By:  Alan Bandes, UE Systems - Elmsford, NY

Saving energy dollars with ultrasonic predictive maintenance

Compressed air is not a free utility. In fact, it may very well be the most costly utility a company uses. It is good business, as well as good management policy to carefully look at the use and efficiency of a compressed air system.

At first glance, the suggestions and procedures mentioned below may seem to be too complex or too time-consuming to consider. This is far from the truth.  In fact, a carefully planned and implemented program can be brought online gradually and absorbed into current preventive and predictive maintenance programs with very little disruption of normal procedures.

Procedures and observations
In order to determine the need for an on-going air loss prevention program, two procedures are usually recommended.  The first part is a brief in-plant review of air-related equipment and procedures such as compressors, air dryers, distribution systems and end users. The second is a comprehensive air leak survey.   It is always advisable to take a road map before leaving on a journey.  With this in mind, it is suggested that an outline or schematic diagram be made of the entire compressed air system.  This should include everything in the system, from the intake of the air compressor to the final production and distribution system.  Your plant may already have blueprints on file.  The next step would be to find out exactly how much compressed air is being utilized.  This can be done with baseline metering information. Each area should be metered to account for the exact amount of air consumption over a set period of time.

In-plant review
During this process, do not be surprised to find many instances in which plant personnel are aware of excessive air leaks.  Some personnel may even remark that they can hear air leaks with their ears, some may have actually wrapped rags around gross leaking fittings in order to muffle the loud noise.  Rags may also be used to stop water and oil dripping through leaking air pipes.  Other common findings may be evidence of uncontrolled air usage, which may include leaking fittings, gauges, pressure regulators with improper adjustments and in some cases sections without pressure regulators.  In other instances one may note an effect referred to as "artificial demand".  This occurs when system pressure is unnecessarily increased, which then increases a demand for more air.  As an example, compressors operating within a 10-psig pressure band will actually increase the demand for air in an attempt to unload. There may be findings of questionable blow-off usage.  Some blow-off usage might unnecessarily waste air.  This may be observed where open blow-off is used.  In many cases, the air supply piping might restrict or limit the air consumption of the blow-off application and reduce the effective air pressure.  Isolation valves may not be closed during some shifts or on weekends.  Often, most air leaks are downstream of isolation valves.  The greatest system loss is not during actual production times—it occurs during the majority of non-production hours.

Measurements
When taking pressure readings throughout the plant, note pressure differences.  Overall, the difference between system pressure and compressor operating pressure is due to uncontrolled demand, leakage, piping system configuration and negligence such as unattended equipment being left on.  In fact, don't be surprised to find that there are often a number of machines using air even while they are out of service.  Dirty line filters may create pressure and flow restrictions.  This is a common reason for improper pressure regulator adjustment.   Line filters generally have a 50-micron element that should be cleaned or replaced on an annual basis.

Air leak survey
An air leak survey may be performed by in-plant personnel or by contracting with an outside company.  In any event, all leaks must be marked, preferably with colored tags stating "air leak".  A list and the location of each leak should be noted and included in a report.  Just as important, a systematic program of repairing the leaks should be implemented as soon as possible, preferably as the leak survey is performed.  An ultrasonic detector should be used.  These instruments electronically translate the high frequency components of leak sounds into the audible range by a principle called "heterodyning".   Some instruments have frequency tuning and a phased array of transducers in the leak pick-up module to allow for leak detection in the typically noisy environments of the operating plant. If leak detection is to be performed around or near hazardous gases, it is advisable to use an instrument that is rated intrinsically safe.  An ultrasonic detector is ideally suited for leak detection. It senses the turbulence of a leak, which is produced as the gas escapes.  The turbulence has high frequency components that can be readily separated from the audible plant noises.  Since ultrasound is a high frequency signal (these instruments typically sense between 20 and 100 kHz) the intensity of the leak signal tends to fall off rapidly as the sound moves from the leak site and is therefore considered "localized".  By scanning along pipes, connections and fittings the leak sound will be heard as a rushing sound.  As the leak site is approached, the sound level will increase.  The exact position of the leak site is determined by moving the scanner back and forth in front of the suspected area.  The sound level will increase as the scanner is moved over the leak and decrease as it is moved away.  Due to the sensitivity of some of these instruments, one will often be able to locate small leaks at great distances, including in the ceilings of some of the buildings.   By using a probe with a metal rod, which acts as a wave-guide, some of these instruments are useful in detecting air leaks buried in underground conduits by touching the earth and tuning to 20 kHz.  The contact probe is also useful in obtaining mechanical information such as potential bearing wear and the status of the compressor valves.  A leak survey will often reveal the most common leak areas to be: (1) pipe connections and valves; (2) abandoned equipment with air left on; (3) hoses and disconnect plugs, traps and drains; (4)  (liquid) manual blow-down valves; (5) filters, gaskets, seals;  and (6)  O-rings, cylinders and controls.

