HEPA/ULPA filter replacement criteria and practical operation methods
The replacement of HEPA/ULPA filters should be based on a comprehensive judgment of scientific testing data and actual operating conditions to avoid cost waste caused by premature replacement or failure of cleanliness due to delayed replacement. The following are the core judgment dimensions, practical operation methods, and precautions, which are applicable to clean scenarios such as pharmaceuticals, electronics, and laboratories:
I. Core Judgment Dimension (Priority Ranking)
(1) Common determination of pressure differences (the most direct basis
Judgment criterion: When the pressure difference before and after the filter screen reaches 1.8 to 2.0 times the initial pressure difference, it needs to be replaced immediately. If the pressure difference suddenly drops (more than 50% lower than the initial value), it may be that the filter screen is damaged or the installation seal has failed. An emergency inspection and replacement are also required.
Reference range: The initial pressure difference of HEPA filters is usually 200-300Pa (@rated air volume), and that of ULPA filters is 300-400Pa. For specific details, please refer to the equipment manual or the factory inspection report of the filter.
Practical operation method
Record the data of the differential pressure gauge every day and establish the differential pressure trend curve.
If the pressure difference remains stably above the threshold for three consecutive days, or rises rapidly in a short period of time (such as an increase of ≥50Pa per week), the replacement process needs to be initiated.
(2) Failure to meet the cleanliness test standards (core performance basis)
The determination criterion: If the cleanliness of the area covered by the laminar flow hood does not meet the design requirements (for example, the number of particles ≥0.5μm in the A-level area of the pharmaceutical industry is ≤3520 per cubic meter, and no microorganisms should be detected), and other factors such as air flow disorder, environmental pollution, and equipment leakage are excluded, it can be determined that the filter screen has failed.
Practical operation method
Regularly conduct multi-point detection with a particle counter (150mm above the operation surface, with one measurement point per square meter);
If the particle count exceeds the standard for two consecutive tests or positive results are found in microbial sampling and culture, the filter screen condition should be checked first and replaced.
(3) Abnormal physical state of the filter screen (Intuitive judgment)
Judgment criteria: If any of the following situations occur with the filter screen, it can be replaced directly without waiting for the pressure difference or cleanliness data:
The filter paper is damaged, torn, wrinkled, or has obvious stains, mold spots, or unpleasant odors on its surface.
The sealing rubber strip at the edge of the filter screen has aged, fallen off, and cracked, resulting in a gap with the installation frame.
The filter screen frame is deformed and corroded, affecting the sealing effect.
Practical operation method
Regularly (monthly) observe the appearance of the filter screen through the inspection panel, or disassemble and inspect it during maintenance.
For environments with high humidity and high dust, the frequency of visual inspections needs to be increased.
(4) Expiration of the service life (preventive replacement)
Judgment criteria: Even if the pressure difference and cleanliness do not exceed the standard, a preventive replacement cycle should still be formulated based on the usage environment
For general environments (such as electronic workshops, regular laboratories) : HEPA filters last 1-2 years, ULPA filters last 2-3 years.
Harsh environments (such as high dust, high humidity, and sterile pharmaceutical areas) : HEPA filters last for 6-12 months, ULPA filters last for 1-2 years.
High-activity/toxic drug production scenarios: The filter replacement cycle is shortened by 50% or adjusted according to the risk assessment results.
Practical operation method: Establish a filter screen installation ledger, clearly recording the installation date and usage environment. Start the replacement plan one month before the expiration date.
(V) Correlation determination of abnormal equipment operation
Judgment criteria: After the laminar flow hood shows the following phenomena and the fan fault and air duct blockage have been ruled out, the filter screen needs to be inspected:
The wind speed remains persistently lower than the design value (e.g., ≤0.3m/s) and cannot be improved by adjusting the speed of the fan.
The operating noise of the equipment has significantly increased (≥70dB (A)), accompanied by overheating due to fan overload.
