718
MODERN GASWORKS PRACTICE
generator is interposed a specially designed throttle which is operated by a valve rod connected to the governor. The latter consists of a cast-iron chamber across the centre of which is stretched a flexible diaphragm. The valve rod is attached at one end to the centre of the diaphragm, and at the other end to the valve in the steam main. Accordingly, any movement of the diaphragm is communicated to
the throttle valve.
Fig. 446.—Steam Contboller for Water Gas. New Type.
The arrangement depends for its operation oii the faet that the pressure in the gas outlet pipe above the fuel in the generator (or, in the case of carburetted water-gas plants, in the short pipe connecting generator and carburet-tor) varies in accordance with the volume of gas passing at a given time. As the temperature of the fuel is at a maximum at the beginning of the gasmaking period, it follows that the pressure also is at a maximum at the same time, for the maximum gas yield is then obtained. In the same way, as the temperature drops so does the yield of gas, and consequently the pressure in the off-take pipe. It is this gradual drop in pressure during the “tuu” which is communicated to the underside of the diaphragm and thence to the valve rod operating the steam throttle. Adjustment of the governor is effected in such a way as to enable it to be ntilized on plants of any capa-city. First, the maximum flow of steam which is required during the early stages of the “ run ” is set at the desired figure by means of an. ordinary valve in the steam main. The next adjustment is for minimum flow—that is to say, the governor, having been set to provide a certain maximum flow at the beginning of the “run,” is then set, by means of a by-pass, to give a definite minimum flow at the end of the “ run.” The third adjustment is for the purpose of varying the time taken by the governor
to effect a reduction from maximum to minimum steam flow, and is carried out by
weighting the diaphragm in accordance with the rate of flow reduction required.
Many water-gas plants are still unprovided with steam-flow meters, so that adjust-ment is, more often th.an not, carried out by guesswork, the steam valve being set to give a maximum gas output with but little regard to the carbon dioxide content of the final gas. The effect of arranging for a gradual reduction of steam flow as the temperature of the fuel falls is well illustrated by the following series of results actually