258
MODERN GASWORKS PRACTICE
of the buckstays is slightly canted over, with. the result that the main may develop a slight twist. All the metliods are, Iiowever, subject to this effect in' some degree. A very fine adjustment can be obtained by the screwed supports shown in Kg. 169.
THE FUNCTIONS OF THE HYDRAULIC MAIN
The principal functions of the hydraulic main are :—
(a) To act as a seal so as to prevent air being drawn up the ascension pipe when the retort doors are open.
(b) To act as a safety seal in the event of sudden heavy back-pressure being thrown by the apparatus following, particularly in the event of the exhauster puIling up.
(c) To form a receptacle for the tar and aqueous vapour Condensed from the gas at the early stages of manufacture.
(d) To act as a primary washer.
The seal on the dip pipes is, therefore, liable to be overcome—
(a) by the vacuum induced by exhausting;
(b) by the stoppage of the exhauster (or from some other cause) and the con-sequent back-pressure thrown on the hydraulic main, in which case the liquor in the main is forced up the dip pipes.
The conditions of retaining the seal are, therefore, dependent upon the relative areas of the hydraulic main and dip pipes. Theoretically, if—
x = the combined cross-sectional area of all dip pipes leading into the main, y — the total cross-sectional area of the main, and
d = the extent to which the dips are sealed when the box is open, to the atmosphere,
then the total back-pressure which the pipes will withstand before letting back gas (neglecting any variation in density of the liquid)
Thus with a J-inch seal, which is frequently found with modern methods of obtain-ing high gas “ makes,” the back-pressure which would be withstood is 6 inches, on the assumption. that the respective areas x and y are as 1 to 12, or 10 inches if the ratio is 1 to 20. At the time when this function. of the hydraulic main, was con-ceived seals were commonly so much as 3 inches in depth, and capable of with-standing a back-pressure of 30 inches.
On the other hand, the amount of “ draw ” which can be applied to the main before the seal is broken forwards is given by the following:—
“ Draw ” must not exceed—
d + (—— x d\ — x /
providing that no overflow for the liquor is employed. That is to say, with a 1-inch seal the vacuum before the seal is overcome may equal 1^ inch, or with a j-inch