Compressed for Success
Ten tips for designing and maintaining optimal instrument air systems
John M. Rattenbury, P.E., Senior Corporate Engineer, R.G. Vanderweil Engineers, Inc., Boston
Compressed air is the motive force for actuation of most control valves in utility and production plants, and is by far a plant's most expensive utility. The energy cost of providing air at pressure-and at suitable quality-usually exceeds the capital cost of the compressed-air equipment within the first two or three years of installation.
In spite of its expense, the compressed-air source equipment and distribution piping can be easily overlooked among the pressing day-to-day maintenance challenges that surface. When a compressed-air system needs attention, maintenance personnel who may be more involved in taking apart a pump or monitoring boiler-water chemistry might be tempted to take a short-term approach to repairs.
For example, particulate and oil filters are sometimes bypassed instead of replaced. Compressed-air loads are added to distribution piping without determining if the pipe diameter is adequate. Supplemental compressors are tied into the piping during periods of peak consumption without adequate oil or moisture removal. Leaks are never fixed. After all, it's only compressed air.
Temporary solutions, however, can develop into long-term problems. The output quality of a compressed-air system is of vital importance to the efficient operation of plant controls. Air containing condensed oils can cause a build-up in actuator mechanisms and lead to sluggish or interrupted response. Water vapor can lead to corrosion. Particulate contaminants will also build up in actuator internals and clog pistons and ports.
The Instrument Society of America (ISA) establishes and publishes instrumentation standards on behalf of the American National Standards Institute (ANSI), including the ANSI/ISA-S7.0.01 series, "Quality Standards for Instrument Air." The purpose of these standards is to establish air-quality values for moisture content, particle size, oil content and other contaminants. The following suggestions for designing and maintaining a compressed-air system are based on this consensus standard-as well as several practical issues that all engineers should consider.
A low dew point
Maintain a low dew point, but not lower than it really needs to be. Dew point-a measure of the moisture content in air-is the temperature at which the vapor in air will condense into liquid. In the case of compressed air, dew point is referenced to line pressure, not the atmospheric pressure that common psychrometric charts are based on.
According to ANSI/ISA-S7.3, "Air Quality Standards for Pneumatic Instruments," the dew point in an instrument air system should meet minimum standards. Where any part of the system is exposed to exterior ambient temperatures, "the dew point at line pressure shall be at least 10
If the entire distribution system is indoors, then "the dew point at line pressure shall be at least 10
The purpose of this dew point specification is to prevent the condensation of moisture and formation of rust or scale in the instrument air system. A refrigerated dryer is usually adequate for interior systems. It can provide dew points to approximately 35°F to 37
However, while a standard heatless desiccant dryer can provide down to minus 100
Clearing the air
The air should be filtered of harmful particulates. According to the ISA, "the maximum particle size in the air stream at the instrument shall be 3 micrometers." Proper particle filtration is easily established by a simple dust filtration element located before other treatment components. A quality general-purpose pre-filter removes particles down to 1 micron. High efficiency after-filters can remove particles down to 0.01 micron.
Bypasses to filters and dryers should be avoided. Instead, any particle filter, carbon filter or dryer should be duplexed-two-piped in parallel with isolation valves-to allow for filter replacement or dryer maintenance without interrupting system operation and to provide for a degree of redundancy should one component fail. Installing bypasses to filters and dryers invites the possibility that an operator will open the bypass to deal with a clogged filter, instead of promptly replacing the filter.
The system designer should consider a differential pressure gauge with a high alarm to alert operators when particle or coalescing filters become clogged. He should also specify replacement filters so that, when necessary, filters will be changed as soon as possible.
As much of the oil as possible should be removed from the system. ISA recommends that the "maximum total oil or hydrocarbon content, exclusive of noncondensables, shall be as close to zero" parts per million (ppm), weight-to-weight or volume-to-volume, under "normal operating conditions." Oil-lubricated compressors can have an oil carry-over of 5 ppm or more. If spray-lubricated compressors are used, a coalescing filter and an activated-carbon filter should be installed after the receiver and the particulate prefilter. A clean activated-carbon filter with appropriate prefiltration removes oil vapor down to a concentration less than 0.003 ppm at 70 before it can reach the dryers. Installing two activated-carbon filters in a "working/polishing" arrangement can help guarantee the capture of aerosol carry-over when the upstream filter becomes overloaded. A complete filter train would consist of:
The next components in the treatment series would be the dryer and after-filter. Oil-coalescing filters should be positioned after a refrigerated dryer, which removes a good quantity of oil through condensation and extends the life of the coalescing filters.
If an old air compressor is to be replaced, the designer of the new system should consider going oil-free. Compressed air is sometimes described as "oil-free" because filters are used, such as the filter bank described above, but filters have limitations. A compressor that is oil-free by design, however, is the only way to guarantee compressed-air delivery with oil content as close to zero as possible. Oil-free compressors use oil to lubricate bearings and gears, but mechanical seals isolate the "air end"-the compression chamber-from any oil. Other compressors below the 25- to 30-horsepower range, termed "oil-less" compressors, have no lubricating oil of any kind. They use sealed bearings and Teflon-coated mating parts. Because there is no oil in the compression chamber of an oil-free or oil-less compressor, the condensate from the inter-cooler, after-cooler, separator, receiver and coalescing prefilters is free of oily waste. The compressed-air output has no oil carry-over, and consequently, aerosol filters and activated-carbon filters are not needed.
