Dew Point Analyzer

FIG. 1 is a schematic drawing of a dew point detection device constructed in accordance with one embodiment of the invention;

FIG. 2 is a partially schematic cross-section through a preferred embodiment of the device; and

FIG. 3 is a graph showing results obtained when measuring the water and hydrocarbon dew points of a gas sample.

Referring to FIG. 1, a dew point detection device includes a zone comprised of an enclosed chamber 3 having an inlet 5 and an outlet 6 to permit a gas sample to be passed into and out of the chamber 3.

A planar mirror with surface 10 is located inside the chamber 3 in operative association with a cooling/heating unit 12, preferably a thermoelectric device. Cooling/heating unit 12 has associated therewith temperature sensor 13 which detects the temperature of the mirror surface 10. The unit 12 is also connected to a heat sink 14. The unit 12 is capable to cooling the mirror from ambient temperature (which may be 40.degree. C.) down to at least -10.degree. C., and preferably to -20.degree. C. In the preferred embodiment the unit is capable of cooling the mirror down to -40.degree. C.

In practice, it has been found that the sensitivity of the system to a decrease in the intensity of scattered light and hence the accuracy of determining the dew point for hydrocarbon in the gas stream is significantly improved if the surface 10 is roughened to increase the intensity of scattered light. However, such roughening of the surface decreases the sensitivity to water dew point determination. Accordingly, the surface 10 preferably comprises a combination of a polished and highly light reflective portion and a roughened light scattering second portion.

A light transparent window 16 is provided in a wall of chamber 3 on the side of the chamber opposite the mirror surface 10. A light beam source 18 is located externally of the chamber 3 to direct a light beam 19 onto the mirror surface 10 at an incident angle so that the reflected light from the mirror surface is out of the field of view of a light detecting device 20 and only the light scattered from the mirror surface is directed onto the light detecting device 20.

Light detection device 20 is located along a light transmittance path which extends perpendicularly to surface L 10, to detect light scattered in its field of view from this surface. Scattered light is focused onto device 20 by lens 21. The light detection device is of the form that it can selectively and independently detect the changes in light intensity scattered from each of the highly polished and roughened areas of surface 10. Preferably, the light detector device is a linear CCD array, having one group of pixels positioned to receive focused scattered light on the smooth highly-polished portion of surface 10 and one group positioned to receive focused scattered light on the roughened portion.

A micro-computer based data acquisition, data processor and control unit 24 is provided to acquire data from the light detection device 20 and to determine the onset of condensation. The unit 24 is also arranged to control the cooling/heating of the mirror surfaces 10 by switching the cooling/heating unit 12 to either cooling or heating mode. It may also control the output light intensity of light source 18 depending on the surface conditions of the light scattering mirror or to compensate for the effect of deposits or contaminants on the surface 10.

A unit 25 is provided either locally to display the dew point temperature and whether it is water or hydrocarbon condensing, or to provide appropriate telemetry signals for transmission of such information to a remote data gathering system.

In operation of the device of FIG. 1, a test gas sample is introduced into the chamber 3. The temperature of the surface 10 is varied by the cooling/heating unit 12 and is measured by the temperature sensor 13. A light beam 19 is directed onto the surface 10 and the light scattered in the field of view of the light detecting device 20 is monitored.

A change in intensity of the scattered light is employed as an indicator of the presence of condensation on the cooled surface. Since the condensation of hydrocarbon and water from the test gas stream produce opposite results in terms of the sense of the associated change in scattered light intensity, as described above, a clear differentiation can be made between the hydrocarbon and water dew points.

At the start of the measuring run, a sample stream of natural gas under test is introduced through the inlet so that the pressure of the gas inside the cell is the same as the line pressure and is maintained at substantially line pressure throughout the measuring run. The temperature of the mirror surface 10 is lowered progressively at a pre-determined and controlled rate by the unit 12 in a single run from a temperature above the dew points of both the hydrocarbon and water vapour contained in the gas sample to a temperature below both of said dew points. The rate of cooling is under the control of the micro-processor control unit 24. The data acquisition system 24 determines independently during such run the intensity of the light scattered from the highly reflective portion of mirror surface 10 and the roughened less reflective portion of mirror surface 10 in the absence of any condensation. The output of the CCD array 20 and the temperature of mirror surface 10 are monitored and recorded by the data acquisition system 24 at selected temperature intervals, namely at 0.5.degree. C. intervals, as the mirror surface is cooled to the minimum temperature. After reaching the minimum temperature microprocessor control unit 24 puts the cooling/heating unit 12 into a heating mode to evaporate the condensate from mirror surface 10 and puts the data processor into dew point calculation mode. The recorded data of the CCD output for each temperature measurement, for each of the roughened and the polished mirror surface portions, is compared with the corresponding value of the previous measurement. Whenever the difference in these values changes by more than a pre-determined amount, the temperature of that measurement is taken as a dew point temperature. The data processor system 24 also determines the polarity of the signal change and indicates whether it is the hydrocarbon or water dew point temperature that has been reached. Both the dew point temperature and the component that has condensed are displayed on the display unit 25, or can be transmitted to a remote data gathering system.

