A mechanical diesel flow meter measures the volume of diesel fuel passing through it using moving parts like oval gears. It provides a direct, physical measurement without electronic components or power supply, often used for its durability and simplicity in various fuel management applications. Fuel can be gasoline , petro, diesel, kerosene,LPG, biofuels and so on. Oval gear flow meter is a kind of typical mechanical diesel flow meter.
Mechanical diesel flow meter principle
A mechanical diesel flowmeter operates as a positive displacement flow meter. It measures fuel flow by capturing and displacing a fixed volume of fuel with each cycle of its moving components. As the internal components move, they displace a set volume of fluid, and the total fuel flow rate is determined by counting the number of displacements.
Each cycle of the moving element displaces
a volume . If the
moving component completes cycles , the total volume of fuel that has passed through the flowmeter is calculated by:
The mechanical movement of the component is
transmitted through a gear mechanism to an indicator, which then moves the
pointer on a dial. This dial displays the total volume of fuel that has passed
through the flowmeter.
Advantages and
disadvantages of mechanical fuel flowmeter
Oval gear flow meter is a kind of positive
displacement flowmeter which is a widely used mechanical fuel flowmeter that
measures fuel oil flow by repeatedly capturing a fixed volume. Known for its
one-directional flow, it is referred to as a "positive displacement
flowmeter" in various regions.
It boasts a long history and broad
applicability, with notable advantages:
1. High Measurement Accuracy: Achieves a
relative error of ±0.1% to ±0.5%. oval gear flow meter accuracy remains
unaffected by fuel oil type, viscosity, density, Reynolds number, or the length
of the upstream and downstream straight pipe sections.
2. Wide Measurement Range: Oval gear flow
meter is capable of achieving an accuracy of 0.5 at a range of 10. It provides
precise cumulative fuel oil measurements, making it suitable for material
measurement applications.
3. Effective at Low Reynolds Numbers:
Measures high-viscosity and low-flow-rate fuel with high precision, even in low
Reynolds number conditions, can be used as high viscosity fuel flow meter.
4. Short Straight Pipe Section when install
mechanical fuel flow emter: Functions effectively on-site with minimal
requirements for upstream and downstream straight pipe sections.
Disadvantages of mechancial fuel flow meter
However, there are several disadvantages
associated with positive displacement type fuel flowmeters that must be
considered:
1. Bulk and Complexity: For the same flow
capacity, positive displacement flowmeters tend to be bulkier due to their
larger volume and the greater number of mechanical components. The assembly
process is more complex, leading to higher manufacturing costs.
2. Sensitivity to Contaminants: These fuel
flowmeters are generally sensitive to particulates and contaminants in the fuel.
Installing a filter upstream can increase pressure loss. Additionally,
components such as the rotor and housing require periodic cleaning, which adds
to maintenance efforts.
3. Susceptibility to Flow Rate Variations:
Frequent changes in flow rate can damage the rotating parts. It is crucial to
avoid sudden opening or closing of valves near the diesel flowmeter, as such
actions can lead to instrument damage if not properly managed by the operator.
Despite these limitations, positive
displacement flowmeters remain a widely used and reliable fuel flow metering instrument due to their high accuracy and long service life. They are commonly employed in
industries such as oil measurement and trading, light manufacturing, food
processing, and other sectors.
Technical specifications
of mechanical fuel flow meter
The main technical parameters of volumetric
flow meters include: nominal diameter, measurable fluid, flow range, accuracy,
nominal pressure, operating temperature range, etc
Mechanical fuel flow meter size: from DN10
to DN250.
Accuracy: 0.5% (standard) option with 0.2%
Fuel temperature range : -20 ~120℃
The max viscosity for the fuel is 4000 cp
Flow meter material: cast iron, cast steel,
stainless steel options.
Pressure rating: standard 232 psi, max fuel
pressure is 914 psi.
Dial can be with cumulative total function,
or option with resettable total function as below picture shows.
Error Analysis of
mechanical fuel Flow Meter
To conduct a precise error analysis of
mechanical fuel flow meters, one must consider several critical factors.
