How to Select and Install Master Meters
Grouped in Two Classes as to Method of Registration—Uses of Recording Venturi—Proportional Flow Meter—How to Detect Leakage—Installation of the Fire Service Meter
THE necessity for determining the total flow of water of the supply of a city or town, in order to check up consumption and waste, is becoming more and more a need for the efficiently managed water department and any information on the subject of the master meter should be of great value. Mr. Coulter’s article sliozvs painstaking care in its preparation and a keen knowledge of the subject treated.
The se1ection and installation of what may be termed master meters, to measure the total flow at some initial point of supply, are matters receiving little attention in water works literature. Types of meters unsuited to the case in hand are not infrequently purchased, and haphazard methods of installation may be encountered in otherwise well designed systems.
Master meters may be installed to measure (a).the pumpage at a station; (b) the flow entering a system from a reservoir; (c) the discharge into a system at one or more points on the corporate limits of a municipality, or at points of delivery of a supplemental supply from outside sources to any system; (d) or the flow through private fire connections.
Grouped Into Two Classes As to Registration
The desirability of initial metered measurements to ascertain the rate and duration of pumpage, efficiency of pumping equipment, water unaccounted for, etc., etc., is generally conceded and will not be discussed here. It is proposed to consider more particularly types of meters and manner of installing.
In general, master meters may be grouped in two classes as to registration; those indicating only the volume measured since the last preceding reading, and those which record upon a chart and show variations of flow as well as the total volume measured.
For the registration of pumpage at a station, it is desirable to have the variations in pumpage recorded, as well as the total output. There are no extremely low flows to be measured by such installations and the Venturi meter, with chart recorder, is well suited for such work. When installing a Venturi meter, a straight length of pipe of the same diameter as the inlet of the meter tube, and containing no valve or fitting to create eddies, should be provided at the inlet, having a length at least six times the diameter of the meter tube for sizes up to 24 inches. This precaution does not apply to the outlet.
Water Hammer Should Be Avoided
In general, a residual pressure of not less than 12 pounds is required for the satisfactory operation of a Venturi meter. If marked pulsations or water hammer are expected in the discharge main, an air chamber should .be provided at the inlet. Pulsations interfere with the action of the meter. This is also true of all other types of meters, and should be borne in mind when installing a meter on a force main. Long pressure pipes from the meter tube to the recording instrument must be increased in size to reduce friction and should, in any case, contain no pockets where air or silt might collect. A special planimeter should be purchased with the recorder, to determine the total flow shown by the circular chart. Such planimeters may be secured from the manufacturers of Venturi meters.
A Simpler and Less Costly Type
Where money is not available for a Venturi meter, a simpler and less costly type may be used to advantage at pumping stations. This is the proportional-flow meter. A small disc meter is installed on a by-pass on the discharge main, the whole being so designed that a certain known proportion of the total flow through the main is registered by the meter on the by-pass. The proportional-flow meter is fairly accurate and offers the same advantage as to unrestricted waterway through the main, and negligible loss of head, as does the Venturi. It does not register the times and duration of pumping, however, but onlythe volume passed since the last preceding reading. The cost is much less than that of a Venturi. The proportional-flow meter, like the Venturi, is not intended to measure low flows through large pipes, but is well adapted for use at pumping stations where such conditions are not encountered.
Measuring Water Delivered to a System
The Venturi meter may be installed at a city line to measure the water delivered to the system, provided there is a large continuous flow. It may also be used on a force main supplying a small town, where the rates of consumption vary between a few gallons a minute at night and a relatively large fire flow, provided there is storage in the system, so that only pumpage to the reservoir or standpipe from time to time is measured. A Venturi meter will not follow all the fluctuations of consumption for a small town and measure fire flows. A minimum flow of approximately one lineal foot a second is necessary for the reliable operation of a Venturi meter. Fig. 1 indicates a Venturi installation at the entrance to a municipal system, the top of the concrete chamber containing the recorder being unusually thick, the manhole provided with a frost stop, and the chamber banked where it protrudes above the ground surface—all to prevent freezing of the recorder and connections. Meter chambers should be provided with means for drainage. Where the ground falls sharply away from the chamber, a Short drain to a lower point will suffice; where this is impracticable, a soak-away may be provided, as shown in Fig. 2, if the ground water level is low enough. When a meter of any kind is installed on a force main at a city line, so located that the rupture of the main may bleed the system, a check valve should be provided near the meter.
A desirable feature of the Venturi meter with recording chart is that complete or partial failure of the meter to register is at once shown by the chart, with time and endurance of the interruption. This information may save a water company money, and prevent disputes.
Master Meter to Detect Leakage
Where a master meter is required to register the night flows or leakage in the system of a small community, or to detect leakage or surrep titious withdrawals through a private fire connection, a different type of meter is required. For such service, a meter should be capable of measuring a fire flow of say 1,000 to 3,000 gallons a minute, with accuracy and a minimum loss of head, and also to register accurately ordinary rates of domestic consumption and greatly reduced night flows. To properly pass and register the maximum fire flow without undue loss of head, a large meter is necessary. A single-unit meter will register the fire flows, but will not also measure the low flows with reasonable accuracy. Sometimes a battery of small single-unit meters is used for such cases, but this arrangement is undesirable from the standpoint of fire protection.
