Improving Air System Efficiency

by: R. Scot Foss, Plant Air Technology

Part 11: Testing and validating the quality of compressed air.

There are many tests available to determine the quality of the compressed air in a system, and a variety of validation equipment types. Measuring quality can range from occasional tests that examine a collected air sample to the use of continuous online testing equipment.

The primary tests used for sampling compressed air quality for sampling compressed air quality are gravimetric analysis and gas chromatographs.  Gravimetric analysis deals with the mass concentration of anticipated contaminants, including: general gas constituents, particulate, and moisture content.  This could be useful for testing plant, instrument, or breathing air.  Gas chromatographs are used to find a specific gas constituent such as nitrous oxide, hydrogen sulfide, or sulfur dioxide in ppm by weight or volume or in µg/m³.  (There are also particulate tests for inlet filter selection.  These screen tests will report the percentage of total particulate by micron size.)

Hygrometers:

Hygrometers help determine the atmospheric or pressure dew point of air.  It is important to decide whether you need a pressure or atmospheric monitor.  The pressure dew point monitor is normally inserted directly into the system’s airflow, while the atmospheric monitor uses a sample tube and a cell, into which a sensor is inserted.  Monitors can handle many different pressure dewpoint ranges; -0^o to –100^oF., 0^o to 75^oF, and –50 to 80^o are common.  Careful installation of hygrometers is critical, because most sensors contain activated alumina or lithium crystal, which cannot handle liquid slugging or exposure.  (There are a few optical hygrometers, which can handle infrequent slugs of liquid.)  Damaging the sensor can be very costly.  It does seem odd that something that measures the presence of moisture cannot handle moisture in a liquid state!

The air system should be dried before installing the sensor.  This may mean several days of running time, to ensure that liquid is completely removed.  (It should be determined whether the sensor is capable of transient or continuous high relative humidity conditions ad, if so, how long the recovery time is.)  Often times, a high dew point alarm is available, and in many cases, 4 –to 20 mA output for trending is an option.  When installing this type of equipment, the goal is to check the quality of the air going downstream to the users – not the performance of the treatment equipment.  If an expander, intermediate control, or other metering device is being used, the test equipment should be installed a the final entry point to the overhead system downstream o this equipment.

Other Monitors:

More sophisticated monitors have been developed to measure contaminants in compressed air, such as total carbon analyzers, particle counters, and nephalometers.  These devices can be delicate and require careful selection and installation.  Some are photometric, with a laser diode or other light source.  Others rely on mirrors along with light.

One problem encountered with these instruments is optical clouding, where the optics can be obscured by heavy contaminant levels.  Another is frequent calibration and cleaning.  This problem typ0ically occurs when a system cools down during depressurization. Sophisticated monitoring equipment might have to be disconnected from the system before shutdown and reconnected after the system is back online and the air quality has stabilized.  Another area to check: whether any limitations exist as to the phase of the contaminant being measured, such as vapor phase or liquid.  Some of these devices cannot distinguish types of contaminant from each other, so care should be take in determining the constituents with a sample test before ordering or installing one of these complex and expensive on-line monitors.

Particular interest in one type of contaminant doesn’t change the fact that other non-specific contaminants may be present that could alter the quality or quantity of information being collected.  Some recently developed photometric monitors can provide a wide range of sample monitoring, such as particulate, hydrocarbons, temperature, pressure, and dew point in the same sample instrument.

Carbon monoxide (CO) monitors are available from many vendors, and typically monitor only CO.  They are relatively delicate devices and need frequent calibration using calibration canisters with CO in the gas stream at the alarm level.  Some can also monitor for other non-specific aggressive gases.  These are a bit more sensitive, but also provide added protection for the air user.  Excessive moisture can negatively affect CO monitors because of the damaging effect water can have on the load cells or sensors.

Compressor Lubricant Detection:

Microimpingers are relatively simple, inexpensive monitoring devices.  They use a litmus-like sampling tube, which is connected to the compressed air piping system.  They take a small (1/32 cfm) sample of he air from the wall of the piping system and indicate the presence of most compressor lubricants or other condensed liquids with carbon material present.  They do this by changing the color of the material in the sample tube.  The tubes have incremental indications that relate to ounces per 100,000 ft³ of air tested.  While this is relatively inaccurate, these inexpensive devices do indicate the presence of condensable hydrocarbons and other fluids containing carbon. Most compressor fluids and condensable gases present at the inlet of the compressor have some carbon in their base stock or additive packages.

