## V2

Note: The word water is used to represent the liquid being pumped. The provisions are applicable to the pumping of other liquids, provided the gages and connecting lines contain the liquid being pumped.

example The pressure at gage connection a in Figure 22 is below atmospheric pressure. The following equation applies:

The negative sign of Zs indicates that the gage zero is located below the datum.

example The pressure at gage connection a in Figure 23 is below atmospheric pressure, and the line between either the discharge or suction pipe and the corresponding gage is filled completely with air. The following equations apply:

-hgs

2g V2

Note: If a connecting pipe is air-filled, it must be drained before a reading is made. Water cannot be used in the U tube if either hdg or hds exceeds the height of the rising loop.

measuring head with mercury gages* The following examples illustrate the use of mercury gages for measuring head in a centrifugal pump arrangement.

example In Figure 24, the gage pressure is above atmospheric pressure and the connecting line is completely filled with water. The following equation applies:

Wm V

h = -m hg + Z + V W g 2g where Wm — specific weight (mass) of mercury, lb/ft3 (kg/liter)

W — specific weight (mass) of liquid pumped, lb/ft3 (kg/liter) hg — suction or discharge gage reading, ft (m) Hg

The quantities h, Z, Y, and V without subscripts apply equally to suction and discharge head measurements.

example The gage pressure in Figure 25 is below atmospheric pressure, and the connecting line is completely filled with air, with a rising loop to prevent water from passing to the mercury column. The following equation applies

Wm v h —-f- hg — Y + V W g 2g measuring head with differential mercury gages* Figure 26 indicates a centrifugal pump arrangement in which a differential mercury gage is used to measure head. When hs

*Note: The use of mercury is restricted because of its toxicity. Alternative liquids or alternative measuring devices are commonly used to avoid mercury contamination.

this type of gage is used and the connecting lines are completely filled with water, the correct equation is

In addition to the differential gage, a separate suction gage can be used, as shown in Figures 22 and 25. The equation in this case is

hs = W hgs - Z + 2g measuring head with bourdon gages An example of a centrifugal pump arrangement that uses calibrated bourdon gages for head measurement is shown in Example 3 of Figure 27, with the gage pressure above atmospheric pressure. The distances Zs and Zd are measured to the center of the gage and are negative if the center of the gage lies below the datum line.

MEASURING HEAD ON VERTICAL SUCTION PUMPS IN SUMPS AND CHANNELS In vertical-shaft pumps drawing water from large open sumps and having inlet passages whose length does not exceed about three inlet opening diameters, such inlet pieces having been furnished as part of the pump, the total head should be the reading of the discharge connection in feet (meters) plus the vertical distance from the gage centerline to the free water level in the sump in feet (meters) (Example 2 of Figure 27).

Power Measurement The pump input power may be determined with a calibrated motor, a transmission dynamometer, or a torsion dynamometer. The Hydraulic Institute ANSI/HI 2000 Edition Pump Standards (Reference 13) are generally used as the basis for most power measurement procedures.

calibrated motors When pump input power is to be determined with a calibrated motor, the power input should be measured at the terminals of the motor to exclude any line losses that may occur between the switchboard and the driver. Certified calibration curves of the motor must be obtained. The calibration should be conducted on the motor in question and not on a similar machine. Such calibrations must indicate the true input-output value of motor efficiency and not some conventional method of determining an arbitrary efficiency. Calibrated laboratory-type electric meters and transformers should be used to measure power input to all motors.

transmission dynamometers The transmission, or torque-reaction, dynamometer consists of a cradled electric motor with its frame and field windings on one set of bearings and the rotating element on another set, so the frame is free to rotate but is restrained by means of some weighting or measuring device.

When pump input power is to be determined with a transmission dynamometer, the unloaded and unlocked dynamometer must be properly balanced prior to the test at the same speed at which the test is to be run. The balance should be checked against standard weights. After the test the balance must be rechecked to assure that no change has taken place. In the event of an appreciable change, the test should be rerun. An accurate measurement of speed is essential and should not vary from the pump rated speed by more than 1%. Power input is calculated as shown later in this section under "Computations."

torsion dynamometers The torsion dynamometer consists of a length of shafting whose torsional strain when rotating at a given speed and delivering a given torque is measured by some standard method. When pump input power is to be determined with a torsion dynamometer, the unloaded dynamometer should be statically calibrated prior to the test. This is done by measuring the angular deflection for a given torque.

Immediately before and after the test, the torsion dynamometer must be calibrated dynamically at the rated speed. The best and simplest method to accomplish this is to use the actual job driver to supply power and use a suitable method of loading the driver over the entire range of the pump to obtain the necessary calibrations. The calibration of the torsion dynamometer after the pump tests should be within 0.5% of the original calibration. During the test runs the speed should not vary from the pump rated speed by more than 1%. The temperature of the torsion dynamometer during the test runs must be within 10°F (6°C) of the temperature when the dynamic calibrations were made. All torsion dynamometer calibrations should be witnessed and approved by all parties to the test. In the event of a variation greater than allowed, a rerun of the test must be made. Power input calculations are shown in the following text.

Speed Measurement The speed of the pump under test is determined by one of the following methods:

Revolution counter (manual or automatic)

Tachometer stroboscopic device

### Electronic counter

In all cases, the instruments used must be carefully calibrated before the test to demonstrate that they will produce the required speed readout to within the desired accuracy. Accepted accuracy is usually ±0.1%. Should cyclic speed change result in power fluctuations, at least five equally spaced, timed readings should be taken to give a satisfactory mean speed.

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