Filtration
Tales & Mystical Solutions
provided
by: Bob Agnew
bagnew@fischer-robertson.com
FISCHER-ROBERTSON
FILTRATION NOTES Issue: 1 Volume: 1
October 2006
Given the time of
year with Halloween around the corner I felt it somewhat appropriate to address the magic
and mystic behind filtration while attempting to travel into the dark side of filter
vessel and media sizing. As you continue reading this document please realize that the
statements, suggestions and rather opinionated format is that of the author only and is
not intended to reflect the views of the filtration profession as a whole.
Being a field filter
applications fellow I see a trend in client side confusion regarding filter sizing,
selection, and their relevance upon acquisition and annual costing dollars. To this end
I've prepared the following in hopes of removing the cloak thus enlightening the
filtration application and selection thought process and its influence on filter costing.
Disappearing
Filter Dollars
"I use way too many filters..."
Being an ex-pump guy I quickly realized when turned filter professional one needs to take
a systems approach to making a filtration recommendation. The vast majority of the time,
"to many filters" is the result of a bad pump and filter marriage. Most rarely
appreciate the dirt loading pressure that must be available to optimize the filter media
(use the filter media to its terminal debris loaded differential pressure) thus the
majority of high annual usage requirements can be contributed to a centrifugal pump which
cannot support the added pressure load of the filter, a positive displacement pump where
the relief vale opens prior a filters terminal pressure, or an air diaphragm pump that
drops below the minimum flow requirements of the process as the filter debris load
pressure rises. Bottom line... the tossing of 30-60% of every filter dollar spent, not to
mention freight to get the things, and the time and money to change and dispose.
Given this set of
operating conditions some elect to increase the size of the filter to obtain longer run
time between change. While this approach may complement the less process interruption
objective truth be known the client continues to experience high annual filter costs as a
larger filter only serves to complement flux rate (flow/filter area) which affords a
higher dirt holding capacity system however, the misconception is this increase does not
serve to optimize the filter dollar as the dirt load window issue remains the constant.
Again, the disappearing of 30-60% or more or every filter dollar.
The return on the
investment for an appropriately suited pump given a filtration requirement can be a
fraction of the cost of annual filter usage.
Unfortunately for the
majority of filter clients high filter usage happens at selection conception. It appears
that while the prime process mover (pump) and filter are key contributors to process
success rarely do these two engineering disciplines discuss or communicate the performance
influence each will have upon the other and the client is left with high post installation
filter usage and the numerous filter sales wizards who promise to reduce these costs start
to appear shortly after start-up.
Once installed annual
filter purchases become somewhat routine, monies are on the table from the budget, and the
client knows what his yearly filtration costs are going to be... deduct 3-5-10% by dueling
vendors and the buyer shows his savings. A clients market is generally as competitive as
the filter suppliers, when small percentage reductions don't exceed a clients
manufacturing increase eventually the buyer and his engineering group gather and take the
risk, they actually select a competent, performance proven filter supplier with a process
approach to his reduction effort in hopes it will prove the realization of a 20-40-60%
total cost of filtration reduction without requiring a capitol expense... this is the
filtration magic only a few performance proven filter wizards can provide.
Many clients
perception of a salesman; not to mention a filter salesman, is that we all fall somewhere
between selling elephant ears at the fair and golf balls at the local sports outlet.
Rightly so, as the majority of filter fellows have little to offer than " I've got
the same thing for less... " feeling this meets their total cost of filtration
reduction claim expressed in the company mission statement. It appears we, with some help
from those marketing fellows created the image thus for a filter professional being taken
seriously presents a daunting task. Truth be known, any filter buyer can buy any filter
any day for less... assuming performance and client side quality aren't to high on the
priority list. Point here is given the high filter usage and pump scenario I've presented
previously a buyer may save 5% on purchase however, should he have a pump and filter
conflict the 30-60% on every filter dollar remains part of the disappearing act.
High annual filter
usage reduction drivers come in a variety of solution approaches depending on the
application. A successful total cost reduction effort generally requires working within
existing installed process components, e.g., filter vessel size and design, pump and
drive, controls, etc., given these process characters the filter guy goes to work,
addresses the flow and pressure influences, cleanliness goal, and identifies current
limitations.
Once the current
filtration process in its entirety is understood a debris load based on current filter
media and change can be somewhat established as a baseline, a hydraulic model can be
assembled and the most suitable filter technology given the task naturally surfaces as the
effort priority pyramid develops.
With the model in
hand, optimal clean and debris loaded filter pressures and flow values established pump
curves and driver horsepower's are reviewed.
It is not uncommon
when working within existing components that positive displacement pumps can handle the
new pressure at flow values by simply increasing the relief valve settings and most PD
pump drivers have been sized to handle the required increase in horsepower load.
