COMPOUND WATER METERS

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Compound meters are essentially two meters within a single housing. They are normally used in situations that require accurate measurement of cold water over a wide range of low to high flow rates. Compound meters are often used for measuring water used at apartment or office buildings, hotels, schools, hospitals, and industrial facilities.

1. Meter Designs: There are two types of compound meter design, parallel and series. The parallel type has two registers and, if either unit fails, the trouble can be detected by stoppage of its register. The series type has only one register. The unit of measurement must be specified as either gallons, liters, cubic feet, or cubic meters. Compound meters are available in nominal sizes of 2 to 10 inches, with a maximum capacity of 2,300 gallons per minute.

OPERATING PRINCIPLES: The main components of a compound meter are maincase, main line measuring chamber (turbine type), bypass measuring chamber (positive displacement type), compounding valve, and one or two registers. Compound meters operate in two modes; at low flows, only the bypass meter operate, as flow increases, the compounding valve opens, allowing the meter to operate at a higher range. In a parallel meter (Figure 4-5), the main line meter does not operate until the compounding valve opens. The bypass meter may or may not continue operating when the main line meter starts up. In the series meter, when the compounding valve is closed, water flows through the bypass meter. When the pressure differential in the bypass meter is great enough to cause the compounding valve to open, the main line meter is already running. The register is driven by a pair of ratchet drives, so that the unit that is producing q ore registration will drive the register. The main line unit is not called upon to start from rest at the changeover point, and thus loss of accuracy is avoided when the valve opens. Changeover usually begins at approximately 5 or 6 percent of maximum rating of the meter. Operating characteristics for compound meters are listed in Table 4-3.

LIMITATIONS: Rated maximum capacity for compound meters is shown in Table 4-3. Normal flow for these meters should not exceed approximately one-half of maximum capacity. Operating at maximum capacity should be limited to short periods or peak loads occurring after long intervals. Maximum pressure loss is 13.3 percent of maximum pressure for all sizes. Mechanical drive and some magnetic drive meters have changeable gears in the geartrain. Changing these gears allows the ratio between the motion of the positive displacement or turbine measuring chamber and the register to be calibrated for maximum accuracy of registration. The turndown ratio for these meters when used for cold water service covers a range of approximately 70:1 to 100:1 depending upon manufacturer, model, and size. Other limitations are as follows:

• Temperature limit is 80°F.
• Pressure limit is 150 psig.
• Installation is permanent.

Failure to observe the manufacturer’s recommendations for minimum lengths of straight pipe to be installed, both before and after the meter, may result in inaccurate measurement and premature component wear.

FIGURE 4-5. Double-Register Compound Meter

TABLE 4-3. Compound Meter Operating Characteristics

INSTALLATION: Compound meters must be installed in the flow line, upstream of the activity or outlet they are monitoring. These meters do require a minimum length of straight pipe be installed before and after the meter. Figure 4-6 shows a typical compound meter installation. When installing a meter, be sure the following checks have been made and the indicated items are available.

(a) Check for meter shutoff valve upstream and downstream of meter.
(b) Check for strainer immediately upstream of meter.
(c) Check for adequate supply of meter connection gaskets.
(d) Ensure that location of meter provides protection from frost, traffic, or other hazards that may be present.
(e) Ensure that check valves or pressure-reducing devices are not installed upstream of the meter.
(f) Ensure that check valves and pressure-reducing devices located downstream of the meter are not located closer than 5 pipe diameters.
(g) Ensure that only full-open ball, gate, or plug valves are used immediately upstream of the meter. Butterfly valves are acceptable if they are a minimum of 5 pipe diameters upstream from the meter. Gate or butterfly valves can be used downstream.
(h) Ensure that installation is in accordance with direction-of-flow markings on the meter maincase.
(i) For optimum performance, ensure that meter is positioned in a horizontal plane.

If remote reading or electronic transmitting devices are used, install in accordance with chapter 10 and the manufacturer’s instructions. A bypass pipe with gate valves is recommended so that service will not be interrupted during maintenance.

FIGURE 4-6. Typical Compound Meter Installation

MAINTENANCE: The following inspection schedules are adequate for most installations.

1. Monthly Inspection: In addition to any instructions provided by the manufacturer, inspect meters monthly for the following conditions:
(a) Meter is operating.
(b) Noisy operation (repair or replace meter as required).
(c) Leaks (repair as needed).
(d) Cleanliness of glass cover on register dial (clean as needed).

2. Annual Inspection: In addition to the inspections in paragraph above, inspect meters annually for the following conditions:
(a) Cleanliness of meter box, housing, or pit (clean as needed).
(b) Adequate protection from freezing (provide protection at least 1 month prior to start of the season).

3. Periodic Inspection: Periodic inspection of meters Is required to determine whether they are measuring accurately. The time interval between inspections should be based on local conditions and the amount of use. The manufacturer’s representative in any particular area should be familiar with local conditions and capable of assisting in the preparation of a schedule for periodic inspection of meters.

ACCURACY: Accuracy limits for water meters have been established by industry. For the meters discussed in this section, limits are based on tests run at four different rates of flow; maximum, intermediate, changeover, and minimum. The accuracy limits for the maximum and intermediate rates are from 97 to 103 percent of the quantity measured. The limits for the changeover rate are from 90 to 103 percent. For the minimum test flow rates shown in Table 4-3, compound meters shall register not less than 95 percent of the actual quantity measured.

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