If one can get past the initial sticker-price shock of oil-free compressors, he may be willing to overlook the premium once he finds out that he can get rid of oily waste separators for the compressor, receiver and filter condensate. The operator need never worry that the condensate going down the drain is in compliance with federal and state environmental protection laws. Lastly, the oil-free systems can save operators from having to replace and dispose of oily coalescing and carbon filters.
Changing the dryer
Consider converting to heat-of-compression (HOC) dryers. HOC dryers-descendants of the older "desiccant wheels"-are by far the most efficient means of drying compressed air. Traditional heatless dryers consume an average of 15 percent of the process air to purge the desiccant media. HOC dryers take hot compressed air directly from the compressor's last compression stage-before the after-cooler-and use it to regenerate a portion of a desiccant wheel. After this air completes the regenerative stage, it is sent through a cooler to remove moisture content and is then injected back into the inlet of the dryer and mixed back with the process air. The only added energy input to the system is a low-wattage motor that turns the desiccant wheel.
While an HOC dryer does not consistently deliver the same dew-point level as a heatless dryer, most compressed-air systems only need a low enough dew point to avoid condensation. Many engineers believe that a -40°F-pressure dew point is a "magic number" critical to compressed-air quality. This isn't necessarily so. It is just the typical level that a traditional heatless dryer happens to deliver. An HOC dryer can provide -20°F to -40°F-pressure dew point depending on ambient air conditions and the mode of cooling used.
If only a small part of the distribution system is exposed to exterior ambient temperatures, consider using a desiccant dryer to lower the dew point to the recommended levels for only that part of the system. Even though dry air is good air, a heatless desiccant dryer uses a lot of compressed air for purging, on average up to 15 percent of the dryer's capacity. Heated desiccant dryers-either steam or electric coil-use only 2-percent air for purging, but they are very expensive compared with the refrigerant dryers. On the other hand, refrigerant dryers do not regenerate, so they do not consume any product air and are more appropriate for drying the interior portion of an instrument air system.
Do not install a refrigerated dryer upstream of a heatless desiccant dryer. This configuration has some advantages for a particular type of desiccant dryer, but the technique has been easily misunderstood. Heated and heatless desiccant dryers work by causing water molecules to adhere to the surface of a desiccant media such as activated alumina powder. When the media becomes exhausted, it is typically regenerated. Heatless dryers divert the dry discharged air from the on-line tower into the off-line regenerating tower in a counterflow direction to purge the adsorbed moisture. This process relies on the heat of adsorption-the exothermic energy generated in the drying process-to efficiently purge the regenerating tower. If the bulk of the moisture is removed by an upstream refrigerated dryer, the heat of adsorption is reduced significantly, and the dryer cannot efficiently regenerate. An externally-heated desiccant dryer, however, uses electrical power or steam as the heat source and is not sensitive to the heat of adsorption to operate properly.
Find and fix leaks. Engineers may be surprised at how much money a client is spending on energy just to keep a leaky system pressurized. Since leakage in a system occurs 24 hours a day, 365 days a year, the total wasted air translates into a substantial amount on the electric bill. Systems can leak as much as 30 percent of the total compressor output. An old system that supplies a nominal total output of 770 cubic feet per minute (cfm) could be losing up to 230 cfm in leakage. Even at an electrical cost as low as seven cents per kilowatt-hour, the compressor would be consuming up to $26,000 per year just to keep the system pressurized. The cost of repairs-or total replacement of piping-can have a very attractive payback
Explore using variable-speed-drive (VSD) compressors. Traditional positive-displacement air compressors regulate pressure of the connected process-air system by loading and unloading. Basically, when the system calls for air, a loading valve closes to pressure the system. When adequate pressure is reached, the valve opens and releases the air to atmosphere. This scheme places the compressor drive under 100-percent power during 100-percent loading, and about 20-percent to 25-percent power while unloaded. The drive motor usually runs constantly, but it can be programmed to shut down during extended periods of no loading.
A VSD compressor controls system pressure by varying the speed of the drive motor. Compressor speed is controlled by the rate of change of the system pressure and delivers only the mass of air that the system is calling for. This type of control allows the compressor to consume the minimum amount of power to deliver the required air with reduced unload power consumption. A VSD system can provide substantial energy savings over a conventional load/unload system. In addition, the nature of this control system allows for a soft-start, thereby reducing power surge.
There is another important reason for maintaining compressed air systems. Manufacturers of control valve actuators and other pneumatic equipment may not stand completely behind their warranties if they know that the system does not meet with minimum quality standards. Maintaining good air quality in a pneumatic-air system will help keep a client's pneumatic controls running much more smoothly.