ROTARY PISTON GAS METERS

It is a positive displacement type of meter, passing pre-determined volumes of gas by means of two counter-rotating impellers. Four times per each revolution a fixed unit of volume is displaced through the measurement chambers. The flowing gas volume is proportional with the number of revolutions of the output. The output of this model Rotary Gas Meter is a special designed magnetic coupling where one part is fitted inside the meter body and the associated part is in the read-out unit. The read-out unit can either be a mechanical index or an electronic index with or without volume conversion and communication functions.

The shape of the meter body, the square shape of the impellers and the rigid supporting front plate makes this meter fully insensitive for pipe-line stresses caused by misalignments. With conventional type of rotary meters, pipe-line stresses often causes the impellers to block or serious degradation in performance.
The typical a-symmetric design of the impeller tips allows considerable increased turn down ratios. Since the shape of the impeller tip is at least two times larger compared to conventional impeller tips, the unregistered leakage through the meter is significantly reduced resulting in an enlarged turn down ratio without making the meter more sensitive to installation stress or dirty gas.

The larger tip also means that the clearance at the tip can be enlarged allowing larger machine tolerances of the measuring chamber of the meter body. Except for the impeller and the timing gears, the remaining parts like meter body, front cover, coupling, etc. can be manufactured locally according to local applicable directives and quality standards. It complies with EN12480 standard. Since tolerances of the meter body become less important due to the use of a-symmetric impellers, the quality of the locally produced and assembled meter will still meet the highest requirements.

The robust design of the RGM is quite visible:

• The shape of the meter body
• The shape of the impellers
• The design of the main shafts and its bearings
• The material selected for the meter body
• The overall dimensions of the meter
• No bending, no twisting, no torque, no vibrations.

The bearings are placed on the outside of the timing gears, which allows the connection between the impellers and the timing gears to be much stronger than conventional type of rotor meters. Consequently, these rotor meters are far less sensitive for improper start up.

RGM series rotary meters are designed to provide maximum comfort for the user. All functions, like index, oil glasses and oil filling plugs are accessible from the front side. This also makes it possible to install the meter with the back against the wall, and as such saving space in, for instance basements installations.
All the major parts are can be replaced on site without special tools. The entire mechanism containing the impellers, timing gears and bearings can be pulled out of the meter and re-installed without losing performance. The front and rear bearings can be replaced in less than 10 minutes.

To eliminate all restrictions for future upgrades of any kind, RGM rotary are equipped with a hybrid magnetic coupling. Using a special designed magnet inside the meter, the reading of the meter can be changed from “mechanical” (magnet is used to drive the follower magnet of the mechanical index) to “electronically” (magnetic field orientation is used to activated pulse wires for detecting the rotation, direction and position of the impellers) by just “one click”.

A typical phenomenon with Rotary meters is the cause of pulsating flow at the outlet of the Rotary Meter as gas passes through the set of impellers. The effect of such pulsations increases with pressure and flow and hence can produce such strong pulsations that their related resonance can become destructive in the metering station. These pulsations actually will limit the achievable Q max of the Rotary Meter. Also the accuracy is affected by pulsations since the pressure in the measuring chambers, and as such the displaced volume, is varying.

Pulsation is indicated as a sine wave (blue and green sine wave). To compensate for the pulsation sine wave, the flow is divided in the meter and measured by two measuring chambers 45 degree (180 degrees in terms of the sine wave) phase shifted. The result is that nearly 100% of the pulsation is compensated (superposition of to 180 degrees shifted sine waves). The result of the compensation is indicated by the yellow sine wave. The residual sine wave is significant smaller compared to pulsation generated by Rotary Meters with just one measuring chamber. The elimination of the pulsation increases the lifetime of the meter reduces the noise and improves the accuracy significantly.

Another advantage of this dual impeller concept is that the meter becomes very rigid by the additional double bearing support between the impellers. The combination of dual impeller, square impellers and the improved position of the timing gears make the meter very suitable for high pressure applications.

The pressure loss relates to the turbine’s running, the friction in the pipe and the speed and director of the liquid.