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How to Select and How to Install Master Meters
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Large single-unit meters have their legitimate place, of course. Where a relatively large sustained flow is to be measured, as at the corporate limit of a large city, one or more large single-unit meters of the turbine type, say, might be satisfactory, unless a knowledge of the variations of flow were desired, in which case a Venturi with recording chart could be installed.
Meeting Conditions of Widely Varying Flows
To meet conditions of widely varying flows, as at the entrance to a small-community system, there are meters on the market which measure the low and ordinary domestic flows through a relatively small disc meter on a by-pass; when flows occur in excess of that which the small meter can register, the main line is automatically opened to present an unimpeded passage, measurement then being made on the principle of the proportional-flow meter. Such meters measure all flows accurately, with very little loss of head, except at the point where the change to the large meter occurs. In the best types the change is made almost instantaneously and the loss of accuracy is confined within very narrow limits. In such types, the loss of accuracy is not great in any case, not falling below 95 per cent., and that within a range of about 35 gallons a minute for a 6-inch meter. The loss of head will reach a maximum of say four pounds at the change point. Some makes show a considerably larger loss of accuracy and head at the point of change and these losses are continued through a considerable range.
Slight Loss of Head in Fire Service Meter
Ordinary domestic flows should not vary back and forth across the change point, where reduced accuracy occurs, but the fire service meter or meters should be of such size that practically all variations of domestic flow will be measured by the by-pass meter. The loss of head in a good fire service meter is so slight that, it offers no obstacle to such an arrangement. Following are the rates of increasing flow at which the change from by-pass to fire service units occurs in one type of fire service meter now on the market:
3-inch meter about 98 gallons a minute at point of change.
4-inch meter about 98 gallons a minute at point of change.
6-inch meter about 173 gallons a minute at point of change.
8-inch meter about 173 gallons a minute at point of change
10-inch meter about 225 gallons a minute at point of change.
Taking these data, if a small town consumes about 260.000 gallons a day on the average, the maximum rate of domestic flow may be taken at a rate of approximately 390.000 gallons a day, or 271 gallons a minute, indicating one 4-inch and one 6-inch fire service meter. An average of 500,000 gallons a day would mean, say, 750,000 gallons a day maximum rate of domestic flow, equal to 521 gallons a minute, which would require three 6-inch meters. Other factors will of course determine whether these meters are also large enough for fire flows in any particular instance.
Full Waterway Needed for Fire Flow
The full waterway of the same size as. the pipe line, at. times of fire flow, has advantages other than unimpeded flow with accurate measurement. The fire flow may entrain leaves, sticks and other matters from the reservoir, if the latter is of the open type, which clog the screen before a single-unit meter, and may practically shut off the flow at a critical time. The writer knows of one case where the screen of a large meter was closed in this manner by a newspaper which had been blown into the reservoir. All such matters pass freely at time of maximum flow through a properly designed fire service meter.
Straight-reading registers should not be ordered for large meters. They are somewhat easier to read than the circular registers, although the latter present no great difficulties, but the complicated mechanism required for straight reading is particularly objectionable in large meters, and is a source of trouble.
Meter Chamber and Its Fittings
Large meters are installed in a concrete chamber, with a covered opening in die top large enough to permit the withdrawal of the meter. When preparing the drawing for the chamber and connected piping. dimensions of the meter and all appurtenances should be secured from the meter manufacturer, not forgetting to obtain the distance from the center of the meter connections to the bottom of the meter. This latter is not ordinarily given by the manufacturer unless specially requested. The writer has known cases where only a plan of the meter chamber was furnished, with no sections, which resulted in the placing of a reinforced concrete floor just low enough to allow laying die pipes through the chamber. When the meter was set, its end connections were a foot or more higher than the pipes as laid, and various makeshifts were necessary to complete the installation. Valves should be provided on each side of the meter, so that the flow may be cut off at time of repairing or replacing. A by-pass, of the same size as the main pipe, should extend around the meter, from a point above the upper valve to a point below the lower valve. The by-pass should, of course, be gated.
Valves Outside of Chambers
Some departments place the valves outside the chamber, with extension top boxes, to be operated by a tee-bandied socket wrench. This permits a less costly chamber. Others place all valves inside the chamber, with wheel handles, even the by-pass valve being housed in an extension of one side of the chamber. Should it become necessary to operate the valves promptly at a critical time, the misplacing of the wrench or the presence of ice and snow, might cause considerable delay in the case of outside valves. Outside valves are, however, common.
Provision for Testing Meter
Provision should always be made for testing the meter in place in the chamber. For sizes up to 4inch, a 2-inch valve on the lower side of the meter will suffice. For larger sizes, a flanged test tee should be provided, with a blank flange plate having a 2-inch nipple, with valve or cock, tapped into the plate. The small unit of the fire service meter can then be tested through the cock. By removing the cover plate from the flanged tee. a connection can be readily improvised for testing fire flow rates.
Fire Service Meter Installation
Fig. 2 shows a fire service meter installation, with valves outside the chamber. Where the valves are placed inside the chamber, it is a good plan to provide a frost stop below the manhole cover, to prevent freezing of the valve bonnets. This liability of valves to freeze, is the principal reason for placing them in the ground outside the chamber. It is desirable to provide the frost stop in any case, as slight seepages do not then form ice masses on the meter, and the cock and nipple for testing are protected.
The type of meter last discussed is also suitable for private fire connections; in fact, it was originally designed for such service.
The members of the Triadelphia, W. Va., volunteer fire department have formed an athletic club, and a room in the firehouse has been secured for five purpose of a gymnasium.