Hong Kong Builder
SANITARY WORKS IN LARGE BUILDINGS
By W. C. Easdale, F. R. San.I., F.I.S.E., M.S.E., and
Donald Easdale, M.S.E., AMISE.
A Paper read at a Sessional Meeting of the Institution held at Caxton Hall, Westminster, S.W.1, on Friday, 14th October, 1938, the President, Lieut.-Col. F. C. Temple, C.I.E., M.Inst.C.E., M.I.Mech.E., F.R.San.I., occupying the Chair.
SCOPE OF PAPER.
The term "large buildings" in the title of this paper refers to large banks and other office buildings in which staffs varying in number from 400 up to 4,000 are employed. In order to keep the paper within reasonable limits, it is confined to soil and rainwater drainage and plumbing works and to special features which do not occur in ordinary domestic sanitation.
DRAINAGE SYSTEM.
Owing to the very high value of the sites of most of these buildings, every effort is made to obtain the greatest possible amount of floor space in the limited area available, with the result that, in addition to eight or more floors above ground level, two and sometimes three floors below ground level are provided. This means that the lowest floor is usually well below the level of the sewer in the street and the drains have to be suspended on walls or from floors.
In preparing the layout of the drains, the Consulting Sanitary Engineer has to work to the preliminary drawings of the Architect and the Structural Engineer, and it often happens that the most suitable line for a drain is obstructed by a beam or by some architectural feature. In some cases a less suitable, but still possible, alternative line can be adopted, but where no alternative line is possible, it becomes necessary to ask the Architect or the Structural Engineer to make alterations in their drawings. An arch may be raised or a sub-ceiling provided by the Architect. A beam may be reduced in depth by substituting two shallow joists for one deep one, and in some cases holes have had to be cut in the webs of deep beams for the drains to pass through them. When it becomes necessary to make the connection to the public sewer before the building is completed, regard must be had to the fact that such buildings at that stage are still liable to settlement although it may be very slight. A settlement of one quarter or one eighth of an inch would be liable to crack even a cast iron drain. It is therefore desirable to provide a flexible joint on the sewer connection immediately outside the building or to insert a copper bend.
Having arranged a satisfactory layout for the drains, the question of the sizes of the pipes has to be considered. In older buildings a daily consumption of 10 to 12 gallons of water per head per day was usual, In recent years, however, there has been a tendency to provide canteens or, in some cases, luncheon clubs on the premises for the use of the staff and the water consumption has risen to 16 and 18, and in extreme cases to 20 gallons per head per day. Another point to be considered is that there are certain rush hours when the lavatories are used for short periods by large numbers of the staff.
In estimating the maximum flow to the drains from the sanitary fittings it may be assumed that half of the fittings are discharged each minute. Hence the total of the numbers of water closets, basins and sinks connected to one drain may be taken as the maximum rate in gallons per minute. Urinals may be ignored. Baths are unlikely to be emptied during peak periods.
Another and usually the larger factor in calculating the sizes of the drains is the provision to be made for taking rainwater. The whole site of the type of building under discussion is normally covered with imper- meable surfaces. In cases of very great area where the drains are of appreciable length and capacity, it is reasonable to calculate the "time of concentration" and use the Ministry of Health formulæ for rainfall intensity. For sites of the order of one acre, the time of concentration is too short for the formule to apply and allowance is made in the capacity of the drains for rainwater run off at the full maximum rate of rainfall, without any allowance for percolation or for time lag in reaching the drains from the roofs. In several instances provision has been made for taking rainwater at the rate of 2 inches per hour and, in one case, at the rate of 4 inches per hour as a rainfall at the rate of 31⁄2 inches per hour for several minutes was on record.
A third component of the flow in the drains is provided by discharges from the heating and air conditioning plants. These are usually inter- mittent but may be at high rates. Where the proposed rate is such as to increase materially the required size of drain, arrangements can usually be made to limit the rate and to prohibit pumping during the peak flow from lavatories and during heavy rain.
With regard to the gradients at which the drains are laid, it some- times becomes necessary to adopt flatter gradients than those usually prescribed for small buildings, in order to avoid obstructions or in order to reach the public sewer by gravitation. Owing to the larger volumes discharged during rush hours, the maximum self-cleansing velocity occurs several times every day and there should be no hesitation in adopting the following gradients if needed, viz.:-4 in.-1 in 60, 5 in.-1 in 90, 6 in.-1 in 150, 7 in.-1 in 200, 9 in.-1 in 280, 12 in.-1 in 400.
SEWAGE PUMPING PLANT.
In buildings where it becomes necessary to provide sanitary accom- modation below the level of the public sewer, suitable pumping plant must be installed. Pumps should always be in duplicate, arranged in such a manner that one is normally in operation while the other acts as a stand-by and automatically comes into operation if the first should fail. The best arrangement is to have two separate complete sets, each consisting of a closed cylinder to receive the sewage, with its outlet at the bottom connected to the suction pipe of an electrically driven centri- fugal pump, automatically controlled by a float in the sewage cylinder. The two cylinders should be connected near the top so that if one pump fails the sewage will overflow into the other cylinder and be dealt with by the stand-by pump.
