Compressed Air Systems: Optimizing Reliability - Minimizing Risk
By
R. Scot Foss
Plant Air Technology
Charlotte, NC
There is a natural assumption that having decent equipment and taking care of it will
guarantee reliability in the system. From this foundation we build on the next error in
thinking which is to believe that this effortful approach will net a minimum risk of
interruption in the facility. Were it only this simple.
Those of us that have operated compressed air systems for any period of time know
that they are one of the most costly and unpredictable of all the utilities in the
modern plant. As we head into this look at systems, my best advice would be: If you
are not going to take a thorough and systemic approach, buy the least expensive equipment
that you can buy. When it fails to meet your expectations, you will not feel nearly as bad
as you would have had you purchased remarkably expensive stuff and netted the same result.
Over the years, we have found that a well-developed system can tolerate less than spectacular
parts and still produce excellent results. Clearly, the best system would be one, which has
been thoughtfully configured with the most reliable equipment available.
One of the dilemmas is that the compressed air industry sells features and benefits. It
is nearly impossible to determine the shortcomings that you may be faced with based on a
discussion with most sales personnel. A basic truth is that it is knowledge of the
shortcomings, not the features that will best facilitate an excellent result.
You can maneuver around the bumps in the road, when you know that they are there.
There isn’t any perfect out there. What usually happens is that the individual responsible
for the plant air systems decisions bases them on experience. This is OK as long as you
don’t define experience as what you got when you didn’t get what you wanted. We have seen
more systems based on bad experience than those based on sound engineering. Often this
approach results in each individual item being evaluated on its own merit, rather than its
ability to integrate into the solution desired.
Risk Planning
One of the first questions is how will we define the term "risk". When we poll
operators, plant engineers, and maintenance personnel in workshops, the majority tells us
that failure in the system is totally unacceptable. Any interruption, curtailment, or
negative impact is failure, therefore a lack of reliability. After all, we are talking
about rotating equipment. It is not a question of whether it will fail. It is a
question of the impact of the failure on productivity. If you develop the system for 100%
reliability and 0% risk, you are going to spend a lot more money on the capital equipment,
which is no guarantee against failure as it has been defined.
Another issue is that production or process could, but seldom participates in managing
the curtailment associated with an element failure in the system. Mature organizations
develop plans to limit the least strategically required demand so that the balance of the
system can be continued in normal service. Another element of this type of strategy is to
have no unit so large that its failure can limit more than an acceptable curtailment. In
many industries, the operating protocol is never to operate a compressor or dryer that is
so large that its failure could cause an interruption of any kind. For most this means you
need at least four or five operating units.
The next step in our failure scenario is to be able to isolate a percentage of the
system equivalent to the worst potential failure. This can be accomplished in an orderly
and automatic format, so to protect the on-line production or process equipment while this
limited portion of the system turns down.
There are a number of systems where the piping is configured so that the various
production areas can be isolated with slow acting valves managed through an SLC that
prioritizes production. Management can change the priorities on a daily basis if desired.
In a just in time environment, inventory on hand can manage a limited curtailment. The
last element of the plan should determine the maximum allowable, time-weighted, curtailments
and the allowable duration. One major industrial corporation plans on curtailment not to
exceed three times per year, not to exceed 25% curtailment with no more than four hours per
incident. This can be planned and managed. We find that a mature and defined approach
towards risk management allows the designer and operator of the system to minimize
capital and operating expense over more traditional protocols.
A common approach to prevent system’s failure is to operate too much part loaded or
throttled power all of the time at as much as 20% pressure than the minimum acceptable value.
This is done so that when a unit fails, the balance will load up preventing curtailment.
The added pressure will support the loss of supply volume and the minimum operating value
during the response. This is done 100% of the time to manage a limited failure event a
slight fraction of 1% of the time. More appropriate solutions will be discussed later in
the article.
Expectations and Operating Philosophies
Nothing can be more frustrating than the attempt to operate a system where the
expectations can rarely be achieved or you operate below minimum acceptable results a
portion of the time. This is all too common a situation. You are operating compressors
designed for 100 psig with a drying and filtering system with an actual differential
pressure of 10 psig. Production expects equal to or more than 90 psig 100% of the time.
