The Service Reservoir and Its Advantages

The Service Reservoir and Its Advantages

It Gives Unvarying Flow—Necessity for Good Initial Pressure—Fire Protection First Consideration—Fallacy of Raising Pressure for Fires

THE employment of the service reservoir in regulating pressures for both fire protection and for domestic sendee is a very common practice and has many advantages to commend it in insuring uniformity of flow. The following paper embodies some important ideas on this subject:

This subject involves two distinct features of water workspractice, features, moreover, which are not always related to each other. Nevertheless, in one important particular they are very definitely connected, as will appear.

It is of course a prime requisite of municipal water supply that its pressure be steady and uniform. This implies the taking of precautions to insure unvarying flow under any and all conditions whether routine or abnormal. And so, as a general rule, the most effective and at the same time least expensive means to overcome the inconveniences attending a sudden increase in draft is the provision of a service reservoir. Such reservoirs are found in many American cities.

Advantages of the Service Reservoir

Usually they are located within or just outside the city limits and are connected with the distribution system by separate mains; that is, by lines other than those which carry the regular supply. Thus, fluctuations in demand can be cared for from the storage in the reservoir instead of by effecting a greater flow in the common supply line, for it is seldom possible to have supply mains of such a size that peak demand does not introduce a considerable reduction in pressure. Where these lines are long, this reduction may be serious for extended periods. Obviously then, service reservoirs which aid in the maintenance of pressure are highly desirable in cities whose water supply source lies at a considerable distance from the community.

As is the case with other features of water works design, the question of the use of service reservoirs and that of the pressure to be carried sometimes overlap. They may, indeed, even conflict; and therefore the advantages of each must be weighed carefully and a proper balance struck for the particular city and condition concerned. Considerations other than necessary pressures, too, must be regarded, such as the most feasible site for a service reservoir, when two or more locations are available; the size of the mains, the proximity to high value districts, the cost of power and the efficient operation of pumping stations.

A reservoir constructed at a high elevation remote from the center of distribution will provide a higher initial pressure than a reservoir at a lower elevation at a less distance, but under maximum draft the relative value of these pressures may he reversed, as the friction losses in the shorter supply mains may permit a greater sustained pressure from the nearby reservoir, than from the one at the higher elevation.

As far as concerns desirable pressures, it is a fact that the demand for good domestic service alone often governs. Many of the older systems in the country were designed in accordance with this theory, with the result that the pressures adopted were relatively low, far too low for present needs. To-day the trend is towards higher domestic pressures. In the past ten or fifteen years several of the larger municipalities have materially increased their normal pressures, ami not only in their business districts, but in outlying sections as well. Notable among these are New York. Newark, Jersey City, Syracuse, Springfield, Mass., Buffalo, Providence, Louisville and New Orleans.

“Largely upon local conditions will depend the size, the kind and the location of service reservoirs. It may be taken as an axiom that service reservoirs are desirable under all conditions, if of such size that the contents will care for material fluctuations in demand or provide for fire flow when other sources are out of commission.”

Necessity for Good Initial Pressure

It is, of course, well recognized to-day that a water system whether privately or municipally owned, is a public utility and that any failure to provide the public with water at all times is certain to draw complaints and create general dissatisfaction. If this handicap to operation is to be avoided, then the water system must furnish an ample supply at pressures sufficient to assure suitable service at all regular elevations, even though excessive peak consumption rates may be reached very infrequently. Assuming, now, that the source of supply, whether gravity or pumped, is ample, it is obvious that the higher the initial pressure the better will a good domestic pressure be maintained at a satisfactory figure during peak demands. Moreover, a high initial pressure will offset local losses due to partial clogging of service pipes to excessive lengths of conveyance because of wide streets, and to other conditions of a similar nature.

