December 20, 2005
By Lucille Davie
THERE is something moving at Egoli Gas and it's big - very big. It is three 50m high tanks, and they rise and drop every two weeks with the flow of gas into and out of them.
They can drop right down to 15 metres, says Jan van der Schyff, operations director at the Egoli Gas plant in Auckland Park. In the old days (before the recent switch to natural gas) the three tanks could hold two days' supply of gas.
Now, with the new gas containing 90 percent methane, the tanks hold four days' gas supply.
The three tanks define the hilltop where they sit, providing a landmark to Joburgers.
The gas plant, now closed and a haunting presence on the site
"To some people they're ugly, to me they're beautiful," says Van der Schyff, an engineer with Egoli Gas for the past 16 years. "It's engineering in motion."
The tanks consist of three or four sections or lifts, resting on a grey steel base. The base is filled with water and each section fits neatly into the one below it by means of a water seal. A gas inlet pipe passes through the base section into the top sections, feeding gas in and out of the tanks.
The reason the tanks rise and fall, says Van der Schyff, is to smooth out demand. Instead of having highs and lows in demand for gas, the gas flow is moderated and the highs and lows level out.
There used to be five gas tanks on the site, but the two smaller ones were demolished. The tanks were built between 1936 and 1952, and the plant on the site was closed down in 1992.
It was built to produce gas from coal, and originally supplied the city with 30 percent of its gas requirements. Sasol came on stream in the 1950s, and supplied the city until this year, when natural gas was tapped in from Mozambique.
The gas stored is measured in cubic feet. The three tanks are of different sizes - the large one stores 5 million cubic feet, the medium one 3,5 million, and the smaller one 1,5 million, making a total of 10 million cubic feet of gas.
The sections turn as they move up and down, in clockwise and anti-clockwise directions.
Do staff have to be careful about smoking near the tanks? No, says Van der Schyff, there's no danger of a fire because the tanks are only filled with gas, and a fire needs oxygen.
Firing an explosive device into them won't cause them to explode either. Although it may cause a hole in the tank, and as the gas leaks out a flame may ignite, but the tank still won't explode.
In the case of a leak, the gas will disperse into the air quite quickly, as it is lighter than air - in fact, it is half the weight of air.
Egoli Gas made the switch to natural gas between July 2004 and August 2005, which Van der Schyff describes as "pretty amazing", taking into account the age and availability of some of the gas equipment being used in the city.
He says there was a customer complaint come-back rate of only 5 percent, although Egoli Gas still gets about one complaint a week.
Because equipment like domestic stove jets was not available, Egoli Gas had to design its own jets, and in all 60 000 pieces of gas equipment were converted.
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