This is a losing proposition. The only way that you can meet this condition is to install
parallel filtration and drying to reduce the differential or operate more capacity part
loaded than what is needed. In the modulating mode, this will cause the system’s pressure
to rise above 90 psig at full demand. Using compressors to raise the pressure to 95 psig
will require twice as much power as necessary, while operating the equipment well off its
optimum design. One result will be premature bearing failure of the compressors and freeze
up of the refrigerated dryers. The system’s pressure will more widely fluctuate and you
will find water condensing downstream of the dryers. Re-rating the compressor to a higher
pressure is very expensive and hardly the appropriate solution. The problem is the
expectation. The situation is the symptom of the problem. Ninety five percent of the
time, we are attempting to correct the symptom without ever defining the underlying cause.
Most utilities providers would prefer to operate the supply system beyond its capabilities
rather than risk challenging the expectation. Most of the time in auditing the system, we
find a $100,000 solution to a $100 problem.
Typical Problems that Reduce Reliability
The following are some of the typical problems with the real underlying root causes:
1. Systems pressure drops below supply expectations.
a. A critical point of use application has a dirty line filter.
b. A regulator at a production point of use has been increased by the production worker.
The higher pressure increases the flow to the air user. The higher flow increases the
differential across all of the point of use components as a square function of flow.
You can increase the regulator pressure and reduce the article pressure at the final point
of use. The solution to elevating the article pressure was to change the regulator and
other installation components to the next nominal size. This would increase use pressure
without increasing the supply.
c. If the article pressure dropped 10 psig during the use of air and you wanted to elevate
the pressure 5 psig, you could double the point of use storage downstream of the regulator.
The pressure drop would reduce in half.
d. You have installed a high rate of flow, short cycle air user in an overloaded
subheader or branch line.
When the new user actuates, a critical pressure user complains about low pressure. You
can dedicate storage with a check valve to the critical user. You could also dedicate
storage to the high rate of flow user with a metering valve to flatten the rate of flow on
the upstream side. This would negate the effect of the new user on the rest of the system.
The traditional diagnosis is insufficient supply with the belief that the solution lies in
adding more supply capacity.
2. The pressure dew point fluctuates to levels well above the minimum acceptable results
with water condensing downstream in the system.
a. There is a receiver upstream of the filter and the dryer. The dryer was sized for
the compressor/s but didn’t include the surge capacity of the storage tank. The velocity on
peak events exceeds the capacity of the dryer. There is not sufficient contact time for
drying.
b. A large transient demand event causes a suction negative rate of change on the
downstream side of the clean up equipment causing a high differential and carryover to the
system.
c. One or more drain traps have failed.
d. The dryer was selected based on theoretical conditions with the coolers clean and
the air temperature and relative humidity at the high average, but not the peak. The normal
condition is dirty coolers. Although the peak conditions only occur for a few hundred hours
per year, it is sufficient to create a reliability problem for production. You must design
for your worst ambient and maintenance conditions.
It would not be unusual to determine that you have the wrong type of dryers leading to
the replacement with lower dew point dryers which can cost up to five times more operating
cost that the original dryers. Without the problem definition, the original dryer which
should produce a 40F dew point produced a 70F dew point. Because complaints of water in
the system have ceased, everyone is delighted.
3. One of your large centrifugal base load compressors frequently surges causing the
pressure in the system to shut down critical production applications.
a. A large volumetric event shuts down from time to time. When it does, the compressor
control valves cannot stroke in sync with the event. The result is a surge and shut down
of the compressor.
b. The compressor was specified based on the manufacturer’s conditions rather than the
actual range of conditions for the site including water temperature, air inlet temperature,
relative humidity, and fouling factors for the coolers, inlet filter, and impeller dirt
loading. The natural curve drops during all off design conditions exposing a lower surge
pressure on the unit.
c. A marginal sized dryer and aftercooler are selected on a price sensitive purchase.
