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Positive Displacement
FAQs Positive Displacement Flow Meter: Questions and answers, professional knowledge directly from the manufacturer:
What is a Positive Displacement Flow Meter?
A positive displacement (PD) meter is a type of flow meter used to measure the volumetric flow rate of a fluid. It measures the volumetric flow rate of a moving fluid by dividing the media into fixed, metered volumes. These meters do not require any straight upstream and downstream pipe runs for their installation.
PD flow meters are particularly well-suited for measuring:
1) High-Viscosity Fluids: PD flow meters can accurately measure fluids with high viscosity, such as oils and thick syrups.
2) Fluids with varying Flow Rates: PD flow meters can handle fluctuations in flow rate without affecting accuracy.
Why is it called positive displacement?
Positive Displacement (PD) flow meters get their name from the way they measure flow. Here is the breakdown:
Positive: "Positive" means definite or exact. It signifies that the meter captures a specific, known volume of fluid with each cycle.
Displacement: This refers to the movement or pushing aside of a fluid. In a PD meter, the internal components physically displace a fixed amount of fluid as they rotate or move.
How Does Positive Displacement flow meter Works?
Positive Displacement flow meters are designed to directly measure the volume of fluid flowing through the meter by physically capturing and displacing the fluid in fixed and measurable quantities. A PD Flow Meter operates when the fluid flows into the chamber and drives the two rotating rotors. As the rotors turn, they trap and displace fixed volumes of fluid. With each rotation, a constant volume of liquid is measured. The rotation of rotating rotors is transmitted via magnets embedded within the rotors. A Hall sensor or a magnetic pick-up, which generates a scaled pulse output also known as K-Factor (e.g. Pulses/Litre) transmits a pulse output.
What types of positive displacement flow meters are available?
There are several types of PD flow meters, including:
Oval Gear Flow Meter
An Oval Gear PD Flow Meter uses two oval-shaped gears that rotate as fluid passes through them. This meter offers excellent accuracy, even for low flow rates and high-viscosity fluids. They are commonly used in industries where precise fluid measurement is required.
Oscillating Piston Flow Meter
Oscillating Piston Flow Meter are also known as direct volume or displacement counters. These flow meters operate by consistently dividing a set volume of fluid in a measuring cell, which is repeatedly filled and emptied. The measuring cell comprises a chamber and a moving piston. The piston is driven by the pressure difference between inlet and outlet of the measured media. The revolution is carried forward via a magnet and a magnet field sensor. The chamber's inlet and outlet are positioned on opposite sides of the wall and are sealed by the piston and a baffle. As the fluid enters, it fills the crescent-shaped space, causing the piston to rotate. This rotation continues until two volumes, V1 and V2, are achieved. As the piston moves, the filled spaces are connected to the outlet and emptied. Since the inner and outer crescent-shaped spaces are offset from each other, the piston moves without reaching a dead center, allowing for continuous operation based on the flow rate.
Screw Spindle Flow Meter
A screw spindle PD flow meter operates using a pair of precisely engineered screw gear rotors, which are fitted within the meter housing. As the liquid flows through the meter, it causes these rotors to turn. The rotations of the rotors are detected by an integrated gear and a non-contact signal pick-up system, which converts the rotational movement into digital pulses. These pulses are then used to accurately determine the flow rate of the liquid. They are specially designed to measure and control viscous media with non-abrasive properties and making them ideal for applications involving thick or dense fluids. Some benefits of these meters are that it doesn't get contaminated easily, pulsation free and has a low pressure drop.
Gear Wheel Flow Meter
The Gear Wheel PD Flow meter been designed for a flow measurement for viscous liquids, such as oils, paints, chemicals, and fuels, because they maintain high precision even at low flow rates and with high-viscosity substances. It consists of a pair of gear wheels which is moved by the flow according to the principle of gear wheel motor. The movement of the gears is sampled without contact by means of two sensors located in the cover. A pressure resistant, non-magnetic separator plate is located between sensor compartment and measuring chamber. The gears are often made of durable materials for long-term reliability. However, these meters typically experience a higher pressure drop compared to other PD flow meters. These flow meters offer an enhanced turndown ratio and a wider measuring span. Due to their moving parts, they require regular maintenance to prevent mechanical wear over time, which can affect performance.