Assuming the machining precision and assembly standards are upheld, the primary
sources of systematic error include leakage or slippage, which arises due to
the clearance between the measuring element (such as a rotor, scraper, or
piston) and the internal cavity of the housing. Another significant factor is
the change in the volume of the metering chamber, which can occur due to
deformation of the housing caused by fluid pressure, mechanical stress, and
temperature fluctuations.
Leakage is influenced by the gap size,
fluid viscosity, and the pressure differential between the flowmeter's inlet
and outlet (which is related to the movement resistance of the measuring
element, the transmission mechanism, and the fluid's flow resistance within the
housing). While minimizing the gap can reduce leakage, it also increases
manufacturing complexity and risks jamming the moving components, or at the
very least, elevating resistance. Thus, the gap cannot be minimized
indefinitely. The rotor's resistance to rotation, which is overcome by the
pressure differential between the inlet and outlet, coupled with the pressure
loss caused by fluid viscosity within the metering chamber, contributes to the
overall pressure differential that drives fluid leakage through the gap.
Factors such as increasing the gap, decreasing viscosity, increasing pressure
differential, increasing density, and heightened rotational resistance all
exacerbate leakage. For high-precision volumetric flow meters, minimizing leakage
is essential, requiring the rotor to rotate freely with minimal resistance
torque, ensuring a small pressure differential, maintaining an appropriately
small gap, and selecting a fluid with moderate viscosity.
To minimize the volume change of the metering
chamber due to shell deformation, one can adopt several strategies: increasing
shell rigidity to reduce pressure-induced deformation, selecting materials with
low and closely matched thermal expansion coefficients for both the shell and
rotor, and ensuring proper installation to avoid deformation caused by
installation stress.
If the viscosity of the fluid being
measured significantly deviates from that of the calibration fluid, the flow
meter's error margin will increase. To mitigate this, calibrating the flow
meter with fluids of varying viscosities—both lower and higher than the actual
fluid—allows for corrections using an interpolation method during actual
operation.
Mechanical fuel flow
meter selection consideration
When selecting volumetric flowmeters, particularly for applications like fuel oil product measurement, trade, and
material accounting, careful consideration must be given to ensure optimal
performance and accuracy. The following guidelines should be observed:
1. High-Precision Requirements: For
applications requiring high accuracy, it is crucial to select a fuel flow meter
with appropriate precision. If the fluid contains impurities, a filter should
be installed upstream of the flow sensor. Additionally, it is advisable to
install a pressure gauge or a differential pressure gauge at both the inlet and
outlet of the filter. This setup allows for timely cleaning of the filter when
a significant increase in pressure differential is detected. Proper selection
of auxiliary equipment, such as filters, air eliminators, and check valves, is
as important as the correct selection of the mechanical fuel flowmeter itself.
2. Flow Direction and Bypass
Considerations: Positive displacement type mechanical fuel flowmeters typically
allow fluid flow in only one direction. Therefore, when reverse flow is
required (e.g., during line sweeping), a bypass pipeline should be engaged to
prevent damage to the fuel flowmeter from reverse flow. To facilitate
maintenance, such as cleaning the filter or flowmeter without disrupting fluid
flow or production, it is recommended to install a parallel bypass pipe that
can be opened as needed. The corresponding accessories must be carefully
selected and procured.
3. Blockage Prevention and Automatic
Switching: Impurities in the fuel can obstruct the rotor of the volumetric
flowmeter, potentially halting fluid flow and leading to process disruptions.
In such cases, an automatic switching system should be incorporated. Installing
a differential pressure alarm between the flowmeter’s inlet and outlet can
alert operators when the rotor is obstructed, triggering the bypass pipeline to
ensure continuous fuel oil flow. Designers should incorporate an automatic
switching system when specifying the flowmeter.
4. Flow Rate and Mechanical Integrity:
Given that this type of flowmeter consists of multiple moving parts, operating
at excessively high speeds can cause mechanical damage. Therefore, when
selecting the fuel oil flowmeter’s specifications, it is important not to
operate the fuel flow sensor at the upper or lower extremes of its flow range,
as this can compromise accuracy. The recommended operating range is between 20%
and 80% of the meter’s upper flow limit. If it is challenging to balance both fuel
flow meter diameter and flow range, priority should be given to maintaining the
optimal flow working range, with adjustments made to the measuring section
diameter if necessary (e.g., using a reducing joint).
