Archive | 2001


4:39 pm
December 1, 2001
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Harmonic Distortion Accelerates Fuse Aging Failures

Often, in the wake of unexpected shutdowns due to costly equipment failures, superficial investigations result in plant engineers assigning fault to mechanical components. In many cases, however, thorough engineering analysis can delve deeper to reveal the causes for malfunctions and prescribe long-term, cost-effective solutions. Such was the case recently at an automotive assembly plant.

Premature failures of pulse-width modified (PWM) variable frequency drives (VFDs) serving supply and exhaust fan motors caused costly interruptions to the plant’s automated paint process. When the company started to investigate these intermittent drive shutdowns in the paint house, 70 VFDs ranging in capacity from 30-400 hp were in service at multiple 2000 kVA low-voltage substations.

Throughout the preceding year, the plant had reported multiple shutdowns of the paint house VFDs with each event costing approximately $300,000 in downtime and lost production. In each incident, maintenance technicians had performed field diagnostic procedures to determine what initiated the shutdowns.

The technicians reported finding one or two blown fuses at the affected drive. They also tested and condemned one or more gate turn-off thyristors (GTOs) on the voltage-source inverters. The condemned GTOs and fuses were replaced and the drives were successfully restarted without further incident. Each event involved a different VFD and none of the drives had experienced more than one shutdown event. With plant maintenance technicians blaming the shutdowns on component failures within the PWM drives, they removed the manufacturer from the plant’s acceptable bidders’ list.

Harmonic problems identified
Subsequent engineering analysis, however, proved that the shutdowns were due to accelerated aging and premature failure of PWM drive fuses. The fuse failures were linked to high voltage distortion levels on low-voltage busses that resulted in erratic drive operation.

A Square D Engineering Services team, led by Blane Leuschner, P.E., identified harmonic problems through onsite measurements and harmonic modeling using the Alternative Transients Program (ATP). ATP is a shareware program developed in Canada and similar to one developed by the Electric Power Research Institute. Using ATP, the team resolved the problem by combining additional line impedance with the application of harmonic canceling techniques.

A Square D Powerlogic monitoring system comprising approximately 500 devices was already in place at the plant. That system was supplemented by temporary measurements with portable test equipment. Permanent electronic meters were located at main breakers in the plant’s 480 V system. In addition to measuring about 200 power system parameters, these meters were capable of capturing simultaneous waveform data on each phase of voltage and current, with sample-rate resolution high enough to detect suspected power quality problems. In addition, portable versions of the same meters were installed temporarily at the input terminals of several drives.

Onsite testing showed that harmonic distortion at the low-voltage bus increased with increasing numbers of VFDs in operation, as expected. “Measurements showed that voltage distortion at the main buses approached 10 percent when normal levels of drives were operated,” Leuschner said. “While this level of distortion exceeds the 5 percent total harmonic distortion (THD) level usually applied to low-voltage buses, it is below the THD level at which VFD problems are usually encountered.”

Further onsite testing showed that VFDs currents measured at the drives were erratic and did not resemble the expected 2-pulses-per-half-cycle signature characteristic of PWM drives. The erratic current signature was unusual for a PWM signature and signaled an anomaly.

“The anomaly was high frequency harmonics, typically around 960 Hz, in the line currents,” Leuschner said. “Analysis of computer simulations showed that the noncharacteristic current harmonics resulted from low circuit inductance and high voltage distortion due to the operation of VFDs on each bus.” The high voltage distortion resulted in severe flat topping of line-line voltage, which limited the ability of the dc bus capacitors to charge. Low circuit inductance compounded the problem by permitting a high-rate-of-change in the VFD line currents.

Harmonics modeling and analysis
In order to analyze the system mechanisms at work in producing the unusual current distortion, the engineering team created an ATP computer model of the VFDs and a low-voltage power system. The simulation included a complex PWM model consisting of about 1000 circuit elements. Actual measurements provided the basis by which the team verified the computer model in order to ensure accuracy of the simulations and effectiveness of the solution alternatives.

“Initially, we based our engineering analysis on the reports by plant technicians that GTOs had failed during each shutdown event,” Leuschner said. “The earlier conclusion reached by the plant was inconsistent with later onsite measurements and computer simulations. Those measurements and simulations didn’t expose a power system event anywhere close to damage levels on the GTOs in the inverter section.”

In addition, the drive engineers determined that drive design prevented GTO failure from drawing enough current to blow an ac fuse. The engineers asserted that the GTO location in the drive was not getting enough available fault current due to upstream circuit components such as diodes, dc bus, etc. None of the empirical evidence could explain why GTOs would fail unexpectedly.

Consequently, the team turned its attention to the GTOs themselves. They set out to determine how the GTOs had been damaged and if the damage was corroborated by laboratory testing under more controlled conditions than existed in the field under emergency circumstances. “We found out that none of the condemned GTOs had been re-tested under laboratory conditions,” Leuschner said. “Neither were the GTOs subjected to forensic investigation that might have revealed their mode of failure.”

He recommended that such additional testing be done. Why, the plant argued, would additional testing be performed, when the GTO/fuse replacements had apparently fixed the problem and returned the affected drive to service? The team replied that the replacements were only a stopgap measure and that, without testing to determine a long-term solution, degradation and premature fuse failure would continue.

Laboratory testing
The plant subsequently authorized more testing, which located only five failed GTOs. Tests were performed under laboratory conditions by the plant and by Square D to determine the mode of failure. Test results showed that none of the five GTOs was damaged, thus confirming Leuschner’s analysis and proving that the GTOs had been condemned by mistake.

The engineering analysis then turned to the drive fuses. These fuses had indeed opened during the shutdown events—there was no question about that conclusion. With the GTO failure theory freshly debunked, plant engineers and Square D investigators wondered if the drive fuses might have been damaged during prolonged operation under erratic harmonic currents.