Suggestions and improvements
Install pressure regulators.  During a survey, identify applications for regulator installation with a tag marked "Regulate." Based on the difference between the system pressure and compressor operating pressure and the number of applications requiring pressure regulators, this uncontrolled demand may easily use approximately 15% of compressor capacity.  If a plant can operate at a set pressure, such as 90 psig, that should be the standard set point for higher pressure applications.  This can be achieved with proper regulation.  Overall system pressure will increase as a result of proper pressure regulation.  The proper operating pressure for each application should be determined.  To be sure these pressures are maintained. Each regulator should be tagged with information showing the proper operating pressure.  Replacing conventional blowoffs with high efficiency air-amplifying nozzles, transvector jets and curtain transvectors is a cost-effective improvement.  It is often suggested that by reducing the compressor's operating pressure, money will be saved.  This is unfortunately not the case.  It actually costs more to run a compressor below its rated pressure.  As an example, using 125 psi", which is common to some of the compressors, let's examine the facts.   (Present Supply) 3,000 acfm @ 100 psig & 70°F x .584 wt/cf .5224 wt/cf @ 90 psig & 30°F = 3.354 scfm.  (Present Demand) 3.354 scfm @ 90 psig & 80°F x .5224 .686 wt/cf @ 120 psig & 0°F = 2.554 acfm (Required Supply).  This example illustrates that the present air demands are approximately 3.354 scfm @ 90 psig and require 3,000 acfm @ 100 psig supply capacity.  Increasing supply pressure to 120 psig would only require 2.5S4 acfm to supply the same demand.  It costs far more to produce 446 acfm than it costs to increase supply pressure by 90 psig.  An added benefit of higher operating pressure would be the useful storage created by the higher system differential pressure.  A 1,500-gallon vessel would have approximately 136 cubic feet of useful storage between 100 psig and 90 psig.  The same vessel would have approximately 409 cubic feet of useful storage between 120 psig and 90 psig.   One of the most important components of any compressed air program is employee education and awareness. All employees, from management to machine operators, should be made aware of the costs of air usage and waste.  Company newsletters should have a column on air use. Employees should be encouraged to report air leaks. Machine operators, must be encouraged to remember to turn off air supply to inactive equipment and to report any air-related problems.  Employee recognition will be important. Positive recognition of employees who have contributed to air loss prevention will go a long way to encourage others to participate.  Recognition can come from mention in a newsletter or public announcement or even by a plaque or notice on the company bulletin board. Establish a routine program of air leak inspection and repair.  Use an ultrasonic leak detector to locate and confirm leaks.  When repairs have been completed, re-check with the ultrasonic detector to be sure the leak is corrected.  In many instances, it will take but a few minutes of labor to tighten a pipe connection or tube fitting.  Use standard plumbing procedures, which should include proper pipe thread sealants.  It may be difficult and possibly impractical to make a system totally leak free; for this reason, it is conventionally agreed that a well-maintained system of compressed air will lose only 596 of its air through leaks.  This does not mean that a detected leak should go un-repaired.  Leaks can increase in size due to abrasive contaminants such as rust and scale present in the system.  The small, un-repaired leak may grow to a level where it may dramatically reduce the ability of a system to maintain adequate pressure.  Typically, leak detection and repair programs provide greater energy savings at substantially lower costs than any other procedure.  It is often in this atmosphere that the daily activities of production demands lead to what might appear to an outsider as organized chaos.  At times, even some plant personnel may feel that they are too busy "putting out fires" to plan ahead.   Communication may be lost; opportunities for increased savings and productivity may be missed.