Airflow vortices and pollutant accumulation occurred in some local areas, affecting production operations.
Ii. Judgment Process and Precautions
(1) Judgment process
Daily monitoring: Record the pressure difference and wind speed data every day and conduct an appearance inspection every week.
Regular testing: Calibrate the differential pressure gauge monthly and conduct cleanliness testing every quarter.
Abnormal troubleshooting: When there is an excessive pressure difference, unqualified cleanliness, or equipment abnormality, verify one by one in the order of “pressure difference → appearance → cleanliness → service life”.
Replacement decision: When any of the judgment criteria are met, the replacement process is immediately initiated, and the judgment basis and data are simultaneously recorded and archived to the compliance file.
(II) Key Precautions
Avoid single-dimensional judgment: Do not replace merely based on the usage cycle; it is necessary to combine the pressure difference and cleanliness data. Similarly, when the pressure difference exceeds the standard, it is necessary to eliminate the interfering factors, such as improper installation of the filter screen and dust accumulation in the air duct, and then determine whether to replace it.
Data traceability requirements: All test data (pressure difference, cleanliness, appearance photos) must be recorded in detail and kept for at least three years after the equipment is scrapped, meeting compliance requirements such as GMP and ISO.
Special scene enhancement determination
Pharmaceutical sterile area: The frequency of microbial testing should be increased. If microbial content exceeds the standard, the filter screen should be replaced first and re-verified.
Highly toxic/highly allergenic environments: The filter replacement cycle must be confirmed through risk assessment. After replacement, leakage tests (such as PAO smoke tests) should be conducted.
Calibration of detection tools: Particle counters and differential pressure gauges need to be calibrated once a year to ensure the accuracy and reliability of the detection data.
Through the above multi-dimensional comprehensive judgment, the replacement timing of HEPA/ULPA filters can be accurately grasped, which not only ensures the clean performance and production compliance of laminar flow hoods, but also maximizes the service life of the filters and reduces operation and maintenance costs.
The replacement of HEPA/ULPA filters should be based on a comprehensive judgment of scientific testing data and actual operating conditions to avoid cost waste caused by premature replacement or failure of cleanliness due to delayed replacement. The following are the core judgment dimensions, practical operation methods, and precautions, which are applicable to clean scenarios such as pharmaceuticals, electronics, and laboratories:
I. Core Judgment Dimension (Priority Ranking)
(1) Common determination of pressure differences (the most direct basis
Judgment criterion: When the pressure difference before and after the filter screen reaches 1.8 to 2.0 times the initial pressure difference, it needs to be replaced immediately. If the pressure difference suddenly drops (more than 50% lower than the initial value), it may be that the filter screen is damaged or the installation seal has failed. An emergency inspection and replacement are also required.
Reference range: The initial pressure difference of HEPA filters is usually 200-300Pa (@rated air volume), and that of ULPA filters is 300-400Pa. For specific details, please refer to the equipment manual or the factory inspection report of the filter.
Practical operation method
Record the data of the differential pressure gauge every day and establish the differential pressure trend curve.
If the pressure difference remains stably above the threshold for three consecutive days, or rises rapidly in a short period of time (such as an increase of ≥50Pa per week), the replacement process needs to be initiated.
(2) Failure to meet the cleanliness test standards (core performance basis)
The determination criterion: If the cleanliness of the area covered by the laminar flow hood does not meet the design requirements (for example, the number of particles ≥0.5μm in the A-level area of the pharmaceutical industry is ≤3520 per cubic meter, and no microorganisms should be detected), and other factors such as air flow disorder, environmental pollution, and equipment leakage are excluded, it can be determined that the filter screen has failed.
Practical operation method
Regularly conduct multi-point detection with a particle counter (150mm above the operation surface, with one measurement point per square meter);
If the particle count exceeds the standard for two consecutive tests or positive results are found in microbial sampling and culture, the filter screen condition should be checked first and replaced.
