SEPARATING DEVICES AND METHODS

Abstract: Separating devices and methods for removing contami­ nating particles of solid and liquid from gas, particularly air for a gas turbine used in a marine environment, employ a liquid spray means (3) by which liquid is sprayed into the gas. The gas is then passed through moisture separating means, e.g. a separating impact filter pad (5) and an inertial separator (6), which remove the liquid and particles en­ trained therein.

Description: TITLE: SEPARATING DEVICES AND METHODS DESCRIPTION This invention relates to separating devices and methods for removing solid and liquid contaminants from gaseous streams without the separator means becoming fouled. The invention relates, particularly but not exclusively, to the cleaning of gas turbine combustion air in such difficult environments as offshore oil rigs or coastal/desert operation or shipboard operation off desert coasts. Sea salt and solid particles contained in intake air to gas turbines have a deleterious effect upon their operation. Particular difficulty occurs in removing a combination of sea salt aerosols and high concentrations of solid particles in the form of mud chemicals, dust and sand. One proposal for protecting gas turbines in marine environments utilizes a three stage system, consisting of hooked vanes, coalescer pad, and additional hooked vanes arranged in series in the direction of the air flow. That three stage system is a compact, high velocity moisture separator having a limited efficiency for removing small solid particles and a very limited dust holding capacity. In the aforementioned.environments, this type of separator is prone to rapid clogging by the high solids content of the air. This invention has for an object to provide for means for self-cleaning, high efficiency removal of sea salt aerosols and solid particles to give protection for gas turbines operating in the above environments. The invention provides a method of removing particles entrained in a gas, in which the gas is passed through separating means for removing said particles, characterised in that a liquid is sprayed into the gas adjacent to the separating means and liquid is removed from the gas by the separating means. This invention also provides a separating device for removing particles entrained in agas having separating means for removing said particles, characterised in that liquid spray means is provided adjacent to said separating means, which is arranged to remove liquid from the gas. Preferably, the separating means includes an impact separating filter pad and the device includes a fluid spray, such as sea water, to saturate an area with droplets of a particular size to best provide a wash for airborne solids. The spray droplets may be optimized to also provide a uniform, in-depth penetration of the filter pad where the pollutants are removed from the air stream. The following is a description, by way of example, of an embodiment of the invention, reference being made to the accompanying schematic drawings, in which Fig. 1 is a sectional plan view of a separating device in accordance with the invention installed in the air duct of a gas turbine, and Figs. 2 and 3 are sectional side and plan views on a larger scale of the separating device of Fig. 1. The separating device shown in the drawings is a self-cleaning device for removing contaminants from gas streams,. particularly gas turbine combustion air. It has in this example, a sea water wash and a sea water impregnated separator for removing sea water from the air stream. The device has initial protecting means at the inlet to the air duct in the form of vertical slats 1 set at an angle to the air flow in such a way as to cause a change of direction of the air flow, thereby eliminating the requirement for snow hoods and also providing protection and separation from snow and rain. In areas of heavy solid pollutants these slats will be irrigated to prevent fouling. The trailing edge of each slat is a vertical pocket which drains liquids from the surface of the slat to a drain box at the base of the slats. The depth of each of the slats in the direction of the air flow, will be between 1 and 12 inches. The slat spacing (pitch) will be in the range of 1 to 12 inches. Preferably, the slats will be 6 inches deep and spaced 6 inches apart. The slats are made of material resistant -to the environment. Preferably, they are made of marine grade aluminium or 316 stainless steel. Immediately downstream of the protecting means is arranged a matrix of fluid nozzles 3, which generate a spray 4 of small droplets of sea water into the air stream, the spray being co-current (as shown in the drawings) and/or counter-current. The spray nozzles can be hydraulically or pneumatically controlled. The nozzles are supplied with sea water from a manifold arrangement 2. The sea water spray generated has a mass mean diameter of 10 microns or greater in order to maximize small particle capture by collision with and encapsulation of contaminants by the salt water droplets. The spray nozzles are also arranged to provide uniform coverage of a knitted mesh pad impact filter 5. The increase in pressure drop due to spray is kept at a minimum by a system which automatically adjusts the spray pressure to a value determined by wind tunnel tests for each inlet velocity. The droplets penetrate deeply into the pad. The floor 7 of the spray chamber is sloped to ensure drainage of the water and pollutants which fall out in this area. High efficiency removal of contaminant is carried by means of the knitted mesh 5. The knitted mesh can be