GB2522460A - Particle counter, particle size estimator, solids mass flow estimator and failed filter detector for inlet air systems - Google Patents

Abstract

An air filter monitoring system comprises means for illuminating a filtered air stream and means for detecting light scattered from particles contained in the air stream. The scattered light is used to determine particle size and particle count for calculating solid mass flow in the stream. Means may be included for determining and displaying, based on the detected particles, locations in a filter array at which particles are bypassing the filter. The illumination means preferably comprise light sources at each corner of a square filter plate or frame, the light sources co-operating to provide a sheet of light through which the filtered air passes. The system is intended to allow optimisation and/or customisation of cleaning and maintenance schedules, for example in gas turbine flows, based on determined values of solid mass flow through the filter.

Description

DESCRIPTION

Particle counter, particle size estimator, solids mass flow estimator and failed filter detector for inlet air systems Invention Precis This invention relates to a real time method capable of counting the number and size of particles present in a filtered air flow and thus estimating the total solids mass flow. As the method is particle position sensitive a filter element which is failing or has failed may also be detected.

Typically devices such as large industrial gas turbines consume in the order of 500kg of air a second.

These machines can be damaged or can lose efficiency if airborne articles are allowed to adhere to the machine surfaces. Thus the air which is drawn into them is filtered to typically prevent airborne particles larger than 5 microns entering the inlet air system. This air is referred to as "clean" air and the ducting through which the filtered air flows as a "clean area". The space before the air is filtered is generally referred to as "dirty". The clean area is typically unlit and entry is prohibited while the machine is in operation. When a machine is stopped entry is strictly controlled and is not normally undertaken unless there are issues to investigate. This ensures that the clean area is maintained clean and free from deposits or foreign objects which may damage the machine if ingested.

Typically particles below bum do not cause erosion and those above 2Oum can cause erosion.

Severe erosion could cause components to break loose and cause catastrophic damage. Any particles which adhere to the machine components can cause fouling or can block cooling passages.

These actions act to reduce the efficiency and the life of the machine which are expensive operational issues.

The inlet filter generally comprises a number of individual filters acting to filter the air in parallel. As the filter bank filters the air the pressure drop across the whole bank increases. This filter differential pressure is used to estimate the filter life and indicate when the filter should be cleaned or replaced. If however one or more filter passes unfiltered air because it is either damaged or not correctly installed then the pressure drop will not correctly reflect the filter status and in particular contaminant will enter the clean area via the damaged or badly fitted filter. Operating the machine under these conditions is not recommended however detecting these issues is not routine on gas turbine power plants and can contribute to loss of efficiency, machine failure and life reduction.

Operators of large gas turbines tend to wash the machine with highly purified water on a daily basis to combat both corrosion and adhesion issues related to particulate matter. The daily routine is based on manufacturer's fleet information and does not necessarily relate to a specific installation.

With knowledge of the particulate contaminants actually entering a specific machine the operating regimen can take appropriate action to wash when economically justified and thus improve operating returns.

This invention provides a method of determining the number of particles and their size distribution down to Sum and thus is able to determine a mass flow and accumulated mass. In addition the particle location is also revealed and thus the point where air is bypassing either a failed or poorly installed filter. Totalisers can reveal total mass flow ingested and can provide data to determine

DESCRIPTION

optimal washing periods for specific machines. These advantages all contribute to increasing equipment productivity.

This invention is encapsulated and thus can be easily retrofitted without risk of liberation of components into the clean air stream, it can be easily removed for cleaning and requires a minimum installation package and maintenance.

Machine manufacturers utilise air flow calculations within the control system based typically on machine geometry and speed. This air flow signal can be provided to this invention to represent air speed at the filter media, thus with knowledge of air and particle speed and size distribution a solids mass flow estimate can be made for use by the operator and manufacturer to feed into their predictive maintenance tools, thus optimise out of operation time and increase productivity.

Outages can be targeted to specific areas of filter or to the bulk filter media Air inlet Filters The following paragraphs and drawings land 2 provide generic background information in order that the reader may better understand the advantages and application of the invention. Filter construction, media and cleaning systems do not form part of this application.

Each final air inlet filter cartridge may be cylindrical or cuboid. For cylindrical cartridges dirty air enters the outer cylinder surface. Particulate matter is entrained in the filter media and filtered air is drawn out of the cylinder axially from one end. The other end is sealed. Each cylindrical filter element is mounted horizontally and is secured to a plate with a hole matching the seal on the open end of the cartridge. A threaded rod passing through the axis of the cartridge secures the cartridge in place through a hole in the sealed end cover place and thus to a securing nut. These can be seen on drawingS Each filter can be 300-400mm in diameter and up to l.Sm long. An air filter installation can contain many rows of these. Typically on one large turbine installation an array of 30 rows by 30 columns Invention Details This invention comprises three main parts. These are the illumination system, the observation system and the computing system. Each is described in more detail below.

