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.
In times gone by there seems to have been no rule, beyond the rule of thumb, for determining the size of house drains, if one may judge from the size of those so often discovered about old houses, where it is not at all uncommon to find a stone drain 4 feet high by 2 feet wide, quite large enough for a man to traverse easily. It is difficult to determine why they should have been constructed so large, but as they are usually open-jointed, the only inference is that they were intended also to serve the purpose of drying the ground by draining off the subsoil-water.
Even in these days, when glazed circular stoneware pipes are almost universally used, it is far too common to find them much too large for the purposes they have to fulfil, and 1 am acquainted with districts where the local sanitary authorities demand that a drain serving two houses shall l>e at least 9 inches in diameter. So many people persuade themselves into the belief that a drain is bound to get stopped up sooner or later, and that by adopting a large pipe they are erring on the safe side. If a pipe is securely and firmly laid, and is of sound materials and kept to the gradient, it will be as true inside as the barrel of a rifle, and any stoppage that may occur will be found to be due to accidental circumstances; if, on the other hand, the pipes are of inferior materials, insecurely and irregularly laid, and also imperfectly jointed, then, of course, they will in a very short time become made up. This imperfect class of work is so common, that it is easy to understand why there are so many upholders of the large-sized pipes. If, however, these people will only consider that, for a great part of the day, only a very thin stream of dirty water is flowing through the drain, and that if it is spread over a large surface - as it must be in the larger pipe - there will be a retardation of the flow, consequent on the increased friction from the surface of the pipe with which the stream is in contact, they will see that the solids brought down will be deposited, and stoppage will ensue. Again, the area of the pipe not taken up by the stream is necessarily filled with foul air, which increases the difficulties met with in providing means of ventilation.
By ascertaining the number of inhabitants of the house, care being taken to fix the probable maximum number so as to include visitors, it is easy to calculate what will be the probable amount of sewage to lie conveyed away. It must be remembered that domestic operations do not extend over more than sixteen hours of the day, and for long intervals in that time no waste-water is being discharged from the house; on the other hand, there are periods when the discharge is rapid. Probably the two hours immediately after breakfast will contribute at least half of the total quantity discharged during the day. The most extreme case, however, will be at the time the baths are discharged; assuming that 45 gallons of water are used in the bath, then the waste would be equal to a discharge of 15 gallons per minute, and in addition there might from other sources be discharged a further 5 gallons. Beyond this domestic sewage, provision may have to be made for the waste-water from laundries, carriage washing, the drainage from stables and cow-houses; the maximum from these sources would be at the time when a pail of water was being discharged over a carriage, and if we allow 2 gallons for this, we have a possible total of 22 gallons discharged in any given minute.
We have also seen what is the possible depth of rainfall that will have to be carried away by the drain, and having limited the area as much as circumstances will allow, it is a very simple matter to calculate the maximum quantity per minute for which accommodation must be provided in the drain. It must be borne in mind too, that this excessive quantity of rain may have to be carried away at the precise moment when the greatest quantity of water is being discharged from the house, - that is, when a bath is being let off, - so that these two quantities must be added together to give us the total for which provision has to be made. Assuming a rainfall equal to 2 inches per hour, this will give a discharge of 2.5 cubic feet per minute from 100 square yards. From these figures, it is a simple matter to calculate the quantity from any required area.
The total maximum quantity of sewage and rainfall being ascertained, the size of drain required to carry it away can be fixed by determining the amount of fall which is available, and by working out the gradient. For house drains a velocity of not less than 3 feet per second should be obtained, as this will serve to remove easily and rapidly from the premises the sewage and the solids carried along with it. This velocity is called a "self-cleansing velocity", and has been fixed upon after a long series of experiments on the flow of water, carried out by Wicksteed, Beardmore, and other well-known hydraulicians. In Beardmore's Manned of Hydrology, it is stated that the following velocities have these effects: -
feet per minute will not disturb sand with clay and stone.
feet per minute will sweep along coarse sand.
„ fine gravel.
„ „ „ rounded pebbles.
„ „ „ angular stones.
Bottom velocity (which imparts the greater motion) differs from the mean velocity in the ratio of .80 to 1, or four-fifths. The greatest discharge from a circular pipe i- when it is not quite full, - that is, when the How occupies rather more than fifteen-sixteenths of the area of the pipe, - and the greatest velocity occurs when it occupies thirteen-sixteenths.
The velocity of the flow depends upon the inclination of the pipe, and what is called the hydraulic mean depth, which is the cross-sectional area of the stream in square feet, divided by the wetted perimeter in lineal feet, the wetted peri-meter being that portion of the surface of the pipe which is in contact with the stream. For instance, in Fig. 309 the wetted perimeter would be the length acb, and the sectional area of that portion of the pipe which is filled up to the horizontal line ab, divided by the are acb, will give the hydraulic mean depth. When the inclination of a pipe remains the same, the greater the hydraulic mean depth the greater will be the velocity. The friction between the particles of water and the surface of the pipe influences the velocity, and this is a factor which has not hitherto received as much attention as its importance deserves. The amount of this friction will he governed more or less by the excellence of the glazing, and freedom from roughness and obstructions on the surface, and also by the accuracy with which the joints are made, as resistance to the flow would be caused by one pipe projecting slightly over the one next above it. It is clear, therefore, that the tatter the finish of the pipe, and the more accurately it is laid and jointed, the higher will be the velocity, and consequently the greater the discharging capacity.