US2989038A - Device for starting-up once-through boilers - Google Patents

Description

June 20, 1961 o. SCHWARZ 2,989,038

DEVICE FOR STARTING-UP ONCE-THROUGH BOILERS Filed. April 19, 1957 INVENTOR.

Onmar Schwarz AT TORNEY United States atent (j) 2,989,038 DEVICE FOR STARTING-UP ONCE-THROUGH BOILERS Ottmar Schwarz, Ratingen, Germany, assignor to Danwerke Aktiengesellsclraft, Ratingen, Germany, a corpoaims. (Cl. 122-406) This invention relates in general to improvements in the construction and operation of power plants having forced circulation vapor generating units of the oncethrough type having vapor generating and superheating sections, and more particularly to improved provisions for starting-up vapor generating units of this type which effeet a simplification of the starting-up procedure and a reduction in the starting-up period, while adequately protecting the vapor superheating section of the unit.

Heretofore steam boiler units of the type described have been usually started-up by circulating water under a progressively increasing pressure serially through the steam generating and superheating tubes by means of a pump, progressively increasing the firing rate and operating pressure, and withdrawing the heated water through a discharge connection from the superheater outlet header. This system c t-flooding the steam generating and superheating sections of the steam boiler has several disadvantages. Efforts have also been made to reduce the length of the starting-up period by reducing the amount of water supplied to the unit or by withdrawing some of the circulated water at a point prior to the steam superheating section. Most prior starting-up procedures have been disadvantageous when applied to a steam boiler of the type described embodying a pendent-type superheater of a non-draining type, as it is extremely difficult to completely eliminate all of the water in such superheater tubes, with the possibility of damage to the turbine by slugs of water when the boiler is connected thereto.

In accordance with the present invention such disadvantages and difficulties are eliminated or greatly reduced by passing only steam through the steam superheating section during the starting-up period. This result is attained by closing the flow connection between the steam generating section of the unit and the steam superheating section, continuously withdrawing the heated water at the outlet end of the steam generating section, and passing the withdrawn water through a pressure re ducing valve into a flash chamber, wherein the steam and Water are separated and the steam conducted to the steam superheating section of the unit and the water returned to the inlet end of the steam generating section.

In the drawing I have diagrannnatically illustrated a steam turbine power plant including a once-through steam boiler of the character described and the auxiliary apparatus required for starting-up the boiler in accordance with the present invention.

In the system illustrated, in normal operation boiler feed water is withdrawn from a hot well or feed water reservoir 1 at a suitable temperature and by means of a feed water pump 2 forced under a substantial positive pressure through the tubes forming serially connected water heating or steam generating sections 4 and 5, preferably located mainly in a radiant heat absorbing section of the boiler 3. From the radiantly heated steam generating sections a mixture of steam and water is discharged through a transition zone, preferably located in a relatively low gas temperature section of the boiler, into convection heated serially connected primary and secondary superheater tube sections 6 and 7 respectively, preferably located in a convection heating pass of the during starting-up and Patented June 20, 1961 boiler. The rate of firing of the boiler is regulated to produce the desired superheated steam temperature and the superheated steam flows through a pressure operated valve 8 to a steam turbine 9. The steam after passing through the various stages of the turbine 9 is condensed in a condenser 10 and the condensate pumped back to the hot Well 1 through a pipe 21.

In accordance with the present invention in starting-up a steam generating unit of the character described, a normally open cut-oft valve :11 positioned between the discharge end of the steam generating section Sand theprimary superheater section 6 is closed, and the mixture of steam and water passed through a pressure reducing valve 12 in a pipe 12 connected to one or more flash drums 13. Due to the lower pressure at the discharge side of the valve 12 and in the flash drum, a portion ofthe Water flashes into steam. The remaining water drains from the bottom of the drum 13 through a regulating valve '16 in a drain line 20 to the condensate return line 21 leading to the hot well 1. A feed Water supply con nect-ion 22, including a regulating valve 18, opens into the drain line 20.

The steam collecting in the drum 13 passes out of the steam space therein through connected pipes 23 and 24, the latter containing a cut-off valve 14, which is open normally closed during operat ing periods. The flashed steam flows through the tubes of the superheater sections 6 and 7, cooling the tubes to a safe metal temperature, and then passes to the condenser 10 through a turbine by-pass line 27 containing a cut-off valve 15. The condensate is returned to the hot well through pipe 21.

