The Student's Column
Part II Drainage
Chapter 15.—Flushing And Testing
[Editor's Note: This information is of historical value only and has been superseded by codes, regulations and standards. Do not use this information as a reference.]
If a drainage system is properly designed and carried out, special flushing appliances are as a general rule unnecessary. There is something radically wrong with a drain which can only be kept clean by the frequent discharge of large volumes of fresh water. Perfect jointing and true alignment of the pipes, good gradients, and exclusion of sand and gravel by means of silt traps, etc., go far to prevent deposits in drains, and it is much better to prevent deposits than to allow them to form, even though a flushing tank is provided to ensure their periodical removal. The discharges from baths and butlers' sinks will in many cases serve to keep the drains clean, if the outlets and wastepipes are of sufficient size. Rainwater-pipes, if the positions are carefully selected, are also of great service in rainy districts, although, of course, the flush obtained from them is irregular both in quantity and time. This disadvantage might be reduced by running the rainwater to a large storage tank from which a fixed quantity might be discharged automatically or by hand at regular intervals.
The simplest form of flushing arrangement is the "tipper" or "tumbler," of which an example is given in figure 71 These are made of stoneware in various sizes, discharging 3, 5, 10, 20, and 33 gallons respectively. They are designed for fixing under waste-water gullies and rain-water pipes, and may be advantageously used in places where the water supply is strictly limited. The collected sewage is discharged with considerable velocity, and has an appreciable influence in keeping the drains free from deposits. The illustration gives a plan and section of Duckett's 5-gallon tipper fixed under a dish and grate. The wastepipes and rain-water pipes may discharge over or under the dish; a trap must, of course, be fixed at or near the outlet of the tipping chamber in order to disconnect the waste-pipes from the drain. The same principle is applied to overhead flushing tanks supplied with fresh water. Oates & Green's tank (figure 72) of this kind is made in two sizes to discharge 5 and 10 gallons respectively. The tipper A is suspended on pivots in the tank B, and revolves within the limits imposed by the rubber buffers C; D is a patent concentrating channel introduced to ensure a solid flash.
Siphon flushing tanks are also made for collecting and discharging sewage, and may be used where the water-supply is inadequate, but only comparatively clean water, such as rainwater from roofs and the wastewater from baths and lavatories, ought to be conveyed to them. Figure 73 shows Adams's flushing tank of this type; it is made of cast-iron, in sizes of from 15 to 100 gallons, and in brown or straw glazed ware from 15 to 30 gallons, Another variety, made by the same firm, requires less depth from inlet to outlet (figure 74), but does not act with a "dribble" supply; an ordinary discharge from a lavatory or butler's sink is, however, sufficient to start the siphon when the chamber is full. Stoppers are provided at A and B, so that the siphon and chamber can be cleaned out if required. Other patent siphons, such as Rogers Field's, are designed for use with sewage or clean water, and can be placed in underground chambers constructed of concrete or brick, or in a galvanised iron tank. The construction of Field's flush-tank (figure 75) is simple. The long leg A of the siphon passes through the bottom of the tank into the trapping-box, B, which is provided with an arm, C, for connexion to the flash-pipe; the dome, D, forming the short leg of the siphon, can be removed for inspection and repair. A handhole is formed at E, so that the trapping-box can be inspected and cleaned. The top of the long leg is so designed that siphonic action is started by a drop-by-drop supply.
The outlets of flush-tanks must be freely ventilated and disconnected from the drains, and all tanks fed with sewage must be periodically cleaned.
A single chamber or tank may be fitted with two siphons so as to flush two drains running in different directions from the same point the siphons can be arranged to discharge alternately.
The size of the flush-tank must be governed by the diameter, length, and inclination of the drain, the velocity of discharge from the tank, and the nature and normal flow of the sewage. Very few experiments appear to have been made, and, consequently, no definite rule can be laid down. Two tests by Mr. S. H. Adams deserve mention. The drain was 9 in. in diameter, and had a fall of 1 in 200. A discharge of 472 gallons started with an initial velocity of 5 ft. per second; at a distance of 250 ft. the velocity had been reduced to 4 it. per second; at 650 ft. it was 3 ft., and at 1,600 ft. 2 ft. per second; and this velocity was slightly reduced during a further flow of 2,000 ft. A discharge of 180 gallons into the same drain started with an initial velocity of about 4 ft. 2 in. per second, or nearly 1 ft. per second less than in the other test, and this difference was approximately maintained throughout the length of the drain. If we assume that a velocity of less than 3 ft. per second is of little use, we must conclude that the larger and more rapid discharge was efficacious for a length of about 7oo ft, and the smaller for about 300 ft.
