That all depends on where it is coming from or forming.
A lot of old installations had a filter just by the meter.
That might suggest its formed with iron pipes and in the days of coal produced gas. But the info below suggests its from copper pipes. In London we must have the same mix of gas everywhere but I only occasionally see it.
Tony
Note this was written for Ireland!
BACKGROUND:
A large proportion of natural gas comes from gas-fields in the North Sea through the two interconnectors.Unlike the gas from the Kinsale fields this gas contains a trace of Hydrogen Sulphide(H2S) which may react with the copper pipe and copper components in appliances. The hydrogen
sulphide causes a black deposit to form on the inner walls of the copper pipe - Copper Sulphide(Cu2S). This is called “Sulphidation”. In some circumstances this deposit is in form of flakes.These flakes can become detached and get carried by the gas flow into appliances, where they can
accumulate and restrict the flow of gas through apertures in injectors, and through valves, filters,pilot lights, etc., causing loss of output, and eventually the failure of the appliance to function.
Sulphidation can occur when the Hydrogen Sulphide content is above 0.4mg/m³, but until thelevels are more than about 1.5 - 2.0 mg/m³ it does not seem to cause many problems. The gaslanded at the St Fergus terminal in Scotland, for example, had a hydrogen sulphide content of only
0.6 mg/m³ when it first came on stream in 1986, but this was found to increase particularly in the90's, and is now at 3.5 mg/m³. In view of this the number of instances of sulphidation in Irelandmay increase in the years ahead.
In rare instances, the effects of sulphidation become immediately apparent but typically it takesabout three or more years before problems arise. The copper has to react with the HydrogenSulphide to form a deposit which might in due course flake off. The gas flow must be fast enoughto cause these flakes to travel with the gas to a collection point, where they may then accumulateand interfere with appliance operation.
Not all copper pipe will react with the hydrogen sulphide in the gas to form the troublesome flakes, and the reasons for this are not yet clear. Based on the experience in the UK (where thesulphidation problem has occurred in Scotland and in the North of England and Wales), it is likelythat less than 0.5% of the domestic sized installations might be effected. In Ireland, this wouldindicate that between 2,000 and 2,300 installations might be affected to varying degrees. The copper sulphide deposits have the appearance of small graphite flakes and are sometimesreferred to as ‘Black Dust’.