How accurate are positive displacement flow meters?
Positive displacement flow meters are known for their high accuracy in measuring flow rates, A Positive Displacement Flow Meter achieves a measurement accuracy of up to 0.1% of reading.
What viscosity range of fluids can a Positive Displacement (PD) flow meters accurately measure?
Positive Displacement (PD) flow meters are well-suited for measuring fluids with a wide range of viscosities. They can accurately measure both low-viscosity fluids, like water, and high-viscosity fluids, such as oils and syrups. Generally, PD flow meters can handle fluids with viscosities ranging from as low as 1 cP to as high as 1,000,000 cP or more, depending on the meter design. As fluid viscosity increases, the flow inside the PD flow meters becomes more accurate because there is less slippage. However, this also causes a higher pressure drop through the meter.
What are the common applications for Positive Displacement Flow Meters?
Positive displacement flow meters find applications in various industries where accurate measurement of fluid flow is crucial. Here are few examples of Applications
Chemical Industry: Metering chemicals, resins, additives, and other fluids for precise control in various processes.
Food and Beverage: Measuring ingredients like syrups, oils, or milk for accurate recipe control and product consistency.
Oil and Gas Industry: Measuring fuel oil flow, metering lubricants, or handling viscous materials like asphalt.
Water and Wastewater Treatment: Metering water flow for billing, monitoring chemical dosing, or managing specific processes.
Pharmaceutical Industry: Dosing precise amounts of medications or ingredients during production.
Can positive displacement flow meters be used with water?
Yes, positive displacement flow meters may also be used to measure the flow of water but caution is required. Water, however, is often not the best medium for positive displacement flow meters, and other technologies usually provide a more accurate way to measure water. Water is a low viscosity media, so medium slippage inside chamber may result in inaccurate readings. The majority of PD flow meters used for this purpose are developed especially for this use, with mechanical counters that utility staff can read on-site.
What are the advantages of positive displacement Flow meters?
Positive displacement flow meters offer several advantages that make them suitable for certain applications. Here are some of the key advantages of positive displacement flow meters:
Accuracy: They are known for their high accuracy, especially at low to moderate flow rates. They provide precise volumetric measurements, making them suitable for applications where accurate flow measurement is required.
Viscosity Independence: They are less affected by changes in fluid viscosity. This makes them suitable for measuring fluids with varying viscosities from thin liquids to more viscous substances.
Repeatability: They offer excellent repeatability, meaning that under similar operating conditions, they consistently provide the same measurements. This characteristic is important for applications that require consistent and reliable flow data.
Wide Range of Fluids: These meters can handle a wide range of fluids. This versatility makes them suitable for diverse industrial applications.
Direct Measurement: The measurement principle of PD flow meters involves directly counting or measuring the volume of fluid displaced. This direct measurement contributes to their accuracy and reliability.
Sturdy Construction: PD flow meters are often robustly constructed, making them durable and suitable for use in challenging environments. This durability increases their longevity and reduces maintenance requirements.
Low Sensitivity to Flow Profile: Unlike some other flowmeter types, PD flow meters are less sensitive to variations in the flow profile. They can provide accurate measurements even in conditions where the flow is not fully developed, as long as the pipe is full of liquid.
Bi-Directional Flow: PD Flow meters are capable of measuring flow in both directions called bidirectional flow measurement. To ensure accurate readings, the flowmeter is calibrated for a specific direction, which is indicated by an arrow on the device. For accurate bidirectional measurements, it is important to use a calibration process that accounts for flow in both directions.
Inlet/ Outlet Pipe Runs: One key advantage of positive displacement flow meters is that they typically do not require straight pipe runs before and after the meter. Unlike some other flow meter types, such as turbine or ultrasonic meters, PD flow meters are less affected by flow disturbances caused by bends, valves, or fittings in the piping. This means you can install a PD flow meter with shorter inlet/outlet pipe runs without sacrificing accuracy, making it a more flexible option in tight or space-constrained installations.