One advantage of the use of light detecting device 20 that is capable of independently detecting the changes in light intensity scattered from each of the polished and roughened areas of surface 10 is that a false indication of the occurrence of condensation of one component due to "puddling" of a previously condensed component can be avoided. Puddling occurs when condensate coalesces on surface 10. Puddling, in the case of hydrocarbons, causes the thin film which has condensed to break into relatively thick puddles which increase the light scattering effect causing an increase in light intensity detected by light detection device 20. Because light detection device 20 can discriminate between the polished and the rough portions of mirror surface 10 and accordingly can indentify whether the changes in intensity are due to hydrocarbon or water condensation, the increase in light intensity due to puddling of the hydrocarbon is not misinterpreted as the onset of condensation of water vapour.

At the completion of the above described measuring run, a new measuring run can be initiated by a command from the control unit 24.

FIG. 2 shows the structure of a preferred form of the dew point detection device without the data acquisition and control system.

A base 30 of heat conductive metal, preferably aluminum, is provided with an outer recess holding a brass flange 32, the flange being firmly pulled into the recess by bolts 34 and springs 36. The flange 32 itself has an internal recess which receives an annular member 38 of heat insulating material such as polycarbonate, the centre of which holds a circular copper cylinder 40. The lower end of cylinder 40 is pressed against, and in good thermal contact with, the upper surface of a thermo-electric cooler 42 contained within an inner recess in base 30, the colder parts of cooler 42 being spaced from the parts 30 and 38 to avoid direct heat transfer from these parts to the cooler. The base 30 is in good thermal contact with a heat sink, not shown. The heat sink may be a finned thermal conductor, for example of aluminum, itself cooled by a fan. Alternatively, a heat sink similar to a small air conditioning device may be used.

The top 41 of cylinder 40, which is co-planar with the top of member 38, forms a circular mirror having two semi-circular side portions. A first side portion 41a is smooth and highly polished, and the second portion 41b is roughened. A suitable degree of roughness is that obtained by sand blasting and then subjecting the surface to a process such as that known as Ebonol C Black (trademark) which provides an oxide coating. Within the centre of cylinder 40 and separated from the mirror surface 41 by a thin wall is a thermistor device 46 having leads which pass through axial and radial bores in the cylinder 40. Base 30 houses a connector (not shown) for the leads to the thermistor device 46 and to the cooler 42.

The annular member 38 is held in place by an upper housing 50 having an internal chamber 51 with a main, lower cylindrical portion 51a, and an upper portion 51b within an inwardly sloping roof portion; both portions are co-axial and their axis is perpendicular to the plane of the mirror 41. The bottom of chamber portion 51a is defined by a cylindrical wall which rests on an o-ring 38' recessed into the outer edge of member 38. The roof portion is stepped to provide a series of alternating cylindrical surfaces and flat surfaces and which face member 38. This formation is designed to minimize internal reflections within the chamber. Stray light is also minimized by making the interior surface flat black. Preferably the housing material is aluminum and this is subjected to sand blasting and an anodizing treatment which produces a black oxide on the interior surface. The housing part 50 is thick walled so that the device can safely receive and maintain, in chamber 50, gas at pipeline pressure. The device is designed to receive and maintain gas at high pressure, for example, 1,000 psi for natural gas pipeline applications.

The upper portion 51b converges towards a bore 60 in the housing, co-axial with the chamber axis, and which includes an outer, inwardly diverging portion 60a, a central cylindrical cavity 60b, and a lower, screw threaded portion 60c. The portion 60a contains a window 62 of transparent plastics material such as polycarbonate which is firmly sealed within the bore, and which is in the form of a plug designed so that internal pressure increases the effectiveness of the seal between the window and housing. Portion 60b contains a converging lens 63 held by spacers 64 which are retained in place by hollow screw threaded member 66 engaging the threads of portion 60c.