House-drains are usually of smaller diameter and quicker gradient, and high velocities can in consequence be maintained for longer distances. Smaller quantities of water will therefore suffice. It is better to place two or more flushing cisterns of smaller capacity at heads of different branch-drains than one large cistern at the head of the principal branch. It is also better to have two discharges daily from a tank of moderate size than a single discharge from a tank of double the capacity. As a rough and ready approximation, the normal capacity of the flushing-tank in gallons (for house-drains) may be found by squaring the diameter of the drain in inches. Thus, a 4 in. drain may have a flush of 16 gallons, a 5-in. 25 gallons, and a 6 in. 36 gallons. These figures must be increased as the gradients are diminished, and as the length of the drain is increased.
Assuming the proper gradient of a 4-in, drain to be 1 in 34 (see Chapter 6), and that the drain to be flushed has a gradient of 1 in 50, add (50-34 =) 16 gallons to the normal capacity, and similarly for other gradients. Add also one gallon for every 20 ft, length of drain beyond 100 ft.
This gives us the empirical formula— F = d2+(g-g1) + (l – 100)/20 —where F = capacity of the flushing tank in gallons, d = diameter of drain in inches, g = length of drain divided by fall, g1 = length of self-cleaning drain divided by the fall (= 34 for 4 in. drains, 48 for 5 in. drains, 65 for 6 in. drains— See Tabl III., Chapter 6), and l = length of drain in feet.
Required the capacity of the flushing-tank for a 5-in, drain 200 ft. long, laid to a gradient of 1 in 60.
Worked example: F = 52 + (60-48) + (200-100)/20 = 42 gallons.
If the drains are much too large for their work, greater volumes of flashing water are required. Thus, if a 9-in. drain is laid where a 4-in, drain would suffice, this 9-in. pipe will require more flushing water than a 9-in. sewer in which the normal flow is, say, one-third of the diameter. The flushing water ought to be sufficient, when added to the normal flow, to increase the depth until the highest velocity is attained—that is to say, the pipe must be from half-full to nearly full.
Flushing siphons may be too large. The rapid discharge of large siphons often forces paper and soil from the main into the branch drains, or siphons the water from the gully traps. We have seen that the velocity is the same whether a drain is flowing full or half-full, and as deposits accumulate on the invert and not on the crown, the depth of the flow need not exceed half the diameter of the drain. Of course, the flushing water enters the drain at a high velocity, which is gradually reduced until the normal velocity due to the gradient of the drain is reached It is impossible to lay down any hard-and-fast rule as to the size of the siphon, but for general purposes the diameter of the outlet-leg ought not be much more than half that of the drain to be flashed. In the case of long outfall drains—which are usually of greater diameter than any of the branches—small tanks may be provided at the heads of the principal branches, and a larger one at some suitable point in the outfall.
Penstocks are sometimes fixed in the manholes of sewers to dam the sewage back; on raising the penstock, the sewage escapes with an increased velocity, and so flushes the sewer below. This method of flushing has obvious disadvantages, and cannot be recommended for house-drains.
Water Tightness Testing
[Editor's Note again: this is historical information. Laws, regulations, standards and practices have changed since this publication.]
However carefully the construction of drains may have been supervised there can be no assurance that they are absolutely water- tight and airtight unless a positive test is applied. The Urban and Rural Model By-laws contain no provisions on this subject; they merely state that the joints of drainpipes must be "watertight," but the onus of proof or disproof appears to rest with the authority, and not with the person who constructs the drain. The London Drainage By-laws are more explicit. The seventh paragraph of By-law 4 states that all drains, except those for the drainage of the subsoil, must be "so constructed as to watertight and to be capable of resisting a pressure of at least 2 ft. head of water. Opinions differ as to whether this provision renders the water-test compulsory, or whether it allows the pneumatic test, with the same pressure per square inch, to be adopted. In our opinion, the wording is clearly in favour of the former of these two readings: the preposition "of" after " pressure" is not synonymous with "equal to," and the statement of the pressure as "head of water" and not in pounds per square inch leaves, we think, no room for doubt as to the meaning intended to be conveyed. When we remember that in the event of stoppage a drain is subjected to water-pressure, it is clear that a test of the same nature will be more satisfactory than any other.
