About the high-efficiency online leak detection solution for air vents

 

1. Product Introduction:

high-efficiency air filter (HEPA) generally refers to a filter with a filtration efficiency of 99.95% or higher for particles 0.3 μm or larger. It is commonly used as the final filter in pharmaceutical cleanrooms to provide clean air. Whether a cleanroom can achieve and maintain its designed cleanliness level is largely dependent on the performance and installation of the HEPA filter. Therefore, leak testing HEPA filters in cleanrooms to ensure compliance is a key measure for maintaining a clean environment.

 

2. Requirements for Post-Installation Leak Testing of HEPA Filters

The FDA CGMP Guide for Sterile Drug Manufacturing, Building and Facility, states that a leak test should be performed after installation of HEPA filters to verify the tightness of the filter gasket, frame, and filter media. Sterile drug production facilities should regularly perform leak testing on HEPA filters.

 

3. Purpose of Leak Testing of HEPA Filters

The filtration efficiency of HEPA filters is generally tested by the manufacturer, and a filter efficiency report and certificate of conformity are included upon shipment. For pharmaceutical companies, HEPA filter leak testing refers to on-site leak testing of HEPA filters and their systems after installation. It primarily examines small pinholes in the filter media and other damage, such as leaks in the frame seal, gasket seal, and filter structure. The purpose of leak testing is to promptly identify defects in the HEPA filter itself and its installation by checking the seals at the connection between the HEPA filter and the mounting frame, allowing appropriate remedial measures to be taken to ensure the cleanliness of the area.

 

4. IV. Principle of the DOP Leak Detection Method

High-efficiency filters are typically leak-tested using an aerosol generator to generate dust upstream of the filter. A photometer or particle counter (OPC) is then used to measure the aerosol concentration upstream and downstream of the filter to determine if there is a leak. This generation of dust is done because the concentration of dust particles upstream of a high-efficiency filter is relatively low in the atmosphere and varies with location and time, sometimes high and sometimes low.

 

 

Artificial aerosols commonly used include DEHS (Sebacic acid-bis(2-ethylhexyl) ester) and PAO (poly-apha olefin), but the test method is still referred to as the “DOP method.” The FDA specifies that the aerosol used for leak detection must meet certain physical and chemical requirements and should avoid aerosols that could cause microbial contamination or growth. Aerosol generators can be categorized as either thermal or cold. Thermal generators utilize the principle of evaporation and condensation. Atomized aerosol particles are evaporated by a heater and condensed into tiny droplets under specific conditions. After removing oversized and undersized droplets, a mist of aerosol approximately 0.3 μm is left to enter the air duct, with a particle size distribution of 0.1 to 0.3 μm. Cold generators utilize compressed air to create bubbles in a liquid, which are then splashed through a laskin nozzle to produce a physical, polydisperse phase (DOP) aerosol with a maximum distributed particle size of approximately 0.65 μm. Cold aerosol generators are often used for filter scanning and leak detection.

 

There are two types of detection instruments: aerosol photometers and particle counters. The aerosol photometer (hereinafter referred to as the photometer) is commonly used for HEPA filter leak detection. It is a forward-scattering linear photometer consisting of a vacuum pump, a light-scattering chamber, a photomultiplier tube, a signal processing converter, and a microprocessor. Its operating principle is as follows: When an airflow is drawn into the light-scattering chamber by a vacuum pump, particulate matter within it scatters light toward the photomultiplier tube. In the photomultiplier tube, the light is converted into an electrical signal, which is amplified and digitized before being analyzed by a microprocessor to measure the intensity of the scattered light. By comparing the signal with that generated by a reference substance, the mass concentration of particulate matter in the gas can be directly measured.

 

Particle counters, on the other hand, measure the number of particles in the airflow and specify a particle size range. They have high sensitivity and are applicable to all dust aerosol sources, providing a wide range of options. It has been proven by ISO14644-3 that for high-efficiency air filters of the same grade, the test aerosol concentration required by the photometer is 100 to 1000 times higher than that required by the particle counter. Therefore, using a photometer to detect leaks will cause greater pollution to clean air conditioning systems and clean rooms.

 

5. V. Requirements and Methods for Scanning Leak Detection Using an Aerosol Photometer

5.1 Aerosol Introduction and Concentration Requirements:

5.1.1 For HEPA filters in HVAC systems, to ensure a uniform concentration upon reaching the HEPA, aerosol can be introduced directly from the negative pressure side of the system fan.

5.1.2 If introduced from the duct, it should be at least 10 duct diameters from the HEPA.

5.1.3 The required upstream aerosol concentration is typically between 10 and 100 mg/m³. Leak detection sensitivity decreases when the concentration is below 20 mg/m³; prolonged testing may cause filter contamination when the concentration exceeds 80 mg/m³.

 

5.2 Result Evaluation and Intervention

5.2.1 The HEPA filter leakage rate should be less than or equal to 0.01%. If the %LEAKAGE (leakage rate) at all points during the HEPA test does not exceed 0.01%, the HEPA is considered qualified. If any point exceeds 0.01%, it is deemed unqualified and marked for repair or replacement.

5.2.2 HEPA filter media leaks may be repaired with special glue, but the area of a single leak cannot exceed 1% of the total area, and the area of all leaks cannot exceed 5% of the total area. Otherwise, the filter must be replaced.

 

6. Requirements and Methods for Scanning Leak Detection Using a Particle Counter

6.1 Aerosol Introduction and Concentration Requirements:

6.1.1 For HEPA filters in HVAC systems, to ensure a uniform concentration upon reaching the HEPA, aerosols may be introduced directly from the negative pressure side of the system fan.

6.1.2 If introduced from the duct, it should be at least 10 duct diameters away from the HEPA.

6.1.3 The upstream aerosol concentration must reach the required concentration, typically Cumax ≥ 3.5 × 10⁷ p/m³, to avoid falsely identifying a leak due to a low concentration.

6.1.4 Calculate the downstream limit value based on the upstream concentration using the formula.

 

6.2 Result Evaluation and Processing

6.2.1 The leakage rate of a high-efficiency filter shall not exceed twice the filter’s factory-qualified transmittance; the UlHepag value shall not exceed three times the filter’s factory-qualified rate. (GB50073-2001)

6.2.2 The following factors should be considered in the actual calculation of HEPA filter leakage rate (ISO14644-3-2005):

Npa = Cc × PL × qVs × Tr

Npa—Acctual particle count corresponding to the leak detection;

Cc—Upstream concentration;

Ps—Maximum allowable overall permeability of the filter under test;

PL—Point-value permeability of the filter under test; PL = Ps × K

K—Coefficient indicating how much PL exceeds Ps; determined by negotiation between the customer and supplier or according to the table below.

qVs—Standard flow rate of the counter, qVs = 472 cm³/s

Tr—Sampling time

6.2.3 HEPA filter media leaks may be repaired with special glue, but the area of a single leak must not exceed 1% of the total area, and the area of all leaks must not exceed 5% of the total area. Otherwise, the filter must be replaced.

 

 

7. HEPA Filter Leak Testing Cycle

The FDA recommends leak testing every six months in its Sterile Drug Manufacturing Guidelines for sterile pharmaceutical production facilities. my country’s GMP Inspection Guidelines generally recommend annual testing. ISO 14644 recommends a maximum interval of 24 months for leak testing of installed HEPA filters.