Patent Application
20100251976
1. Technical Field
The present disclosure relates generally to a boiler system and, more particularly, to a boiler system including a steam generator having a start up system that re-circulates fluid back to the steam generator with an ejector.
2. Related Art
Generally speaking, start-up of a steam generating system is a stepped process as steam from the generator is not immediately available. Similarly, during low load conditions, steam may not be sufficiently available. Typically, a start-up system is incorporated into the steam generating system to protect components of the generating system during these low steam conditions, e.g., at start-up and low load conditions. For example, the start-up system protects water walls, which include a plurality of tubes, of a steam generator from overheating when the steam generator is below a minimum once-through load by providing an additional flow of feed water. The minimum once-through load is a load (approximately twenty to about fifty percent (20%-50%) of the full load flow) where the flow (e.g., a required flow) through the water walls is enough to protect the tubes from overheating without the need for the additional flow. As shown in
As is generally known, substantially all of the feed water 24 and 28 that is not converted into steam within the water walls is drained off by the start up system 20 and replaced by the feed water 24 flow. This operating mode is referred to herein as a drain mode. The steam generator 30 has to fire at a rate sufficient to bring the feed water up to saturation temperature and then to generate steam. Initially during the start up of the steam generator, none of the feed water is converted to steam and all the feed water provided to the generator must be drained off (e.g., in the drain mode). As steam is generated, less water has to be drained off until the boiler reaches the once-through mode where substantially all of the feed water flow is converted into steam. In the once-through mode, the flow of feed water for steam generation is referred to herein as a required flow. Until the once-through mode load is attained, a minimum flow of feed water is sent to the generator for water wall cooling and, as noted above, is referred to herein as the additional flow. Accordingly, prior to attaining the once-through mode load, both the required flow and the additional flow are provided to the steam generator.
As noted above, when operating in a once-through mode, the steam generator converts substantially all of the feed water into steam. Therefore, during the once-through operating mode, there is substantially no flow in the conduit 50 from the separator 40 to the flash tank 60. During operational loads below the once-through load, for example, at loads below a minimum once through load, which typically ranges from about twenty percent (20%) to about fifty percent (50%) of a full load flow, the additional flow is required for water wall cooling. During these low load conditions, a flash tank valve 62 modulates to allow flow to the flash tank 60 thus allowing the feed water pump to deliver the minimum required flow for cooling.
The inventors have recognized that this method of cooling the water walls of steam generators during start-up and low load conditions is inefficient and wastes water and thermal energy, which in turn translates into increased cost to operate the steam generation system. Accordingly, a need exists for a reliable and economical method of cooling components of a steam generator during low load conditions.
A steam generating system includes a steam generator, a separator coupled to the steam generator, a feed water supply providing feed water, a start-up system coupled to and receiving the feed water from the feed water supply, and a recirculation system coupled to the start-up system. The steam generator operates in a plurality of operating modes. In at least one of the operating modes the steam generator generates a flow of steam and fluid. The separator receives the steam and fluid flow and separates components of steam and fluid from the steam and fluid flow. The recirculation system receives the feed water from the start-up system and provides a required flow to the steam generator during at least one of the plurality of operating modes. The recirculation system includes an ejector. The ejector induces at least a portion of the fluid from the separator into the recirculation system, mixes the induced fluid with the feed water to provide a recirculation flow, and the recirculation system provides the required flow to the steam generator including the recirculation flow.
In one embodiment, the recirculation system further includes a bypass valve and a block valve. The bypass valve and the block valve are selectively operated during the operating modes of the steam generator to provide one or both of the required flow and the recirculation flow. For example, when the steam generator is operating in a once-through operating mode, the bypass valve opens and the block valve closes to isolate the ejector and to provide the feed water from the feed water pump through the recirculation system to the steam generator as the required flow. In a low load condition operating mode of the steam generator, the bypass valve selectively restricts or prohibits flow and the block valve opens to permit the ejector to induce the fluid from the separator, mix the induced fluid and the feed water (e.g., to provide the recirculation flow), and provides the required flow to the steam generator including the recirculation flow for steam generation and to cool the water wall. In one embodiment, the low load condition includes a start-up operating mode of the steam generator. In another embodiment, the low load condition includes operating modes below about twenty percent (20%) to about fifty percent (50%) of a full load flow of the steam generator.
In one embodiment, the recirculation system includes a throttling valve coupled to the ejector. The throttling valve receives the mixture of the induced fluid and the feed water (e.g., the additional flow) from the ejector and provides the required flow to the steam generator including the recirculation flow. The throttling valve controls the feed water pressure and flow through the ejector as loads vary during ejector operation.