These devices will also percolate liquid water form the top of the exhaust port if liquid water is present in the compressed air sample location.  The installation of these devices (downstream of filters and dryers) serves as a good predictive indication of performance of upstream treatment equipment designed to remove the tested contaminant.  

Microimpingers do require visual inspection on a regular basis.  Manufacturers recommend that a 15-minute sample be taken on a daily or shift basis.  Alternately, these devices can be left on continuously when installed downstream of treatment equipment that is intended to remove the subject contaminants.  Noting the presence of these contaminants can serve as an alert to predictive maintenance needs, long before the detrimental effects will be noticed downstream by the end user.

In most industrial or process environments, microimpingers serve as a good early-warning system.  It is more important to know that a contaminant has worked its way downstream of treatment equipment at specific locations, than it is to measure the quantity of contaminant once it has entered production or processes.

Temperature and Pressure:

Temperature and pressure are important to monitor it the system for a variety of purposes.  The following is just part of the data that may provide a more effective predictive maintenance program for better, more consistent air quality:

·         Aftercooler discharge pressure and temperature,

·         Cold temperature differentials,

·         Inlet conditions for aftercoolers, which would indicate compressor maintenance needs,

·         Filter dirt loading and channeling,

·         Overloading of dryers with mass flow or heat load, and

·         Under loading of dryers with heat load.

Here’s a situation we find in some facilities.  Maintenance personnel are assigned to take pressure and temperature readings at 40 to 50 monitoring points at least once per shift.  (Collecting this data can consume several hours.)  The fallacy with this exercise is that data is collected, but then just hung on the wall or stored for posterity.  The individuals collecting it don’t understand its significance, and while someone else in maintenance or plant engineering may know how to diagnose the pressure and temperature information, they never see it.

What’s more, the accuracy of the pressure and temperature instruments used in many plants is of relatively poor quality and uncalibrated.  An audit frequently reveals that these gages are as much as +/-7% off metrology standard for test purposes.  Even if the information gathered was diagnosed by a knowledgeable individual, or if he or she had manufacturer’s data indicating benchmarked levels to watch for, the quality of the information would not provide dependable feedback.  Furthermore, if the gages were consistently unreliable, their data would at least be relative.

It is the instrument manufacturer’s position that this is a consumer-driven market, and if the consumer doesn’t ask for dependable gages, why would manufacturers supply them at extra cost when they would be unable to recover the cost?  The OEM certainly has a point.  On the other hand, they do not report the standard quality of the gages that are applied to the equipment, nor the impact that this may have on diagnostic results.  (This includes analog as well as digital instruments.)

Another problem frequently seen is that the minor divisions used on these gages are so rough that maintenance personnel can barely use the information, as inaccurate as it may be.  One option is to specify high quality, instrument grade gages that can be calibrated, and with 1 psig or 1^oF minor divisions on compressors, dryers, filters, and cooling equipment.  However, because of the standard mark-ups by equipment manufacturers for non-standard options (and the delay) in manufacturing to support these changes), a better alternative is to install instrument taps and tees so that sophisticated pressure and temperature monitoring equipment can be used to collect individual and differential information.

In a relatively small system with four compressors, one or two dryers, and two to four filters, there are probably 22 to 26 data points for pressure and temperature.  That would require between 44 and 52 quality pressure and temperature gages.  The cost for this instrumentation could range from $4,000 to $6,000!  Installing instrument taps at all data points would allow using one tap for both pressure and temperature, and with 1/3 to 2/3 of the money, invest in a few high quality pressure and temperature instruments that con monitor air, water, and refrigerant.  On the high end of the test equipment, multi-channel, handheld instruments are available that could accurately determine ∆T, and cold temperature differentials.

It may be wise to use permanently installed pressure and temperature monitoring equipment with downloadable trending on treatment equipment inlet and outlet temperature and pressure.  This could be used for top-of-the-line predictive maintenance by observing trends in collected data.

Reprinted with permission from R. Scot Foss, president of Plant Air Technology, Charlotte, N.C., a company specializing in system auditing and design.  This article is based on his book, "Compressed Air System Solution."  A portion of the proceeds from sales of the book is donated to children’s charities.  To order a copy of the book, please contact Southern Corporation.