“Our team searched published articles on the subject in search of a precedent, but found none,” Leuschner said. “Yet, evidence suggested that the substantial high frequency content and dramatic fluctuation in peak current magnitude of the drive currents subjected the fuse elements to abnormal stresses and resulted in accelerated aging.” That theory was supported when Square D measured other circuits in the plant and found that no fuse failures had occurred on a circuit where drive operation was stable. Investigators used two 400 hp drives already equipped with line reactors—which constituted the majority of the circuit load—to confirm stability.

Solution options
Following detailed computer simulations of the VFDs and power system, the team issued its prescription. The report indicated a combination of line reactors and phase-shifting isolation transformers would provide the most cost-effective solution to the problem.

Other typical solutions, such as harmonic filters and hybrid filters at the main 480 V buses, were considered and eventually discarded. “The harmonic filter option, a typical solution of choice in such a situation, encountered a common dilemma when harmonic filters are being considered for transformers with a high percentage of VFDs,” Leuschner said. He explained that conventional shunt filters contain capacitance that increases displacement power factor on the applied bus. VFDs, however, typically operate at high displacement power factor, while producing high levels of harmonic current. Many VFDs on a bus means high levels of capacitance that can result in leading displacement power factor and overvoltage.

Further, ATP modeling revealed that harmonic filters would not resolve the erratic drive current phenomenon. Low system inductance was the major factor contributing to that erratic drive current, yet harmonic filters would appreciably change system inductance. Low inductance allowed the dc filter capacitors inside the VFD to charge erratically, resulting in the noncharacteristic ac current.

Simulation identified the optimum simulated-voltage and current distortion reduction and proved that the best technical solution was to increase system inductance seen by the VFDs. While line reactors alone could provide this inductance, the investigators also modeled delta-wye transformers to assess additional benefits. “We knew that even though harmonic current passes through line reactors and wye-wye or delta-delta transformers without appreciable phase shifting, delta-wye transformers have a different effect,” Leuschner said. “Fifth and seventh harmonic components, which comprise a significant portion of VFD currents, are phase shifted by 30 deg of the fundamental by delta-wye transformers.”

The resulting phase-shift of these two dominant harmonic components comprises currents that are 180 deg out of phase with fifth and seventh harmonics from nonphase-shifted drives. The combination of line reactors and delta-wye transformers contributed to significant cancellation of the aggregate fifth and seventh harmonic current contribution of all the drives.

Of the 66 drives in the paint house not already equipped with inductive isolation—four 400 hp drives had line reactors—only seven were equipped with delta-wye isolation transformers, while 50 received line reactors. Delta-wye transformers were reserved for VFDs with ratings of 200 hp and above, while 100 and 125 hp drives were provided with open-style reactors in the existing drive enclosure. Inductive isolation was not required on drives under 100 hp. Fuses that had not failed and had been replaced were changed due to suspected deterioration.

Results and conclusions
During its Christmastime shutdown, the plant implemented the recommendations for line reactors and delta-wye transformers. All equipment was installed and operating within a month. Bad, erratic voltage and waveforms were corrected into clean sinusoidal voltage and double-hump current waveforms. Harmonic voltage distortion was reduced to less than 5 percent. Drive currents returned to their normal signature.

“Our recommendations also identified the need for improved training of plant maintenance technicians,” Leuschner said. “Much of the early confusion about the cause of the shutdowns could have been avoided by more accurate assessment during in situ testing of the GTOs.” While the GTOs are difficult to test in situ, the plant implemented procedures to improve this testing, and to require that any electronic devices suspected of damage would be subjected to laboratory testing.

No further fuse failures have occurred since the modifications were completed, and the drivers’ manufacturer was returned to the plant’s acceptable bidders’ list. MT

Information supplied by Square D/Schneider Electric, Palatine, IL; telephone (800) 392-8781

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4:03 pm
December 1, 2001
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From Top to Bottom and Side to Side


Robert C. Baldwin, CMRP, Editor

“Plant floor to the top floor” and similar slogans are being used by analysts, pundits, and the larger players in the automation and control system arena to push investment in enterprise information systems and automation and process control systems.

To me, their promotional materials invariably conjure up a picture of an executive peering at a computer terminal to find out what is happening. What is he looking at? An idiot light on the enterprise dashboard?

Perhaps you get another image when you hear “plant floor to top floor,” but my first vision is of the big boss trying to manage a company from a computer terminal. What should top managers be doing? I think they are getting paid to focus on shareholder value and enterprise strategy.

If the leader has competent associates, then things run smoothly. A plant manager needs a competent operations manager and a competent maintenance and reliability manager. If they are doing their jobs, their boss can focus on the future and not have to get mired down in day-to-day activity.

The boss doesn’t need an idiot light to tell him that his production engine needs servicing. He needs competent people who can operate it and service it effectively. And there lies the rub. Most of them don’t have the information and systems to make them as effective as they need to be.

So what is really needed is horizontal communication on the plant floor. Without it, there isn’t much to send up to the top floor.

Plant information typically resides in functional islands: operations, control, predictive maintenance, asset management, etc. They are all components of the information matrix needed for competent decisions. If they are not readily available, how will the plant floor know when to turn on the idiot light, or what color it should be?

Open standards will play an important role in building the information network for the plant floor and the enterprise. They free the builders to select components based on performance rather than pedigree. (We expect MIMOSA to be an important contributor to open information standards.)

In spite of my vision of “plant floor to top floor,” I’m embracing it because it promotes action. Whether communications is best established top to bottom, bottom to top, or side to side, you have to begin somewhere. And once you start, you find that the value of the network increases as the product of the number of nodes, and everybody wins. MT


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4:01 pm
December 1, 2001
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Can Six Sigma be applied to Maintenance Efficiency (wrench time)?