Drains ought to be tested before they are Covered with concrete, so that defects can be easily repaired. Long drains must be tested in sections, so that the head of water in any part does not exceed 8 or 10 ft. Excessive pressure may burst the joints, as the test must obviously be applied long before the cement has attained its ultimate strength. The resistance of neat cement to direct tension will in a few hours be much greater than the water-pressure here recommended, but it is the adhesive rather than the tensile resistance which is called into play, and the smoothness of many drainpipes prevents the cement adhering firmly. in a short time; for this reason, the spigots and sockets of drainpipes ought to be roughly scored before being burnt. A quick-setting cement allows the water-test to be applied almost as soon as the drain is laid, but such cement may expand in setting and burst off the collars of the pipes. It is better to use a cool slow-setting cement, and to allow as long a time as possible (say, two days) to elapse before the test is applied. Admixture with sand retards the setting of cement, and it is often convenient to use a rich mortar (say 1 part of cement to ½ or ¼ part of sand) in order that the test may be carried out and the trench filled up without loss of time.
To apply the water test, the lower end of the length to be tested must be stopped, usually by means of a drain-plug, and a bend and stand-pipe not less than 2 ft. high must be fixed at the upper end, and the drain must then be filled with water. The water should remain in the drain at least half an hour, and if at the end of this time it retains its original level in the stand-pipe, the drain may be approved. If the level is lowered, the leaks must be discovered, the water drawn off, the joints repaired, and the drain again tested. Manholes ought to be tested in a similar manner, after plugging the drain-pipes and traps. The London by-laws clearly specify that every means of access to a drain must be watertight up to the level of the adjoining ground-surface or roadway.
When a length of drain has been tested with satisfactory results, the concrete covering can be laid, and the water may be allowed to remain in the pipes till this is done. If the level of the water remains constant, no disturbance has taken place, and the filling of the trench can be completed. Where the drains are not covered with concrete, the trench must be filled very carefully, the material being well packed around the pipes. In many cases the test may with advantage be repeated some time after the trenches have been filled, so that defects caused during this operation or by settlement of the foundations may be discovered before the building is occupied.
Drain plugs or stoppers are of two principal kinds—disc-plugs and bag-stoppers. The disc-plug or stopper shown in figure 76 is known as Jones's patent expanding screw stopper. It consists of two discs of galvanised iron or brass, between which a moulded rubber ring is fixed. By turning the handle in one direction the discs are brought closer together, and the indiarubber is thereby compressed and forced outwards from 1 in. to 3 in., according to the size of the stopper. The illustration shows a centre outlet fitted with an indicator; if the latter is suspended from a fixed point, the slightest leakage in the drain can he detected by observing the gauge. Bag-stoppers are more portable. The bag is cylindrical in shape (figure 77), and after being placed in the drain is inflated by means of a small air-pump until the required pressure is attained. The escape of air is prevented by turning the tap at A. Bag-stoppers are also made with central outlets, so that part of the testing water can be allowed to escape before the air is released from the bag.
Very long outfall drains without manholes may require one Or more access-pipes at convenient places for the insertion of the stopper, so that the drain can be tested in two or more sections. A cross section of Winser's access-pipe for this purpose is given in figure 78 It has an arched stoneware cover resting In a deep oblong socket. When the test has been applied, the cover is placed in position and entirely covered with cement-mortar. The drain between an intercepting chamber and a sewer can be easily tested if an access-pipe of this kind is laid in the drain near the sewer.
The Smoke Test has been very largely adopted, but is not as certain as the water test. After stopping all openings, pungent smoke is forced into the drain by means of a smoke machine connected with one of the plugs. This method is not without its advantages, but the water test is now generally preferred by sanitarians. Smoke rockets are useful for the preliminary testing of old drains, and may reveal sufficient defects to induce the building owner to decide upon a new system of drainage, but the smoke test, however applied, is not always satisfactory.
The pneumatic test consists in pumping air into the stopped drain until a certain pressure is attained. This pressure is recorded on a gauge, and any reduction after pumping has ceased shows that there is a leak, but the position of the leak cannot be located as easily as with the water test or smoke test. This method has not been generally adopted.