2. 旦entur庐
tubes. These consist of a well-rounded convergent section atthe entrance， a throat of constant diameter， and a divergent section.Their accuracy is high; however，installation， unless planned for inadvance， is very difficult in the field.
3. ASMEjlow nozzle. These nozzles provide for accurate measurements. Their use is limited because they are not easily placed in a processplant; however， they are excellent for shop tests. Venturi meters and nozzles can handle about 60% more flow than orifice plates with varied pressure losses.
4. Elbow jlow meters. The principle of centrifugal force at the bend is used to obtain the difference in pressure at the inside and outside oftheelbow， which is then related to the discharge pressure.
. Turb庐ne jlow meters . The principle of this flow meter is the computa-tion of the revolutions of the turbine wheel in a given time frame.
Other techniques for measuring flow through the compressor include:
1. Calibrated pressure drops from the inlet flange to the eye of thefirststage impeller in centrifugal compressors， when such data is avail-able from the manufacturer.
2. A
flow trace technique in which Freon is injected into theconstream， and flight time between two detection points is measured.
3.
Velocity traverse techniques must be usedwhen， due to the config-uration inpiping，nozzles， or orificeplates，etc.， cannot be used.
These techniques have been described previously in the pressure measure-ment section.Usually， one of the flow-measuring devices and the required instrumentation is incorporated as a part of the plant piping. The choice of technique depends on the allowable pressuredrop， flowtype， accuracyrequired， and cost.
Nozzle arrangements for various applications vary considerably. For subcritical flow measurement at the outletend， where nozzle differential pressure pis less than the barometric pressure， flow should be measured with impact tubes and manometers as shown in Figure 20-3.
.00 Gas Turbine Engineering Handbook
NOZZLE PRESSURENOTE d NOT MORE THAN 6 D NOZZLE TEMPERATURE ONE IMPACT TUBE FOR d < 5°
FOR ANY NOZZLE
2 - MEASURING STATIONS TWO IMPACT TUBES FOR d > 5° SPACED 90 DEG.
ARRANGEMENT
FLOW EQUALIZER AND d
STRAIGHTENER (FIG. 17 D) NOT GREATER THAN 20
THROTTLE
VALVE
Dd 6 D
D APPROXIMATELY (23°)
10 D MINIMUM
Figure 20-..Flow nozzle for subcritical flow. (Power Test Code10， Compressors and Exhausters， American Society of MechanicalEngineers， 1965.)
NOZZLE TEMPERATURE NOZZLE PRESSURE 2 - MEASURING STATIONS 2 - MEASURING STATIONS SPACED 90 DEG.
SPACED 90 DEG.
BY B.T. GAGES OR HG. MANOMETER
d 6D
D
10 D MINIMUM
Figure 20-4.Flownozzle， for critical flow. (Power Test Code10， Compressors and Exhausters， American Society of MechanicalEngineers， 1965.)
NOZZLE TEMPERATURE NOZZLE PRESSURE4 - MEASURING STATIONS
2 -TAPS, SPACED 90 DEG. THROTTLE VALVE
d
d
D
PROTECTING SCREEN
2 5 D MINIMUM
Figure 20-5..ozzle for e.hausters. (Power Test Code10， Compressors and Exhausters， American Society of MechanicalEngineers， 1965.)
For critical measurement， where the drop p is more than the barometricpressure， flow should be measured with static-pressure taps upstream fromthe nozzle as illustrated in Figure 20-4. For exhaust measurements， differ-ential pressure is measured at two static taps located downstream from the nozzle at the inlet as shown in Figure 20- .
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