Patent Application
20080083221
Systems according to this invention are designed to produce a fan-like action in opposition to the negative pressure effects of the existing Induced Draft and Booster Fans of the particular plant to which the present invention is applied. This is fundamentally accomplished by inserting momentum (mass flow with a significant velocity) in a direction generally opposite to the normal gas flow. In essence, the systems and methods of this invention involve inserting an element or a set of elements at one or more appropriate equipment locations in order to be functionally equivalent to a fan blowing flue gas in the opposite direction at such location(s), thereby canceling the negative pressure transient conditions during a MFT or similar incident. The systems of this invention can operate with virtually no time delay, and they are designed to be able to produce a counter-pressure in the draft system as large as may be necessary to protect the ductwork and equipment from vacuum conditions. Accepted redundancy and control logic practices adapted to this invention allow the systems to have a reliability which exceeds other active control methods in this field. Specific novel design features of this invention provide systems which readily meet the reliability, maintenance and functional verification requirements typical to the power industry.
In a specific representative embodiment, a system according to this invention consists of a compressed air (or other suitable gas or material) storage tank, standing ready as an energy and counter gas source for the system, together with an array of jet nozzles appropriately positioned and oriented within the flue gas ductwork. When a system according to this invention is put in place and actuated, the jet nozzles produce an ejector (jet pump) action, which acts as a substantially equivalent fan acting in opposition to the fan(s) that are operating to produce negative pressure in the ductwork. This produces a “reverse pumping effect”. By selecting and/or adjusting the size of the compressed air (or other suitable material) storage tank, its initial pressure, the sizes of connecting flow paths between the storage tank and the jet nozzles, and the configuration and orientation of the jet nozzle array, the effect of this invention can be tailored substantially to match and counteract a shutdown MFT-induced vacuum effect, or to control and/or moderate other actions taken to reduce the vacuum-creating capacity of the normal draft system fans.
Compressed air is envisioned as the preferred operating medium for typical power plant installations, however virtually any momentum material (other gasses, liquids, or even powdered solids) could be used to produce the desired result. The fundamental technology produces a “reverse pumping effect” by injecting momentum and additional mass flow in a direction generally opposite to the normal exhaust flow direction, and transferring this momentum to the flue gas, thus tending to arrest or reduce the flue gas flowrate. In addition, this method rapidly adds additional materials to a region experiencing low-pressure or vacuum conditions thus also helping to counteract such conditions. Alternative momentum materials to compressed air would be considered based on their relevant properties. For example, inert gasses may be selected to preclude unwanted reactions with the flue gas. Liquids could be selected to suppress temperatures in addition to providing the “reverse pumping effect”. Powdered solids, fine enough to effectively transfer their momentum to the flowing flue gas, could be used to provide greater momentum transfer if desired.
As described above, however, under fuel interruption conditions or another sudden, unexpected plant shutdown incident, the continuing operation of fan(s) 14 for a short time even following shut-off exacerbates the condition of flue gas contraction in the ductwork upstream of the fan(s) and can lead to at least a partial vacuum which can damage the equipment and ductwork. In accordance with the present invention,
The implosion interruption jet system as shown in
In this Example, based on several furnace implosion studies, implosion interruption air jet systems in accordance with this invention were evaluated to assess the appropriate implementation and performance parameters. Draft system pressures (or vacuum conditions) are typically measured as “inches water column” (WC). Standard sea-level atmospheric pressure is typically 14.7 PSIA, which equals 407 inches water column (WC), for comparison. For a 750 megawatt coal-fired power plant used in this Example, the transient ductwork pressures resulting from a main fuel trip were as follows:
Based on the data in Table 1, it is clear that the electrostatic precipitator unit would require extensive reinforcement, likely costing several millions of dollars, to withstand the transient pressure (vacuum) conditions created by a fuel interruption if a conventional passive protection approach was adopted. Although the furnace equipment design just barely meets the transient pressure conditions, there is no margin for error. As a result, this equipment as well would likely require expensive reinforcement if this system were using passive protection to guard against transient vacuum conditions.
Instead of using passive protection, however, the power plant of this Example could be equipped with an implosion interruption jet system according to this invention at a fraction of the cost of passive protection. Such a system according to this invention would be at least as effective as the passive protection in preventing implosion failures as well as being adjustable to accommodate different power plant loads, future plant modifications, and other operational variables.
As draft system fans (which cause the high negative pressures, i.e., vacuum conditions, in the flue gas ductwork) become larger and more powerful, the available options to achieve sufficiently rapid control actions become increasingly limited. For example, with the smaller duct fans used in the past, it might have been possible to control fan output (e.g., by shutting off a power supply) within a relatively short period of about 5 to 7 seconds and thereby reasonably quickly eliminate the dangerous negative pressure conditions in the ductwork.
For the fans typically used in new power plant retrofit applications, however, it can take as much as 20 to 25 seconds to obtain a sufficient fan control response. Some fan vendors have suggested that special control options may be capable of reducing that fan response time to about 15 seconds, but there is no current suggestion that a faster control response is achievable. Because of such slow fan control response, additional expensive reinforcement of equipment may be the only viable alternative to the implosion interruption jet systems of this invention.
