Thermal Power Plants
The Safety Assessment and economical operation of Power Plants (Fossil fuel, Oil and Gas, Combined Cycle, Hydro and Thermal) and process control equipment requires continuous regular monitoring to ensure integrity and sustained operations. To ensure this predictive maintenance inspection is needed to identify fatigue failures and crack detection for propellers, reciprocating engines, gas turbine engines, boilers, boiler tubes, pressure vessels, storage tanks, welds, heat exchangers, turbine bores, headers, steam lines, feedwater heaters, condensers and in plant piping.
The reason for inspection depends on the component and its effect on plant operation. Boiler tubes and feedwater heater tubes are inspected to avoid forced outages. Inspection of turbine components is done for safety and operational reasons. Steam lines are inspected for safety reasons. The inspection of condenser tubes is primarily done to assess condition for replacement decisions.
With digital radiography the inspection can be conducted even while the plant is operational. No surface preparation is required. With Se 75 gamma isotope based radiography due to much softer energy than Iri192 it is possible to detect the inside product and any blockages.
Computed Radiography (CR) uses very similar equipment to conventional radiography except that in place of a film to create the image, an imaging plate (IP) made of photostimulable phosphor is used. The imaging plate housed in a special cassette and placed under the body part or object to be examined and the x-ray or gamma exposure is made. Hence, instead of taking an exposed film into a darkroom for developing in chemical tanks or an automatic film processor, the imaging plate is run through a special laser scanner, or CR reader, that reads and digitizes the image. The digital image can then be viewed and enhanced using software that has functions very similar to other conventional digital image-processing software, such as contrast, brightness, filtration and zoom. The CR imaging plate (IP) contains photostimulable storage phosphors, which store the radiation level received at each point in local electron energies. When the plate is put through the scanner, the scanning laser beam causes the electrons to relax to lower energy levels, emitting light that is detected by a photo-multiplier tube, which is then converted to an electronic signal. The electronic signal is then converted to discrete (digital) values and placed into the image processor pixel map.Advantages
> No silver based film or chemicals are required to process film.
> Reduced film storage costs because images can be stored digitally.
> Computed radiography often requires fewer retakes due to under- or over-exposure which results in lower overall dose to the patient.
> Image acquisition is much faster - image previews can be available in less than 15 seconds.
> By adjusting image brightness and/or contrast, a wide range of thicknesses may be examined in one exposure, unlike conventional film based radiography, which may require a different exposure or multiple film speeds in one exposure to cover wide thickness range in a component.
> Images can be enhanced digitally to aid in interpretation.
> Images can be stored on disk or transmitted for off-site review.
Ever growing technology makes the CR more affordable than ever today. With Chemicals, dark room storage and staff to organize them, you could own a CR for the same monthly cost while being environmentally conscious, depending upon the size of the Radiographic Imaging Plates can theoretically be re-used thousands of times if they are handled carefully. IP handling under industrial conditions, however, may result in damage after a few hundred uses. An image can be erased by simply exposing the plate to a room-level fluorescent light. Most laser scanners automatically erase the image plate after laser scanning is complete. The imaging plate can then be re-used. Reusable phosphor plates are environmentally safe but need to be disposed of according to local regulations.
Common applications for computed radiography include:
> Corrosion surveys on pipes, often through insulation;
> Examination of valves for erosion;
> Information shots on industrial components; e.g. checking to see if a valve is closed properly, or checking for obstructions in valves and pipes
> Examination of boiler water walls
> Weld examination for certain ASME code applications
> Automotive casting inspection
> High pressure braze joint inspection (aerospace)
> Wax pattern core integrity verification in investment casting foundries
Appropriate selection and application of NDT techniques are key to the inspection of non-nuclear power plants.
The NDT digital computed radiography can be used for detection of various flaws such as outlined below:1.0 Boilers
1.1 OD Erosion, Corrosion and Overheating
1.2 Hydrogen Damage, Caustic Corrosion, Chemical Attack
1.3 Cracking - Corrosion Fatigue, Stress Corrosion and Thermal Fatigue
1.4 Creep - ID oxide Scale2.0 Headers3.0 Steamlines4.0 Turbines
4.2 Solid Rotor
4.3 Disk Keyway Cracking
4.4 Disk - Blade Attachment Area
4.7 Retaining Rings5.0 Condensers6.0 Feedwater HeatersFor further details contact CIT LTD.