We know there are two principal inputs to the maintenance cost equation: Reduce or eliminate the need to do maintenance (reliability of equipment), and improve the effectiveness of the resources needed to accomplish maintenance (people, parts, and outside services).

Leadership and crafts both want greater productivity in the execution of work. Leadership definitely wants to reduce waste and the dollars associated with it. Crafts want to utilize their time and skills effectively to accomplish equipment tasks professionally. Eliminating wasted time is good for business and good for general morale.

Typical wrench time ranges from 25 to 35 percent while benchmark levels range from 50 to 60 percent. What time-wasting issues make up this gap? From past Viewpoint articles you know that I see things through Six Sigma glasses. Using Six Sigma methodology, we can discover the set of circumstances that represent the gaps. Fundamental to Six Sigma thinking is that: Y (wrench time) = f (x1, x2, x3, x4, &).

What are some of the xs? How about stores delays, equipment preparation, work permitting, travel time, incomplete or wrong diagnosis, inadequate tools, waiting on the cherry picker, bad or wrong spare parts, inadequate work plans, and more.

The trained Black Belt or Green Belt will summon the disciplined Six Sigma roadmap: Measure Analyze Improve Control (MAIC). A team of cross-functional stakeholders will drive the effort to eliminate these daunting wastes. Some key MAIC tools include:

  • Map the as-is process with operations and maintenance stakeholders. (There is an as-is process whether it is controlled or uncontrolled.)
  • Define the inputs that make up the existing process. Are they controlled or uncontrolled?
  • What outputs are really important to the process (in addition to wrench time)?
  • Measure the defects and the current pro-cess capability (sigma, defects per million opportunities).
  • Analyze the failure modes of key inputs.
  • Mitigate the failure modes.
  • Improve the process by mapping the should-be process.
  • Install mistake-proofing tools at key steps (a.k.a. poka-yoke).
  • Validate the improvements and measure the new process capability (sigma, defects per million opportunities).
  • Establish measures and controls to ensure sigma capability is improving and sustained. Yes, statistical process control (SPC) can be used for work processes.

Of the bullets above, measuring becomes a key element of the effort to improve. How is the current process performing? Where are the wastes? How often do these wastes occur (frequency, or MTBF)? How severe are the waste events (time wasted, or MTTR)?

To answer these questions, a data collection method needs to be designed and deployed. Work sampling is one method. Another approach is to “follow the babies,” as we say in Six Sigma; maintenance events (work orders) are literally followed from start to finish. It sounds simple, almost juvenile; but the team will be amazed by what it learns about how the current process works and what comprises the defects (wastes).

Once the measuring phase has been completed, a cadre of immediate quick fixes and opportunities (greater analysis required) will emerge. The opportunities likely will point to broken supporting processes (job plans, work permits, tool crib, outside services coordination).

None of these are simple processes with easy fixes. They likely will require a new, more detailed set of work-process improvement projects.

One of the truisms of Six Sigma is “you don’t know what you don’t know.” Once you have data (now you know), it becomes very apparent that the interactions of maintenance activities are quite complex. If your wrench time is estimated in the 35 percent range, then your maintenance work process is broken.

What supporting processes are broken? You know that the brokenness is impacting the margin on the products you produce, and therefore can be classified a business driver for your company.

Go find out by measuring and converting to estimated dollars wasted (cost of poor quality in Six Sigma terminology). Then begin a structured MAIC path to uncover the defects, mitigate the failure modes, and install the should-be processes to control and sustain to benchmark wrench time. MT
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8:54 pm
November 1, 2001
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Managing Compressed Air Energy Part III: Supply Side Issues

Eleven factors that affect a system’s energy requirements.

Energy management in compressed air systems can be divided into two sectors: demand side and supply side. Once the energy constituents of demand in the system have been determined (MT 9/01, pg. 21, and MT 10/01, pg. 21), we must determine how effectively we are using energy to support the usage. In most systems, much of the demand usage is a constituent of the supply energy.

There are a number of ways that energy is consumed in an industrial air system besides the obvious. Some of these are very interactive and difficult to isolate, but they must be addressed in a typical plant compressed air system. Remember that air systems are extremely dynamic.

In auditing most systems, there is a common problem that always surfaces in the supply side of the system. The issue is ownership. Responsibility in the system is broken up between supply and demand, often with no one responsible for the demand side of the system. Preventive maintenance normally is performed by maintenance personnel. Overhaul and annual maintenance often is contracted to compressed air service organizations. It is assumed that these personnel are adjusting the equipment for its efficient operation. Often, this is not the case. The following problems are typical.

Problem: A service contract is issued with no discussion or understanding of the objectives of the owner. Preventive action: Establish service objectives.

Problem: The owner, out of a lack of knowledge, assumes that the right things will be done. Neither party understands that the way the system operates must be owned by someone. Preventive action: Designate a compressed air system manager.

Problem: The equipment is adjusted to factory-recommended set points and signals when it is installed. This has nothing to do with either how the system will work or efficient operations. In most cases the equipment is never readjusted for its useful life unless the controls are replaced. Preventive action: Periodically audit system performance and adjust set points as needed.

Problem: No one, including the service vendor and the operators, has any records of how the equipment was originally adjusted. No discussion occurred about how the system should work, other than meeting a minimum acceptable pressure. Preventive action: Document system history.

Problem: “Keep the equipment running” is a vague protocol that assures energy waste and high operating cost. Preventive action: Establish a rational approach to system management that can allow unused equipment to be taken off line and adjust signals, set points, and control philosophy accordingly.

These preventive measures must be revisited each time demand changes or a piece of supply equipment is added, deleted, or replaced, and the operating approach adjusted appropriately. If not, the system energy efficiency and system effectiveness will suffer.