The mechanical contractor installs the signal line downstream of dryer. The start up
service adjusts the controls for the compressor discharge pressure rather than downstream
of the dryer. The differential across the dryer and the aftercooler are absorbed upstream
of the signal location at the elevated control pressure. The results are a final stage
compression pressure which is equivalent to the design pressure of the compressor plus
the clean up equipment differential. The control panel on the compressor reads "line pressure"
which is actually the signal pressure downstream of the dryer. No one suspects a problem,
but the unit surges on a regular basis. You can move the signal, but the best choice is
to lower the control pressure to the desired system’s pressure. Make sure that you create a
process flow diagram with all of the process values on it including the set points. Show
the signal line and make sure that you know what the desired results need to be. Also
request "0-0%" performance curves for the compressor based on your metrics and site condition
extremes so that you can properly evaluate the best choice of compressor for the application.
It is not unusual for the original equipment supplier to suggest that this problem can be
solved with a PLC control retrofit or a total automation system ranging in price from $25-$150K.
In some situations, adding storage to the system can control the rate of pressure rise in the
system to synchronize with the valve stroke speed of the compressor controls. In some cases,
the event application in production that causes the problem can be retrofitted with a slow
acting shut off valve that eliminates the rapid rise in system’s pressure. "
Good Trouble Shooting Rules
There are dozens of scenarios like the previous examples of missed diagnosis. The
following are good rules of thumb for trouble shooting a compressed air system:
a. Whatever the problem appears to be, its probably something else. Air systems always
display the symptoms of the problem. You have to dig to find the problem. It can be much
like peeling an onion.
b. The first solution to the problem is seldom the best solution. Air systems are
very interactive and offer nu-merous approaches to each critical issue. If you don’t
have at least three optional approaches to the problem, you haven’t investigated adequately.
c. You have to draw a picture of the problem in order to understand what is actually going
on. The dynamics of the system are too complex to grasp between your ears.
d. The only “Bigger is Better” in the air system is storage capacitance. Bigger
compressors, dryers, and even piping are not better.
e. Whatever you do in the system, do it slowly, other than responding to demand.
Satisfy demand with potential energy and demand controls and then wait as long as possible
to replenish the storage. The longer you wait the least amount of energy you will use.
When the pressure drops in the system, control the rate of decay so that you can wait as long
as possible to add the next available compressor. The longer you wait, the more likely
the event that caused the decay or another air user will stop allowing the pressure to
recover without the add. Wait, wait, wait!!! This ability is available through a
combination of software, control storage, and improved control set points.
Rules for Reliable & Efficient Systems
The following are a series of excellent rules for the development and operation of
compressed air systems:
a. The larger the individual compressor relative to the total demand, the higher the
likelihood that the system will fail and cause production interruption. Never select a
compressor that is so large that its failure can cause the system to fail.
b. Select your base load compressors for the best displacement to power efficiency.
Select your trim compressors for cold start permissive speed to full load and automation
flexibility. Remember that the faster the trim compressors, the longer you can wait to add.
c. Make sure that the total trim capacity of the system is equal to or larger than the
largest base load compressor. This will assist in allowing a back up base compressor to come
up on line in the event of another base failure.
d. Know all of the shortcomings of the equipment you plan on using before buying or
applying it.
e. Write a number of detailed failure scenarios including the elements of supply,
demand, events, temperature, pressure, capacitance, and time. When you know what can
go wrong precisely, you can develop a plan to manage failure without a production
interruption. You can’t buy reliability, but you can plan and design for it.
f. Parallel as much compression and treatment equipment as possible so that the failure
of any part of the system will not take out an entire compressor and dryer-filter train.
g. Benchmark and trend all variables and deltas against design values. This is the
best method of predicting problems in advance of failure.
Know the Right Questions to Ask Before You Buy
Reliability is certainly an important issue. It is important enough to ask the questions
of potential suppliers to make the best possible decision regarding what you may be faced with
if you select their equipment.
a. How much running maintenance downtime is required per year?
b. Does the manufacturer require annual inspections? Can you train in-house people to
do this work? How much out of service time is required?
c. Will anything that can occur as a result of normal wear and tear or fouling cause the
performance of the machine to degrade? How does this occur? What kind of performance loss is
possible?
d. What other compressor capacities and pressure are available in the same frame as the
one that is being presented? We would strongly recommend caution in selecting a unit above
mid-frame unless you are keen on maintenance.
e. What tread able information can we measure and what tests can be conducted that will
anticipate performance degradation?
f. What is the expected life between major overhauls or repairs? What do most overhauls
cost in today’s dollars?
g. What are the ways that this equipment will fail? What is the typical root cause
for these failures? Is there any option, which can offset or correct these potential
problems? What costs can I expect for these options? Are there any applications or
operations advice which will extend the potential for these somewhat predicable failures?
