714 MODERN GAS WORKS PRACTICE
Fio. 443.—Detail of Fig. 444.
Explosions in the biast main, occasionally resulting in the wrecking of the fan, are by no means an unknöwn. occurrence. They are essentially the outcome of a badly fitting hot-gas valve, i.e. the valve between biast main and generator, which may permit a small portion of water gas to get by during the “ down runs.” This gas meeting with the air in the biast main, forms an explosive mixture, which may be ignited by a spark from the fan. A safety appliance, designed to mini-mize the damage caused by such a
contingency, is shown in Figs. 443 and 444. It consists of a cast-iron flap swinging in a special box which is attached to the outlet of the fan. During the time when the blower is at work the flap remains in the position shown. Should any explosion occur in the biast main the force of it drives back the flap on to its
Fig. 444.—Steam Turbine Blowing Plant fitted with Automatic Shut-off.
seating, t'hus protecting the fan from damage, and expends itself by breaking through the canvas panels of the box.
As stated above, it is customary on water-gas plants for the blower engines to