Illumination system: The illumination system provides short duration light pulses, designed to illuminate a narrow field directly behind and parallel to the filter bank in the clean area, thus illuminating any particles which may be present having defeated filtration. This arrangement is depicted in drawing 4. The illumination system has a number of illuminators. Typically each illuminator is placed at a corner of the filter bank and provides high intensity short duration light pulses of differing frequencies from infra-red to ultra-violet. The light sources within each illuminator are solid state being either LED or laser, and the lensing system produces a narrow beam of light fanning out to 9odeg. The light pulses are produced in strict duration and sequence with no moving parts. Each illuminator is fitted with a

DESCRIPTION

specific frequency light sensor which detects the light pulse from other illuminators. The first energised illuminator, sensing no light pulses starts to emit pulses in accordance with its base schedule. The second energised illuminator on sensing light pulses from the first energised illuminator synchronises with it. Any other illuminators synchronise in a similar fashion and thus the only connection to the illuminators is a power supply using conventional cable systems. The net result is that once energised the illuminators automatically synchronise to produce a sheet of light parallel to the filter plane in the clean air area and the illumination frequency changes in strict sequence.

Illuminators are installed and sealed into a 2 inch pipe fitting in such a way as to eliminate the chance of material from them being entrained in the clean air and thus contaminating it.

As particles pass through this sheet of light they are illuminated and scatter the light in relation to their size and thus become capable of detection and reveal their size and thus mass estimate. The amount of Rayleigh scattering that occurs for a beam of light depends upon the size of the particles and the wavelength of the light. Specifically, the intensity of the scattered light varies as the sixth power of the particle size, and varies inversely with the fourth power of the wavelength.

Observation System The observation system comprises a high definition optical detector with optics tightly focussed on the illuminator lit volume directly behind the filter bank. This system arrangement is as per drawing 5. The observation system can be likened to a high resolution digital camera capable of detecting the light scattered from any particles in the clean air area. The observation system has no moving parts as the focus is fixed and shutter is not required as the optics will always be open. Any particles traversing the illuminator lit air volume are thus detected in terms of number and from the scattered light frequency their size as well. This information is used by the computing system.

The detector synchronises automatically to the light pulses being generated by the illuminator array.

The detector is installed and sealed into a 2 inch pipe fitting in such a way as to eliminate the chance of material from it being entrained in the clean air and thus contaminating it.

Computing System The computing system receives video input from the detector array. Colour and brightness data of each scattered particle flow reveals particle size, particle mass and thus entrained mass flow. Based on local ambient air particle levels, size distributions and quantity analysis and an input proportional to mass flow from the turbine control system or other air flow controlling system the computing system calculates total and instantaneous number of particles detected at each size category defined by scattered light wavelength, estimates mass of each particle and particle mass flow total per particle size category and overall.

DESCRIPTION

The computing system is a conventional PC running either a Microsoft or Apple operating system, software prepared for this application which may contain third party modules associated with video rendering and the necessary I/O hardware and software to communicate with the scattered light detectors.

This information is of use to the turbine or downstream equipment operator and OEM as it can be used to determine equipment wash cycles. Optimisation of the wash cycle can maintain efficiency without the cost of washing materials and load reduction.

The computing system presents to the operator a graphic of the inlet filters as seen from the clean air side and superimposed on this the particle detection points and the mass flow at each detected location. The operator can adjust the sensitivity of this displayed data to better pinpoint the dust ingress location. This allows the operator to determine the actual point of ingress and have additional information on the quantity and quality of the entrained particles. This reveals the point at which the particles are entering the clean air system and the operator can prepare and decide when to implement the outage to remedy the situation. Either by replacing the failed filter or adjusting or replacing the filter seal.

The system allows the operator the confidence that other differential pressure monitoring equipment and software is correctly reporting the overall filter condition and executive action can be taken as advised by those systems

Claims (2)

1. Claims 1) A system which monitors the frequency of light scattered from dust particles in the airspace downstream of an air filter and so estimates particle size and solid mass flow.
2. 2) A system according to claim 1 which uses multiple light frequencies to differentiate particle size and the mass flow according to particle size 3) A system according to claim 2 which locates and displays the location of the area in a filter bank where particles are bypassing the filter