Some really large cities, such as Chicago and Detroit, have furnished a supply at a low initial pressure for many years from which circumstance it might be argued that the absence of general complaint in these cities—and in others wherein pressures average from 20 to 40 pounds—proves that pressures as low’ as these arc suitable for domestic consumption. Yet this is no proof. As in so many things, so in the matter of water pressures, what the general public knows nothing of, that the general public does not miss. Low pressures, however, do result in forcing owners of high buildings to install special pumping equipment and local water tanks, if they tire to obtain a proper supply, and “this self-service,” as it were, is costly. Furthermore, relatively low pressures often necessitate on the part of the w’ater department, establishment of “booster” or special high service to take care of elevations above the average or to serve outlying sections where a satisfactory supply cannot be sent when pressures drop 10 or 20 pounds.

Fire Protection First Water Works Consideration

It is conceded to-day that one of the essential services performed by a water system is the suppression of fire, and few communities are any longer so purblind as to provide a system adequate for domestic purposes only. In point of fact, almost everywhere the first consideration relates to the potential demand for fire protection. It is therefore evident that the question of pressures, as well as other features of the design of a water system cannot be decided solely from the standpoint of domestic service. Probably, in the average smalt town and in fact in any community where few buildings exceed in height 3 stories, 30 pounds of pressure is ample for domestic service. Yet successful fire combat always entails the concentration of enormous quantities of water about individual buildings, even when the blaze occurs in a residential section. The amount of water so diverted far exceeds any normal domestic draft and to make it obtainable, either pipe of large size must be laid or else, if smaller mains arc used, pressures must be raised high enough to overcome friction loss. Between these two requirements stands a mean, and this mean affords the ideal condition.

Fallacy of Raising Pressures for Fires

In the past many cities, particularly those in the West, to which water had to be pumped, attempted to meet the situation by increasing pressures during fire emergencies. This measure, throughout the long period of the horse-drawn, steam fire engine, with its high cost of maintenance and operation, was satisfactory enough, since, with pressures at from 90 to 100 pounds, fire companies with hose wagons could run their lines direct from the hydrants. The modern fire department, however, with its motorized apparatus, uses a pumping engine instead of a hose wagon, and this can be purchased at a figure which is hardly prohibitive even to small towns. Times literally without number automobile pumpers have proved eminently dependable; in fact, they far surpass the old steamer in their ability to deliver a large quantity of water continuously. It is therefore held, nowadays, that the raising of pressure for fire fails to bring an appreciable economic advantage, while it does unquestionably introduce a material element of unreliability. Increase in pressure for emergencies means added stress, and other troubles are sure to follow the change in direction of flow following concentrated draft. This, and the water ram due to sudden shutting down of hose lines has resulted, in a number of instances in broken pipe lines.

Height of Buildings Make High Pressures Necessary

The growing height of buildings in America is making it ever more difficult to furnish water direct from the hydrant at pressures which will give satisfactory fire service, unless special fire mains are laid. To reach throughout structures which are four stories or over in height, a pressure of at least 60 pounds at the hydrant is necessary during such times as the full fire demand is being drawn. With the most advantageous layout of mains this means a normal pressure exceeding 75 pounds. As the number of buildings of four stories and higher increases, the fire flow is required at stronger pressures to give satisfactory service, and even then pumper capacity in the fire department must be provided for extreme heights, unless the remaining pressure attains to 150 pounds or above as in a special fire main system.

Advantages of Good Hydrant Stream

Though the fire department may have equipped all companies with pumping capacity, it nevertheless is true that ability to deliver at least 10 per cent, of the required fire flow direct from hydrants materially increases protection; if the fire apparatus can respond direct to the building and use a chemical stream from the large tank, after laying a line of hose from the hydrant to the fire. With suitable pressures this line can be used direct from the hydrant to back up the chemical line in case it is not sufficient to extinquish the fire. There is always delay when the fire engine must be connected to the hydrant before a good stream can be obtained. Therefore, the ideal fire service condition is one where pressures will permit a few hose lines to be used direct from hydrants, and which also will assure a continuously satisfactory flow under the heaviest demands, both domestic and fire, wothout dropping locally to less than 20 pounds. Twenty pounds is regarded as the minimum to which fire draft should be allowed to reduce pressures, for at 20 pounds considerable friction loss can be overcome in the hydrant and suction hose and still have a positive pressure and not a vacuum on the pump of the fire engine.

