Causes of capacity errors in skid-mounted fuel station dispensers

When using Skid-mounted fuel stations, we may sometimes find a significant discrepancy between the displayed capacity and the actual fuel output. This indicates a potential equipment failure, so we need to investigate the entire system. Before doing so, we need to identify the primary causes. The causes of capacity errors in skid-mounted fuel station dispensers are complex and often result from a combination of factors. We can analyze these factors from multiple perspectives, including the equipment itself, the external environment, human factors, and management. Today, we will focus on the causes of capacity errors in Skid-mounted fuel station dispensers.

I. Equipment Causes (Core Factors)

1. Flowmeter Wear and Aging:

• Cause: The flowmeter is the core metering component of a gas station dispenser. Over extended use, its precision components, such as the piston, cylinder liner, and connecting rod, wear and increase clearances.
• Impact: This leads to "internal leakage" (i.e., increased "slippage") of oil, allowing oil to escape the metering chamber without passing through it. This results in the actual delivered volume exceeding the displayed volume, creating a "negative error" (a disadvantage for the gas station and a benefit for the customer).

2. Sensor Failure:

• Cause: The flowmeter's sensor (such as an encoder), which converts mechanical motion into an electrical signal, has failed or has decreased in accuracy.
• Impact: The flowmeter's rotations cannot be accurately converted into pulse signals, resulting in counting errors and random errors.

3. Solenoid Valve Failure:

• Cause: The solenoid valve responsible for opening and closing the oil circuit and maintaining the preset mode is not closed tightly or operates slowly.
• Impact: This may cause a small amount of oil to pass through the valve without being measured when opening and closing the valve, or may cause "overshoot" (i.e., exceeding the preset amount/liter) during preset refueling, resulting in errors.

4. Inaccurate Factory Commissioning and Verification:

• Cause: During factory delivery or initial mandatory inspection, the metering regulator (such as the gear that adjusts the seal) was improperly set, or the standard metal measuring instrument itself had errors.
• Impact: This can cause the dispenser to have systematic positive or negative deviations from the start of operation.

II. Improper Installation and Maintenance (Main Cause of Accumulated Errors After Long-Term Use)

The installation accuracy and routine maintenance frequency of Skid-mounted fuel station equipment directly impact metering stability. Many errors are not due to sudden failures but rather to accumulated problems caused by long-term lack of maintenance.

1. Installation accuracy is substandard.

• The connection between the meter and the oil pipeline is not perpendicular or aligned: This causes "eddy currents" when the oil enters the meter, disrupting the piston's trajectory and causing metering errors (mostly negative errors).
• Poor grounding: Static electricity buildup on the pump housing or internal circuitry interferes with the sensor's electronic signals, resulting in inaccurate counting (with random errors).

2. Lack of maintenance or improper operation

• Failure to regularly calibrate the meter: According to the "Fuel Dispenser Calibration Regulations" (JJG 443-2015), fuel dispensers must be calibrated every six months. If calibration is not performed beyond this period, wear and aging of the meter will gradually amplify errors.
• Failure to clean filters promptly: If the filters at the fuel inlet and before the meter are not cleaned for a long time, impurities will accumulate, leading to unstable fuel flow and, in turn, metering fluctuations.
• Failure to reset after repair: If the meter or sensor is replaced without recalibrating (i.e., adjusting the meter's "pulse coefficient") and then directly put into use, errors will inevitably occur.

III. External Environmental Factors (Easily Overlooked Indirect Influences)

Changes in ambient temperature and humidity, as well as the oil's inherent properties, can affect actual fuel output through "physical effects." While not considered equipment failure, this can manifest as "capacity errors."

1. Temperature Change (the most significant environmental factor)

• Principle: Oil has the characteristic of thermal expansion and contraction. When the temperature rises, the volume of the oil expands and its density decreases; when the temperature drops, the volume contracts and its density increases.
• Error manifestations:
  • If the oil temperature in the tank exceeds the calibration temperature of the dispenser meter (usually 20¡ãC), the actual "volume" of oil discharged will increase due to thermal expansion, but the dispenser will count according to the calibration temperature, resulting in a negative error (the customer receives more oil, resulting in a loss for the gas station).
  • If the oil temperature is below 20¡ãC, the volume of the oil contracts, reducing the actual amount discharged, while the displayed amount remains unchanged, resulting in a positive error (the customer receives less oil, resulting in a profit for the gas station).
• Note: Skid-mounted fuel stations often store oil tanks outdoors or semi-outdoor. Summer exposure to sunlight and winter temperatures can exacerbate this error.