Centrifugal pumps
present more of a challenge, on occasion a larger diameter impeller can be installed with
ample horsepower available to accommodate change, more often however, a new pump with
steeper performance characteristics is required and in many instances this pump can use
the existing drive arrangement with little change in plant real estate.
Air-diaphragm pumps
require a different approach, generally a larger pump operating slower will accomplish the
task. Acquisition costing value of a new pump would be considered low opposed to the total
reduction savings and operating a larger AOD pump slower reduces the scfm @ psi compressed
air values further complementing the reduction effort with residual savings in compressor
air drying, reduced load on an already stressed plant air system and lower maintenance /
refurbish cost of both pump and air compressor.
Marketing
& Magic
Filter Technology Makeover...
We all love marketing guy's, on occasion they come up with pretty cool commercials, nifty
incentives to promote products, and killer filter buzz words. At times one might think
these fellows work in a high rise in lower Manhattan where top models have photo shoots
and lunch is 9-5 at a Cheers want to be. Some take it a step further making the
"Single Source Filter Store" appear as an attractive filter partner. It would
most likely benefit the filter guy and his client more should these creative individuals
move into R&D where the real solutions to difficult filtration challenges evolve as a
result of out of the box thinking creative engineering.
In the fancy
brochures, Power Point presentations and the web based information highway, it should be
what a client doesn't see, read, or view that makes him suspicious of the various filter
technology claims to fame.
The melt blow
polypropylene depth cartridge... nowadays we hear meltblown this meltblown that when in
fact the majority of times this filter technology is marketing opposed to performance
driven. Its pristine white appearance photos nicely, looks great in a bathing suite when
placed next to a string wound or ugly duck resin bonded cartridge, and is priced to drive
off the showroom floor. Sounds all to familiar, until you get it home.
What isn't shared
with John Q filter public is the fact that on the average it would require 33% more of the
melt blow filter cartridges to equal the task of the old string wound filters that anyone
over the age of 35 is familiar with, and surprisingly it would require and average of
71% more of the melt blow cartridges to equal the performance of the ugly resin
bonded cartridge. Realize the price of the later resin bonded cartridge will be somewhat
higher that 71% works more to between 30-50% taking note that the interruption and yield
loss for change is not included within these figures.
As with many products
new or improved doesn't necessarily equate to better, it all boils down to the right tool
for the job thing. After all, for most of our needs that 25 year old Craftsman wrench
works just as well, if not better (I prefer the thicker handles myself) as those expensive
new ones shown in the extraordinary display case at Sears.
Myth: Surface
Area & Pleated Media where more is better...
As filter guys we're brainwashed when attending Filter School 101 whereas, more surface
area equates to longer filter run times and fewer pleated cartridges required this is not
followed with providing test solution and media remain constant. In the real world
of filtration things just aren't that simple. Take off the blinders and one realizes its a
surface area media combination that largely influences filtration costing values. The key
to selecting a pleated cartridge is to look deeply into flow profile (psid / gpm), and DHC
(debris holding capacity) thru terminal pressure.
A pleated
polypropylene cartridge may provide 7.2'2 media whereas a
microglass pleated cartridge 5'2 however it would require 80%
more of the pleated polypropylene cartridges to equal the life cycle of the micro glass
cartridges. Better yet, acquisition of the absolute rated microglass cartridge will be
less than a comparatively rated pleated polypropylene. Generally, this type of performance
difference is experienced when discussing 0.45µ-5µ filter cartridges. Point here is
filter guys are trained to say more area for the dollar after all that's what it says in
the nicely done brochure, what isn't in the brochure is the fact the micro glass pleated
cartridge with less area wins the flow profile comparison which complements flux and the
technology itself provides a higher DHC.
Urban Legends
Bag Filters...
Not to be outdone by their pleated and depth cartridge counterparts, the filter bag
marketing guys in upper Manhattan have been hard at work as well. Scribbling on napkins at
the 19th hole has provided us filter guys with high-efficiency, high-dirt holding capacity
and pleated filter bags all promising to reduce the total cost of filtration. Some have
even gone so far as to dress up the meltblown polypropylene technology to fit into a Size
#2 suite.
Fact is the age old
question remains "... which is better for me, bags or cartridges" and the age
old answer is, "... if it walks like a duck, and quacks like a duck, then a duck it
is" Back to the right tool for the job thing, if the application is best suited a
duck, then install a duck. Recognizing when to employ filter bag technology is not as
difficult a challenge as some might have others believe. Upon addressing all the
application variables the more appropriate of the two e.g., filter bags, cartridges will
naturally surface.
This happens during
the filtration process priority pyramid construction and influence phase of a well thought
out filter selection process. Unfortunately during process conception many a
recommendation and selection is based on limited options in, meaning the bag filter vendor
may not have a suitable cartridge offering, or decisions are based solely on capitol
expense opposed to annual filter costing, change, and yield loss values.