Automatic Sprinklers in Fire Protection

Outstanding as a feature of modern fire protection in America is the automatic sprinkler. Without its aid the high conflagration hazard prevailing in most of the business districts of the towns and cities of this country cannot be lowered. Buildings of large area and of the high combustibility common to ordinary wooden joist construction cannot be made safe for store or manufacturing occupancy, especially if three stories or higher, except by the installation of automatic sprinklers; the same is also true to a lesser degree of fire proof buildings if the contents are readily combustible. Mandatory requirements for such installations are embodied in the labor laws of numerous states and cities, and in the past year legislation has been introduced in one of the eastern states for compulsory sprinklering of apartment houses and hospitals.

Minimum Pressure Necessary for Sprinklers

Recognizing, then, that automatic sprinklers are essential, and that to be of value an ample and dependable water supply must be provided for them, and further, that for the individual to provide this water supply in each case the cost would be excessive—in view of all this, it is plain that modern water systems must be so designed as adequately to feed these systems.

Regulations for automatic sprinkler systems state that:

“One or more connections from a reliable public water system of good pressure and adequate capacity furnishes an ideal ‘primary supply.’ A high static water pressure should not, however, be a criterion by which the efficiency of the supply is determined. The supply from city main should not be considered standard unless a hydrant test of the main capacity with at least 500 gallons a minute flowing indicates sufficient residual underground pressure to give at least 12 pounds under the roof; this requirement of flow is the minimum for equipments having 200 heads or less in one fire area; larger areas will require additional flow in gallons per minute.”

Analyzing this stipulation, it is apparent that the minimum pressure permissible, allowing five pounds to a story, is, for:

To this must be added an allowable drop, due to the flow in the local mains; for a 6-inch main, 600 feet long and fed both ways, the loss caused by a 500 gallon flow to a central point would be only about 1 pound, indicating that the above figures might be adopted as the needs of the system if sprinklers are to be supplied. But to be of full value this minimum of 12 pounds on the top line of sprinklers, with a flow of 500 gallons, should be available during periods of maximum consumption draft ; an ideal condition would be to have this sprinkler supply available even at a time of heavy fire draft in an adjacent building. It is not always possible to build a distribution system such as to assure sprinkler supply under all conditions, but in determining upon the pressure to be carried it is reasonable to allow a material increase to care for losses due to peak consumption demand, or moderate fire demand, and also to care for a greater flow than this minimum of 500 gallons; this increase should be at least 100 per cent, in the average city or town, giving required true static double the pressures indicated above for buildings of various heights.

Where Six Atlanta Firemen Perished This mass of burning baled cotton was precipitated upon the firefighters below without warning, when the floors of the Jass Manufacturing Company's warehouse at Atlanta, Ga., gave way in a fire on May 6, as reported in the May 13 issue of FIRE AND WATER ENGINEERING. The great weight f the water soaked bales proved too great for the floor beams and joists, burned through by the flames. Besides the six men who were killed, five others, more or less injured, were rescued by their comrades.

(Continued on page 1123)

Advantages of the Service Reservoir

(Continued from page 1096)

Suggests Normal Static Pressure of 60-75 Lbs.

It is not argued that pressures should be such as to care for all buildings in a community. It is reasonable, however, to provide for a building of normal or average height, which in most cities means four or five stories. It is therefore evident that a normal static pressure of from 60 to 75 pounds is desirable for automatic sprinkler protection; such a pressure will also furnish good first aid hose stream protection from standpipes in buildings of 4 or 5 stories, and from yard hydrants about manufacturing plants.

Considering all features it appears that a pressure ranging from 60 to 75 pounds is desirable as the normal static pressure in American cities, since this pressure will:

  1. Supply domestic consumption to all except the highest buildings (over 10 stories).
  2. Permit considerable fluctuation or lowering of pressure locally during peak demand without leaving extensive sections poorly supplied with water.
  3. Insure quicker eperation of the fire department, by allowing direct hook-up of hose lines with hydrants.

Assure effective sprinkler supply to a building of 4 or 5 stories.

  1. Enable smaller service pipes to be utilized.

Afford more satisfactory flow for manufacturing processes requiring large quantities of water, for use in lawn sprinkling and for other needs.