The systems and methods of this invention, on the other hand, can eliminate or at least reduce the need for ductwork reinforcement by applying strong and rapid corrective draft system pressures during periodic transient vacuum or negative pressure conditions. The systems of this invention can be designed to respond with corrective pressure in time frames ranging from several seconds to as little as a fraction of a second, as needed for the condition being corrected. For typical utility boiler system applications, correction response times will be required within a few seconds to prevent equipment damage. The corrective pressure response will typically need to last from about 10 to 30 seconds, that is, until the normal fan controls have time to become effective.
Thus, the pressure correction provided by the implosion interruption jet systems of this invention cancel out excessive fan vacuum during a transient condition. The magnitude of the pressure correction provided by the jet systems of this invention can range from a fraction of an inch WC to hundreds of inches WC, depending on the application. For typical utility boiler systems, correction response pressures will be in the 10 to 30 inch WC range. For some atypical applications, the jet systems of this invention could be designed to provide corrective pressures ranging to as high as several atmospheres (thousands of inches WC), although this would require an air supply at a higher pressure than is typically found in utility plant systems. In this case, special equipment would be required to recharge the air storage tank following a use of the system.
The size of equipment for the implosion interruption jet system of this invention needed for a large utility boiler can be roughly estimated as follows: The compressed air storage tank is about the size of typical compressed gas tank trucks seen on the highway, i.e., about 7000 to 9000-gallons in size. This tank is about 7 feet in diameter×28 feet long, carrying a pressure of about 125 psig. No special compressed air supply system is required because normally existing plant air systems can charge or recharge such a storage tank. The air tank is preferably located close to the plant's induced draft or booster fans. The piping and valves leading from the air tank to the ducts at the fan inlets might typically be 6 to 8 inches in diameter, with several branches of smaller size. In one embodiment, the air jet grid attaches around the perimeter of the interior of the duct, with typical duct size being on the order of 25 feet×25 feet. Control of the air jet system can be achieved through the plant control system (DCS), or by dedicated electronic controls typical to this industry. Electric or pneumatic valve actuation in the event of a fuel interruption incident is preferably automatically implemented by the air jet system controls. Dotted lines in
An implosion interruption system according to this invention has its greatest effect when initially actuated because at that point the air storage tank is fully charged. The magnitude of the reverse pumping effect decays, however, as the pressure in the air tank gradually decays, which more or less coincides with a gradual reduction of draft fan vacuum-inducing capability resulting from use of the normal fan control methods that, in a preferred embodiment, are employed in conjunction with the methods and systems of this invention. Thus, the transient vacuum-creating condition is managed initially by the implosion interruption jet system of this invention, and finally by the normal draft fan controls.
A system according to this invention is capable of producing corrective draft system pressures for all power plant retrofit conditions. The most severe draft system implosion transients typically occur when a power plant has been retrofitted with new air quality control equipment, coincident with a conversion from a forced draft furnace to a balanced draft unit. The system of this invention, however, can readily accommodate even the large counter pressures that are required to address this type of vacuum control situation.
The grid of air jets which are disposed in the ductwork in accordance with this invention produce only a minimal pressure drop in the ductwork during normal plant operation, in particular less pressure drop than a large damper unit would produce if it were installed in the ductwork to control furnace implosion conditions.
The capital costs associated with this invention are low, and system equipment essentially comprises standard power plant components such as piping, valves, small reservoir tanks, etc. Thus, in general, the present invention can be implemented with off-the-shelf components, and no special elements need to be designed, sized, tooled and/or fabricated specifically for this invention.
By using inexpensive series/parallel solenoid valve arrangements in conjunction with the power plant's DCS, the implosion protection methods and systems of this invention provide much better reliability of action than what is realized by any other active implosion protection schemes. Two-of-three or two-of-four logic schemes, as known in this art, can be employed with this invention to provide the highest reliability of function.
By appropriate selection and/or adjustment of the air reservoir size and charge pressure, the desired implosion protection effect produced in accordance with this invention is variable according to the unit load, cleanliness condition of the gas path, and other relevant operating parameters. In this way, one can assure that the proper amount of pressure compensation is available, and such pressure compensation is adjustable on a real time basis.
In a preferred invention embodiment, the jet nozzles are provided with simple hoods which help to assure that furnace ash conditions will not cause plugging of the jet nozzles. The nozzles are accessible for inspection and cleaning while the plant is in operation. Inspection, system functional verification, and cleaning can all be performed even while the plant is operating. The implosion interruption system of this invention may be continuously operable while maintenance-related functions are performed.
The implosion interruption jet systems of this invention can usually be charged by the already existing power plant compressed air system (for example, using either plant service air or plant instrument air). Charging the air storage tank in this way further reduces capital costs because no new compressors are required, and there is no new auxiliary load at the plant.
The implosion interruption jet systems and methods of this invention also can be designed for installation with the potential for periodic high temperature excursions, with virtually no significant additional expenses.
Analysis and simulation of furnace/power plant draft system transient condition responses can be used to tailor system parameters according to this invention to any furnace/power plant operation. Such analysis and simulation is routinely performed, for example, in connection with major draft system retrofit planning. Scale model testing, as known in this art, can also be used to verify and/or optimize the parameters and performance of this invention.
Although this invention has been described by reference to specific embodiments thereof, it will be understood by those skilled in this art that various changes and modifications may be made in the apparatus components, configuration of the components, and other invention details without departing from the spirit and scope of this invention.
| Number | Date | Country | |
|---|---|---|---|
| 60849270 | Oct 2006 | US |