(3) Abnormal physical state of the filter screen (Intuitive judgment)
Judgment criteria: If any of the following situations occur with the filter screen, it can be replaced directly without waiting for the pressure difference or cleanliness data:
The filter paper is damaged, torn, wrinkled, or has obvious stains, mold spots, or unpleasant odors on its surface.
The sealing rubber strip at the edge of the filter screen has aged, fallen off, and cracked, resulting in a gap with the installation frame.
The filter screen frame is deformed and corroded, affecting the sealing effect.
Practical operation method
Regularly (monthly) observe the appearance of the filter screen through the inspection panel, or disassemble and inspect it during maintenance.
For environments with high humidity and high dust, the frequency of visual inspections needs to be increased.
(4) Expiration of the service life (preventive replacement)
Judgment criteria: Even if the pressure difference and cleanliness do not exceed the standard, a preventive replacement cycle should still be formulated based on the usage environment
For general environments (such as electronic workshops, regular laboratories) : HEPA filters last 1-2 years, ULPA filters last 2-3 years.
Harsh environments (such as high dust, high humidity, and sterile pharmaceutical areas) : HEPA filters last for 6-12 months, ULPA filters last for 1-2 years.
High-activity/toxic drug production scenarios: The filter replacement cycle is shortened by 50% or adjusted according to the risk assessment results.
Practical operation method: Establish a filter screen installation ledger, clearly recording the installation date and usage environment. Start the replacement plan one month before the expiration date.
(V) Correlation determination of abnormal equipment operation
Judgment criteria: After the laminar flow hood shows the following phenomena and the fan fault and air duct blockage have been ruled out, the filter screen needs to be inspected:
The wind speed remains persistently lower than the design value (e.g., ≤0.3m/s) and cannot be improved by adjusting the speed of the fan.
The operating noise of the equipment has significantly increased (≥70dB (A)), accompanied by overheating due to fan overload.
Airflow vortices and pollutant accumulation occurred in some local areas, affecting production operations.
Ii. Judgment Process and Precautions
(1) Judgment process
Daily monitoring: Record the pressure difference and wind speed data every day and conduct an appearance inspection every week.
Regular testing: Calibrate the differential pressure gauge monthly and conduct cleanliness testing every quarter.
Abnormal troubleshooting: When there is an excessive pressure difference, unqualified cleanliness, or equipment abnormality, verify one by one in the order of “pressure difference → appearance → cleanliness → service life”.
Replacement decision: When any of the judgment criteria are met, the replacement process is immediately initiated, and the judgment basis and data are simultaneously recorded and archived to the compliance file.
(II) Key Precautions
Avoid single-dimensional judgment: Do not replace merely based on the usage cycle; it is necessary to combine the pressure difference and cleanliness data. Similarly, when the pressure difference exceeds the standard, it is necessary to eliminate the interfering factors, such as improper installation of the filter screen and dust accumulation in the air duct, and then determine whether to replace it.
Data traceability requirements: All test data (pressure difference, cleanliness, appearance photos) must be recorded in detail and kept for at least three years after the equipment is scrapped, meeting compliance requirements such as GMP and ISO.
Special scene enhancement determination
Pharmaceutical sterile area: The frequency of microbial testing should be increased. If microbial content exceeds the standard, the filter screen should be replaced first and re-verified.
Highly toxic/highly allergenic environments: The filter replacement cycle must be confirmed through risk assessment. After replacement, leakage tests (such as PAO smoke tests) should be conducted.
Calibration of detection tools: Particle counters and differential pressure gauges need to be calibrated once a year to ensure the accuracy and reliability of the detection data.
Through the above multi-dimensional comprehensive judgment, the replacement timing of HEPA/ULPA filters can be accurately grasped, which not only ensures the clean performance and production compliance of laminar flow hoods, but also maximizes the service life of the filters and reduces operation and maintenance costs.
