = 11266 metre/hour or 11.3km/hour, so an average runner could out-run it.

In conclusion, in a number of everyday gas-burning applications, the gas travels through household pipes at quite “human” speeds.

The results for copper pipes are summarised in the table below. To get the speed for a gas appliance, multiply the speeds in the table by the gross input in kW. 