How much lead time is required for parts needed and repair work that needs to be done?
h. Does the local service dealer maintain an inventory of parts for the potential for
both running maintenance and first level failure repair?
i. If you have to change any major components or perform a major overhaul, what are
the risks associated with loss of original performance? If you change out a major stage
part such as an airend, do you have to change out the other stage? Are they matched?
Will the unit need to be tested before it is restarted after the repair? What test
protocol will best determine the performance?
There isn’t any perfect out there. Get familiar with the downside of the equipment.
Tell the sales representative that you want the good and the bad. It isn’t an unreasonable
request. It will help you with instrumentation and maintenance planning.
Summary
Whether you are designing a new system or retrofitting an existing compressed air
system, you can control the reliability, and manage limited curtailment at the precise
level you desire. You can control the front end, maintenance, and operating costs of the
system. Remember that all undefined problems are not insufficient supply. This does not
have to be dealt with as an art form. It is a science. With the knowledge of good system’s
technology coupled with the up and downside of the components applied, compressed air can
contribute in a positive way to improved productivity at lower costs.
July-August 2000
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Installation and Trouble Shooting Air Filters
By
J.G. Barry
Director of Sales and Marketing
Dwyer Instruments, Inc.
Introduction: The Application of Air Filters
Air filters used in ventilating systems are called upon to remove an
extremely wide variety of contaminants from the air, ranging from soot
and smoke and the common dusts to mold spores, bacteria and pollen.
Particle sizes vary from less than a micron (.000039") to insects and
leaves. The concentration will vary hourly, daily and seasonally.
These variables combined with filtering requirements that range from the
simple needs of a household furnace to the absolute filtration required
for a clean room make air cleaning a complex science. Filter selection
should, therefore, be made by a qualified engineer or in cooperation
with the filter manufacturer.
The same variables that affect filter selection also contribute to the
problem of determining when a filter has reached the tolerable limit of
its effective life span as a result of the accumulation of contaminants.
Because of wide variation in contaminant load in the air being filtered,
time is an uncertain measurement. Visual inspection of the condition of
the filter tells little or nothing. The most widely used method of
checking filter condition is to measure pressure drop across the filter
by means of an air filter gage.
The Function of the Air Filter Gage
A clarification of terminology is essential to understanding the
function of an air filter gage. The air filter gage measures pressure
drop across the filter for the purpose of determining whether or not the
filter is operating within its design range of effective utilization. It
does not measure filter efficiency.
Filter efficiency as defined by the ASHRAE GUIDE, "is synonymous with
the term arrestance and measures the ability of the air cleaner to
remove particulate matter from an air stream." Several methods of
determining filter efficiency exist, because of variations in filters
and application requirements.
One method is by weight, in which a carefully compounded mixture of
standardized dust is fed into the air stream and the amount trapped by
the filter related to the amount released in the test determines the
filter efficiency in percentage. A second method, the dust spot method
originated by the National Bureau of Standards, compares the opacity of
the dust collected on filter paper from two air samples of equal flow
rate, one of which samples filtered air. The efficiency of the filter is
thus evaluated on the basis of the darkness of the dust spots. A third
method measures arrestance of DOP (Dioctyl-phthalate) smoke, a
homogenous aerosol of 0.3 micron size, to test a special class of high
efficiency filters used to control particulate matter of high toxicity
or where extreme cleanliness is required.
The "dust holding capacity" of an air filter is defined by ASHRAE GUIDE
as, "the amount of a particular dust that an air filter can hold and
determines the operating life of the air cleaner which is fixed mainly
by some figure of tolerable resistance." Thus, an air filter gage,
sensing the differential in static pressure across an air filter, will
permit the air filter to be used to its maximum dust holding capacity as
determined by the filter manufacturer and specified in terms of maximum
filter resistance. It should be noted that servicing a filter when it
has reached its point of specified maximum resistance is the only method
which will guarantee that the filters are being used to their full dust
holding capacity and at the same time not being used beyond the point
where their increased resistance could affect balance of the system,
increase air leakage and, for some types of filters, begin to pass
contaminants.