2. Oil quality issues

• Excessive water or impurities in the oil: Water has a much higher density than gasoline/diesel. If it mixes with the oil, the "actual oil volume" will be reduced (water occupies part of the volume), but the dispenser will still count the total volume, resulting in a negative error (the actual amount of oil delivered to the user is less).
• Abnormal oil viscosity: Diesel oil easily forms wax in low temperatures, which increases viscosity, increasing flow resistance in the pipeline and slowing the actual oil delivery rate. If the meter's counting frequency is mismatched, a negative error will occur.

3. Air pressure effects

• Clogged tank breather valve: When the oil in the tank is reduced, the breather valve cannot properly admit air, resulting in negative pressure inside the tank and preventing the oil from flowing smoothly into the dispenser. The actual oil delivery volume decreases, but the displayed amount remains unchanged (a negative error).
• Air leakage in the oil pipeline: Poor seals in the pipeline joints allow air to be drawn in during refueling, which is counted as part of the "oil volume," causing the displayed amount to exceed the actual oil delivery volume (a positive error, with the user receiving a mixture of "oil and air").

IV. Human Operation and Management Factors (Subjective or Mishandling)

Some errors are not due to equipment or environmental issues, but rather to improper operation or misadjustments, which are avoidable human errors.

Improper Operation

• Frequently Starting and Stopping the Fuel Gun During Refueling: Frequent actuation of the fuel gun switch causes the metering piston to start and stop frequently, resulting in "pulse signal loss." Some oil is not counted, leading to a negative error.
• Incompletely Inserting the Fuel Gun into the Fuel Tank: During refueling, the fuel gun is tilted or not fully inserted, causing oil to splash out of the tank (not received by the user), but the displayed amount is included, resulting in a negative error.

Improper Adjustment of Metering Parameters

• For profit, some gas stations use technical means to modify the "metering pulse coefficient" of the fuel dispenser (such as through motherboard jumpers or software settings). Lowering the pulse coefficient results in "less oil output for the same amount" (a positive error), which is known as "oil theft." Raising the coefficient results in a negative error (excess oil output).
• Note: Such adjustments are illegal and violate the Measurement Law. Investigations will result in significant fines.

Summary and Recommendations

To ensure accurate metering for skid-mounted fuel station dispensers and maintain fairness between buyers and sellers, comprehensive measures must be taken:

• 1. Legal and Compliance: Purchase qualified products with type approval certificates and manufacturing licenses for measuring instruments (CMC mark).
• 2. Mandatory Verification: Actively accept and cooperate with market regulatory authorities' mandatory verification every six months to ensure the integrity of the calibration seals.
• 3. Daily Inspection: Station personnel should conduct daily self-inspections, using a 20L standard metal graduated cylinder to verify dispenser accuracy. Any abnormalities should be immediately decommissioned and repaired.
• 4. Regular Maintenance: Establish a comprehensive equipment maintenance plan, regularly replace filters, and inspect the performance of the oil-gas separator to maintain fuel quality.
• 5. Temperature Monitoring: Where conditions permit, prioritize dispensers with temperature compensation to ensure fair trade.
• 6. Strictly Prevent Fraud: Strengthen internal management, regularly inspect seals and equipment integrity, and eliminate any fraudulent activity.

The above are the reasons why Skid-mounted fuel station dispensers may experience capacity errors. We hope this explanation will be helpful when using Skid-mounted fuel stations. If capacity errors occur during actual use, the best approach is to have a professional metering technician or equipment supplier handle the issue. If you have any questions, please contact us.

Written by

TAIAN SHENGDING METAL CONTAINER MANUFACTURING CO., LTD.

Editor Wang

WhatsApp:+86 152 5486 3111

Email:shengdingtank@126.com