Filter Bleed-Through, the Myth, the Reality and the Solution:
Filter “Bleed-Thru” 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 cleanroom off-line, can literally be “bloody” (possibly the real history for the term “Bleed-Thru”). Filter “Bleed-Thru” 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-Thru”. This is extremely troublesome to end users where down-time can very quickly translate into tens of thousands of dollars in lost production.
There are several key factors that can have an effect on and/or result in filter “Bleed-Thru”:
Inappropriate Filter Specifications
Filter Face Velocity
Test Particle Size
There are misconceptions within the industry concerning the true cause of filter “Bleed-Thru”. This article will review these misconceptions (myths), provide insight on the true mechanisms resulting in “Bleed-Thru” and recommend solutions.
"Bleed-Through", The Myth:
There is a general opinion, within the industry, that filter or media manufacturers have made a substantive change that has caused “Bleed-Thru. In most cases, the blame is directed towards 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 medias 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 causes of filter “Bleed-Thru”? As mentioned earlier, the primary causes are related to Inappropriate Filter Specifications, the Filter Face Velocity and/or the Test Particle Size. Let’s explore each of these possibilities to understand how they impact filter “Bleed Thru”:
Inappropriate Specifications: This is the start or origin of most filter “Bleed-Thru” problems. The typical Face Velocity specified to filter manufacturers for HEPA filters being 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 at the filter manufacturer to guarantee compliance to 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 hence specified 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 micron 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 Thru” problems by design. Since more of these ‘smaller’ challenge aerosol particles will ‘penetrate’, the filter will, therefore, 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. The reality is such that 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 at 120, 140, 150 or even as high as 180 FPM in the field. This upward shift in velocity has a rather dramatic negative impact on filter efficiency.
As an example, on the following chart:
Face Velocity vs. Efficiency
As you can see 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 and the result is “Bleed-Thru” in the field. Keep in mind, that if a laskin nozzle generated challenge aerosol is utilized, the possibility of “Bleed-Thru” 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 hence specified 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 0.3micron diameter thermal DOP. In Class A areas (fully filtered ceilings), field certifiers utilize portable thermal generators in order to achieve sufficient upstream concentrations. The problem with these generators, is that they are generating a particle size in a size range very close to or at 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 will likely exhibit “Bleed-Thru” 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-Thru” 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 stated, 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.
Although filter “Bleed-Thru” has been thought of as a mystery caused by Media and/or Filter Manufacturers, it is evident that the root of such a problem stems from many possibilities. It is clear that a key factor for filter “Bleed-Thru” 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-Thru”, 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 will 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.
Filter Classifications Table