Type of Filters and Recommended Resistance for Servicing
All filter manufacturers supply technical data which will permit full
utilization of their filters. This data will include initial resistance
in inches of water column for the filter at its rated air flow and a
recommended resistance at which point the filter should be replaced or
serviced. The following describes the broad classifications of air
filters: Viscous Impingement Filters have the filter element treated
with an oil or adhesive which holds dust particles coming in contact.
The initial resistance of a typical filter will usually run from .08 to
.15" water, and servicing will be called for at .50" water. In the
automatic moving curtain type, resistance will usually be maintained
between .40 and .50" water by means of a pressure actuated switch or
timer with overriding pressure switch operating an electric drive. Dry
Type Filters are available in a multiplicity of materials, in varying
thicknesses, in batts, woven or bonded materials, natural or synthetic,
and in pleated form or in tubes or bags to obtain greater filter area.
Efficiencies are usually at least equivalent to the viscous impingement
type and may run to virtually 100%. Dust holding capacity is high and
resistance values vary so widely no rule of thumb figures are possible.
The manufacturer should always be consulted for proper initial and final
pressure drop figures for these filters.
Electronic Air Cleaners using the electrostatic precipitation principle
have no characteristic increase in pressure drop as they accumulate dust
and must therefore be serviced on a pre-set schedule. The mechanical
filter normally used with such equipment should, however, be serviced on
the basis of increased pressure drop.
Significance of High or Low Initial Resistance Readings Due to the many
variables inherent in an air distribution system, the initial pressure
drop in some systems may be found to be below that specified by the
filter manufacturer. If the system has been carefully checked over and
found to be in order, this need not be a source of concern, as it simply
indicates that less than rated air volume is passing through the filter.
Generally, this will mean increased filter life before the filter
requires servicing. Conversely, if initial pressure drop exceeds the
filter manufacturer's rating, it indicates that a greater volume of air
is being handled than the filter is rated for and filter life will be
shortened.
Excessive variation from the filter manufacturer's published initial
resistance figure will probably indicate one or more of the following
problems:
Low Pressure Drop
1. Less than rated air volume being handled due to over design of filter
area, improper air balancing of system, open bypasses, etc.
2. Incorrect filter installed.
3. Velocity influence.
4. Air leakage around filters, past frames, and possibly through
damaged filter.
IMPORTANT. An initial pressure drop 30% to 40% or more below the rated
value for the filter means that approach velocities may be sufficiently
low to impair the efficiency of some types of filters.
High Pressure Drop
1. Incorrect filter installed.
2. System handling more than rated air volume. Suggest thorough check of
system to be certain all controls are properly set and operating. If
unable to reduce pressure drop to less than ten percent or fifteen
percent above rated value additional filter area should be provided or
air volume reduced.
IMPORTANT. An initial pressure drop 10% to 15% or more above the rated
value for the filter, means that approach velocities may be sufficiently
high to impair the efficiency of some types of filters.
Installation Check and Trouble Shooting
Before putting your air filter gage into service or in the event of
initial pressure drop readings that don't agree with the filter
manufacturer's specified pressure drop, make the following checks:
1. Check zero adjustment of the gage. Remove both tubing leads or open
vent valves to atmosphere.
2. Check all tubing connections for tightness from gage to the static
tip or fitting connection.
3. Check static pressure tips or fittings to be sure they are not
plugged.
4. Check installation of static tips or fittings. Angle type static
pressure tips must point directly into the air stream. Flange type
static pressure fittings should be mounted on a duct wall in such a
location that the opening is at a right angle to the moving air
stream. A velocity pressure error can be created if the air blows
directly into the opening.
About the author: J.G. (Jerry) Barry is the Director of Sales
and Marketing at Dwyer Instruments, Inc. He has been with Dwyer
Instruments for 10 years in this position and can be reached at
info@dwyer-inst.com.
May-June 2000
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