The following 11 items are issues that affect supply energy in the tyical system.

1. Demand usage

Demand usage is the amount of energy necessary to support consumption assuming no inefficiency in the system. Factors affecting demand were discussed in previous articles (MT 9/01, pg. 21, and MT 10/01, pg. 21).

2. Temperature and relative humidity of intake air

Using standard conditions as a normalized value, higher temperatures at the inlet of a compressor provide less dense air and result in less compressed air mass. Because the compressor produces less results in terms of mass or work energy, more energy is required to produce the identical results systemically achieved at lower temperatures. Dynamic and positive displacement compressors respond differently, but the systemic results are comparable. When the inlet air represented is based on volume with no regard to density, one can easily overlook this issue and the corresponding energy required.

When the temperature drops, the air is denser and the compressors will produce more mass to the system using more energy. As work energy in the system is our objective, inlet density or mass is a necessary component of determining the power required and the number of compressors to support all conditions relative to the system. At 0 F without the effect of relative humidity, you will produce close to 12 percent more air by mass and an equivalent amount of energy than at standard conditions. At 100 F without the effect of relative humidity, you will produce at least 7 percent less air by mass and an equivalent amount of energy.

Higher relative humidity implies that there is more water present in the air at the compressor intake. Because water does not compress, it reduces the amount of net air that can be compressed. Relative humidity can influence the net result by as much as 3.5 percent less displaced mass.

When effects of temperature and relative humidity are combined, there can be as much as a 22.5 percent swing in performance on the compressors in the system from 0 F at 1 percent relative humidity to 100 F at 100 percent relative humidity.

When you compare the demand requirements including trim and conditional loads against the inlet condition profile, the amount of energy needed will be a function of the size of the compressors used. The larger the compressors as a percentage of the total requirement, the more part loaded a large unit will be depending on conditions and load. The smaller the compressors are, as a percentage of the total requirement, the less part loaded any one compressor can be. When properly controlled, the arrangement and size of the units vs. the needs profile can represent as much as 33 percent less power systemically at the lowest temperature and load.

3. Compressor optimization

The primary objective of compressed air system management is to get the most mass per kilowatt of electricity. The more mass you can achieve in any one compressor in the system, the fewer compressors you will need to accomplish the same results. Throughout this article, mass refers to pounds mass (volume at density).

On positive displacement compressors, the volume is fixed within the operating pressure of the compressor. As you elevate the pressure within this range, the volume will remain constant as the pressure and power increases. This is an increase in mass. As you exceed optimum, the mass will become constant and/or the motor efficiency will begin to drop. The pressure and energy will increase, but the mass will remain constant. In our experience, optimum will be achieved within a compressor frame range at a higher pressure in the bottom of the frame and at a lower pressure in the top of the frame. This is a result of packaged losses that increase in the top of a frame.

On centrifugal or other types of dynamic compressors, optimum is achieved in a slightly different manner. There are zones on the performance curve. A portion of the curve will produce constant mass. This implies that as the pressure rises and falls the volume will change inversely and maintain the amount of mass.

Above and below this zone of performance, either the pressure will drop faster than the increase in volume or the volume will diminish faster than the rise in pressure. Both of these zones will produce less mass. The power does not diminish as quickly as the mass in these zones. You would have to determine at what pressure you would displace the most mass per kilowatt of electricity by carefully examining the curve of the compressor at the typical and extreme inlet conditions of intended operation.

The confusing issue with centrifugal and dynamic compressors is that the performance curve moves with changes in inlet temperature. It will drop and move to the left as the inlet temperature increases, and rise and move to the right as the inlet temperature drops. This would require adjusting the operating pressure and the current limit adjustments on temperature change to stay within this optimum range. If the curve is generous enough for your conditions, you may be able to operate at relatively little performance change despite conditions.

The number of stages and the design will determine at what range of pressure this can be achieved. Optimum is normally achieved at the lowest pressure in the constant mass zone of the curve, which uses the least amount of input power for the mass achieved. We have found that factory-supplied curves only show performance between maximum stable flow and the surge pressure of the unit. More than half of the compressors of this type that we evaluate are operating at least a portion of the year below the pressure and temperature where you will find maximum stable flow (not mass).

If you continue down the curve from this point, the volume will become constant as the pressure drops and the energy either remains constant or diminishes. This is referred to as the choke zone. In the choke zone, you lose the advantages of this type of compressor. Further down the curve the volume will diminish as the pressure drops. This more-than-linear loss of mass is called stonewall where you will achieve sonic velocity through the unit.

Neither the choke zone or stonewall are shown on factory-supplied charts. We would suppose that this is not shown because you should not operate in this manner. Nevertheless, we frequently find compressors that have set points of operation below maximum stable flow and well below optimum. We also seldom find compressors operating on the curve. Most of the time they are operating in modulation or throttle. If the curve were extremely vertical, where there is very little volume turndown on the curve, you would have to operate at a low relative pressure in throttle to prevent surge. It should be obvious that examining the curves in terms of mass at pressure including minimum stable mass at various pressures and inlet conditions is the only way to determine the best operating set points for efficient performance.

You also should request that the curves show throttled performance at pressure and power at these various pressures and inlet conditions. Despite this need, we did not encounter one facility in the past 250 audits that had enough information to determine the best way of operating the compressor. In fact, most owner/operators have never seen curves, even during the selection process. Since this information is obviously needed and has never been produced, one must be curious how factory required service technicians adjust the units in the field. Our experience is that keeping them running is the approach taken, not optimization. Factories need to train their service personnel and the operators in performance optimization of both the individual equipment and the system using actual performance data and curves rather than typical factory set up suggestions.