Many existing water systems are functioning at materially lower pressure than the desirable one of from 60 to 75 pounds. Where supplied by direct pumpage, an increase is readily feasible; in some instances this would mean the cutting out of a standpipe or tank, but in a majority of cases these devices are of small capacity and are located near the pumping station; their main function has been to cushion the pump discharge. The need for them is slight, and they have been abandoned by many cities.

Effect of Raising Pressure Upon Plumbing

Experiences, in respect to effect upon house plumbing and fixtures, in various cities where water pressures have been increased are highly significant. Here is some of the testimony:

In Springfield, Mass., pressures were increased in 1910 from 35 to 135 pounds in the business district, and by the same or a somewhat lower amount throughout the city. Following completion of the readjustment the water commissioner reported for 1910 and 1911 in part as follows:

“Three serious breaks, one in old cement-lined pipe and two in mains and connections recently laid. Including the above, there was a total for the year of 73 leaks in mains and 226 service pipe leaks, with average cost of repairs of $51 and $5, respectively.”

“During 1911, thirty-two leaks in mains and 113 in service pipes; average cost per leak $19 and $7.50, respectively. Leaks generally attributable to the increased pressure which in many cases was where cement-lined or odd light weight pipe had not been removed.”

And these are extracts from a statement by Water Registrar Hathaway:

“The lower level, or down-town portion of Springfield was formerly supplied by the old low-service system which gave a maximum pressure of about 35 pounds. The large number of old tenement premises were located in this section. At that time the high-service supply from Ludlow gave a maximum pressure of about 135 pounds in this lower portion, but was not used for these tenements because of the old and weak plumbing.

” When the supplies were both superseded by the new supply from Little River, with a maximum pressure of about 145 pounds in the downtown section, it became necessary to replace the service pipes for this class of property with new and stronger pipes from the street main to the building. The cost of these services was equitably divided between the property owner and the water department. Pressure regulators were also installed between the new services and the old plumbing, the consumers paying only the actual cost of same to the department. In this manner, we overcame the objection spoken of, and the consumers having become accustomed to the new situation, we now have very little trouble.”

In Syracuse, N. Y., which increased pressures in 1894 from about 45 to 90 pounds in the business district and similarly elsewhere, the water department states that, while no record of breaks was kept, not more than 3 or 4 ruptures could be charged to the increased pressure. Leaks in services and plumbing were too infrequent to be of much concern; troubles were practically over in less than a year. Similarly increase in pressure was effected in the districts of Him wood Avenue and University Hill, early in 1910, by feeding from a new standpipe under an increased head of 47 pounds. In the nine months following this change there were 4 breaks in mains and 4 service blow-outs, and there is considerable doubt as to two of the former and three of the latter being due to increased pressure, as they occurred about six months after the change was made. With inferior light weight plumbing, as was originally installed in these districts, the trouble experienced was surprisingly slight.

Elntira, N. Y., advanced pressures in 1912 from about 25 pounds to 75 in the business district and similarly elsewhere. F. H. Shaw, engineer in charge of changes, reports only three cases where cast iron pipe gave way and in all three there were indications of previous cracks. Numerous leaks or blow-outs in plumbing, but no serious damage, were reported. The alterations were made during very cold weather and for two weeks many frozen services developed, some of the bursts doubtless being attributable to frozen pipes rather than to increased pressures.

In the Borough of Manhattan a number of changes from low to high service have been made during the past few years in order to secure a better supply to certain districts. Notice was given to house-owners of the proposed increase in pressure, so that plumbing might be looked over and, if necessary, repaired. Very little trouble resulted. Similar changes since have been made in Brooklyn and the Bronx.

Albany, N. Y., in its business section increased pressures in 1910 from 60 to 90 pounds, with no resultant trouble; while Louisville, Ky., jumped its poundage from 40 to 70 in 1908 and reported similar freedom from difficulties.

Size of Service Reservoirs

Largely upon local conditions will depend the size, the kind and the location of service reservoirs. It may be taken as an axiom that service reservoirs are desirable under all conditions, if of such size that the contents will care for material fluctuations in demand or provide for fire flow when other sources are out of commission. But where the action is solely that of cushioning, the value may be negative, particularly if the elevation of the flow line is such as to preclude the carrying of suitable pressures.

