Where is oval gear flow meters used?
Owing to its precision metrological characteristics, oval gear flow meter as a kind of positive displacement flow meters are deployed across petroleum, chemical, coating, pharmaceutical, food, and energy industries for fiscal/custody transfer measurement of high-value fluids. Typical process applications include pharmaceutical injection/extraction, chemical additive dosing, food flavoring injection and quantitative coating supply for painting lines. The primary application of oval gear flow meters (Positive displacement flow meters) lays in custody transfer and distribution measurement of petroleum products, such as used as petrol flow meter , diesel flow meter and so on. These flow measurements serve as legally binding data for financial reconciliation, taxation compliance and contractual execution.
Oval gear flow meter for diesel flow measurement
Certain national and regional regulations mandate the exclusive use of Positive Displacement Flowmeters (Positive displacement flowmeters) and a limited number of alternative flow measurement technologies in such applications. A prime example is the European Directive 71/319/EEC ‘Measuring Instruments for Liquids other than Water’, which explicitly restricts device selection to approved meter types.
High quality oval gear flowmeters with high precision, long-term performance retention, and good repeatability are used as standard flow meters (also known as calibration flow meters, reference flow meters, or master flow meters) for flow value transmission in comparative flow standard devices, such as rigid scraper plates, screw types in liquid instruments, and rotary drum types in gas instruments. Rotary vane gas flow meters are also used as standard flow meters abroad.
High accuracy oval gear flow meter
However, oval gear flowmeters require periodic maintenance, making them unsuitable for restricted-access environments such as radioactive fluid systems and toxic chemical processes. It is best to choose maintenance-minimized flowmeters with no moving parts, such as electromagnetic flow meter or ultrasonic flow meters.
val gear flowmeters can perform on-site total volume measurement without the need for external energy sources, it is mechanical flow meter . When equipped with a preset control totalizer and a control valve assembled together, it can be used for quantitative product delivery or batch mixing control. In the field of process detection and control, by adding a signaling and remote transmission module, it can output pulse or analog signals for flow control or pipeline mixing ratio adjustment.
Pulse output oval gear flow meter
Oval gear flowmeters are relatively bulky and heavy, especially high-flow/large-size flow meters .They’re being partially replaced by turbine flow meter , electromagnetic flowmeters, vortex flowmeters , and Coriolis mass flowmeters.However, their strengths—like excellent repeatability and long-term accuracy stability ensure they’ll remain irreplaceable in many applications for the foreseeable future. Positive displacement flowmeters are widely used for LPG (liquefied petroleum gas) overseas,while in China, adoption is still in its early stages.
What needs to be considered when choosing a proper type oval gear flow meter?
Given the unique characteristics of oval gear flow meter, the following factors should be carefully evaluated:
1. Purpose of use of elliptical gear flowmeter.
Is it for process control/engineering management or custody transfer/fiscal accounting?
2. Operation Mode.
Is the continuous or intermittent flow measurement? What are the maximum, typical, and minimum flow rates?
3. Operating Conditions.
What are the maximum, typical, and minimum temperature & pressure, and what is the allowable pressure drop?
4. Fluid Characteristics.
Including fluid type, viscosity, corrosiveness, and presence of particulates (including particle size).
5. Calibration Method.
Is the off-site calibration or on-site calibration?
6. Pumping System.
What are the type, capacity, and pulsation of pipeline pumping?
7. Installation Space.
Available space constraints for oval gear flow meter at the installation site.
Why need to consider flow range, accuracy, precision, repeatability?
Manufacturers typically determine flowmeter ranges based on fluid type (primarily viscosity), operation mode (continuous vs intermittent) and required accuracy.
In order to maintain the good performance and long service life of the flow measurement instrument, it is recommended to choose the maximum flow rate during continuous use at 80% of the highest (upper limit) flow rate of the instrument.
Flow range should be considered when choosing proper type oval gear meter
If the flow meter manufacturer does not clearly specify appropriate flow ranges based on fluid type and operating characteristics, the following selection principles may be applied: The upper limit flow rate of instruments used for medium viscosity and lubricating oil products is 100%. When used for non lubricating low viscosity liquids (such as gasoline and liquefied petroleum gas), the upper limit flow rate is reduced to 70%~80%. When used for water at around 1100 ° C, it is 40%~60%, and for high viscosity liquids, it is 75%~85%; The maximum flow during intermittent use can be 100% of the upper limit flow; The maximum flow rate during continuous use is 80% of the upper limit flow rate for medium viscosity liquids, 50%~60% for low viscosity liquids, and 50%~60% for high viscosity liquids.