4. Compressor cooling temperature

All compressor performance is influenced to some extent by the temperature at which the unit is cooled. There is a considerable difference in types of compressors. The displacement can be influenced by 0.5 percent to 3.5 percent of rated capacity for every 10 deg F increase over rated cooling media temperature. The inherent inefficiency, combined with the range of cooling media temperature and the maintenance condition of the coolers on the compressor, can effect the displacement efficiency by as much as 25 percent on the most temperature-sensitive types of compressors. Centrifugal compressors should be tested for natural surge and throttle surge at least twice a year to determine the performance decay of the unit as a result of cooler fouling.

You must record the inlet conditions during these tests so you can compare actual performance against theoretical performance for these conditions. Factory service personnel should either teach operating personnel how to perform these tests or provide this test data to maintenance personnel on a regular basis if you wish to minimize onboard energy and perform maintenance as required.

5. Systems storage vs. rate of flow of the largest event in the system vs. loading time required for the next available compressor

The more storage capacity in the system, the less the pressure will fluctuate on any demand event. This will allow you to maintain all of the compressors that need to be operating closer to optimum. The slower the speed that it takes to turn on the motor and load the next compressor, the more the pressure will drop. You should be able to add trim compressors to the system with a minimum delay and pressure drop. When this process doesn’t work well, and the pressure fluctuates too much, the normal reaction is to put all compressors in modulation and keep them on line regardless of demand. This will avoid the fluctuation or pressure decay, but not without a considerable increase in energy and operating cost.

Another anomaly is that as demand increases, the supply pressure drops. When the pressure drops on all of the compressors that are base loaded, there is a loss of isothermal efficiency proportional to the decay in the density of the compressed air. In most systems, as the demand increases and you require more mass, the compressors that are operating produce less mass. This causes the pressure to drop exponentially. As you need more, you can become less efficient with pressure decaying at an accelerated rate.

Careful design of control storage and thoughtful selection of set points, signal locations, and operating logic is necessary to achieve any relative efficiency from the system. Single set point, rate of change automation is the best approach to maintaining optimum system performance.

6. Resistance to flow in the system’s piping and downstream point of use components

The highest point of use pressure requirement is determined by the highest article or inlet pressure on the air-using equipment plus the highest installation differential across the point of use transmission components such as filters, regulators, lubricators, disconnects, hose, and fittings.

Original equipment manufacturers install smaller transmission components with high differential pressures to control manufacturing costs. The user of the equipment must compensate by providing high initial pressures from the plant air system. The tradeoff between high operating costs and the price of equipment with lower operating pressures or differentials is an important but rarely considered issue. Typical pressure drops across accessories on air-using equipment have increased substantially in the past 15 years. As long as this is a non-issue among the purchasers of this equipment, you may be assured that manufacturers will continue to use differential pressure as a tool for controlling manufacturing costs.

The highest differential is achieved at the highest flow, highest inlet temperature, and the lowest pressure. All specifications should incorporate this information in performance queries and specifications with maximum differential being the desired response. Compressor manufacturers report that elevating the pressure 1 psig will increase power by 0.5 percent of the total connected onboard energy. If you are operating with the compressors in load-no load mode and the elevation of pressure does not increase the demand in the system, then this is true.

Unfortunately, most systems are in modulation and do not have a demand control or expander. The elevated pressure then will cause demand to increase. The demand increase will be a function of the percentage of unregulated demand including leaks and points of use with regulators adjusted to the maximum setting. The power will increase proportional to the pressure increase adjusted for the percentage of unregulated demand plus the 0.5 percent per 1 psig rise. If the increased demand does not require an added compressor, the influence on energy will be between 0.5 percent to 1.575 percent of the total connected brake horsepower (bhp) from 100 percent regulation at the point of use and no leaks in the system, to 0 percent regulation plus leaks respectively per 1 psig rise in operating pressure.

If you are in modulation, and must add another compressor in order to increase the pressure, the new compressor will support a portion of the added load, but the total volume will be shared across all modulating compressors. This can be so inefficient, depending on the degree of part load prior to the add, that the effect of a pressure increase can be 25 percent or more for a 1 psig pressure increase if another compressor must be added. We haven’t seen a system without leaks, nor 100 percent regulated below the lowest compression pressure. Compressor manufacturers do not field test how their units are influenced in systems, only packaged results. So much for the 0.5 percent per psig of pressure rise.

7. Differential pressure across supply components downstream of the compressor control signal location

Differential pressure influences system energy in the same manner as in item 6. Filters that degrade will cause the downstream pressure to drop. Typical of these components would be aftercoolers, separators, filters, and dryers when the compressor signal is located upstream of the aftercooler. Specified performance never includes the influence of differential. This must be determined at the highest flow, lowest pressure, and highest temperature as with point of use equipment. What is unique is that the differential will ride on the system’s pressure and drive backwards into the compressor’s operating pressure signal.

If the operating philosophy is to turn on compressors to maintain a system pressure of 100 psig, the pressure drop across the clean-up equipment will drive the control signal up accordingly. If the clean up differential pressure is 10 psig, the signal pressure would have to be maintained at 110 psig to maintain a system pressure of 100 psig. As you need more air, the differential will increase at a higher rate of rise than the dropping system’s pressure.

The more you need, the harder it will become to satisfy the demand, and the more likely you will turn on the next available compressor to share the load.

8. Higher internal pressures resulting from differentials across components upstream of the compressor control signal

This is where the compressor control signal is downstream of all or some of the components as in item 7. In this case, the differentials increase the internal pressure in the compressor. In this case only, the compressor energy will increase 0.5 percent of the total connected bhp per psig for this internal pressure rise.

The same would be true of an air to lubricant separator on a rotary screw compressor. As the filter/separator dirt loads, the upstream internal pressure rises, increasing the motor bhp. The separator is upstream of the control signal. The increase in energy we are discussing is only true as long as there is motor capacity available. Once you have consumed all of the available energy, either the displacement will diminish, the motor will overload depending on the compressor type, or you will have to add another compressor. The differential and the energy will rise and fall with the change in the compressor volume, but the system will not see the effect of this, only the drive motor.

