Filter Bleed-Through, the Myth, the Reality and the Solution

Preface:

Filter “Bleed-Through” is a condition existing primarily in the Bio-Pharm marketplace within Class A areas (fully filtered ceilings). Although a Band-Aid is not required, the outcome of such encounters, when dealing with end users that have a clean room off-line, can literally be “bloody” (possibly the real history for the term “Bleed-Through”). Filter “Bleed-Through” can be defined as the measurement of background filter penetration exceeding the leakage specification during field certification.

For example, if the percentage (%) penetration over the entire face of a filter measures 0.02% and the maximum percentage (%) penetration leakage specification is 0.01% , you are experiencing Bleed-Through. This is extremely troublesome to end users where downtime can very quickly become extremely costly in terms of lost production.

Several key factors can have an effect on and/or result in filter Bleed-Through:

• Inappropriate Filter Specifications
• Filter Face Velocity
• Test Particle Size

There are misconceptions in the industry concerning the true cause of filter Bleed-Through. This article reviews these misconceptions (myths), provides insight on the true mechanisms resulting in Bleed-Through, and recommends solutions.

Bleed-Through, the Myth:

It is generally thought within the industry that filter or media manufacturers have made a substantive change that has caused Bleed-Through. In most cases, the blame is directed at the media. The claims being made are:

• The filter manufacturers are using cheap media
• New medias are thinner than MIL-SPEC media, resulting in higher penetration.

Certainly, the newer standard media are less expensive and thinner than MIL-SPEC media. The standard media grades utilized by Camfil Farr typically have the same percentage (%) penetration specification as the MIL-SPEC media grades *(Remember: percentage (%) penetration is percentage (%) penetration regardless of how you measure it). In identical configurations, these different media grades would perform the same, with respect to percentage (%) penetration. Therefore, media thickness, in this case, has no impact on penetration performance. It does, however, have an impact on pressure drop and its capability to stand up to very harsh conditions.

As a consequence of higher tensile strength, MIL-SPEC grade media has a pressure drop penalty of nearly 20%.

Bleed-Through, the Reality:

What is the reality or true cause of filter Bleed-Through? As mentioned earlier, the primary causes are related to Inappropriate Filter Specifications, Filter Face Velocity and/or Test Particle Size. Let’s explore each of these possibilities to understand how they impact on filter Bleed-Through:

Inappropriate Specifications: This is the start or origin of most filter Bleed-Through problems. The typical Face Velocity specified to filter manufacturers for HEPA filters used in Class A application areas is 90 or 100 FPM. These specifications do not usually set the maximum utilization velocity that the filters will be subjected to in their actual application (in situ). Since velocity has a significant impact on penetration, the maximum utilization velocity should be the actual test velocity used by the filter manufacturer to guarantee compliance with field testing conditions. Another specification issue is attributed to the efficiency and leakage specification. Most specifications are written referring to industry-recommended practices such as IEST (Institute of Environmental Science and Technology) or utilizing the verbiage contained within such a document. Most, if not all bio-pharm facilities specify a “type C” or performance indicative of a “type C” filter. The performance level specified here is a minimum global efficiency of 99.99% on 0.3 micron particles and a fully leak-tested (scanned) filter with a maximum leakage rate of 0.01% (which is identical to the global efficiency minimum penetration). The recommended practice of IEST recommends laskin nozzle generated aerosols for leak testing due to this issue of the maximum leakage penetration value being identical to the minimum efficiency value. This helps because the mass mean particle size diameter of a laskin nozzle generated oil aerosol is in the order of 0.7 microns in diameter. This eliminates problems with background penetration and allows you to look only for leakage *(Note: a leak is not particle size selective). If thermal aerosols are utilized, the mass mean particle size becomes much smaller, resulting in potential filter Bleed-Through problems by design. Since more of these smaller challenge aerosol particles will penetrate, the filter will have a lower filter efficiency versus these smaller particles when tested in situ.

Specifications do not address this issue, and leave the field testing requirements up to the certifier. In fact, in many cases, field testing requires the use of thermally generated aerosol (which generate smaller challenge aerosol particles by design) to achieve sufficient concentrations, which in turn will lead to a higher penetration/lower efficiency filter when tested in the field.

Filter Face Velocity: As stated above, Filter Face Velocities are typically specified at 90-100 FPM in bio-pharm applications. However, the actual velocities in situ are usually significantly higher. It is not unheard of to see Filter Face Velocities of 120, 140, 150 or even up to 180 FPM in the field. This upward shift in velocity rather dramatically impairs filter efficiency.

As an example, on the following chart:

As shown in the table, if the in situ application subjects the filter to a higher than specified velocity, the filter efficiency drops below the 99.99% level, resulting in Bleed-Through in the field. Keep in mind that if a laskin nozzle generated challenge aerosol is utilized, the possibility of Bleed-Through due to a high application velocity is greatly diminished.

Test Particle Size: As stated previously, most, if not all bio-pharm facilities specify a “type C” or performance indicative of a “type C” filter. The performance level specified here is a minimum global efficiency of 99.99% on 0.3 micron particles and fully leak tested filter with a maximum leakage rate of 0.01% (which is identical to the global efficiency minimum penetration). The “type C” requirements specify efficiency testing with 0.3 micron diameter thermal DOP. In Class A areas (fully filtered ceilings), field certifiers utilize portable thermal generators to achieve sufficient upstream concentrations. The problem with these generators is that they generate a particle size equivalent or very close to a typical filter’s MPPS (Most Penetrating Particle Size). If a factory-tested filter just meeting the 99.99% @ 0.3 micron efficiency specification is then tested with thermal aerosol in the field, it is likely to exhibit Bleed-Through since the efficiency in the field tested MPPS range will always be lower than at the 0.3 micron factory efficiency testing *(likely in the range of 99.996% -99.98%). This is typically not a problem for Bio-Safety Cabinets or Terminal Housings since a laskin nozzle generator is utilized.

*NOTE: you significantly compound the Bleed-Through issue when testing in situ at higher face velocities utilizing smaller sized (MPPS range) particles.

Filter Bleed-Through, the Solution:

The solution is quite simple. The filter specified/purchased by end users should be rated at an efficiency/particle size and maximum velocity to guarantee acceptance when tested with a thermal generator in the field. Simply speaking, Camfil Farr would recommend a filter efficiency purchasing specification of H14 per EN1822 (a minimum efficiency of 99.995% @ MPPS). This performance level would be specified at the maximum velocity to be encountered in situ. The leakage threshold would be set at a maximum of 0.008% at the factory to guarantee 0.01% scanning results in the field.

Summary:

Although filter Bleed-Through has been thought of as a mystery caused by media and/or filter manufacturers, the root of the problem clearly stems from many possibilities. It is evident that a key factor for filter Bleed-Through is related to particle size. The particle size issue stems from the use of portable thermal generators. The use of these generators is typically restricted to Class A areas to achieve sufficient concentrations. Bleed-Through, therefore, generally occurs in these applications and not in applications such as Terminal Housings or Bio-Safety Cabinets. It is vitally important that both end users and filter manufacturers develop an appropriate filter specification, as proposed in the solution section, to guarantee that all filters purchased meet the field testing requirements.

Filters are and will remain a critical part of the installation to maintain the cleanliness required in bio-pharm manufacturing and packaging facilities. Camfil Farr is proud to be the leading manufacturer supplying clean air solutions to this industry.