At opposite sides of housing 50 radial bores 68 and 69 are provided which have a common axis perpendicular to that of the housing and located just below upper portion 51b. Radial bore 68 has an outer, inwardly diverging portion 68a, a central cylindrical cavity 68b, an an inner screw threaded portion 68c. The portion 68a contains a plug-like window 70 similar to window 62, and cavity 68b contains a converging lens 71 held by spacers 72 retained by hollow member 73 in portion 68c. Radial bore 69 has a cylindrical cavity containing a hollow cylindrical holder 74 having opposite sides recessed to receive pivot pins projecting from opposite sides of a flat circular mirror element 76 which has an inner surface angled downwardly towards mirror 41. The arrangement is such that a light source outside the housing can be arranged to direct a light beam through window 70 and lens 71 radially into the chamber, which beam is reflected by mirror 76 to meet mirror 41 at an oblique angle, for example, of about 45 degrees. This angle can be adjusted by rotating mirror 76 about its pivot pins. Light scattered from mirror 41 can pass out of the housing through lens 63 and window 62.

Housing 50 also includes a gas inlet port 78 which communicates with the upper chamber portion 51b. The bore 69 serves as a gas outlet port by virtue of leakage space which is provided between mirror element 76 and the holder 74. Both ports 78 and 69 are threaded to receive standard threaded pipe elements. The element engaging part 69 retains holder 74 in place

Plate 80 is fixed to the top of housing 50, and supports a light sensor housing 81. This contains a mounting plate 82 carrying an array 84 of charge coupled devices (CCD) arranged to detect light scattered by mirror 41 in a substantially perpendicular direction and which has been focused by lens 63 through window 62. The lens 63 focuses an image of a representative linearly extending area of mirror 41 onto the CCD array, in such manner that a first part 84a of the array 84 can receive light scattered from a narrow, elongated portion of the polished surface portion 41a of mirror 41, and a second part 84b of the array can receive light scattered from the roughened surface portion 41b of mirror 41, both these portions being aligned. The two parts 84a and 84b of the array 84 are connected independently to a data acquisition and processing computer, which is also connected to the thermoelectric cooler 42 and the mirror thermocouple 46. The computer is arranged to monitor sequentially each of the elements (pixels) of the CCD array 84.

Operation of the device is similar to that described with reference to the first embodiment. The computer controls the cooling of mirror surface 41, and processes signals received from the thermistor 46 and from the CCD array 84. The computer averages the output for the pixels of each array part 84a and 84b separately. The computer displays on a monitor screen continuous indications of temperature intervals and of the average intensity of the scattered light received by the pixels of each of the CCD array parts 84a and 84b as the mirror surface 41 is cooled. A sample display obtained for a full cooling run is shown in FIG. 3. In contrast to some automated devices shown in prior patents referred to above, cooling is continued after the dew point of the first condensate has been reached until the second dew point has been determined.

As shown in FIG. 3, the bottom line shows the output from array 84a. The point (W) indicated as the water dewpoint shows the temperature (about -25.degree. C.) at which scattered light caused by ice crystals condensing on the polished mirror portion shows a sharp increase. The top line in FIG. 3 represents light scattered from the roughened portion of the mirror, and the hydrocarbon dewpoint (H) is recorded as the temperature (about -16.degree. C.) at which this scattered light shows a marked decrease. It will be apparent that with this system the two dew points are readily distinguishable and can be determined to an accuracy of about .+-.1.degree. C.

When used with natural gas samples, the conditions of water content and pressure are such that the water dew point will usually be below freezing point so that the water will condense as ice crystals. In contrast to some prior art proposals, this fact does not adversely affect operation.

The light beam source is shown as outside the chamber since the chamber is subjected to high pressure as discussed above. However, if the device is designed for use at low pressures then a light source within the chamber can be used. Also, in certain applications, it may be convenient to use fibre optics to transmit light into the chamber.

While the invention as described in FIGS. 1 and 2 has been schematically shown with the mirror horizontally disposed, in practice it has been found that the invention may be advantageously installed such that the mirror surface is vertically disposed. When vertically disposed, the adverse effect of puddling is minimized due to run off of excessive condensate.

The invention has been described as using two portions of a mirror surface, it will be understood that separate mirrors, one reflective and highly polished and one roughened and scattering, may be used. Separate light beams may be used for such separate mirrors.

While the invention has been described for use in sampling natural gas in pipeline applications, the invention is useful for measuring dew points in other applications such as dielectric gas in high voltage electrical utility transformers.