9. Resistance to flow on inlet filters and their effect on reduced inlet pressure to the compressors

As the inlet filter becomes loaded with dirt, the dry inlet throat pressure drops proportionately. If the compressor is not fully loaded, it will increase in load to achieve the desired result as a consequence of the controls set point at the reduced inlet pressure. The effect will be different depending on how the compressor is operating.

In load-no load mode, the increase will be in more load time. The energy increase will be proportional to the ratio of the atmospheric pressure divided by the added differential. This particular differential is measured in inches of water and needs to be corrected to pounds/sq in. for the ratio (27 in. static pressure water gauge equals 1 psi).

If the compressor is in modulation, the effect on energy can be more or less than linear depending on the load on the compressor at the time that the inlet pressure reduction occurs. Power is anything but linear in modulation. Based on manufacturers’ recommendations for filter changes, most systems will increase power by 2.3 percent to 3 percent during the time between when the inlet filter is clean and when it is sufficiently dirty to require change. If this causes the need for another compressor, the influence increases the power dramatically.

10. Inefficiencies resulting from how the compressor controls are set up and their effect on unit performance

The effect of improperly set controls can increase energy consumption by a modest amount to as much as 33 percent of the total connected kilowatts. A system control setting is a very complex matter requiring a great deal of understanding. Previous discussion outlined the general effects of the demand system and the supply components on energy usage. This actual influence is specific to the interrelationship between the compressors, their signals, set points, and differentials within the system. This complex subject is outside the scope of this article but is covered fully in a 90-page section of the author’s “Compressed Air Systems Solution Series.”

11. Fouling of internal components on the air path of the compressor

Fouling of internal compressor components is specific to dynamic compressors such as centrifugal and axial types. The dirt and condensibles from the inlet loading on impellers and diffusers can cause considerable performance losses. As this occurs, the surge pressure usually drops in the compressors. Natural surge testing can assist in determining whether this condition exists. You can also observe whether the unit is performing in terms of mass at pressure and power comparing test results against rated performance on its curve.

Measuring and managing energy
For the most part, compressors have no power monitoring equipment on them. In the infrequent case where there is, the compressor is monitored with an amp meter measuring current to the motor. Current is not an accurate means of monitoring power on a compressor because of the relationship between mass and input power.

Monitoring and trending total and individual input power is perhaps the best means of trending operating cost and predictive maintenance issues. The only accurate method we have observed is using kilowatts, relative to full and part load performance.

Compressors and systems do not usually fail. They degrade. If you trend individual compressor input power vs. status and system efficiency, you can easily avoid an interruption without the extravagant application of power as an alternative. You need to monitor the total mass at pressure on the demand side of the system and trend the demand mass divided by the total input kilowatts.

Although many of the individual compressed air energy issues have been understood for some time, it has only been in the past five or six years that the interrelationship of a system’s supply and demand has begun to be understood. The improvement in and quality of information applied and trended systemically has provided the best basis for separating the theoretical from the actual. The commercial emphasis in compressed air has always been with the equipment. Much more emphasis must be placed on systems and their operations at all levels including system operations, production usage, sales engineering, contract field service, and equipment manufacturing.

The remarkable opportunities available for operating cost reduction and quality improvements in production are a strong indication that there is much work that needs to be done in this area of plant asset management. Many of the poor decisions that are made are out of fear of not satisfying production. Fear is only present in the absence of knowledge. Education, ownership, and the application of information are the beginning of more effective plant air systems. MT

R. Scot Foss is president of Plant Air Technology, P.O. Box 470467, Charlotte, NC 28277; telephone (704) 844-6666. He is the author of “The Compressed Air Systems Solution Series,” 1994, Bantra Publishing; (704) 372-3400.

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8:24 pm
November 1, 2001
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Handhelds Speed Work at Corning Cable Systems

In February 2000, Corning Inc. acquired Siemens AG’s interest in their worldwide fiber-optic cable ventures and merged the businesses into one operation called Corning Cable Systems. The new Corning Cable Systems then announced plans to double the size of its Hickory, NC, specialty cable plant to meet rising customer demand.

For the maintenance staff of the Hickory plant, the expansion posed an enormous challenge. Corning technicians were faced with the task of maintaining more than 300,000 additional sq ft of plant space without compromising the speed or quality of their work. In addition to tackling the mounting workload, the Hickory plant’s management needed to improve its ability to capture performance data to generate more accurate and timely records.

To manage its maintenance operations, Corning had been using MAXIMO from MRO Software, Bedford, MA. “Typically, our technicians would walk up to a terminal, write down work order information on a sheet of paper, and then walk off to complete the job. Sometimes technicians would be lined up two or three deep at a terminal to receive their next work orders. We realized there had to be a more efficient way of getting that information into their hands,” Lawrence Bugielski, CMMS coordinator at the Hickory plant, said.

Mobile technology is the solution
Corning searched among several technology providers for a solution before selecting SMART from Syclo, Barrington, IL, which is built on the company’s Agentry platform and works as a companion to MAXIMO.

Tradesmen use Windows CE-based handhelds as electronic clipboards to send and receive work orders and other workflow data to and from the CMMS. SMART provides data at the point of performance, eliminating reliance on time-consuming paperwork and allowing technicians to accomplish more maintenance tasks in the field. It also combines a variety of communication optionsùwireless, dial-up, or docking cradleùwith synchronization capabilities to ensure that users can work effectively in and out of wireless network coverage.

“After researching the available technology, we recognized that this was the best solution to replace our system of fixed-point access to the CMMS with a more efficient mobile operation,” Bugielski said.

