CO882-(3-4) — Page 500

PUBLIC RECORD OFFICE

Reference :-

PELLIC.O. 882

PUBLIC RECORD OFFICE, LONDON ALLY WITHOUT PERMISSION OF THE COPYRIGHT PHOTOGRAPH—NOT TO BE REPRODUCED PHOTOGRAPHIC-

Self-cleans-

ing velocity gradient being limited, dia- meter is

letermined

of emuent to be dealt

48

For this reason also 6-inch pipes are often used for house drains where 4-inch pipes would fully suffice to carry off the sewage and even the rain water. This is specially so where the work cannot be executed under close supervision.

There can be no doubt that a 6-inch pipe is much less liable to casual obstruction by bits of rag, &c. than a 4-inch, but it is not less liable to deposit, rather the reverse.

It is unfortunate that 5-inch pipes are not usually made. They would probably combine the advantages of both sizes.

If ordered in large numbers, or manufactured in China, pipes of any desired diameter could be obtained.

With careful workmanship 4-inch pipes will suffice for house drains.

Although a 4-inch pipe is fixed as the minimum size, it is neither necessary or desirable to augment the size of a drain pro rata to the number of 4-inch house drains which it receives. The following calculation shows this.

We may assume that the volume of the affluent from any dwelling is about equal to the water supply. In this case 10 gallons a head a day will be a most liberal estimate. In the poorer class of houses there will be at least 16 inhabitants per house, so that the discharge will amount to 160 gallons a hoïse a day, of which about one half will

pass off in 8 hours. The maximum rate of discharge will be 10 gallons per hour per house. Now, a 4-inch pipe at a fall of 1 in 30, running half full, delivers about 6,000 gallons per hour, so that such a pipe would carry off the sewage of 600 such houses with a margin of 100 per cent. for casual increase. Even if the gradient be reduced to 1 in 90, a 4-inch pipe will suffice to remove the sewage of nearly 300 houses, whilst the velocity of flow will still be sufficient to prevent deposit. There is, therefore, a direct advantage in concentrating the effluent of a large number of houses in a pipe no larger than that used for a single dwelling.

As the rate of flow from single houses or even from small of dwellings cannot

groups be predicted with accuracy, and because of the difficulties as to accuracy of form and the obstruction to ventilation already mentioned, it may not be desirable to reduce dimensions, and especially gradients to the ultimate limit indicated by such calculations. They serve, however, to show the very moderate dimensions that will suffice, dimensions far less than those employed at present.

The discharge from large districts may be more readily estimated, for the individual irregularities tend to equalise one another, and therefore in fixing the proportions of main sewers the results of calculation may more be closely adhered to than in the case of small areas.

6. To secure freedom from deposit, the velocity in sewers should be from 2.5 feet to 3:0 feet per second according to various authorities. It is reasonable to suppose that a higher velocity is requisite to break up and remove deposit than to prevent its formation

Hence, in branch sewers where the flow is intermittent the higher velocity should be adopted, whilst in main sewers, where the movement is more regular, a lower velocity may be admissible.

The velocity of flow through a pipe running full or half full is a function of two variables, the diameter and the gradient. The velocity being fixed by the conditions just stated, it is obvious that, when, as is frequently the case, the available fall is also fixed by local circumstances, the diameter is determined by these considerations alone, apart from the question of the volume to be conveyed.

Inasmuch as the proper velocity is only obtained when the pipe is half full, and irrespective diminishes as the depth diminishes, slowly at first, but more rapidly afterwards, in order that a sewer may be self-cleansing, it is necessary that the volume of water passing tbrough it should at intervals, at least, be sufficient to fill it to the depth that will produce the desired velocity.

with.

fundequate

quantity of

fauent necessitates

flushing.

For example, with a fall of 1 in 440 a 20-inch pipe running balf full has a velocity of 3 feet per second, that required, and discharges about 3 27 cubic feet per second. With a depth of 2 inches only, in the pipe, the velocity is reduced to 1.7 feet per second, and the discharge to about 0.19 cubic feet per second. To prevent or remove deposit, therefore, it would be necessary that once a day at least the rate of discharge must reach 3.27 cubic feet.

If the natural flow does not reach the required amount, then it must be supplemented by occasional flushing. Field's flush tank affords a most simple means of doing this, as it enables any given quantity to be discharged automatically at any desired rate or interval of time into a sewer or drain.

Moreover, it is pot necessary to use clean water for this purpose, sewage from a higher district may be collected and thus employed to flush lower levels.