The basic error of most structural models of liquid instruments is ± 0.5% R; the basic error of high-precision instruments is ± (0.1~0.2)% R, such as rigid scraper, screw, elliptical gear, etc. Some manufacturers claim to achieve ± 0.05% R (such as Dresser Wayne’s scraper); The basic error of lower precision instruments is ± (1-1.5)% R (such as elastic scraper type). The accuracy of gas instruments is slightly lower, with most structural models being ± (1-1.5)% R (such as waist wheel and CVM), higher ones being ± 0.5% R (such as rotary drum), and lower ones being ± (2-2.5)% R.
The repeatability error is generally 1⁄5 to 1⁄2 of the basic error.
Among all flowmeter types, oval gear flowmeters offer relative high measurement accuracy. The basic error specified in silverinstruments.com data sheets is derived under ideal laboratory calibration conditions. However, real-world operating conditions often deviate from these reference standards, introducing additional errors.
The turndown ratio of flowmeters typically ranges between 5:1 to 100:1, with most models operating within 10:1 to 20:1. When the same oval gear flow transmitter is rated at a higher accuracy level, the range obtained is lower. To obtain a larger range, the accuracy level needs to be reduced. For example, when the range of various rotary liquid instruments is 5:1, the basic error is ±0.2% R; when the range is 10:1, it is reduced to ±0.5% R.
Why happen pulsation for liquid measurement?
With the exception of a few designs like helical gear flow meter and vane-type flowmeters that operate smoothly without significant measurement-induced pulsation, most Positive displacement flowmeters ——including oval gear flow meter, Roots-type, and rotary piston meters ——generate flow pulsations due to their non-uniform rotational motion. At high flow rates, these pulsations can lead to significant noise, even vibration.
The Waseda University Institute of Engineering (Japan) conducted a seminal study on pulsation characteristics of Positive Displacement Flowmeters (Positive displacement flowmeters), analyzing angular velocity variations in Roots-type and oval gear designs, as illustrated in Figure 1.
In the figure, θ1 and ω1 respectively represent the rotation angle and angular velocity of one rotor in a pair of rotors (average angular velocity ω1 =1), ρ1 and ρ1 are the instantaneous pitch radii of the two rotors, and J1is a coefficient related to the rotational inertia I1
of the rotor, the cross-sectional area A and length L of the pipeline behind the instrument, the liquid density γ, and the gravitational acceleration g, that is, J1=γ L/I1 gA. b is the eccentricity of the elliptical gear.
Figure 1: Angular Velocity Variation
What is Pressure Loss in oval gear flowmeters
Oval gear flowmeters require fluid energy to drive the measuring elements, resulting in significant pressure loss. Elliptical gear flowmeter exhibit higher pressure loss than turbine flow meter or other obstruction flowmeters of equivalent size and flow rate. For liquid service meters at maximum flow (1~5 mPa·s viscosity), pressure loss ranges 20~100 kPa.For low-pressure gas meters ,roots-type ranges 200 ~500 Pa, and diaphragm-type ranges 130 ~ 400 Pa.
Proper flow sensor selection must prevent unacceptable pressure loss, particularly when measuring high-vapor-pressure liquids where excessive pressure drop may induce cavitation. Persistent cavitation will damage critical components, requiring special precautions for meters rated to handle 120% overload capacity.
Fluid corrosiveness
Fluid corrosiveness is the primary determinant of oval gear flow sensor material selection. For various petroleum products, cast steel and iron are used for manufacturing; For chemical liquids with mild corrosiveness and cold/warm water, they are made of copper alloy; For pure water, high-temperature water, crude oil, asphalt, high-temperature liquids, chemical liquids, food or food raw materials, stainless steel flow meter is used for manufacturing.
Stainless steel oval gear flow meter for water or mild corrosive liquid
While oval gear flowmeters are not inherently corrosion-resistant, the food and biopharmaceutical industries mandate stainless steel or other sanitary materials for wetted parts due to frequent cleaning, sterilization, and hygienic requirements. Designs must allow easy disassembly and eliminate liquid-trapping areas. In China, food-grade variants of oval gear and rotary piston flowmeters are now commercially available
Sanitary oval gear flow meter
What is influence of liquid viscosity on positive displacement flow meter?
Gas viscosity exhibits minimal variation across different types, exerting negligible influence on meter performance. In contrast, liquid viscosity significantly impacts meter behavior, particularly for fluids with substantial viscosity differences (e.g., 1~500 mPa·s). To accommodate high-viscosity liquids (up to 500 mPa·s), specialized oval gear flowmeters with enlarged clearances and other design adaptations are manufactured. Among flowmeter technologies, oval gear flowmeters boast the most extensive field experience with high-viscosity liquids. While oval gear flowmeters are affected by liquid viscosity, their performance deviation is markedly smaller compared to differential pressure, variable area flow meter (rotameter), or turbine flowmeters. Viscosity influences oval gear flow meter performance in three key aspects: measurement error, pressure loss, and operable flow range.
