Rotating mechanical meters for liquids

Rotating mechanical flowmeters derive a signal from a moving rotor that is rotated at a speed proportional to the fluid flow velocity. Most of these meters are velocity-measuring devices except for positive-displacement meters, which are quantity or volumetric in operation. The principal types are: positive-displacement, rotating vane, angled propeller meter, bypass meter, helix meter, and turbine meter.

Positive-displacement. Positive-displacement meters are widely used on applications where high accuracy and good repeatability are required. Accuracy is not affected by pulsating flow, and accurate measurement is possible at higher liquid viscosities than with many other flowmeters. Positive-displacement meters are frequently used in oil and water undertakings for accounting purposes.

The principle of the measurement is that as the liquid flows through the meter, it moves a measuring element which seals off the measuring chamber into a series of measuring compartments which are successively filled and emptied. Thus, for each complete cycle of the measuring element a fixed quantity of liquid is permitted to pass from the inlet to the outlet of the meter. The seal between the measuring element and the measuring chamber is provided by a film of the measured liquid. The number of cycles of the measuring element is indicated by several possible means including a pointer moving over a dial driven from the measuring element by suitable gearing and a magnetically coupled sensor connected to an electronic indicator or "flow computer.”

The extent of error, defined as the difference between the indicated quantity and the true quantity and expressed as a percentage of the true quantity, is dependent on many factors, among them being:

(a) The amount of clearance between the rotor and the measuring chamber through which liquid can pass unmetered.

(b) The amount of torque required to drive the register. The greater the torque, the greater the pressure drop across the measuring element, which in turn determines the leakage rate past the rotor. This is one reason why electronic readout devices have become much more common in recent years, as it eliminates this error factor.

(c) The viscosity of the liquid to be measured. Increase in viscosity will also result in increased pressure drop across the measuring element, but this is compensated for by the reduction in flow through the rotor clearances for a given pressure drop.

The accuracy of measurement attained with a positive-displacement meter varies very considerably from one design to another, with the nature and condition of the liquid measured, and with the rate of flow. Great care should be taken to choose the correct meter for an application.

The most common forms of positive-displacement meters are: rotary piston, reciprocating piston, nutating disc, fluted spiral rotor, sliding vane, rotating vane, and oval gear.

Rotary piston. The rotary-piston flowmeter is most common in the water industry, where it is used for metering domestic supplies. It consists of a cylindrical working chamber that houses a hollow cylindrical piston of equal length. The central hub of the piston is guided in a circular motion by tw'o short inner cylinders. The piston and cylinder are alternately filled and emptied by the fluid passing through the meter. A slot in the sidewall of the piston is removed so that a partition extending inward from the bore of the working chamber can be inserted.

This has the effect of restricting the movement of the piston to a sliding motion along the partition. The rotary movement of the piston is transmitted via a permanent-magnet coupling from the drive shaft to a mechanical register or electronic readout device. The basic design and principle of operation of this meter is shown diagrammatically in Figure 1.16.

Figure 1.16.Rotary-piston positive-displacement meter. Courtesy, ABB Instrument Group. 1. Lid. 2. Hinge pin. 3. Counter housing complete with lid and hinge pin. 4, Counter with worm reduction gear and washer. 5. Counter washer. 6. Ramp assembly. 7.Top plate assembly comprising top plate only; driving spindle; driving dog; dog retaining clip. 8. Piston. 9. Shutter. 10. Working chamber only. 11. Locating pin. 12. Strainer-plastic. Strainer-copper, 13. Strainer cap. 14. Circlip. 15. Non-return valve. 16. Oring.17. Chamber housing. 18. Protective caps for end threads

Reciprocating piston. A reciprocating meter can be either of single- or multi-piston type, this being dependent on the application. This type of meter exhibits a wide turn-down ratio (e.g., 300:1), with extreme accuracy of ±0.1 percent, and can be used for a wide range of liquids. Figure 1.17 illustrates the operating principle of this type of meter.

Figure 1.17. Reciprocating-piston meter

Suppose the piston is at the bottom of its stroke. The valve is so arranged that inlet liquid is admitted below the piston, causing it to travel upwards and the liquid above the piston to be discharged to the outlet pipe. When the piston has reached the limit of its travel, the top of the cylinder is cut off from the outlet side, and opened to the inlet liquid supply. At the same time the bottom of the cylinder is opened to the outlet side but cut off from the inlet liquid. The pressure of the incoming liquid will therefore drive the piston downwards, discharging the liquid from below the piston to the outlet pipe. The process repeats.

As the piston reciprocates, a ratchet attached to the piston rod provides an actuating force for an incremental counter, each count representing a pre-determined quantity of liquid. Newer devices use magnetically coupled sensors—Hall-effect or Wiegand-effect types being quite common, or optical encoders to produce the count rate.