Method and Apparatus for Determining Oil Mist in Compressed Air

Air compressors and vacuum pumps are typically oil sealed or lubricated and thus compressors and pumps generate a heavy concentration of oil both in the form of oil vapor and oil-mist particles in the exhaust air during the pumping cycles. Typically the oil-mist droplets are very small, often well below 5 microns in average size. The oil mist, which is mostly composed of finely divided liquid droplets dispersed in air and visible typically as a haze, a fog, and the droplet size represent a health hazard in the environmental working place. Typically such oil mist is removed by employing a suitable filter, such as a glass-fiber filter, which removes the oil mist by a process of coalescing that permits the coalesced oil to drain from the filter.

There is a need for a means of testing on an easy semiquantitative basis the amount of oil mist in compressed air in order to determine the type and nature of the filters to employ in the compressed-air line. Present techniques to determine the amount of oil mist in compressed air are often quite cumbersome, expensive, complex or tedious. Such present techniques include the sampling of air from the compressed-air line, which air sample is then subject to test by a gas chromatograph, a mass spectrometer or hydrocarbon analyzer to determine the amount of oil in the air. However, such techniques ofter measure not only the oil-mist particles captured by the glass-fiber filter, but also the oil vapor in the air and in addition such tests do not provide for the testing of the compressed air under operating-line conditions and pressure. Such test methods also represent problems in getting an accurate representative sample of the compressed air, particularly where the oil-mist particles are so small and therefore, there are problems associated with the isokinetic sampling of compressed air.

One test method in use is a modification of a DRAEGER tube wherein a glass tube is filled with a selective oil absorbent and the gas to be tested is then passed through the tube. The absorbent contains a visual color indicator and the change in color and the intensity of the color change are observed as an indication of the presence of a particular gas. This test is typically employed for toxic gases, such as hydrogen sulfide, rather than for oil mist from compressed air or vacuum pumps. In the test modification oil absorbent with a color indicator is employed in a tube and compressed air bled through the tube at a known flow rate and the color change observed as an indication of oil mist in the air. This test is still quite insensitive, since the amount of air flow depends on the line pressure and the time factor involved in the test. In summary the present mentods and related techniques to determine oil-mist particles in a compressed-air stream are quite limited, insensitive, complex, expensive, tedious, hard to perform and unsatisfactory.

My invention relates to a method of and a kit apparatus for the determination of oil mist or suspended-oil particles in compressed-air streams and the standard plates used therein. In particular my invention relates to a direct, semiquantitative test method and technique wherein the amount of oil mist in a compressed-air stream can be determined by comparison of test means with a set of standards whereby the amount of oil mist in the compressed air may be determined.

My test method comprises in general the sampling, such as by bleeding, of the compressed-air stream containing the oil mist whose concentration is to be determined from the compressed-air source or line to be tested through a defined orifice, directly impinging the compressed-air streams so bled for a defined time period onto a coated plate containing a UV radiation fluorescent material, which plate captures at least a portion of the oil-mist particles in the compressed-air stream impinging on the plate surface, and then comparing the test plate under UV radiation with a standard plate to determine by the degree of fluorescent quenching between the test plate and the standard plate the amount of oil mist present in the tested compressed-air stream.

For example, my method comprises bleeding a compressed-air stream through a known orifice, for example, a 0.010" orifice, in a pipe plug with a fitting in the air line, which directs the bleeded compressed-air jet against the flat surface of a coated microscope glass slide with the slide coated with an active material which fluoresces under radiation, such as a slide coated with silica and a UV fluorescent indicator. Such glass-covered slides are commonly employed in thin-layer liquid chromatography, although not for the purposes of this invention. The glass slide is permitted to become covered with oil over a defined time period, for example, from 10 seconds to 30 minutes, more typically for example 30 seconds to 10 minutes, with the compressed-air line pressure often ranging from 80-125 psig.

The test slide then coated with the impinged oil-mist particles is then removed and placed in a UV radiation box together with a standard test slide and both subjected to ultraviolet radiation and the amount of fluorsecent quenching of the test slide compared with the fluorescent quenching of the standard slide is visually observed to determine the amount of oil-mist particles in the tested compressed air either by the area of the nonfluorescing spot or in the lower concentration of oil by visually observing a difference in the degree of darkness of the spot.

A standard test slide is separately prepared and distributed and illustrates the amount of fluorescent quenching on the test slide for varying degrees of known concentration of oil mist. The test slide is calibrated in relation to the percent oil or parts per million of oil mist. The oil mist on the test slide absorbs the UV radiation and quenches the fluorescence of the indicator on the plate. The amount of quenching of the fluorescent indicator is an indication of the percent of oil in the compressed air over the defined time period. The standard slide contains a series of spots or other forms of different diameters and/or intensity corresponding to different amounts of quenching due to the presence of different amounts of oil in a compressed-air stream composed during the same time over roughly the same pressure through the same defined orifice. This test procedure is simple, direct, easy and semiquantitative enough to permit a determination of the correct filters to be employed in the compressed-air line.