For the village or small town, service reservoirs, standpipes or tanks may permit intermittent operation; the minimum storage under such a condition is that which will be necessary for fire demands during the time necessary to return the main source to operation. With allowance for depletion arising from domestic draft during the normal shut-down period, it is usually sufficient to provide for a 1-hour fire draft. Where, however, the storage is intended to provide part of the fire draft, even with the main service in operation, greater quantity should be provided. The minimum desirable in any service reservoir is a capacity equal to the fire demands for a period of 10 hours in places of over 2,500 population and 5 hours for smaller places; to this should be added a quantity equal to the peak hourly demands over the normal maximum day.

With further increase in size of service reservoirs various features of reliability of supply are at the same time cared for. Under all but the most exceptional conditions a storage equal to 5 days’ consumption at a maximum 24-hour rate, will assure a supply—especially with proper operation to conserve it— during the time required to repair a pump or other essential piece of equipment, or to fix a main pipe line. To offset the hazard of a non-fireproof pumping station, however, storage should equal 10 days’ maximum consumption.

Element in Economical Operation of Plant

Service reservoirs are an element in the economical operation of a plant, but no definite statement can be given as to the saving which may result. With elevated storage equal to the fire demand and the excess due to peak consumption, the number of wells, in a system depending upon them for a source, need only to be sufficient to furnish the maximum 24-hour rate of domestic consumption and provide for a sufficient number to be out of service for cleaning. In like manner, where the supply is filtered, the capacity of the filtration plant need only be equal to that of maximum daily consumption with one unit out of service. Equalizing storage enables the filtration plants to be operated at a uniform rate, which assures a better quality of water than when they are operated at varying rates. Where the amount of water in storage is sufficiently large, the pumps can be operated at a capacity giving their maximum economic operation.

Elevated storage becomes almost a necessity where pumping depends upon electricity for motive power. It is theoretically possible to design and install generating equipment, transmission lines and other appurtenances to provide non-interrupted service, but records indicate that this is seldom accomplished. Where a service erservoir is not part of the system, reserve equipment, depending upon steam or internal combustion engines, must be provided.

Should be Built in Two Bays

Obviously, service reservoirs must be built substantially, and they should preferably be in two bays to permit cleaning and repairs. The location of the service reservoir is of vital importance. It may be taken as another axiom of water supply that elevated storage shall be either in the high value section of a city or beyond it. as viewed from the source of supply, since under these conditions fire and domestic demands at their peak will receive supply from two directions, with a minimum drop in pressure. Also it is desirable that this storage be as close to the higher values as the topography will allow. Equally true is it, moreover, that a single place of storage, connected by large mains to the high value district, is preferable to smaller segregated reservoirs with mains from each of sufficient size only to care for peak demands locally and a small percentage of fire demands in the high value districts.

Many cities and towns are built on relatively flat ground, with no adjacent elevation upon which to set a service reservoir with a high pressure gravity feed. But this need not interpose an insuperable obstacle if such a reservoir is really needed. Recent advances in the construction of steel tanks and reservoirs. particularly in the oil trade, where containers of from 1,000,000 to 5,000.000 gallons’ capacity are becoming very common, makes it possible for these level cities to provide storage. With a height of 35 to 40 feet, a tank of 2,300,000 gallons capacity, which is a standard size, can be erected for approximately $40,000; and this includes a steel top, which may not be necessary for water-works use. Such a tank, erected even 100 feet above the main values of the community, will assure a degree of service well worth the expenditure.

In conclusion let it be repeated that desirable pressures range from 60 to 75 pounds; that the raising of pressure above these figures for fires is undesirable and introduce a hazard; and that the introduction of service reservoirs add to the adequacy and reliability of a system—although it must be remembered, the use of auxiliary reservoirs can never justify carrying on the regular water supply a pressure lower than 40 pounds.

(Excerpts from paper read before the Annual Convention of the American Water Works Association at Louisville, Ky.)

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