Rotary piston meter
Rotary piston meters are direct volume meters, also called displacement meters . The eponymous component is the annular piston. It is a cylindrical cap with a slit in the jacket. The movement of the annular piston is guided over this slot with a separating web in the measuring chamber. There is a hole for the measuring shaft journal in the center of the cap cover. When the measuring liquid flows through the measuring chamber, the center of the piston rotates and drives the measuring shaft like a crank drive . The amount of liquid that has flowed through and the number of revolutions of the measuring shaft are proportional.
It is crucial that all cavities in the measuring chamber parts are always completely filled and emptied when flowing through. Gap losses on the outer wall and the divider guide must also be taken into account.
Only incompressible media with low viscosity can be measured. A common example is the domestic water meter .
function
The function of the annular piston is based on the continuous delimitation of defined partial volumes of the volume flow in the measuring mechanism by continuously filling and emptying the measuring space.
The measuring chamber consists of the walls of the measuring chamber and the moving part, the annular piston. The annular piston is driven by the pressure difference between the inlet and outlet side of the medium. The speed is transmitted by means of a magnet and magnetic field sensor. The annular piston, which is U-shaped in cross section, is guided with its piston or guide pin in an annular space on the measuring chamber bottom and cover and also with its slot on the partition. There are entry and exit openings on both sides of the wall. They are sealed by the annular piston and the partition.
The fluid flowing in fills the crescent-shaped spaces and turns the annular piston. As you move on, this filled space with the exit area becomes free and emptied again. Since both crescent-shaped spaces, the inner and the outer, are offset from one another, there is no so-called dead center during the piston movement . The piston moves continuously according to the measuring current. Magnets in the ring piston activate a reed switch or a semiconductor relay to detect the flow rates.
The advantage of the rotary piston meter is that even at very low flow rates (drop quantities), a display is provided. The accuracy is generally better than with multi-jet water meters. Due to their structure, however, rotary piston meters are more sensitive to dirt particles (abrasion, blocking). The rotating piston makes audible noises at high flow rates.
Executions
According to the state of the art, there are three variants of how the medium to be measured is to be fed through the rotary piston meter.
In the first variant, the water flows in through a sickle-shaped opening in the bottom of the measuring chamber and out again through a further sickle-shaped opening in the bottom of the measuring chamber. The water flow exerts unilateral forces on the annular piston, so that it experiences increased friction and wear . Both affect the measurement accuracy.
In the second variant, the water to be measured flows in through a sickle-shaped opening in the bottom of the measuring chamber and out again through a likewise sickle-shaped opening in the cover of the measuring chamber. The inlet and outlet openings can also exchange their positions. Here too, one-sided forces are exerted on the annular piston, which have an unfavorable effect on the measurement accuracy.
In the third variant, an inflow opening is provided both in the bottom and in the cover of the measuring chamber, through which half of the water to be measured flows into the measuring chamber. Rectangular slots in the jacket of the annular piston and in the jacket of the measuring chamber form the outlet openings. Due to this water flow, the axial flow forces acting on the annular piston largely automatically compensate each other.
application
Water supply
Use as a calibrated domestic water meter. Usually only the measuring insert is exchanged when changing the meter, so that the housing can be left in the line.
Type WVG
Dry runner for cold water.
- RTK
- Nominal flow rate Qn 1.5 m³ / h Nominal diameter DN 15 (1/2 ")
- Nominal flow rate Qn 2.5 m³ / h Nominal diameter DN 20 (3/4 ")
- Metrological class: C
- Operating pressure: 16 bar
- Operating temperature: max. 40 ° C
- Display, counter rotatable through 360 °
- Counter evacuated
Industry
The rotary piston meter has all the design-related advantages that have been proven in many process applications with both low-viscosity liquids such as solvents and chemicals and higher-viscosity liquids such as oils, fats and basic foodstuffs.
The construction materials used in industrial applications are made of stainless steel . A wide range of chemical applications can be covered with stainless steel . There are various corrosion-resistant sealing materials for this purpose. For most oil products, crude oils and also refined products, a cheaper aluminum variant can also be used. The maximum operating temperatures are normally 120 ° C. Special designs up to 300 ° C are available for even higher operating temperatures.
For special applications, versions for high operating pressures of up to 350 bar are available, which are used in hydraulic systems, for example.
The measuring span of the individual sizes is up to 1:70. The time interval between the pulses is a measure of the flow rate and counting gives the total amount. On-site flow indicators, counters, dosing controls etc. are available for the evaluation, which are also possible in explosion-proof design.
advantages
- Measurement of liquids of high and low viscosity
- Regardless of the flow profile , conductivity or damping
- Measurements at high operating temperatures
- Measurements and dosing without auxiliary energy sources
- Mobile operations, e.g. B. also on vehicles
- available with external heating
- Easy installation, as no undisturbed inlet and outlet sections are required
- Long durability
- Rotary piston meters are characterized by their reliable function in all installation positions and excellent measuring accuracy. The measuring accuracy is given as 0.5% of the measured value.
- If you want to read from a distance, appropriate modules can be attached at any time. This guarantees the reaction-free recording of the consumption quantities and magnetic manipulation security.
- Good acceptance probability in the case of sampling procedures to extend the calibration period if none of the tested meters are blocking.
- Fast maintenance possible thanks to easy disassembly
- In contrast to impeller meters, with rotary piston meters there is no lag, which can reduce the accuracy of an impeller meter, especially with small flow rates.
disadvantage
- more sensitive to foreign bodies (e.g. grains of sand) compared to single-jet or multi-jet meters. If such foreign bodies are washed in, they can block the annular piston.
- Noise development, especially at higher flow rates.
literature
- European Water Framework Directive 2000/60 / EC
- Martin Bantel: measuring device practice; Functions and use of measuring devices . Hanser Fachbuchverlag, September 2004
- Karl Walter Bonfig: Technical flow measurement with special consideration of novel flow measurement methods , 3rd edition, Vulkan-Verlag GmbH, Essen, 2002