In the foregoing it has been assumed that no sanitary fittings will be fixed on the lowest floor of the building. This is very desirable, as if drains have to be laid in the lowest floor, the necessary sewage and pump wells have to be constructed below the foundations, and it is often extremely difficult to make them watertight.
The delivery pipe from the sewage pumps is usually connected into the main gravitating drain from the upper floors. This connection should be made preferably into an inspection chamber which is well ventilated or a separate vent pipe taken up off the top of the delivery main.
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The two sewage cylinders should also be connected to a separate ventilating pipe carried up to the roof or into a conveniently situated soil vent pipe.
SOIL SYSTEM.
For the sizes of soil and vent pipes, as for drains, calculations might be based on the flow capacities given in the United States Plumbing Code. These figures are based on the theory of probabilities and confirmed by practical observations. While covering soil pipes up to 8 in. diameter and vent pipes up to 10 in. diameter, they permit the connection of 64 water-closets with flushes of not less than 4 gallons to a 4 in. soil pipe. Hence, in buildings subject to the height limits of by-laws such as those of the London County Council and with the normal 2 gallon water-closet flush, the standard 4 in. soil and 3 in. vent pipes are capable of dealing with the discharge from many more water-closets than are likely to be connected to one stack.
WASTE SYSTEM.
Except where only two or three basins are connected, it is usual to use a 3 in. waste pipe and a 2 in. vent pipe. Although the number of basins connected is usually only a small proportion of the 72 permitted by the United States Plumbing Code, the use of these sizes is justified by freedom from stoppages.
Gullies for the wastes from wash-basins, sinks and baths are usually situated in areas, and any odours arising from them are liable to rise and be drawn into the open windows of offices on the upper floors. It is therefore necessary in many cases, especially for kitchen wastes, to provide gullies with sealed covers and separate ventilating pipes carried up to the roof. Rainwater pipes from eaves gutters should not be connected to such sealed gullies, as this may defeat the object of keeping smells away from windows.
ONE-PIPE SYSTEM.
The one-pipe system, which is now legal in the Metropolis, should be seriously considered in all future plumbing projects. In addition to eliminating the troublesome gulley and its drain connection, its practical effect is to dispense with the whole of the vertical piping of the waste system, including the vent pipes from sealed gullies.
The 4 in. soil pipe and 3 in. vent pipe are normally required except in the occasional case where no waterclosets are connected to a stack, when the appropriate waste pipe size may be used.
RAINWATER SYSTEM.
The question of the rate of rainfall to be allowed for was discussed in considering the drainage system.
For the eaves gutters and down pipes of an ordinary dwelling- house, a principle sometimes applied to storm water sewers may be used. In order that a self-cleansing flow may occur with sufficient frequency, the capacity is made inadequate for the two or three heaviest storms of the year. While such а storm is in progress, overflow from eaves
gutters is of no importance.
In considering the drainage of rainfall from the roofs and areas of city buildings, this principle must be entirely forgotten. Provision must be made for taking the whole of the precipitation at the maximum rate on record. Architectural and structural considerations usually indicate likely positions of down pipes, and design consists not so much in deter- mining pipe sizes as in providing a sufficient number of pipes. Except for very small areas or in positions where pipes of special shape are required by the Architect, 4 in. diameter down pipes are normal. From an examination of the plans, the largest arca draining to one outlet is noted and the run off from this area at the maximum rate of rainfall is checked to be within the capacity of the outlet.
It is to be noted that the capacity of the outlet is normally very much less than that of the down pipe. With a suitable entry, the maximum free flow down a 4 in. vertical pipe is over 800 gallons per minute. With an entry of the pipe size only, as commonly cast on eaves gutters, a head of 2 in., wasting 2 in. depth of the gutter, discharges only 100 gallons per minute into a 4 in. pipe. Thus, except in cases where the gutter and pipes carry little water and are primarily an architectural feature, an eaves gutter should have an open end dis-
charging freely into a rainwater head of which the bottom tapers down to the pipe spigot. Outlets from flat roofs and cornice gutters should be of trumpet shape and, for a 4 in. pipe, not less than 12 in, clear diameter at the top. To reduce the risk of stoppage, the grating should be a high dome. To avoid heading up on the roof, the lip of the trumpet should be set about 3 in. below the local surface, which may slope steeply to it.
In some cases portions of the building may have flat roofs, overlooked by windows the cills of which may be not more than 6 in, above the roof. In such cases, and in fact for every separate portion drained, it is essential to provide two or more outlets for the rainwater in case one should be choked.
Where a drain or the upper part of a drain receives rainwater only, it is preferable to connect the down pipes untrapped and to use trapless gulleys for surfaces at ground level. Ventilation is thus provided and air locking prevented. A trap is of course interposed before joining the soil drain or sewer. For connections inside the building of rainwater pipes to soil drains, unvented sealed gulleys may be used. Rainwater by heading up in the down pipe will break down any air-lock caused by surcharge in the drain.
MATERIALS.
Cast iron pipes may be used for soil pipes where the arrangement is straightforward, but in these large buildings this is not always the case. Occasionally these pipes have to be fixed inside the building, in odd corners, or carried across ceilings, or along corridors in sub-ceilings, and in such cases lead pipe is much more convenient than cast iron.
In any case the upper portions of soil pipes above the topmost connection, acting as vent pipes, should always be of non-ferrous material. (Continued on page 15)
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