The system was up and running quickly for Corning, whose technicians found the solution easy to learn and simple to use. Beginning in August 2000, it was deployed for both Corning’s break-fix team and its preventive maintenance (PM) operation.

Swift productivity gains
The installation at the Hickory plant produced swift, measurable results. Using Hewlett Packard Jornada handheld PCs and synchronizing with the CMMS via conveniently located docking cradles, Corning technicians soon recorded a significant increase in the amount of work they were able to complete.

By providing work orders and other important data to technicians at the point of performance, the system eliminated the need for Corning technicians to walk back and forth between terminals to handle data entry, saving them an average of 1 hr per shift. With more time in the field to complete their maintenance tasks, Corning’s tradesmen are able to help the newly expanded plant run much more smoothly. The system also eliminated the plant’s reliance on costly, time-consuming paperwork, which had slowed its productivity in the past.

“Our decision to combine handhelds with our CMMS has resulted in so much more completed work that it’s almost as if we doubled our staff of tradesmen,” Bugielski added.

In addition to boosting the amount of work completed, the deployment also has helped Corning ensure that the Hickory plant’s operations meet manufacturing standards set by the International Organization for Standardization (ISO). The plant’s maintenance staff is using the system to perform condition monitoring and rounds-and-readings activities to make sure equipment is running at peak efficiency.

“Our customers are investing a lot of money in the fiber-optic cables we produce and it is essential to assure them that our manufacturing facility meets all of the quality checkpoints,” Bugielski said. “By collecting more accurate and timely data, our PM operations have improved and we are better able to meet and exceed our ISO requirements. The system helps us maintain the quality that our customers expect and rely on.”

Corning Cable Systems has achieved a number of benefits since deploying the mobile technology:

  • Technicians are saving an average of 1 hr per shift by eliminating their reliance on paper work orders.
  • The plant maintenance staff has dramatically increased the square footage supported by each maintenance technician.
  • After the full rollout, the Hickory team was able to maintain equipment for a plant expansion of 300,000 sq ft with the same number of technicians.
  • Tradesmen perform condition monitoring and rounds-and-readings activities to make sure equipment is running at peak efficiency. By capturing more timely and accurate data, management has improved its PM operation and ensured its adherence to ISO operating requirements. MT

Information supplied by Syclo LLC, 1250 S. Grove Ave., Suite 304, Barrington, IL 60010; (800) 567-9256.

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3:58 pm
November 1, 2001
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Let's Undo the Confusion


Robert M. Williamson, Strategic Work Systems, Inc.

If I’ve heard it once I’ve heard it a thousand times: “I’m getting confused” with all the talk about Lean Manufacturing, Lean Enterprise, Six Sigma, equipment reliability, Total Productive Maintenance (TPM), ISO/QS9000, the learning organization, and on, and on. “What’s it going to be?

From my perspective it’s not an either-or question. Why does it have to be one improvement program at a time, fully implemented, over three to five years? What we really need is to systematically identify and eliminate the causes of poor performance using the appropriate tools or techniques—in a sustainable manner of course. I’ve said it before, and here it is again: Focus on results and change the culture along the way.

Our culture has a history of looking for the “silver bullet” or the “secret ingredient” to successful equipment management. We tend to single out one improvement program and go whole hog to implement it. A giant leap of faith—hoping that equipment will perform better, last longer, and operate at lower costs. Then the “new program” comes along and interrupts what we started. We never seem to fully realize the fruits of our former labors before we have to shift gears. Or, in some cases, we start seeing the results but are unable to sustain them because the new initiative-of-the-month has priority.

Can it be that the top decision makers are so desperate for improvement that they keep looking for another, and another, and another silver bullet?

In the late 1980s the term Lean Manufacturing was coined by a researcher in the international motor vehicle program at MIT when comparing many of the mass-production approaches with the Toyota Production System (TPS). But since then, as Lean concepts started catching on, they often were taken out of historical context and the maintenance elements fell by the wayside. The silver bullet syndrome again?

Just last month, Portland, OR-based Productivity, Inc. hosted its 6th annual Lean Management and (12th annual) Total Productive Maintenance Conference in Detroit. This was the first time that a maintenance improvement theme (TPM) was brought into the context of Lean Manufacturing, Lean Production, and Lean Enterprise that I can recall. This combination of two previously separate, and seemingly unrelated, improvement strategies has laid another big foundation stone for yet another breakthrough in Lean thinking and equipment and reliability improvement. But the significance of this combination may be overlooked by the Lean consultants.

Yet, just a little over 30 years ago Japanese automotive supplier Nippondenso realized that until you address and systematically eliminate the causes of poor equipment performance you cannot deliver to your customers just in time, nor improve quality levels, nor lower operating costs, nor improve profits. In 1969 the ideas of TPM facilitated by Seiichi Nakajima helped take the TPS to the next level. Since the TPS was focused on the absolute elimination of waste to reduce manufacturing cost, TPM was designed to systematically identify and eliminate equipment losses (downtime, inefficiency, defects).

We have an opportunity. As maintenance and reliability professionals we need to help our “Lean thinkers” understand the relationship between getting Lean and equipment reliability and performance improvement. It’s not one or the other, it’s both. You cannot have a Lean manufacturing facility without reliable equipment. Conversely, you can have reliable equipment without Lean. But the key to sustaining equipment reliability comes in building a capable infrastructure—one that not only supports and encourages equipment reliability at all appropriate levels but links reliability to the needs of the business, to deliver business results. That capable infrastructure is essential for sustaining any improvement initiative—including Lean.

Until we help lead maintenance and reliability as a core business strategy, our efforts will remain just another maintenance program. It’s up to those of us who understand maintenance and reliability methods to collaboratively build the bridges between our reliability improvement efforts and Lean transformation efforts in our plants and facilities. Planned, preventive, predictive, proactive, total productive, and reliability-centered maintenance are known and proven.