49

The use of the flush tank is, therefore, recommended in those cases where the gradient

is limited, and where there is doubt as to the adequacy of the natural flow.

tions to be

7. For the realisation of the proper velocity due to the fall it is essential that pipes Accuracy of should be even, true, and straight, and that there should be no roughness or irregular work requi- projections on its inner surface. Depressions also are most objectionable, for in them site, precau- deposit takes place during the intermissions of flow which may not afterwards be removed. observed in

Care is, therefore, necessary in laying the pipes. To secure true line, as long a length laying pipes. of trench as possible should be excavated at once, and its bottom carefully graded, before putting in the pipes. If practicable, the whole length between two bends or junctions should be excavated at once. By the ordinary plan of laying the pipes, and filling in the earth as the excavation proceeds, it is quite impossible to secure the necessary accuracy. When the trench is completed, the pipes should be laid to a line stretched along, and before jointing, they should be firmly bedded behind the socket in their final position, leaving just room to make the joint. The joints should be made with Portland cement, and the greatest care should be taken to prevent a ragged fringe of cement from protruding from the joint, inside the pipe. To remove any cement that may exude from the joint into the pipe, a long rod with a round head covered with rag is used to wipe out the pipe.

When drains have to pass through the walls of buildings, especially when newly erected, abundant clearance must be left both above and around the pipe. If firmly connected to the wall, the least settlement will cause fracture and leakage.

When the ground is soft, the pipe should be bedded in concrete.

able to

8. Standford's patent joints make excellent work, and require no cement or other Standford's composition for jointing. The spigot and the inside of the socket are provided with joints favour- bands of a certain composition, carefully shaped, so that they mutually fit each other. accuracy, All that is necessary is to press the spigot slightly greased into the socket, and a but expen- perfectly water-tight joint is at once formed. They cost, however, nearly 40 per cent. sive and more than ordinary pipes, and experience is wanting as to the behaviour of the com- hitherto position under high temperature, such as that to which they would be exposed in the untried in

tropics. hold of a ship. It might be well to send out a small number as an experiment.

9. When a pipe drain is broken for any purpose, such as to remove an obstruction, it is Pipe drain almost impossible to restore it to its original condition of accuracy. Obstructions usually once broken take place at bends, junctions, or syphons. At such places, therefore, special means of restored with

difficulty. access should be provided. Between these points the pipe should be absolutely straight, Necessity for so that a jointed rod may be passed up to remove obstructions if they do occur, which manholes, should be exceedingly rare in well constructed work.

turning and The most complete method of doing this is to make all bends and junctions in the inspection form of open channels, in the floor of a chamber of sufficient size to admit a man (man, give access hole).

chambers, to

without

disturbance.

In street sewers this should always be done, but the cost will often be too great in case necessity for of house drains. In these cases either a smaller turning or inspection chamber should be used, or an inspection cap, or small lid on the top of the pipe which at least allows examination to be made without breakage, and in many cases they can be so arranged that a rod can be passed down the pipe from them. At all syphons also either manholes or inspection chambers should be used.

10. The price of pipes will be a most serious factor in the cost of the works. I, Prime cost therefore, made inquiries as to the possibility of obtaining suitable pipes in China, of pipes Through the kindness of Dr. Kew, of Canton, I found out and visited the potteries of serious item.

Possibility Shek-Wan, a town situated on a creek of the Canton river, about 20 miles from that of manu. city. Some pipes were made to a pattern which was sent, one of which is now in my facturing possession. They appear to be of excellent quality, and with a little improvement, as to pipes in regularity of dimensions, would be quite suitable for drainage purposes. The price paid for these pipes, 10 in number only, was $0.20 each, or about 5d. a foot, or rather less than what English pipes would cost delivered in Hong Kong, including freight. For larger orders a much lower price would be asked, so that I believe, with a little trouble, pipes may be procured in China at little or no greater cost than in London,

China,

Description

11. Plates 1. aud II. show methods of draining some common types of houses, of drawings, according to the principles just enunciated.

Removal of slop water

Provision is made for the removal of slop water only, but, as stated in the report, only pro- drains fit for that purpose are fit for the conveyance of excreta also, so that when the vided for, time comes, they can easily receive the additions necessary to fit them for this additional but drains duty. No provision is made for removal of rain water which should be dealt with equally separately for the reasons stated in the report. The dimensions are, however, ample to receive a considerable quantity of rain, to carry off all would be clearly impossil lę.

G

R 3737.

suited to CORVET

exoreta if

desired.