Determining the filtration accuracy and flow rate parameters for the hydraulic oil tank breather valve filter requires a two-step calculation based on the tank specifications and system operating conditions. The key is to ensure the filter element "can filter without clogging." The specific method is as follows:
1. First, determine the filtration accuracy: Based on the system cleanliness requirements, infer the filter element's filtration grade.
The core of filtration accuracy is to block foreign particles and prevent them from entering the tank and contaminating the hydraulic oil. Consider two key dimensions:

Hydraulic System Cleanliness Standard (NAS Grade)

Different systems have different oil cleanliness requirements. The required NAS grade must be determined first (e.g., NAS 8-9 is commonly used for construction machinery, while NAS 6-7 is required for precision machine tools). Then, consider the filter element precision based on the grade:
NAS 8-9 (medium-low precision requirements): Select a 10-20μm filter element, which can filter most dust and metal debris.
NAS 6-7 (medium-high precision requirements): Select a 3-5μm filter element, suitable for components such as servo valves and precision pumps.
If the system has special requirements (such as food processing), select an ultra-high-precision filter element (e.g., fiberglass) with a filter fineness of 1μm or less.
The pollution level of the fuel tank's surrounding environment. The environment is a key source of impurities and requires additional adjustments:
Dusty environments (such as mines and construction sites): Increase the base precision by one grade (e.g., from 10μm to 5μm) to prevent excessive dust ingress.
Humid or fumesy environments: Prefer filters with a hydrophobic/oil-repellent coating, with a precision of at least 10μm, to prevent clogging due to moisture and oil mist. Second, calculate flow parameters: Based on the tank's "breathing volume," ensure the filter element does not restrict exhaust/intake. The key to flow parameters is to ensure that the filter element's ventilation volume matches the tank's "breathing requirements" under operating conditions, avoiding abnormal tank internal pressure due to insufficient flow. This calculation involves three steps:
Step 1: Calculate the tank's "maximum breathing volume" (Qmax). Tank breathing volume is determined by both "oil volume change" and "temperature change." The simplified formula is:
Qmax = (tank effective volume × oil volume change rate) + (tank total volume × temperature coefficient)
Tank effective volume: 60%-80% of the tank total volume (the tank should not be completely filled to allow for expansion).
Oil volume change rate: This is determined by system operating conditions. For example, systems with frequent cylinder expansion and contraction should use 5%-8%, while static systems (such as oil storage tanks) should use 1%-2%.
Temperature coefficient: 0.03-0.05 for large ambient temperature fluctuations (such as outdoor equipment) and 0.05 for constant temperature environments. 0.01-0.02 (e.g., for equipment in a workshop).
Example: For a 1000L oil tank (800L effective volume), frequent cylinder actuation (6% volume change rate), and outdoor operating conditions (temperature coefficient 0.04), Qmax = (800 × 6%) + (1000 × 0.04) = 48 + 40 = 88 L/min.
Step 2: Select the filter element's "rated flow rate" (Qe).
The filter element's rated flow rate must be greater than or equal to 1.2 times Qmax (allowing a 20% margin to prevent short-term blockage and insufficient flow), i.e., Qe ≥ 1.2 × Qmax.
Continuing with the previous example: Qe must be ≥ 1.2 × 88 = 105.6 L/min. Select a filter element with a rated flow rate of 110 L/min or higher. Step 3: Adjust based on tank interface specifications
Filter flow is also limited by the size of the mounting interface. Ensure that the filter interface diameter (e.g., G1/2, G3/4 threads) matches the tank vent to avoid "flow restriction" due to an undersized interface. Even if the filter element has a sufficient rated flow, a narrow interface can reduce actual airflow.

III. Additional Adjustments for Special Operating Conditions

High-temperature operating conditions (oil temperature > 60°C): Filter flow should be increased by 10%-15% as high temperatures reduce air density, requiring more airflow for the same volume.

Negative-pressure-sensitive systems (e.g., systems that rely on gravity return): Prioritize low-resistance filters (e.g., metal mesh) to avoid excessive filter resistance, which can lead to negative tank pressure and affect return efficiency.

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