manufactured from either metal or synthetic welt able fibre of diameter greater than .001 and less than .006 inches knitted into a mesh of between 5 and 25 stitches per inch. The knitted mesh contains a crimp to ensure separation of the superimposed layers which go to make up the pad. The pad is made up of layers of crimped knitted mesh material with the crimp of alternative layers oppositely handed. The layers of the pad are supported in a frame which is fixed in the duct on the upstream side of the final separating stage. Preferably, the fibres forming the layers of the pad are comprised of Monel wire. Money is especially useful in marine applications since it is especially resistant to salt water corrosion. The fibres have been knitted together to form a mesh or grid having the fibres joined or tied together. Preferably, the diameter of the fibres is in the order of .002 inches but the diameter can range from greater than .001 inches to less than .006 inches. With .002 inch diameter fibre it has been found with conventional knitting apparatus that there may be between 10 to 12 stitches or joints per inch of mesh. That is, in one inch length of the fibre mesh there are approximately 10 to 12 fibres. The wires are crimped so that each of the layers does not lie completely flat. Thus, when the layers are combined to form the pad, the layers will not lie flat against one another but, instead, will provide a cushioning or lofted medium. Preferably, the crimp is sufficient to allow no more than 20 to 24 layers per inch of depth of the pad. Without the crimping of the fibres, it has been found there would be approximately 70-75 layers of fibres per inch of pad depth. The reason that crimping of the fibres is desirable is that, for a given number of layers the resistance to flow will be lower and the self-cleaning properties increased in the distance between adjacent layers is increased. Droplets of sea water penetrate the pad, wetting the fibres, and providing a matrix of adhesive surfaces to collect and capture small particles of dust and salt aerosols. The spray droplets penetrate deeply into the knitted mesh pad ensuring a depth of collecting media and, hence, high efficiency and the capability to handle high concentrations of contaminants. The continuous stream of droplets penetrating into the pad ensures that the fluid wetting the fibres is being continuously changed, i.e., it is washing the fibres. The property of the device provides for selfcleaning of the pad. The small particles are encapsulated within the fluid and are washed out of the pad. Devices without this self-cleaning property rapidly become clogged by the build-up of solids. The combination of pad physical properties, of mesh density, fibre diameter and the droplet size distribution of the spray, ensure that while the spray penetrates the pad, no droplets are small enough to go through the pad without capture. The fluid within the pad wets the fibres and forms droplets within the interstices of the media. The mesh size and layering is such that these droplets grow large, several millimeters in diameter, before the aerodynamic drag forces entrain them into the air stream. The droplets re-entrained are large and are removed by a following arrangement of hooked vanes providing the final separating stage 6. The quantities of sea water collected by the hooked vanes is sufficient to ensure they are always running wet, thus preventing the dustcarrying droplets from drying within the system and causing fouling and blockage. The knitted mesh pad 5 operates wetted with sea water at high face velocity, between 15 and 35 ft. per 2 second (400-2100 cfm per ft. of media frontal area). The wetted pad, when used this way, is an excellent heat exchange media. When operating as gas turbine protection where warm sea water can be made available, such as on oil rigs, the system can anti-ice itself by wetting with warm sea water all surfaces susceptible to icing. This ability to anti-ice itself reduces the size and complexity of the separator. The prior art requires the addition of hot air to the inlet air to anti-ice the gas turbine inlet with all the problems of availability and capital equipment, size and costs of providing this form of anti-icing. The same heat exchange properties can be made use of as an evaporative cooler for gas turbine operation in hot climates. Significant increases in power and fuel efficiency can be gained from the use of these evaporative cooling properties of this separating device. The final stage 6 of the separating device is an arrangement of hooked vanes to remove by inertial separation all droplets entrained from the pad. The inertial droplet collector consists of a plurality of multi-chevron vertical vanes parallel to each other at a pitch between 0.5 to 1.5 inches so as to remove all entrained droplets. The droplets are removed from the air stream between the vanes by impaction with the vane surfaces at points of momentum change caused by the tortuous path between the vanes, the impacted water droplets are removed from the vane surfaces and possible re-entrainment into the air stream by vertical pockets on the vanes which direct the collected water vertically to a drain box at the base of the vanes. The vanes are made of rigid corrosion resistant material such as plastics, e.g. P.V.C., aluminium or steel. Preferably, they are made of marine grade aluminium or Cres 316 stainless steel. The vanes penetrate into the drain box to a greater vertical depth than their depth in the direction of the air flow in order to ensure they cannot be bypassed via the drain