Given the competitive
climate client side the filter user is becoming much wiser and realizes significant
manufacturing cost reductions can be afforded the bottom line thru wiser choices in
filtration technology and filter vendor.
To avoid market share
loss bag filter fellows have introduced high-efficiency and high dirt holding capacity
filter bags and pleated cartridges. Actually pretty innovative and resourceful but the
fact remains, if its a duck, its a duck and should the application require high-efficiency
and high dirt holding capacity then chances are its a cartridge filter requirement. This
is not to say applications don't exists where this technology would prove a wise choice,
rather its practical and cost effective application is somewhat limited.
Given a 25µ
requirement where 90%+ efficiency is the cleanliness target and 25gpm the flow constant
one might consider a high-efficiency / high-DHC single Size #2 filter bag. An option would
be to consider a 6 or 8-round suitable cartridge vessel/media combination. Realizing the
cost of the respective vessels will be close an annual filter usage and change comparison
would be 2.3 interruptions for bag change opposed to a singular change of a 6-round
30"L cartridge system and 3.5 bag changes in comparison to an 8-round 30"L
cartridge system. Costing values will vary, for sake of this discussion we'll consider a
slight edge to the cartridges. Point is, time for change and yield loss can present a
significant price adder to the filtered/pound of product.
Bag filters perform
extremely well in batch application and those tasks where the solution is low viscosity
and the density of the debris to be removed is heavy while the debris characteristics are
irregular and hard. This combination affords a separation affect within the bag whereas
the debris falls below critical carrying velocities settling within the void are of the
bag. Unfortunately, most bag filters are somewhat undersized and flow velocities are high
as with the subsequent bag change and purchase values.
Rheological
Magic
Where the fluid is the filter medium...
Should you have gotten this far then I assume your associates perceive you as
being a bit on the creative, if not goofy side. This is actually a good thing, these folks
left the page after the animation loaded when they realized most of what's being discussed
is rather spooky to them, the topics go against what has been professed by others and
frankly, they most likely don't posses the ability to get up from the campfire and take a
walk around the dark side.
Viscous filtration is
a highly regarded secret within the filtration wizards society. The rules are cloudy, and
the media employed mystical. Having personally been involved in classifying 90,000cPs
solutions, and removing gels from 1,200,000cPs fluids, I feel these successes, among other
10,000 and 100,000cPs applications entitles me to a paragraph or two on the subject...
Most filter fellows
will walk away or provide large expensive solutions to viscous filtration challenges.
Doesn't need to be this way. As I've discussed previously, it all starts at conception
when the priority pyramid is in the development process. Most size a filter system based
on clean differential pressures remaining below 1-2psi for filter bag technology and those
that dare may size a cartridge system with a clean pressure differential target of
20-25psi or less. Thus the beginnings of a large expensive filtration system not to
mention post installation costing drivers.
A process system is
somewhat easy to bracket at conception whereas, a 150psi system would be the least
expensive capitol baseline, move up to a 200-300psi system the price just increased, and a
1000psi+ obviously the company best inherit some capitol from another project.
For the benefit of
this discussion I will use the 150psi system. When filtering viscous products it's pretty
much common place 50% or more of the positive displacement pump work available is required
simply to transport the product thru the system and required components. Size a filter
system with a low clean pressure differential and ample room exists to optimize the filter
media. Filter systems sized for these conditions represent the large capitol intensive
vessel/media combinations.
Example (whereas the
solution is 10,000cPs, flow @ 25gpm, filter media is a constant):
Clean Pd = 10psi: 80 pieces of a
30"L cartridge costing value: 19,000
Clean Pd = 25psi: 32 pieces of a
30"L cartridge costing value: 11,000
Clean Pd = 52psi: 16 pieces of a
30"L cartridge costing value: 7,000
Clean Pd = 100psi: 8 pieces of a
30"L cartridge costing value: 4,000
Obviously, the
8-round 30"L vessel/media combination represents the least capitol required. Should
the yield of the 8-round satisfy the established change driver objective then the 8-round
vessel/media surfaces as the optimum filtration system. It is not uncommon with post
installation applications to find vessel/media combinations sized for much lower clean
Pd's, where the change driver is batch driven and 60-80% of the filter media remains
unused... back to the disappearing filter dollars.
In filter school we
as filter fellows are taught to size systems within the clean differential pressure
guidelines, those presented previously. We are also taught to meet the clients cleanliness
target request upon recommending and selecting the appropriate filter media. As with any
educational institution we become somewhat hypnotized and accept these guidelines as one
of those don't cross the line things. These guidelines present two extremely costly client
side issues, high vessel acquisition costs and perhaps the more disturbing higher
annual filter usage. Other oversizing costing influences include yield loss, change time,
operator ergonomics, and plant real estate required.