My apparatus also comprises a test kit or assembly for use in determining the amount of oil mist in compressed air, which kit comprises in combination a standard fitting for insertion in the compressed-air line of an air compressor or vacuum pump or other pressurized air source, which fitting contains the defined orifice, for example, from 0.010" to 0.020" in diameter; a holder for a test plate, the holder adapted to be placed adjacent to and opposite the orifice so that the sample bleeded air stream from the orifice will impinge on one surface of the test plate; a test plate which is adapted to be inserted into the holder and which contains a coating on at least one side thereof (preferably both sides) which absorbs or retains oil-mist particles and which coating also includes a UV fluorescent indicator compound; a standard test plate which contains oil at defined concentration ranges on the plate; and an UV radiation fluorescent test chamber adapted to permit the insertion of a test and a standard slide side by side or in a comparison manner and a source of ultraviolet radiation on the surfaces of each slide. The tester views the test slide together in the UV radiation chamber with the standard slide and determines the amount of quenching which is proportional to the amount of oil-mist in the tested compressed-air line. By a comparison of the amount of quenching of the test plate and the standard plate the tester determines the amount of oil mist in the compressed air.

For the purposes of illustration only my test method and apparatus will be described in connection with the preferred embodiment and in connection with the determination of oil mist in a compressed-air line. However, it is recognized that various changes and modifications may be made in the described method and apparatus by those persons skilled in the art, all within the spirit and scope of my invention.

FIG. 1 is a front plan view of the test kit apparatus.

FIG. 2 is a scale plan view of the apparatus of FIG. 1.

FIG. 3 is a schematic illustration of the comparison between standard and test plates in determining the amount of oil mist vapor.

Referring in particular to FIGS. 1 and 2 there is shown a test apparatus 10 of my invention which comprises the test apparatus to be installed on a compressed-air line whose compressed air is to be tested for oil-mist concentration with the test apparatus installed through the NPT threaded Nipple 12. The test apparatus includes a pressure gauge 14, a slide holder 16, and a slide 18 mounted essentially perpendicular and spaced a short defined distance about 1" from a fitting 20 containing a defined orifice 24, for example, 0.010 inches, through which air is admitted through the employment of a valve 22 so that the air sample 26 impinges on the surface of plate 18. Test holder 16 is a piece of aluminum square tubing to form a box-like shape with aluminum ridges on each side with a groove in each edge to support a test plate inserted in the groove.

In operation the apparatus test kit 10 is installed on a compressed-air line and a test slide 18 then placed in the slide holder 16 with the coated or oil-absorbent surface of the slide toward the fitting 20 containing the orifice 24 so that the air sample 26 will impinge on the coated surface of the test slide essentially perpendicular to the slide surface. The line pressure is then observed and recorded on the pressure gauge 14 and then the valve 22 opened for a defined time period, for example, 1 to 5 minutes. Where the compressed air has a low amount of oil mist, for example, if oil-mist filters are installed in the air line; that is, the air is essentially clean air, it may be necessary to leave the valve open for greater periods of time. After the defined time period, the valve 22 is closed and the slide 18 removed from the holder 16. The test slide 18 is then placed under a source of ultraviolet light and compared with a standard plate to determine the amoun of oil concentration.

FIG. 3 is an illustration in comparison of a test plate and a standard plate wherein the standard plate contains a series or typically different diameter spots showing increasing oil-mist concentration in the compressed air. The oil mist on the test slide may be identified as to the concentration range by a comparison between the darkened spot on the test plate in comparison to the darkened spots on the standard plate. This provides a semiquantitative determination of the amount of the oil mist in the compressed air. The standard plate can be marked if desired adjacent each area or spot in parts per million of oil mist represented under the defined test conditions for that spot. At lower concentration levels where there is not much difference in the diameter of the test spots, the determination may be made through the degrees of intensity of fluorescent quenching of the darkened area on the test plate compared to the fluorescent quenching on the standard plate or a combination of spot diameter and the degree of fluorescent quenching.

The test apparatus of FIGS. 1 and 2 was installed on a 1/2-inch compressed-air line, the air line containing in series; a lubricator to supply oil mist, a trap, such as a type 92 Balston filter housing without the filter tube which would capture gross oil-liquid droplets not in aerosol form in the bowl of the trap, the test apparatus and a flow meter with a valve. Tests were carried out with both standard no. 2 compressor oil and a synthetic diester oil as contaminants representing the oil mist. The No. 2 oil and diester were tested separately with line pressure of the compressed air line regulated to 100 psig with a 400 standard cubic feet per hour flow being observed.

A silica gel chromatography slide containing a UV-254 NPT phosphor indicator was placed in the slide holder. The valve on the test apparatus was then opened for a period of one minute and closed. The test slide was then removed and examined under 254 NM ultraviolet light. In both test cases the results showed a dark spot had appeared on the test slide where the oil impingement had occurred. The darkness was a result of fluorescent quenching of the phosphor indicator on the chromatography test slide. A nonwoven glass-fiber bonded filter tube was then installed in the filter tube housing and the tests repeated with the results that the quenching of each test contaminant was smaller and less intense in color than without the filter, which indicated that the tests were semiquantitative in nature.

A standard test plate may be prepared under the same test conditions by inserting a test slide into the test holder and exposing each portion of the plate to different defined time periods to obtain, for example, a standard plate of 0, 1, 5, 10, 30, 50 and 100 parts per million of oil mist as a standard plate. The standard plate may be prepared for lower concentrations, such as 0.1 to 5 parts per millions, by exposure for longer periods of time to show the difference in the degree of darkening or quenching.

My test method and test apparatus have been described in particular in connection with the determination of oil mist in compressed air or air under pressure. However, it is recognized that my test method and apparatus may be used to determine generally contaminants in any source of compressed gas wherein the contaminant serves to quench the UV indicator in the test plate, such as for example, testing for various hydrocarbon mist or chemical-mists in other compressed gases.