Positive Displacement Meters

Positive Displacement Liquid Meters:

• Nutating Disk Meter
• Oscillating Circular Piston Meter
• fluted rotor meter or rotating impeller meter or helical rotor meter
• Oval Gear Meter
• Sliding Vane Meters or (rotating vane)
• Reciprocating Piston Meters
• Rotating lobe
• Precision Gear Flow meters

Positive Displacement Gas Meters:

• Wet Gas Meter 
• Diaphragm Meter
• Rotary Positive Displacement Gas Meter

Nutating disk meter:
          In this type of meter, an inlet chamber is formed by the housing, a disk, and a partition between the inlet and outlet port. Water is prevented from leaving the chamber by the disk, which maintains line contact with the upper and lower conical surfaces of the housing. When the pressure is reduced on the outlet side by a demand of water, the difference causes the disk to wobble (but not rotate) about the vertical axis and thus provide a passage for the flow around the partion. The wobble of the disk causes a small pin attached to its spherical mount to trace out a circular path about the vertical axis of the device. This motion of the pin is used to drive the recording mechanical.

Oscillating circular piston meter:
           It is similar to the nutating disk meter. Its center is constrained to move in a circle by the transmission so that the radius of the cavity is essentially the sum of the radii of the rotary piston and the circle on which its center moves. When the center of the rotary piston is at the top of its travel, the piston forms a closed compartment with the cavity. One rotation of the shaft will cause the rotary piston to return to its starting place and so to discharge the volume of one compartment.

Fluted rotor meter or rotating impeller meter or helical rotor meter:
         The axial and radial fluted rotor meters work on the same principal. The axial fluted rotor meter makes use of two aluminium spiral fluted rotors working within the same measuring chamber — with the rotors maintained in a properly timed relationship with one another by helical gears. As the product enters the intake of the measuring unit chamber, the two rotors divide the volume being measured into segments; momentarily separating each segment from the flowing inlet stream and then returning them to the outlet of the measuring unit chamber. During this ‘liquid transition’, the segments of flow are counted and the results are transferred to a totalising counter or other flow recording device by means of a gear train.

Oval gear meter:
            The oval gear meter is a special form of a multiple rotor meter in which each oval rotor is toothed, and sealing between the rotors is enhanced by the resulting labyrinth. Each rotor transmits fluid from inlet to outlet and forms a closed compartment when its major axis is aligned with the flow direction. The volume passed per revolution of each rotor is four times the volume between the rotor and the oval housing when the rotor is confining liquid [Figure 9.4(a)]. In place of the leakage paths between the rotors of multi rotor meters, there will be, for this meter, extremely small leakage where the rotors mesh, and the tolerances for the other surfaces are likely to be to a high standard, giving a very small value for the overall leakage.

Sliding vane:
            Consists of a cylindrical rotor from retractable vanes protrude. The fluid flow against the vanes causes the rotor to rotate. As the rotor rotates, the trapped fluid between vanes is swept around and out of the chamber. The number of revolution of the rotor is thus a measure of the amount of fluid that has been passed through the meter. Accuracy is high, about 0.1%.

Reciprocating piston meter:
            Reciprocating piston meters are probably the oldest PD meter designs. They are available with multiple pistons, double-acting pistons, or rotary pistons. As in a reciprocating piston engine, fluid is drawn into one piston chamber as it is discharged from the opposed piston in the meter. Typically, either a crankshaft or a horizontal slide is used to control the opening and closing of the proper orifices in the meter. These meters are usually smaller (available in sizes down to 1/10-in diameter) and are used for measuring very low flows of viscous liquids.

Lobed impeller or Rotating lobe:
              In the rotating lobe design, two impellers rotate in opposite directions within the ovoid housing (Figure 3-3B). As they rotate, a fixed volume of liquid is entrapped and then transported toward the outlet. Because the lobe gears remain in a fixed relative position, it is only necessary to measure the rotational velocity of one of them. The impeller is either geared to a register or is magnetically coupled to a transmitter. Lobe meters can be furnished in 2-in to 24-in line sizes. Flow capacity is 8-10 gpm to 18,000 gpm in the larger sizes. They provide good repeatability (better than 0.015% AR) at high flows and can be used at high operating pressures (to 1,200 psig) and temperatures (to 400¡F). The lobe gear meter is available in a wide range of materials of construction, from thermoplastics to highly corrosion-resistant metals. Disadvantages of this design include a loss of accuracy at low flows. Also, the maximum flow through this meter is less than for the same size oscillatory piston or nutating disc meter.

Precision Gear Flow meters:
           The spaces between the gears and the chamber wall form the fluid transfer compartments. In this version, rotation is sensed by two electromagnetic sensors operating through a pressure-resistant and nonmagnetic element in the housing. Two sensors can be arranged to allow better resolution than one and to determine flow direction. Measurement uncertainty of ±0.1% rate is claimed. It is also claimed that rapid flow reversal can be followed (e.g., 801/min in about 0.01 s). Meters for flow range of as low as 0.001 1/min and as high as 1,000 1/min may be available with temperature ranges of -30 to 150°C and pressure up to 300 bars or more

Wet Gas Meter:
            The wet-type gas meter (Figure 2.11) comprises a gas-tight casing containing a measuring drum, with four separate compartments, mounted on a spindle that is free to revolve. The casing is filled to approximately 60% of it’s of volume with water or light oil. Under normal operation the gas passes through the measuring drum so that each compartment of the drum must, in turn, be emptied of water and filled with gas ⎯ thus forcing the drum to rotate. In an alternative arrangement the gas is introduced into the space above the water in the outer casing and then passes through the drum to the outlet of the meter. The spindle on which the measuring drum is mounted is connected through gears to record the quantity of gas passing through the meter. Such meters are available in capacities ranging in size from 0.25 to 100 dm3 with accuracy down to ±0.25%.