box which would cause salt water to be re-entrained from the drain box. Each stage is drained separately (D1-D4) via manometric traps. A subsidiary system consisting of either an ejector or a suction device prevents the build-up of water and accumulation of dust by flushing one or more of the drain boxes and manometric traps. Where water is not freely available the unit can operate on a limited self-replenishing water supply. The drained effluent can be filtered in a water treatment plant and recirculated. To provide protection for a gas turbine 14 from combustion air contaminants, the device is preferably situated at the first part of the gas turbine inlet housing, thus protecting the entire inlet configuration which could include inlet silencer splitters 11, acoustically lined plenum 12 and inlet flare 13. The small dimensions of this device make this preferred arrangement possible. The device, as described, is self-cleaning and can be readily anti-iced and should, therefore, not give rise to levels of high differential pressures which may jeopardize gas turbine operation. However, it is standard practice in the majority of gas turbine operations to include emergency bypass doors to ensure the option of continued availability of gas turbine power. In the prior art, doors known as "blow in" doors have been provided adjacent to the separator. These doors automatically open, if the differential pressure increases above a predetermined level, allowing unfiltered air to the gas turbine. However, with the separator device described above, it is possible to have the whole or part of the separator device itself comprise a "Blow in" door. Preferably, the combination of knitted mesh pad 5 and inertial vane stage 6 a module that is movable. This module is supported within the separator device and locked in position. A high differential pressure across the separator device causes the release of the locking device allowing the module to move out of the airstream allowing unrestricted flow of air through the aperture, thus produced, to the gas turbine. To ensure there is no possibility of the spray fluid being ingested by the gas turbine, a liquid flow cut-off device is operated coincidentally with the separator device bypass. Although in one desirable embodiment shown in the drawings, the water spray is shown as co-current with the air flow, it may as previously indicated be countercurrent or a combination of co-current and countercurrent spray may be used. As shown in the drawings, the spray means (3), the filter pad (5) and the inertial separator (6) are arranged in that sequence in the direction of air flow. However, if desired the sequence could bespraymeans (3), inertial separator (6) and filter pad (5). The illustrated embodiment has only two separating stages (5,6), but if desired one or more separating stages could be added, or only a single stage might be provided. Also other kinds of moisture separators than those shown in the drawings might be used. All of the above possibilities fall within the scope of the invention. In referring above to only two separating stages (5,6), it will be understood that the slats (3) are not considered to constitute separating means as used in the present invention for removing small particles.

Claims: CLAIMS 1. A method of removing particles entrained in a gas, in which the gas is passed through separating means for removing said particles, characterised in that a liquid is sprayed into the gas adjacent to the separating means and liquid is removed from the gas by the separating means. 2. A method according to claim 1 characterised in that the liquid is sprayed into the gas upstream of separating means comprising an impact separating filter pad (5). 3. A method according to claim 2 characterised in that the gas is also passed through an inertia separating means (6). 4. A separating device for removing particles entrained in a gas having separating means for removing said particles, characterised in that liquid spray means (3) is provided adjacent to said separating means, which is arranged to remove liquid from the gas. 5. A separating device according to claim 3 characterised in that the liquid spray means (3) is disposed upstream of the separating means and the separating means comprises an impact separating filter pad (5). 6. A separating device according to claim 5 characterised in that the separating means also comprises an inertia separating means (6). 7. A separating device according to claim 5 or 6 characterised in that the liquid spray means (3) is disposed upstream of a first separating means in the form of an impact separating filter pad (5), which itself is disposed upstream of a second separating means in the form of an inertia separating means (6). 8. A separating device according to any of claims 4 to 7 characterised in that the liquid spray means (3) is arranged to spray liquid co-current with the gas flow. 9. A separating device according to any of claims 4 to 8 characterised in that the separating means comprises only an impact separating filter pad (5) and an inertia separating means (6) disposed upstream or downstream of the filter pad, the liquid spray means (3) being disposed upstream of both the filter pad and the inertia separating means. 10. A separating device according to any of claims 4 to 9 characterised in that the separating means includes an inertia separating means (6) of the vane type.

Inventors: CARY, BOYD BALFORD, MAVROS, STRATOS
Applicants: PARMATIC FILTER CORPORATION
Application year: 1983
Main classification: B01D47
IPC codes: F23D14, B01D45, B01D50, B01D47, F02C7