By means of the described arrangement it is possible to restrict the tube cooling fluid in the radiantly heated steam generating sections 4 and 5 mainly to Water, and in the superheating sections 6 and 7 to steam. The pressure maintained in both sections can be suitably regulated by the valves 12, 15 and 8. During the starting-up period the pressure maintained in the steam generating sections 4 and 5 will be substantially higher than the pressure initially in the steam superheating sections 6 and 7. This pressure differential facilitates the operation of the pressure reducing valve 12 and flash chamber 13. Some of the steam can be used for warming up the turbine during the starting-up period and in that case the valve 8 is partly open.

As the unit is brought up to pressure the pressure in the steam generating and steam superheating sections will tend to equalize and when a balanced pressure exists the cut-off valve 11 is opened and the boiler is ready for operation. The valves 12 and 14 are then closed to cut out the flash drum 13.

If it should be desirable to protect the tubes in the steam superheating sections during the initial portion of the starting-up period during which little or no steam is generated in the steam generating sections, steam may be regulably supplied from an outside source through a valve-controlled pipe 17 connected to the pipe 24 leading to the steam superheating sections. The feed water make-up pipe 22 may be also utilized for introducing a chemical solution through the steam generating section of the boiler during a boiler cleaning period.

The described starting-up provisions permit starting-up with a variable amount of water in the steam generating sections of the boiler, thus avoiding the difliculties which tend to occur when the steam generating sections and steam superheating sections are flooded as heretofore. Restriction of the fluid in the steam superheating sections to steam during starting-up avoids the introduction of water into that section and the special water discharge fittings which would be otherwise required. The present atrangement reduces the temperature stresses in the high ferrous alloys usually employed for the tubes of the steam superheating section. In case of needed repair to the flash drum 13, the boiler can still be started up as heretofore. It has been found that the starting-up time is considerably reduced with the described provisions.

While in accordance with the provisions of the statutes I have illustrated and described herein a specific form of the invention now known to me, those skilled in the art will understand thatchanges may be made in the form of the appartus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features. The invention is applicable to starting-up boilers of this type to be operated at supercritical or sub-critical pressures.

I claim:

1. In a forced flow once-through type steam boiler having a steam generating section and a steam superheating section serially connected thereto, means for supplying feed water to said steam generating section, means for starting-up said steam boiler with only steam flowing through said steam superheating section comprising a cutoff valve arranged to control fluid flow between said steam generating and steam superheating sections, a flash drum having a steam space and a water space and at a pressure substantially below the pressure in said steam generating section, a conduit directly connecting said steam generating section to said flash drum and arranged to convey the entire discharge from said steam generating section to said flash drum when said cut-ofl valve is closed, a pressure-reducing valve in said conduit arranged to cause water discharged from said steam generating section to flash into steam, and a valve-controlled conduit directly connecting the steam space of said flash drum to said steam superheating section for supplying the flashed steam to said steam superheating section.

2. In a forced flow once-through type steam boiler having a steam generating section and a steam superheating section serially connected thereto, means for supplying feed water to said steam generating section, means for starting-up said steam boiler with only steam flowing through saidsteam superheating section comprising a cutofl valve arranged to control fluid flow between said steam generating and steam superheating sections, a flash drum having a steam space and a Water space and at a pressure substantially below the pressure in said steam generating section, a conduit directly connecting said steam generating section to said flash drum and arranged to convey the entire discharge from said steam generating section to said flash drum when said cut-off valve is closed, a pressure-reducing valve in said conduit arranged to cause water discharged from said steam generating section to flash into steam, a valve-controlled conduit directly connecting the steam space of said flash drum to said steam superheating section for supplying the flashed steam to said steam superheating section, and conduit means connecting the water space of said flash drum to the inlet end of said steam generating section.

References Cited in the file of this patent UNITED STATES PATENTS 2,405,573 Frisch Aug. 13, 1946 2,700,353 Schoenfeld Jan. 25, 1955 2,811,837 Eggenberger Nov. 5, 1957 2,869,517 Lieberherr Jan. 20, 1959 FOREIGN PATENTS 693,330 Great Britain June 24, 1953 716,524 Great Britain Oct. 6, 1954 762,556 Great Britain Nov. 28, 1956

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