How to optimize an instrument air system

Short-term solutions to maintain compressed air source equipment and distribution piping can become long-term problems in your plant. Here are a few areas deserving attention to sustain the quality of compressed air systems for the actuation of control valves.

06/25/2001


COMPRESSORS
INSTRUMENTS

Feature: How to optimize an instrument air system

Sidebar 1:
Advantages of oil-free compressors

John M. Rattenbury , PE, R.G. Vanderweil Engineers, Inc., Boston, MA

  • Compressed air quality must be maintained for instrument and control use, not general plant applications.

  • Either remove oil or use oil-free compressors.

  • Be careful where and what kind of dryer is used.

    • The primary motive force for the actuation of most control valves used in plants is compressed air. However, maintenance of compressed air source equipment and distribution piping can be easily overlooked on a day-to-day basis due to more pressing maintenance challenges. The quality of a compressed air system can be compromised by the principle of "out of sight, out of mind."

      Common problems
      Short-term solutions can be long-term problems. Here are a few areas deserving attention.

      • Particulate and oil filters are sometimes bypassed instead of replaced.

      • Compressed air loads are added to the system without determining if pipe diameters and compressors are adequate.

      • Supplemental compressors are turned on during periods of peak consumption without adequate oil or moisture removal.

      • Leaks are never fixed.

      • Old equipment is maintained—not replaced—sacrificing efficiency and energy savings.

        • The quality of a compressed air system is of vital importance to the efficient operation of plant controls and in minimizing energy costs. Air that contains condensed oils can build up in actuator mechanisms and lead to a sluggish or interrupted response. Water vapor causes corrosion. Particulate contaminants can build up in actuator internals and clog ports.

          11 tips for a better instrument air system
          1. Maintain the required dew point.
          Dew point, which is the measure of the moisture content in air, is the temperature at which the water vapor in air condenses into liquid. In the case of compressed air, dew point is referenced to line pressure, while common psychrometric charts are based on atmospheric pressure.

          According to ANSI/ISA-S7.3, Air Quality Standards for Pneumatic Instruments , where the instrument air system is exposed to exterior temperatures, "the dew point, at line pressure, shall be at least 18 F below the minimum local recorded ambient temperature at the plant site."

          If the instrument air system is indoors, "the dew point, at line pressure, shall be at least 18 F below the minimum temperature to which any part of the air system is exposed to at any season of the year. In no case should the dew point at line pressure exceed 39 F."

          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. For interior systems, a refrigerated dryer is usually adequate, since it can provide a dew point of about 35%%MDASSML%%37 F.

          If any part of an instrument air distribution system is exposed to exterior ambient temperatures, a desiccant-type air dryer must be used to maintain a low enough dew point. A standard heatless desiccant dryer can provide a dew point as low as -100 F, but the expense of purge air is usually not justified. Desiccant dryers with dew point monitoring systems can save energy.

          2. Consider using a dryer. If only a small part of the distribution system is exposed to exterior ambient temperatures, use a desiccant dryer to lower the dew point to recommended levels for only that part of the system. Even though dry air is preferred, a heatless desiccant dryer uses a lot of compressed air for purging—up to 15% of the dryer's capacity. Heated desiccant dryers use significantly less air, about 2%, for purging, but are expensive. Refrigerant dryers do not regenerate, do not consume any product air, and are appropriate for drying an instrument air system ( Fig. 1 ).

          3. Do not install a refrigerated dryer upstream of a heatless desiccant dryer. A heat-regenerable dryer works better with reduced inlet moisture. A heatless desiccant dryer needs moisture to regenerate. A desiccant dryer causes water molecules to adhere to the surface of its media. When the media becomes saturated, it is regenerated. Heatless dryers divert dry discharged air from an online tower into an offline regenerating tower to purge the adsorbed moisture. This process relies on the heat of adsorption 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 uses electrical power or steam as the heat source and is not sensitive to the heat of adsorption.

          4. Consider heat-of-compression dryers. Heat-of-compression (HOC) dryers are the most efficient means of drying compressed air. Heatless dryers consume an average of 15% of process air to purge the desiccant media. HOC dryers use hot compressed air directly from the last compression stage to regenerate a portion of a desiccant wheel. The air is then routed through a cooler to remove moisture, and then directed to the dryer with the rest of the process air.

          The only energy required is a low-wattage motor to turn the desiccant wheel. While an HOC dryer does not deliver a consistent dew point level, most instrument air systems only need a dew point low enough to avoid condensation.

