Positive displacement flow meters are mechanical instruments that measure the volumetric flow rate of a fluid. The main advantage of these types of flow meters is their ability to handle high viscous and corrosive fluid, and they typically do not require filtration prior to processing. Here is a guide to positive displacement flow meters which includes an overview of the basic design, operation and benefits of this type of mechanical flow meter.
General Components and Operation
Though the details of each design vary, positive displacement flow meters all contain these major components:
- Inlet and outlet ports
- Measuring chamber
- One or more rotors
Fluid enters the measuring chamber through the inlet port and imparts energy on the rotor. The rotation of the rotor corresponds to the volume of fluid entering the chamber, and a signal is collected by the connected flow meter transmitter. The fluid then exits the chamber through the outlet port.
Two gears are commonly used as rotors for this type of flow meter. The gears can be aligned vertically or horizontally, and they can either be circular or oval shaped. In either case, the purpose of having two gears is to create a seal within the measuring chamber so that all fluid is measured before passing through. Other positive displacement flow meter designs may utilize two helical or screw-shaped gears, while some use one or more pistons or diaphragms to physically push fluid through the chamber.
Benefits and Concerns
Positive displacement flow meters are known for their accuracy, mainly due to the tight seal created within the mixing chamber. They can also measure a wide range of flow capacities and fluid characteristics. Gear meters perform well in lower flow applications, but can handle higher rates with accuracy. They also respond quickly to changes in flow conditions, so the time it takes to collect and transmit real-time data is relatively short.
The simplicity of the design makes these flow meters relatively low maintenance and easy to disassemble and reassemble. Disassembly typically only requires disconnecting electrical wiring, removing a few screws, lifting off the cover, removing an O-ring and unscrewing the rotors. Bearings may also be installed on the rotor shaft to support the load.
Since positive displacement flow meters contain mechanical components, the parts are susceptible to wear and damage over time and require replacement. Bearings typically do not require relubrication, or they are lubricated by the fluid being pumped. Gears and bearings may need to be replaced eventually depending on the solids content and type of solids present in the fluid.
Though the flow meters can be used for a variety of fluids, if large particles are present in the fluid, they should be filtered upstream to protect the mechanical components from damage. Also, for systems where cavitation is an issue or where air pockets are common within pumped fluid, air eliminators should be installed upstream of the flow meters to prevent damage and provide more accurate volumetric readings.
Pressure drops do occur across positive displacement flow meters, which must be accounted for in system calculations. Such pressure drops are usually minimal and therefore relatively insignificant. However, larger pressure drops can damage internal bearings and seals. Manufacturers typically define a maximum operating pressure value to prevent premature wear and potential flow meter failure. If the viscosity of the pumped fluid changes, check the pressure drop to ensure it still falls within the acceptable range.
Positive displacement flow meters can be used in a variety of industries. Some common applications include:
- Oil and gas
- Chemical and petrochemical transport
- Water and wastewater treatment and infrastructure
- Food and beverage manufacturing
Such types of mechanical flow meters are also useful for batching, lubrication oil blending and hydraulic test stands.
Calibrating Positive Displacement Flow Meters
Calibration is an important component of flow meter maintenance. A well-calibrated flow meter provides accurate readings and helps operators keep the system running efficiently. Though calibration procedures for gas and liquid flows follow the same underlying principles, the methods for calibration are different. For liquid applications, there are various ways to calibrate a positive displacement flow meter such as:
- Standing start and stop method, also known as the bucket and stopwatch method
- Flying start and stop method
- Dynamic methods that calibrate during continuous flow conditions
To calculate the volume and mass of collected liquid during the calibration procedure, either gravimetric calibrators or volumetric calibrators are used.
Other types of calibration devices can be installed along the pipeline to measure the flow meter performance in-situ. For example, pipe provers can be installed in sealed pipelines to measure the accuracy of the flow meter without interrupting the process. This is a dynamic calibration that continuously compares the flow meter measurement to a standard.
Gas applications require different calibration methods due to the compressibility of the fluid. Examples of gas calibration methods include:
- Prover methods, which work like pipe provers for liquid applications
- Gas displacement methods
- Sonic nozzle methods, which use a Venturi-style nozzle to determine flow accuracy
Tracer methods are also commonly used for both liquid and gas applications when provers cannot be installed and the flow meter cannot be removed from the system.
Professional Calibration Services
Though calibration should be incorporated as part of regularly scheduled instrumentation maintenance, it is difficult to determine the best frequency of calibration. Therefore, it is helpful to hire a third-party who specializes in flow meters and flow meter calibrations to determine a schedule that it appropriate for each system.
Professional flow meter calibration services use advanced methods to more accurately determine the performance of flow meters. Such professionals are knowledgeable about the tolerance levels for each type of flow meter and can identify and interpret errors in the calibration procedure. They can also help identify system issues from the calibration results. For example, calibration data may indicate leaks or damaged internal components that need to be replaced. Professional services can also look at the history of calibration to identify significant system changes or make recommendations based on changes to the characteristics of the flow.
Overall, positive displacement flow meters last for many years when the right flow meter is chosen for the application, proper flow conditions are maintained and the flow meter is calibrated at regular intervals.