(1) Impact on Measurement Error
Unlike most flowmeters whose accuracy degrades with increasing viscosity, Positive Displacement Flowmeters exhibit a unique characteristic: rising viscosity reduces clearance leakage, thereby improving measurement performance. Figure 2 demonstrates the effect of liquid viscosity on the basic error of a Roots-type flowmeter.
Figure 2: Example of Error Characteristics for a Roots-Type Flowmeter at Different Viscosities
Within the viscosity range of 0.8–11 mPa·s, the viscosity effect is significant. When viscosity decreases from 5.65 mPa·s to 0.8 mPa·s, the negative error increases by approximately 0.5%. For viscosities between 11–51 mPa·s, viscosity still exerts a noticeable influence on meter error. Above 51 mPa·s, the viscosity effect on error becomes negligible.The above data represents one test case; actual effects vary with clearance dimensions.
Thus, higher-accuracy measurements require stricter viscosity control to mitigate error effects.For Class 0.2 Positive displacement flowmeters, viscosity must remain stable during operation to maintain specified accuracy.Calibration fluid viscosity should match operational fluid viscosity.If unachievable, calibrate with two bracketing viscosities and interpolate using:
In the formula,
E———Error at actual viscosity, %；
E₁、E₂————Respectively the errors calibrated using liquids with viscosity higher and lower than the measured liquid, %；
μ————Actual operating viscosity, mPa·s;
μ₁、μ₂————Respectively refer to the liquid viscosity that is higher or lower than the measured liquid viscosity, mPa·s.
Figure 3 Example of Pressure Loss vs. Viscosity for an Oval Gear Flowmeter
(2)Pressure loss impact
If the liquid viscosity increases, the pressure loss (Δp) of a Positive Displacement Flowmeter will also rise. Figure 3 illustrates the flow rate–pressure loss curves of an oval gear flowmeter for liquids of different viscosities. The graph clearly shows that, at the same flow rate (q), higher viscosity leads to a greater increase in pressure loss.The relationship between pressure loss (Δp) and flow rate (q) can be expressed as Δp=kqn (where k is the coefficient and n is the exponent).
When the viscosity is below 0.005Pa ⋅s (=5mPa⋅s), n=2; Above 0.5Pa ⋅s (=500mPa⋅s), n=1; Between the two, n=1.9~1.1. When measuring high-viscosity liquids, the load on moving components increases significantly, leading to elevated pressure losses. To address this, specialized high-viscosity oval gear flowmeters employ enlarged clearances, with some designs featuring gaps up to 0.5 mm.The oval gear flowmeter reduces the squeezing load of liquid between teeth by opening several grooves on the gear for unloading (when ≥ 150 mPa⋅s). When it is greater than 500 mPa⋅s, an undersized oval gear is used.
Viscosity affects oval gear flow meter measurement
(3) Impact on Flow Range
As previously noted, the operable flow range of a oval gear flow meter is viscosity-dependent. Increased viscosity elevates pressure loss (Δp), necessitating a reduction in the maximum flow rate (qₘₐₓ) for systems with Δp constraints—effectively narrowing the flow range. Conversely, the minimum measurable flow (qₘᵢₙ) decreases with rising viscosity, thereby expanding the operable range.As a rough estimate, with a 10 fold increase in viscosity, the lower limit of flow rate decreases to to 1⁄10 of 1⁄3 the original value.
Why need to consider pressure and temperature for oval gear flow meter?
All flow meters are specified with an operating temperature range and maximum working pressure (MWP). The MWP denotes the pressure tolerance under ambient temperature and surge conditions. When operating at elevated temperatures, the MWP must be derated—a critical adjustment often omitted in product manuals.
Rapid valve closure or opening generates surge pressures. Such surge pressures may exceed the rated working pressure. Surge pressures can also induce false flowmeter readings. Installation of surge vessels is recommended to mitigate these effects when necessary.
Temperature not only affects the compressive strength of flow sensor, but also changes the measurement chamber and gap size due to thermal expansion of instrument measuring components, affecting measurement accuracy, reducing gaps, and even causing moving parts to get stuck. Therefore, special size gaps should be reserved to compensate for higher temperatures, especially when using different material combinations, and attention should be paid to the differences in thermal expansion coefficients. Temperature changes can also alter liquid viscosity and cause changes in flow rate readings.
Temperature-induced dimensional changes in measuring components alter the measuring chamber volume. For instance, in an oval gear flowmeter where both the measuring chamber and gears are cast iron, the measurement value varies by +0.33% per 10°C.When the measuring chamber is cast iron and the gears are aluminum alloy, the variation becomes +0.14% per 10°C. Automatic temperature compensation may also be employed to correct the chamber volume variation to a defined standard temperature (e.g. 20°C).This can be achieved using devices with adjustable output transmission ratios for correction purposes.
