Mechanical diesel flow meter

What is Mechanical diesel flow meter?

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.

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.
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• 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.