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MODERN GASWORKS PRACTICE
point of reliability. The merits of tower systems are too well known to need repetition here. The more common defects which may go unnoticed for a time may be briefly summarized as follows
(a) In many cases an external or internal seal is employed, so that the liquor flow from the box may be readily seen. The warm liquor flowing out in. this way is responsible for appreciable ammonia losses, and the liquor should preferably be led to a closely covered seal-pot.
(b) Choked or partly choked equilibrium pipes may go unnoticed for some time, witli the result that the seal on the dip pipes is not what it is thought to be.
(c) It must be remembered that a tar tower and. the hydraulic main, connected together by the tar inflow pipe, in reality form a U-gauge, and the weights per unit of area of the two columns balance one an.oth.er. Consequently, fluctuations in specific gravity in the liquid in the two columns will be responsible for a rise or fall in level, heuce alteration of the seal. Differences in. temperature are also followed by similar effects. It has been shown that if a variation of 30° C. occurs in the temperatures of the liquid in the tower and hydraulic main (for instance, if the temperature of the liquor in the tower rose from 20° C. to 30° C. and. that of the liquor in the hy-draulic main feli from 70° C. to 60°
C.), this alone may be responsible for a variation of 14-tenths in the seal. To minimize this effect as far as possible it is advisable to connect the main tar pipe to the tower as high up as can reasonably be done, and to ensure that the tower is flushed down before the accumulated tar reaches the level of this pipe.
(<Z) A special means should be provided for guarding against the possibility of unsealing the dip pipes when the tower is discharged. It is usual to run liquor into the tower whilst the tar is being run off; but even. under such conditions the outward flow may be greater than the inward flow, thus the level will fall. A simple means of seeing at a glance whether or not the seal is being maintained is to flt a 1-in.ch test pipe, as shown in Fig. 180. When. the tower is flushed the J-inch test cock is opened, and so long as liquor passes down the pipe a seal is ensured.
(e) The only questionable drawback to the liquor take-off from the tower as