This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The Miller syphon has lately been introduced into this country from America. The following description of it is taken from Engineering, February 1, 1895: -"The flushing syphon is one which has been largely adopted in the United States, having obtained the highest award at Chicago in 1893. Previous syphons have l>eeu brought into action by the simple release or rarefaction of the air confined in the syphon, or by the sudden removal of such air by special subsidiary devices, which are entirely absent in the 'Miller' syphon." As shown in Fig. 413, "it consists of two simple castings, a U tube (or trap) and mouthpiece, cast in one- pine, and a cast-iron bell which is placed over the longer leg of the syphon, and is held in place by brackets cast on the trap. The action of the syphon is as follows: As the water entering the tank rises above the lower edge of the bell, it encloses the air within, the lower portion of the (J or trap being, of course, tilled with water. As the water-level in the tank rises, the confined air gradually forces the water out of the long leg of the trap, until a point is reached when the air just endeavours to escape round the lower bend. Now, as the difference of water-level in the two legs equals the difference of the levels between the water in the tank and the water within the bell, it will be seen that the column of water in the short discharge-leg has practically the same depth as the head of water in the tank above the level at which it stands in the bell. The two columns of water therefore counterbalance each other at a certain fixed depth in the tank. As soon as this depth is increased by a further supply, however small, a portion of the confined air is forced around the lower bend, and by its upward rush carries with it some of the water in the short leg, thus destroying the equilibrium. But the secret of this invention is the free projection of the overflow edge, which allows of the instantaneous eacape falling away of the heaved-up water. Thus, if the discharge-mouth were formed as an ordinary bond, the syphon would not act (although the confined air rushes around the lower bend), for the simple reason that the heaved.1 -up water has no means of instantaneous escape, and therefore the equilibrium is not sufficiently disturbed. It will thus be seen that the action of the syphon depends not on the escape of air, but on the sudden reduction of a couuter balancing column of water.
Fig 413 - Details of the Miller Syphon In Manhole.
Repeated trials have shown that a 6-inch Miller syphon will discharge full-l>ore a 500-gallon tank, fed so slowly as only to be filled in fourteen days. There. being no internal obstruction, the discharge is extremely rapid. There is, it will l>e seen, a deep-water well between the flashing tank and the sewer, which is of course an advantage. We have had the opportunity of seeing one of these syphons at work in the excellent sanitary museum at Hornsea, and though severely tried, the syphon worked perfectly. As will be seen by a reference to Fig. 413, the syphon-chamber can be very neatly combined with a manhole. No special mouthpiece is required; the mouth of the discharge-pipe stands quite clear, and delivers the water into a concrete basin, from which it rushes down into the sewer."
In long lengths of drain it is not necessary to go to so great an expense as would be occasioned by the construction of intermediate Hushing tank-, but it is sufficient to re-use water discharged from the flushing tank at the lead of the drain, and this can be accomplished by fixing penstocks in the manholes; when the penstocks are securely closed, the sewage or water is dammed back in the sewer, and when the penstock is opened again, the liquid rushes forward with great force. The "screw-down" penstock, shown in Fig. 414, is raised or lowered by a key fitted on to the top of the spindle from the surface. This type of penstock is the most suitable where absolute water-tightness is desired, and where the pressure behind it is considerable; but there are circumstances where a penstock of the disc pattern, as shown in Fig. 415, would answer the purpose. This, as will be seen, is self-locking, and works from a side pivot, and is raised or lowered by working the spindle attached. Whenever a penstock is fixed in a manhole, it is desirable, on account of the sudden rush of water when the penstock is raised, which is apt to rise over the invert on to the platform at the wde of it, that the outgo of the manhole should be enlarged into the form of a swallow, or shoot (as it is sometimes called), as shown in Fig. 416. For some years 1 have made it I practice in fixing a penstock to place a relief-pipe from the drain immediately behind the wall of the manhole, turning it over into the manhole at a level that will prevent the escape of sewage on to the surface of the ground through the gullies or nian-hole-gratings farther up the drain, in case any accident should prevent the penstock from being raised. I have generally found that gardeners and other servants, who are set to attend to the flushing of drains, have a knack of going away in the middle of the operation and of forgetting to raise the penstock. The relief-pipe also acts as an escape for the air displaced by the incoming body of water. A penstock should always be fitted upon the inlet-pipe of the inter-cepting manhole, as it will be found not only useful for drain-cleaning purposes, but also whenever it is desired to apply the hydraulic test to the drain.-.
Fig. - 414. Screw-down Penstock.