          A -40-F pressure dew point is not a magic number critical to compressed air quality. It is the typical level that a heatless dryer happens to deliver. An HOC dryer is capable of providing -20 to -40-F pressure dew points, depending on ambient air conditions and the mode of cooling.

          5. Filter out harmful particulate. According to ISA, "the maximum particle size in the airstream at the instrument shall be 3 microns." Proper particle filtration is easily established by a simple dust filtration element located before other treatment components. A quality, general-purpose prefilter removes particles down to 1 micron. High efficiency afterfilters can remove particles down to 0.01 micron.

          6. Avoid bypasses to filters and dryers. Particle filters, carbon filters, or dryers should be piped in parallel with isolation valves to allow for filter replacement or dryer downtime without interrupting system operation and to provide for a degree of redundancy should one component fail prematurely. Bypasses allow operators to pass untreated air into the system.

          7. Equip filters with differential pressure gauges. Install a differential pressure gauge with an alarm to alert when filters become partially clogged ( Fig. 2 ). Keep replacement filters on hand to ensure substitution as quickly as possible. Dirty filters are inefficient and reduce air quality and pressure.

          8. Get all of the oil out. ISA recommends that "the maximum total oil or hydrocarbon content, exclusive of noncondensables, shall be as close to 0 ppm by weight or volume as possible; and under no circumstances shall it exceed 1 ppm under normal operating conditions." Oil lubricated compressors can have an oil carryover of 5 ppm or more. If lubricated compressors are used, a coalescing filter and activated carbon filter should be installed after the receiver and particulate prefilter. A clean activated carbon filter with appropriate prefiltration removes oil vapor down to a concentration of & lt;0.003 ppm at 70 F.

          Oil removal is particularly important if desiccant dryers are installed for moisture removal. Oil removal must occur before the dryers to protect the desiccant media. Installing two activated carbon filters in series, if the pressure drop isn't too great, in a "working/polishing" arrangement helps capture oil carryover when the upstream filter becomes overloaded.

          A complete filter system should consist of a particulate prefilter, water/oil coalescing filter, aerosol filter, activated carbon filter, and another activated carbon filter followed by a dryer and afterfilter. Install oil coalescing filters after a refrigerated dryer. The dryer removes much of the oil through condensation and extends the life of the coalescing filter.

          9. Consider going oil-free if replacing an air compressor. Compressed air is sometimes described as oil-free because filters are used, but filters have limitations. An oil-free compressor is the only way to guarantee compressed air delivery with oil content as close to zero as possible ( Fig. 3 ). Oil-free compressors use oil to lubricate bearings and gears, but mechanical seals isolate the compression chamber from any oil. Compressors below the 25%%MDASSML%%30-hp range, termed oilless, have no lubricating oil of any kind. They use sealed bearings and PTFE-coated mating parts.

          Since there is no oil in the compression chamber of an oil-free compressor, condensate from the intercooler, aftercooler, separator, receiver, and coalescing prefilters is free of oily waste. Because the compressed air output has no oil carryover, aerosol and activated carbon filters are not needed, which minimizes energy losses through filter pressure drop.

          10. Find and fix leaks. It is surprising how much money is spent on energy just to keep a leaky system pressurized. A system with an output of 770 cfm and leaking 30% can waste $26,000/yr with an electrical cost of $.07/kWh. The cost of repairs can have a very attractive payback—so much that this could become a very high priority in any energy savings campaign.

          11. Consider a variable speed compressor drive. Traditional, positive-displacement air compressors regulate pressure in the air system by loading and unloading. Such a control scheme places the compressor drive under 100% power during 100% loading and around 20%%MDASSML%%25% power while unloaded, with the drive motor running at constant speed. A variable speed drive (VSD) compressor delivers only the mass of air the system requires. This type of control allows the compressor to consume the minimum amount of power required to deliver the required air with reduced unload power consumption.

          These 11 suggestions are just some of the ways to get instrument air systems to work reliably and efficiently. Even though reliability and energy efficiency are top priorities, there is another important factor behind maintaining proper air dew point, oil content, and particulate filtration. Manufacturers of control valve actuators and other pneumatic equipment may not stand completely behind their warranties if they know that an instrument air system does not meet minimum quality standards.

          —Edited by Joseph L. Foszcz, Senior Editor,
          630-320-7135, jfoszcz@cahners.com





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