Flow Measurement

What is a positive displacement flow meter and what are its types?

The positive displacement meters measure the flow rate by separating the stream into segments of known volume, counting segments, and multiplying by the volume of each segment. Measured over a specific period, the result is a value expressed in units of volume per unit time. This type of flow meter doesn’t require power and its motion is only dependent on the differential pressure from upstream and downstream side of the inlet and outlet port of the chamber.

The operation will be according to the continuous filling of the empty chamber with a known volume. In this meter, the fluid will go to a chamber with a unit and it repeatedly fills and discharges a fixed volume. The total flow rate is calculated from the rate of filling and discharging the discrete volumes. The rotational speed of this meter’s impeller is a function of the process flow. There would be an internally coupled counter it could be electronic or mechanic type, and this would check the measuring elements rotations to provide a volumetric recording of the total flow. Positive displacement flow meters are used in residential and small commercial applications. This type of flow meter is very accurate at low to moderate flow rates. The positive displacement flow meters can be used for any non-abrasive fluid such as heating oil, Freon, printing ink, or polymer additives.

How does a positive displacement meter works?

This is the only type of the flow-meter that directly measures the volume of fluid that passes through it. The operation of the positive displacement flow meter is based on the continuous filling and discharging of the measuring chamber. This device has a rotor and it would revolve on a ball bearing and carries evenly spaced blades. So when the liquid flows through the meter the rotor and blades revolve around a fixed cam and this would cause the blade to move outwards. So when the rotor turns, isolated chambers are formed between blades, rotor, base, covering, and housing. It would be like a revolving door, the known segments of the fluid would pass through the chamber and they are counted.

So this device would trap a preferred amount of incoming fluid, displacing it to the discharge side of the meter. In order to determine the total amount of fluid displaced, we must count the number of discharge and recharge cycles. We must check the clearance between the impeller and its casing so that leakage can be prevented. The positive displacement meters have electronic pickup coils and they would generate pulses according to the volume of the fluid that has passed through the meter, and this ratio of pulses per unit volume is called the meter factor of the PD meter. The positive displacement meters are not affected by asymmetry and swirl, but it is affected by the changes in viscosity.

What are the types of positive displacement flow meter?

Rotary vane meters

This type of PD meters has impellers with two or more equally divided compartments. A fixed volume of liquid will be removed from each of the meter’s compartment when the impeller rotates. The revolution of the impeller can be counted. The flow rate would be proportional to the rotation of the rotor. These meters are suitable for gas applications.

Helix flow meter

This type of flow meter consists of two helical rotors and they are geared together with a small clearance between the rotor and casing. So when the fluid flows to this meter the two rotors would move the liquid axially from one end of the chamber to the other.  This type of PD meter can accurately measure higher viscosity liquids.

Nutating disc meter

This type of PD meter has a movable disc and it would be mounted on a concentric sphere. So in this meter, the displacement element is the disc, the lower part of the disc is in contact with the bottom of the chamber on one side and the upper part of the disc is in contact with the top of the chamber on the opposite side so the chamber is separated into two compartments. So the liquid will enter the meter through the inlet connection on one side and leaves through the outlet on the other side. So the compartments are filled and emptied also the disc is moved around the pivot.

Oval gear meters

This type of PD meters is composed of two identical oval rotors and both of these rotors are connected together with the help of gear teeth. The rotor would rotate on a stationary shaft and it would be fixed in a measuring chamber. The inlet will be sealed from the outlet flow with the help of meshed gears and this would create a differential across the meter and this would make the oval rotor move. For each revolution, a fixed amount of fluid would pass through the meter. In order to get the flow rate, we can monitor the shaft rotation.

Lobed impeller flow meter

This is a type of PD meter that is constructed with two machined lobe inlet-outlet that has close clearance with each other and the material housing. This type of meter can be used for either gas or liquid flow measurement. This meter maintains an accurate fit because the impeller and the case are carefully machined. So due to this the fluid that flows to the meter will always get trapped between the rotors and the fluid will be displaced to the outlet because of their rotation. We can determine the volumetric flow rate by checking the number of revolutions of the rotors.  

Piston type meter

This type of PD meter has a rotating piston and it is attached to the center of the chamber. This chamber has a partition wall and it would block the inlet and outlet port from each other. So when the fluid reaches the inlet port the fluid will exert pressure on the piston and this would make the piston move in the chamber and because of this, the piston slot would move up and down in this wall. This type of PD meters are used for residential water service

Diaphragm type meter

This type of PD meter has several diaphragms and it would take the fluid when it passes through the meter. Because of the differential pressure across the meter, one diaphragm would expand and the other one would contract. The gas can move smoothly through the meter due to the rotating crank mechanism. This mechanism is connected by the help of gearing to the index, and this would determine the amount of fluid that passes through the meter. This type of PD meter is mostly used for gas applications.   

How to do the installation of the positive displacement meter?

The positive displacement meters are affected by environmental changes so this must be considered while the installation. The meter should be protected from sand, dust, and rain. The meter should be mounted in a proper base, the piping should not create any strain in the meter. The meter must be shielded from the effect of thermal expansion. We must use an air eliminator so that we can remove air and vapor out of the meter. The meter must not be calibrated with water.

How to select a positive displacement flow meter?

  • It must be selected according to the media like viscous liquids, or clean gases
  • Flow range
  • Pressure
  • Connections
  • Viscosity rate
  • Temperature
  • Accuracy
  • System characteristics

What are the advantages of positive displacement flow meter?

  • High accuracy
  • It is suitable for non-abrasive fluids
  • It has good turndown ratios
  • Low maintenance because of the less moving part
  • There are no upstream and downstream requirements
  • There are no Reynolds number constraints
  • Good repeatability
  • There won’t be any leakage
  • There is no connection restriction it can be directly connected to elbows or valves
  • The PD meters has a good turndown ratio

What are the disadvantages of a positive displacement flow meter?

  • Very expensive when compared to the other type of flow meter
  • It has a high-pressure drop
  • It measures discrete fluid flow instead of the actual flow rate
  • This meter is not small so it would take some space
  • The PD meters has mechanical parts so they are prone to wear
  • This device would use some energy from the system because they are self-powered.
  • Corrosion  could happen
  • It should be used for clean fluid applications
  • It can only be used for a limited range of pressure and temperature
  • Maintenance cost is high

What are the applications of positive displacement meters?

  • It is used in refinery gas and liquid services
  • It can be used for clean, non-abrasive, and also for medium to high viscosity liquids
  • Good for batch operation, low tech plants
  • Chemical industries
  • Pulp and paper industries
  • Container filling
  • Water and wastewater
  • Pharmaceutical
  • Metal and aerospace

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