Plant description
There are three main sections within the plant: preprocessing and storage for raw materials, processing and the loading/product storage area.  The boiler house supplies utilities such as steam, hot water and air to the various sections of the plant via above-ground and underground conduits.  Equipment description. There are 4 primary compressors, supplemented by 6 satellite compressors.  Two of the primary compressors are two cylinder, reciprocating types rated at 1,875 acfm @ 100 psig free air delivery; the two other primary compressors are four cylinder, reciprocating types rated at 1,985 acfm @ 125 psig free air delivery.  The 6 satellite compressors are single stage rotary screw types rated at 625 acfm @ 100 psig free air delivery. There are also three primary air dryers for the primary compressors and 6 smaller air dryers for the satellite compressors.   The three primary air dryers are non-cycling direct expansion refrigerated dryers with a capacity of 5,000 SCFM @ 100 psig, 100°F ambient and inlet air temperatures producing a 35-39°F pressure dewpoint.  The satellite air dryers are non-cycling direct expansion refrigerated units with a rated capacity of 625 cfm @ 100 psig, 100°F ambient and inlet air temperatures producing a 38°F pressure dewpoint.  Each air compressor has its own capacity control system for individual control.  Primary compressors are manually turned off and on during the day depending on shift needs.  As a possible alternative to help alleviate the drop in air pressure, there was a suggestion made to purchase additional compressors. It should be noted that no action, fortunately, had been taken on this suggestion.  Although this is a common practice in many plants, it is often a costly miscalculation.  By performing a survey as Mr. Franks had started, correction of inefficiencies found within the present system will produce more economies and be less costly.  There were 673 air leaks located.   The estimated loss is approximately 1,200 cfm during the first shift, with an annualized cost of $31,680.  Air loss is approximately 1,000 cfm during the remaining hours of operation, with an annualized cost of $89,232.  Air costs are based on the system full burden operating cost of $.22/1000 cubic feet.  In perspective, it takes two-125 horsepower and one-60 horsepower compressors (1,530 cfm) in operation on a Sunday just to maintain 109 psig system pressure.  The magnitude of system loss during non-production hours of operation (6,760 hours) should never be tolerated.  Air leak loss/costs add no value to the finished product.

Look at the big picture
This is where management can take a step back to look at the "big picture" and call for help.  This is exactly what Mr. Bill Franks, The Maintenance Engineering Supervisor did when he read an article not too long ago detailing the savings that can be achieved through a compressed air management program.  "I was not sure what we could accomplish," Franks said," but I knew we had to do something.  Our energy bill was way up, maintenance crews were preoccupied with their daily work orders and we couldn't take the time to see how to produce savings.  "A brief meeting with an air compressed air survey company representative encouraged Mr. Franks to initiate a plant survey of the compressed air system.  The problem was how to get corporate approval and monies.  A proposal was prepared to justify the program to management.  In it were included data and quotations from a study by the United States Department of Energy titled, "Compressed Air Systems - A Guidebook on Energy and Cost Savings".  Some of the quotations included: "In most factories, there were many opportunities for energy conservation in the generation distribution and use of compressed air" and "In some factories there was found to be an outrageous waste of energy associated with compressed air systems."  Charts indicating how the loss of air relates to the loss of dollars were also included as were estimates of potential savings.  As an example, one of the charts indicated that a leak of 1/4" at 100 psi wastes 104 cfm and at a cost of $.06 KwH, the loss, without an active survey program would be $10,130 per year.  The average air leakage in plant operations is typically around 24%. Charts were then produced to demonstrate potential loss, project cost and potential savings.  A preliminary study was authorized.  The results indicated nothing unusual.  As in many plants, air was looked on as a "free" commodity.  There was little realization that this attitude is exactly what makes air one of the most wasted utilities in plants today.