Let’s start undoing the “Lean confusion” while it’s still in its infancy. The foundation stones are in place. MT
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3:56 pm
November 1, 2001
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CMRP: A Maintenance Milestone


Robert C. Baldwin, CMRP, Editor

Fifty-nine members of the reliability and maintenance community became Certified Maintenance and Reliability Professionals (CMRP) last month when they passed the comprehensive examination given by the SMRP Certifying Organization at the Ninth Annual Conference of the Society for Maintenance & Reliability Professionals (SMRP).

The examination is designed to validate the examinee’s skills and knowledge in five interrelated work processes: Equipment reliability, manufacturing process reliability, work management, business and management, and people.

Individuals who pass the test and agree to abide by certain guidelines for professional conduct become certified and can proudly add the initials CMRP after their names. They have the right to a large measure of personal pride because their proficiency in maintenance and reliability management is at a professional level certified by their peers as represented by SMRPCO.

Because I participated in the development of the examination, I was not eligible to obtain certification by examination. However, through SMRPCO “grandfather” provisions, I can use CMRP in my byline, and I am proud to do so. Not having sat for the examination, my sense of pride is different from those who did. I feel proud to have the privilege of working with SMRPCO in the development of the CMRP process.

In my estimation, the person deserving the greatest sense of pride in CMRP is Brad Peterson, a fellow founding member of SMRP. Without his vision, leadership, sense of purpose, and years of hard work as chairman of the committee, there would be no CMRP. His insistence on rigorous process development and excellence of execution was essential to making CMRP unique in several ways:

  • Independence: SMRPCO is a practitioner-based organization without ties to any commercial venture.
  • Body of knowledge: SMRPCO recognizes management and manufacturing skills as well as the technical aspects of maintenance and reliability.
  • Validation: SMRPCO validated each step of the process with input from a broad cross section of the maintenance and reliability community.
  • Certified process: SMRPCO work was conducted according to National Organization of Competency Assurance guidelines with the intent to have the process certified by that organization.
  • Continued enhancement: SMRPCO has a plan to continue to enhance the value of certification to practitioners who have become certified.

I believe SMRPCO’s CMRP is a profession milestone in which all present and future participants can take pride. MT


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8:42 pm
October 1, 2001
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Modifying Hydraulic Systems for Maintainability

Filtration pump and hydraulic reservoir modifications can reduce maintenance effort and increase reliability.

Preventive maintenance of a hydraulic system is basic and simple and if followed properly can eliminate most hydraulic component failure.* However, many hydraulic systems are not designed to facilitate maintenance work. A properly designed hydraulic reservoir and the use of a filtration pump can increase maintenance efficiency and increase equipment reliability.

Modifications to an existing hydraulic system need to be accomplished professionally. Here are some recommendations on what should be included.

Filtration pump with accessories
The use of a properly designed filtration pump will reduce contamination introduced into the hydraulic system when fluid level is topped off or when new fluid is added.

Hydraulic fluid from the distributor is usually not filtered to the requirements of an operating hydraulic system. This oil is typically strained to a mesh rating but not filtered to a micron rating. However, hydraulic fluid must be filtered to 10 microns absolute or less for most hydraulic system (25 microns is the size of a white blood cell, and 40 microns is the lower limit of visibility with the unaided eye).

Many maintenance organizations add hydraulic fluid to a system through a contaminated funnel and may even use a bucket that has had other types of fluids and lubricants in it previously, without cleaning them.

Recommended equipment and parts:

  • Portable filter pump with a filter rating of 3 microns absolute.
  • Quick disconnects that meet or exceed the flow rating of the portable filter pump.
  • A ¾-in. pipe long enough to reach the bottom of the type of container the distributor uses to deliver fluids.
  • A 2-in. reducer bushing to ¾-in. npt to fit into the 55 gal drum, if you receive your fluid by the drum. If you receive fluid in larger quantities, mount the filter pump assembly to the supports of the double wall tote tank.
  • Reservoir vent screens should be replaced with 3/10-micron filters and the openings around piping entering the reservoir should be sealed.

Designing a frame that will allow the filter pump and fluid drum to be transported by fork truck could further enhance the fluid handling operation. Regulations require that secondary containment be addressed. The assembly should include a catch pan so that any fluid any spilled fluid would “leak” into the pan.

Hydraulic reservoir features
A well-designed hydraulic reservoir will minimize the risk of introducing contamination when oil added to the system or contaminates being allowed to enter through the air intake of the reservoir. A valve should also be installed for oil sampling.

The air breather strainer should be replaced with a 10-micron filter if the hydraulic reservoir cycles. (The breather should be sized to the output of the reservoir.) A quick disconnect should be installed on the bottom of the hydraulic unit and at the ¾ level point on the reservoir with valves to isolate the quick disconnects in case of failure. This allows the oil to be added from a filter pump as previously discussed and would allow for external filtering of the hydraulic reservoir oil if needed. Install a petcock valve on the front of the reservoir that will be used for consistent oil sampling.

Recommended equipment and parts:

  • Quick disconnects that meet or exceeds the flow rating of the portable filter pump.
  • Two gate valves with pipe nipples.
  • One 10-micron filter breather.

Do not weld on a hydraulic reservoir to install the quick disconnects or air filter.

Maintenance of a hydraulic system is the first line of defense to prevent component failure and thus improves equipment reliability. These equipment modifications can enhance that effort. MT

Ricky Smith is president of Technical Training Div., Life Cycle Engineering, Inc., 4360 Corporate Rd, Ste. 100, North Charleston, SC 29405; (843) 744-7110

*Preventive maintenance issues were discussed by the author in a previous article “Developing PMs for Hydraulic Systems”.

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