Venturi flow measurement
The venturi effect refers to a level readings based on mathematical algorithms and the flow measuring device in conjunction with a venturi channel. Venturi flow measurement is named after the Italian physicist Giovanni Battista Venturi (1746–1822).
Working principle
The Venturi channel is a channel section with a lateral cross-sectional constriction and an optional elevation on the channel floor. It is used to measure the discharge and is one of the hydraulic flow measurement methods (such as the measuring weir). Venturi canal measurements were most frequently used in the inlet or outlet of sewage treatment plants.
As a result of the narrowing of the cross-section, hydraulic discharge control occurs in the flow. This is synonymous with the occurrence of the critical depth or limit depth. The underflow of the cross-section with the limit depth is a piece with a running outflow if there is no backwater (is a basic condition for the function of the Venturi canal). This rushing runoff keeps the influences of the underwater at bay, so that a steady relationship between the headwater level and runoff applies through the runoff control. This relationship is called the discharge curve of the Venturi canal (English rating curve or head-discharge relation ). It is used in discharge measurement to derive a discharge from the measured upper water level. The ideal measuring range is 10 l / s to 5 m³ / s.
The entire measuring flume usually consists of 5 parts, the inlet or calming section, the constriction or warping section, the bottleneck cross-section (also called throttle section, can be short or longer), the widening section and the trailing section.
The water level is typically measured without contact using ultrasonic echo sounders upstream of the start of the warping. The reference level for the water level is the bottom height in the bottleneck cross-section (not the bottom below the sensor!). The discharge curve is stored in the transducer of the ultrasonic measuring device as a formula or polygon (hQ value pairs in table). The converter converts the measured height with this discharge curve (also called linearization curve) into a discharge. As a rule, the converter also contains an integration function so that the converter integrates the instantaneous values of the discharge (Q e.g. in l / s) into quantities (e.g. m³). As a rule, the converter outputs a pulse (usually as a relay contact) to an external counter for each unit quantity reached. The unit quantity for the pulse output is also called the pulse value and can be specified by the installer in the converter (usually 1 pulse per cubic meter).
Different basic types are used for the Venturi ducts, which can be subdivided:
- Standard channels according to DIN 19 559, part 2
- Type-calibrated channels, which are installed as prefabricated parts, in different shapes Khafagi-Venturi, Parshall-Gutter, Palmer-Bowlus-Gutter etc.
- Special forms
The most important effect of this typification concerns the discharge curve. For standard venturi channels according to DIN 19559, the discharge curve can be calculated using a formula given there. For the other standard formers, the discharge curves are given by the manufacturer as a formula or curve (table).
Applications
Mainly sewage treatment plants:
- Raw sewage
- purified wastewater
- Muddy water
Relief measurement at rain relief
Measuring systems
In the past, a large number of different systems were used to measure the water level, ranging from analog systems with floats and potentiometers to air bubbling and the first simple ultrasonic systems. Today only microprocessor-controlled devices with non-contact ultrasonic sensors are in use.
advantages
In the case of wastewater flow measurement, the Venturi duct is gradually being replaced by magnetic-inductive flow meters. For details, see also: DIN 19 559. However, it still has advantages due to the accessibility and verifiability of the measuring section and the good verifiability of the system components. If a height control point was created in the headwater of the Venturi Canal, z. B. the flow can be measured at any time with a meter stick and the discharge curve.
disadvantage
- The entire measuring channel (see above) requires a considerable length of approx. 15 times the channel width.
- In the upstream water there must not be any built-in components that would interfere with the flow (e.g. sampling suction systems).
- Under no circumstances should backwater from underwater occur over a long period of time.
- The relevant cross-section is reduced by the sewer skin growth in the constriction, so that systematic and considerable measurement errors occur in the direction of an overestimation of the runoff. Regular cleaning is therefore necessary.
- For raw sewage, the smallest measurable flow is around 5 l / s. The minimum water depth for determining the runoff is 3 cm.
- Danger of deposits in the headwater channel, which must also be countered by cleaning.
literature
- WH Hager: Modified Venturi channel . In: ASCE, Journal of Irrigation and Drainage Engineering , Vol. 111, 1985, pp. 19-35
- DIN 19 559 parts 1 and 2, flow measurement of waste water in open channels and gravity lines.
- ÖNORM B 2402: Flow measurement in open measuring channels; Venturi channels
- Self-Monitoring Ordinance - EÜV; Ordinance on self-monitoring of water supply and sewage systems
- ATV-DVWK regulations, GfA Hennef