When working with
elevated viscosity applications the filter media selection will prove one of the more
challenging tasks for even the most seasoned filter professional. Communication between
vendor and client is critical, and identifying the cleanliness retention required becomes
a must realizing the system as a whole is somewhat pressure driven. Generally, at
conception the client has a cleanliness target available, for sake of discussion we'll
choose 25 micron. Should this retention be clarity based (no visual debris) or debris size
driven when working with viscous fluids experience has proven rheology influences filter
media retention and subsequent performance. To employ a 25 micron filter cartridge when a
75-100µ will meet the cleanliness objective only serves to increase the size of the
system itself and annual filter usage.
Retention statements
with many clients is taboo... a 25µ means a 25 micron filter must be employed without
regard to the added total cost of filtration should a 75-100 micron serve the cleanliness
goal. While generally a non-issue discussion with 30cPs and similar solution, viscous
filtration gets help from the product being filtered in forms of the boundary layer of
product within the filter media itself. The higher the viscosity value the
"thicker" the boundary layer. Thus, as a 25 micron particle that would generally
pass a 75-100 micron media is now removed from the bulk flow trapped within the boundary
layer of the solution surrounding each fiber of the media matrix thus the porosity value
of the cartridge at 1cPs decreases dramatically when handling 10,000cPs, 100,000cPs ...
and so on.
There are numerous
books discussing rheology, viscosity, and boundary layer theory. Nothing better proves the
discussion than experience and what we know for fact:
*At 90,000cPs a 90%
75µ filter cartridge will remove visual debris
*At 1,200,000cPs a
150µ filter cartridge will remove gels while extrusion of the gels will occur should
the viscosity be reduced to 750,000cPs at which time a 75µ cartridge serves the
filtration task
*At 10,000cPs a 50µ
filter cartridge will meet a specific client cleanliness target of 25µ
*At 100,000cPs a 75µ
filter cartridge will meet a client specific cleanliness target of 25µ
Note: few
cartridges at these retention ratings can meet the demanding challenges each of these
application possess. Each requirement has a unique set of process requirements and
limitations. Should you have a viscous application extreme caution should be applied
during the sizing and selection process to ensure success.
Unique opportunities
present themselves for those having existing viscous filtration installations. Generally,
at conception the pump drive e.g., gear reducer, motor, coupling, etc., has been sized to
handle the pump manufacturers recommendation for non-overloading performance. Thus, the
existing PD pump can handle a work load to 150-200psi. Should the system currently be
operating where filter change occurs anywhere less than 150psi a portion of every filter
dollar is disappearing.
To realize the
benefits of an increased pressure system simply raise the pump operating pressure to the
manufactures limit. This can generally be accomplished by changing the relief valve spring
or adjust the RV setting. Once the pressure at flow increase is available select a
cartridge that meets the process requirements ensuring it can meet the maximum new system
pressure as this now becomes the filter media max design differential, point where the
pump RV opens. With this new work available the debris loading (pressure) window of the
filter has increased and the filter cartridges meet their maximum loading, thus optimized
dollar and a significant total filtration cost reduction.
Providing the filter
cartridge itself can handle the 150psi differential several process improvement options
present themselves:
System flow can be
increased to 100Pd clean filter pressure allowing a 50psi debris loading window providing
the pump has the ability to meet the new 150psi work target. Thus time to accomplish the
task is decreased without the cost of capitol.
Should flow remain
constant and clean differential unchanged, the increased filter debris window will
optimize filter use resulting in fewer annual cartridges required, less operator
involvement for change, and reduced yield loss.
Depending on the
desire affect, the client can elect to increase flow to a point where the new clean system
filter differential pressure meets the new flow value while affording an ample debris load
window to optimize the filter media.
When discussing
viscous filtration successful and impressive total filtration cost reduction efforts can
prove tremendous result. This type of effort is an ideal Six Sigma Black Belt project and
in most cases the effort is accomplished without the use of capitol and requires nothing
more than intelligent engineering combined with experience and a performance proven filter
media and filter vendor.
RELATED:
Case Study In
Viscous Filtration
I appreciate
this document is a rather unorthodox, opinionated communication of a filtration
discussion. I have alienated everyone I know in marketing, and various filter
manufacturer's look at me a bit sideways (although that was always the case). The purpose
of the discussion was to get the thought process started. I would appreciate hearing your
opinions, should you wish to expand the discussion on a specific topic or application
please feel free to call or mail.
Bob Agnew
bagnew@fischer-robertson.com
November's
Issue: Flow Profile Sizing & Selection Considerations
December's Issue: Flow / Flux / DHC Influences
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