Compressibility
The compressibility of liquids is generally negligible, but it must be considered in high-precision measurement of petroleum products. As listed in API Standard 1101, the compressibility factor of petroleum products ranges from (5 ~ 20)×10⁻⁴/ MPa. For instance, heavy oil undergoes a 0.45% volume reduction when pressure increases from 0.5 MPa to 6 MPa. Liquefied petroleum gas (LPG) exhibits significantly higher compressibility.
Gases are highly compressible. At low pressures, their volume reduction is directly proportional to the pressure increase, and most Positive displacement flowmeters (Pressure-Density Functions) under low-pressure conditions allow direct conversion.However, under high-pressure conditions, the volume reduction deviates from proportionality with pressure, and the rate of change decreases. In such cases, the gas compressibility factor must be taken into account.
Selection of Gas PD flow meter
Pressure-Density Functions (Positive displacement flowmeters) are applied in gas flow measurement devices such as diaphragm gas flowmeters, wet gas flowmeters, rotary lobe gas flowmeters, and rotating vane gas flowmeters.
1)Diaphragm gas flowmeters.
Diaphragm gas flowmeters are suitable for total consumption measurement of gaseous fuels such as coal gas, natural gas, and liquefied petroleum gas (LPG). Known for high reliability and low cost, they are the primary type of residential gas meters in urban households. Additionally, large-capacity models are widely used in canteens, hotels, and industrial workshops.
2)Wet gas flowmeters
Wet gas flowmeters (also known as rotating drum gas flowmeters) feature high accuracy, wide turndown ratio, and stable performance. However, they require trained operators to maintain these superior characteristics. These instruments are primarily used in laboratories as reference standards for gas flow calibration systems, as well as in other applications demanding precise total flow measurement.
3)Rotary Lobe Gas Flowmeter
The rotary lobe gas flowmeter is suitable for measuring small to medium gas flow rates.Its main technical parameters are:flow range: 2.5 ~ 2000 m³/h , accuracy: (±1% ~ ±2.5%) R , nominal diameter (DN): 50 ~ 300 , operating pressure: Typically ≤1 ~ 1.6 MPa (higher versions support ≤10 MPa).
Compared to other inferential gas flowmeters, the rotary lobe gas flowmeter offers : high accuracy , wide turndown ratio (5:1 ~ 10:1) , and immunity to flow profile distortions, eliminating the need for long straight pipe sections. Limitations include: sensitivity to dirty media (requires an upstream filter) and vibration and noise during operation (requires rigid mounting). Operational caution: Avoid rapid flow changes (e.g., quick-opening valves), as the rotor’s inertia generates excessive forces that may cause impact and damage.
4) Rotating Vane Gas Flowmeter
The main technical parameters of the rotary vane gas flowmeter in the domestic market are as follows: flow Range:≤160 m³/h (International models available up to 1200 m³/h) , nominal Diameter: DN50~80 , accuracy : ±1.5% R ,turndown Ratio: 20:1.
The rotary vane gas flowmeter is suitable for measuring various gases. Compared with the gas waist wheel flowmeter, the flowmeter works more smoothly, without vibration or noise. Due to its excellent metrological characteristics, standard meters have been used abroad as gas flow standard devices.
Applied to liquefied petroleum gas (LPG flow measurement)
Oval gear flow meters are commonly used for liquid LPG metering during tanker truck loading and refueling station transfers. LPG composition varies by source (propane/butane ratios), remaining gaseous at atmospheric pressure. Liquefaction requires pressures exceeding its vapor pressure. Thus, oval gear flowmeters for LPG require minimum rated working pressures of 2MPa (20 bar).LPG exhibits low density (0.51–0.58 g/cm³), varying with composition, pressure, and temperature.
LPG has extremely low viscosity (0.10–0.17 mPa·s), significantly lower than gasoline (0.7 mPa·s). When a PDF calibrated with water (~1 mPa·s) measures LPG, viscosity effects alone can cause -0.5% measurement deviation and increased minimum flow rate. Additionally, poor lubricity exacerbates these issues. To compensate, LPG-grade oval gear flowmeters require integrated lubrication systems with external lubricant supply.
LPG exhibits significantly higher compressibility compared to other petroleum products, with volumetric compression reaching (0.44-0.73)%/MPa under pressure. For flowmeters requiring 0.25% or 0.5% accuracy, this represents a non-negligible static pressure effect that must be accounted for in measurement systems.
In LPG systems, all instrumentation and pumps must remain fully liquid-filled at all times, even during shutdown. Avoid partial filling or dry piping, as vaporization of residual liquid can lead to deposit formation on internal surfaces. These deposits cause accelerated wear and significantly reduce service life.