5. Special Media Handling: For special
media, such as two-phase flows or corrosive fluids, a specialized volumetric
flowmeter should be selected. However, it should be noted that the available
specifications for such meters are often limited. In cases where standard
options do not suffice, it may be necessary to consult with the manufacturer to
discuss customized solutions.
Installation, use and
maintenance of mechanical fuel flow meter
When installing and maintaining volumetric
type fuel oil flow meters, particularly for applications involving precise
measurements such as trade or standard delivery, adherence to the following
guidelines is essential to ensure accurate performance and longevity:
1. Install Site Selection: The
installation site should conform to the fuel flow meter's operational
guidelines, ideally being indoors. If outdoor installation is necessary, a
protective enclosure should be used to mitigate the effects of environmental
exposure. For sites with explosion-proof requirements, select a flow meter that
meets the necessary explosion-proof ratings.
2. Installation Precautions: Ensure that
the flow direction indicated on the fuel flow meter aligns with the actual
fluid flow, such as gasoline, diesel. If needed, install a check valve to
prevent reverse flow, except in cases where a bidirectional flow meter is
specifically designed for such purposes. Before installation, the upstream
piping must be thoroughly cleaned, followed by the installation of a filter and
the flow meter. In certain cases, an air eliminator may be required. The valve
used to regulate the diesel flow should be positioned downstream of the flow
meter to maintain a fully filled pipeline during operation. When connecting the
flow meter to the pipeline, avoid applying mechanical stress to the meter's
housing that could cause deformation. The pipeline should be securely supported
to prevent movement, and sufficient clearance should be provided around the flow
meter for maintenance access.
3. Pre-Installation Testing: For fuel flow
meters used in trade or precision measurement applications, metrological
performance should be verified before installation. The diesel flow meter
should only be installed after confirming its accuracy and ensuring operation
within the optimal flow range, as indicated in the calibration certificate.
4. Commissioning: When putting the PD type
fuel flow meter into service, open and close the valves gradually to avoid
sudden pressure fluctuations that could damage the metering instrument. During
initial use, verify that the actual fuel flow rate is within the specified
range for the flow meter, and adjust the flow meter device specifications if
necessary.
5. Compensation for Standard Conditions: Volumetric
flow meters measure fuel volume flow under actual operating conditions
(pressure and temperature). However, measurements often need to be reported as
volume or mass flow under standard conditions. This requires simultaneous
measurement of temperature and pressure, with the signals fed into a
compensating display instrument to calculate and display the corrected flow
values under standard conditions (typically 20°C and 1 standard atmosphere).
Users should be particularly cautious when interpreting flow meter readings for
gases, as accuracy is compromised without temperature and pressure
compensation.
6. Maintenance: Due to the presence of
numerous moving parts, regular lubrication of bearings and gear meshing points
is essential to maintain the transmission mechanism's flexibility, accuracy,
and service life. Over time, filters and air eliminators should be cleaned of
debris and sediments to maintain their effectiveness.
7. Calibration and Monitoring: To maintain
measurement accuracy and detect potential issues early, the flow meter should
be calibrated regularly according to the requirements of the measurement
authority or the validity period of the calibration certificate. If feasible,
clean the interior of the flow meter periodically to ensure it remains in
optimal working condition.
8. Rotor Monitoring: The rotor components
of volumetric flow meters are in direct contact with the fluid and rotate at
high speeds, making them susceptible to blockage, contamination, jamming, and
wear. To monitor these conditions, a differential pressure gauge should be
installed at the flow meter’s inlet and outlet. Use the differential pressure
reading at maximum flow during normal operation as a baseline for monitoring.
If the differential pressure becomes excessive at maximum flow, the fuel flow
meter may require inspection.
9. Fuel flow meter manufacturer support:
Given the diversity of volumetric flow meter designs and potential failure
modes, it is crucial to thoroughly review the user manual. In the event of an
unidentified fault, contact the fuel flow meter manufacturer for assistance
with repairs and calibration.
10. Measurement Dispute Prevention: In
trade measurement applications, to prevent disputes, the metrology department
should secure the flow meter’s error adjustment mechanism with a seal or lock.
The verification period should be clearly defined, and the meter should be
regularly calibrated against a higher accuracy standard, potentially at a
certified metering station.
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