In another embodiment, the steam generating system further includes a downcomer conduit coupling the separator and the recirculation system. The recirculation system further includes a check valve that provides one way flow from the downcomer conduit to the ejector and thus prohibits reverse flow back to the downcomer conduit.
In still another embodiment, the steam generating further includes a flash tank coupled to the downcomer conduit through a flash tank valve. When the steam generator is operating in a once-through operating mode, there is no fluid flowing from the downcomer conduit to the flash tank and nothing is drained from the system.
Referring now to the Figures, which are exemplary embodiments, and wherein the like elements are numbered alike.
Initially, in a start-up mode, the bypass valve 210 opens to receive the heated feed water 128 and pass the heated feed water 128A as the required flow 132 (including the additional flow) to the steam generator 130 and, in particular, to water walls of the steam generator 130, to protect the water walls from damages caused when tubes of the water walls overheat. As with the steam generating system 10 of
At the ejector 240, the recirculation fluid 180 mixes with the heated feed water 128B pass through the block valve 230 to form the recirculation flow 270 passed through the throttling valve 250. The throttling valve 250 controls the feed water pressure and flow as the load varies when the ejector 240 is in operation. That is, the throttling valve 250 provides control for optimal differential pressure and improved ejector performance. During start-up and/or low load conditions, the recirculation flow 270 mixes with the heated feed water flow 128A from the bypass valve 210. The portion of heated feed water 128A passed through the bypass valve 210 is controlled based on, for example, a volume of the recirculation flow 270. For example, the bypass valve 210 is operated in one or more of the aforementioned modes (e.g., the fully open mode, the partially closed mode, and the fully closed mode) to regulate the flow 132 to the steam generator 130.
As can be appreciated, at start-up and low load conditions, all, none or a percentage of the fluid 180 (e.g., water) flowing from the separator 140 may be selectively recirculated to the steam generator 130 within the required flow 132. At a predetermined time period or occurrence of a desired event such as, for example, attainment of the once-through mode, the fluid 180 (if any) is provided to the flash tank 160 where it may be drained off or stored. In one embodiment, at once-through mode of the steam generator 130, the recirculation system 200 is controlled to directly pass the heated feed water 128A through the bypass valve 210 as the flow 132 to the steam generator 130 and to inhibit the heated feed water flow 128B to the ejector 240 and throttling valve 250 by the block valve 230. Similarly, the recirculation system 200 is controlled via the bypass valve 210 and the block valve 230 to respond to low load operation (e.g., below the minimum once-through flow rate equal to a range of about twenty percent (20%) to about fifty percent (50%) of the full load flow) to maintain a minimum rate of the required flow 132 to the water walls.
The recirculation system 200 and the ejector 240 selectively recirculate a percentage of the recirculation fluid 180 (e.g., the recirculation flow 270) to increase a temperature of the required flow 132 (including the additional flow) entering the steam generator 130. The heat recovery realized by the recirculation process reduces an amount of fuel that, for example, the steam generator 130 consumes to heat the feed water 124 during the start-up process and low load conditions. The recirculation of fluid from the separator 140 back to the steam generator 130 also reduces a loss of feed water 128 as less water needs to be drained off. As can also be appreciated, a savings in terms of the feed water 124, 128 supplied also yields a saving in the cost of chemical treatment of the feed water and thus operational savings and greater efficiencies in operating the system 100. An additional benefit in the recirculation system 200 as described herein is seen to be reduced operational costs as ejector systems, such as the ejector 240, have no moving parts, proven reliability, easy operation and maintenance. Additionally, it should be appreciated that should the ejector 240 be disabled or require maintenance, the generating system 100 can be started and operated in the drain mode (as in
At least some features of the steam generating system 100 including the start-up system 120 and the recirculation system 200 as described herein, include the addition of the ejector 240 and the throttling valve 250. For example, the throttling valve 250 allows the feed water pump 122 to operate at a higher pressure than the steam generator 130 and to deliver the motive force for the ejector 240. In one embodiment, using the head provided by the feed water pump 122, the ejector 240 induces recirculation of the fluid in the downcomer 150 from the separator 140 back through to the steam generator 130. Additionally, since the ejector 240 has no moving parts, the recovery of heat through the recirculation system 200 gives rise to only minimal concern for additional maintenance and/or repair.
Moreover, as described above, conventional systems (such as the steam generating system 10 of
While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
This patent application claims priority benefit under 35 U.S.C. §119(e) of copending, U.S. Provisional Patent Application Ser. No. 61/166,045, filed Apr. 2, 2009. The disclosure of the aforementioned U.S. patent application is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61166045 | Apr 2009 | US |