What are the Disadvantages of Positive Displacement Flow Meters?
While positive displacement flowmeters offer several advantages, they also have certain disadvantages and limitations that should be considered when choosing a flow measurement solution. Here are some of the disadvantages of positive displacement flowmeters:
Limited Turndown Ratio: PD Flow meters often have a limited turndown ratio. In applications with wide-ranging flow rates, other types of flow meters may be more suitable.
Pressure Drop: PD Flow meters can create a noticeable pressure drop in the system, depending on the gear types used. This may require additional pumping power.
Size and Weight: Positive displacement flow meters, particularly those designed for large flow rates, can be bulky and heavy. This may limit their use in applications where space and weight are critical considerations.
Maintenance Requirements: PD flow meters may require more maintenance compared to some other flowmeter types without moving parts. Seals, moving parts, and other components may need periodic inspection and replacement.
Cost: These meters can be relatively expensive compared to some other flowmeter technologies. The initial cost and potential maintenance expenses should be considered when evaluating flow measurement options.
Media Compatibility: PD flow meters may only be used for clean media. They are generally not suitable for applications involving solid particles in the fluid.
Which types of liquids are compatible with Positive Displacement flow meters?
Positive displacement flow meters are versatile devices that can be used with a wide range of liquids across various industries. Here are some examples of suitable media types:
Oils: It is used in lubricating systems and industrial machinery.
Detergents: It is Commonly used in cleaning and manufacturing processes.
Fuels: It is essential in the energy sector for monitoring fuel consumption and distribution.
Deodorizers: Utilized in industries where odour control is essential.
Caustics: Employed in chemical processing and industrial cleaning.
Acids: Critical in various industrial applications, particularly in chemical manufacturing.
Honey and Molasses: Applicable in the food and beverage industry, especially for measuring viscous fluids.
Brake Fluid: Utilized in automotive industries for accurate measurement during manufacturing.
Adhesive Coatings and Glues: Crucial in manufacturing and packaging industries.
Corrosion Inhibitors: Important in extending the life of equipment by preventing corrosion.
Inks: Essential in the printing industry for accurate ink flow measurement.
Pastes, Resins and Creams: Used in cosmetics, pharmaceuticals, and industrial applications where viscosity management is important.
What should be considered when installing a positive displacement flow meters?
Positive displacement flow meters are generally less sensitive to the orientation of installation compared to some other flow meter types. However, it is recommended to follow the manufacturer's guidelines.
PD flow meters can be used both horizontal and vertical flow measurements. For vertical flow installations it is recommended that the liquid flow should be from bottom to top. These meters are suitable for Bi-directional flow measurements. To identify the flow direction, in which the flowmeter was calibrated, the flowmeters are marked with arrow.
It is recommended to install a filter or strainer upstream of the flow meter to prevent debris or particles from entering the meter and causing damage or clogging. The axis should be parallel to the ground to avoid the rotary wheels / gears scratching the flow meter body base. Flow is not required since the PD flow meters does not require any straight pipe runs before or after the flowmeter.
Are Positive Displacement Meters Suitable for Use in Hazardous Areas?
Yes, Positive Displacement meters can be suitable for use in hazardous areas, provided they are certified for such environments. Explosion-Proof Certification indicates whether the meter is safe to use in areas with flammable gases and vapours. Please check the manufacturer's product approvals before selecting a PD flow meter for your hazardous area applications.
What maintenance is needed for Positive Displacement flow meters?
How much do Positive Displacement (PD) flow meters cost and what factors affect their pricing?
The cost of Positive Displacement (PD) flow meters may vary widely depending on several key factors. These include the line size, flow and pressure capabilities, and the material of construction. Generally, larger line sizes and higher flow and pressure requirements result in higher costs. Additionally, PD flow meters made from metal are typically more expensive than those made from plastic, due to the enhanced durability and performance of metal components. This range in materials and configurations means that pricing for PD flow meters can accommodate various industrial needs and budget considerations. Please contact the manufacturer for more details.
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