US3127747A - Desalting a forced flow steam generator - Google Patents

Description

April 7, 1964 G. PETER DESALTING A FORCED FLOW STEAM GENERATOR Filed 001:. 17, 1960 .7 n ven f 0 r: 60 TTFR/ED PETER PA rcwr Anogyay m fir MW MP 7 a g Q f MP 0 4 cox N no N a m: 9 m. J 87 33 77 7 u 4 w Tu Ar 4 7 8 \w g W W w. aw L m, a m H W 3 m 35w 2 a f? WWW 1 7/. 6 7 Pd Bl 7 7 H 3 r 8 M H M 3 4m ,7 w 5 mm A a 5i 6 m 0 M All. A 5:: 1' L 7 United States Patent 0 F 3,127,747 DESALTTNG A FURCED FLOW STEAM GENERATOR Gottfried Peter, Wlnterthnr, Swizerland, assignor to Sulzer Freres, S.A., Winterthur, Switzerland, a corporation of Switzerland Filed @et. 17, 15960, Ser. No. 63,133 Claims priority, application Switzerland Oct. 3%, 1959 1 Claim. (Cl. 60-67) The present invention relates to a method and apparatus for desalting a forced flow steam generator, more particularly a forced flow steam generator operating at the critical or supercr-itical pressure of water whereby portions of pipe lines forming the tubular heat transfer system of the steam generator and subject to salt deposits are periodically washed out by the operating medium of the steam generator. if the steam generator is operated at critical or supercrit-ical pressure, the pressure in the pipe line which is washed out is reduced to subcritical pressure in order to produce Wet steam whose liquid phase dissolves the salt deposits.

In conventional forced flow steam generators operating at subcritical pressure salt depositions in the tube system of the generator are impeded and the water circulating through the steam power plan-t of which the steam generator forms a part is simultaneously desalted by providing a liquid separator at the outlet of the portion of the tube system of the steam generator wherein the water is converted into the steam, the liquid phase of the operating medium being blown down from the separator. Since the major part of the salts present in the operating me dium is in the liquid phase, the salts are removed from the circuit together with the blowdown liquid.

This desalting method is not suitable for a steam generator operating at the critical or at supercritical pressure, because in these generators there is no definite difference between the liquid and the steam phase of the operating medium and the transition from the liquid phase to the steam phase is not sudden but continuous. Salts contained in the feedwater of a forced flow steam generator operated at critical or supercritical pressure are usually deposited in the zone of the high pressure tube system where the operating medium reaches critical conditions and where there is a relatively great increase of the spe cific volume of the operating medium. The salts separated in this transition zone from the liquid to the steam phase and deposited on the inside of the heating tubes must be periodically removed by washing out. done according to present operating practice by providing a check valve at the outlet of each of the tube lines which are arranged in parallel relation with respect to the flow of the operating medium and form the tubular heating sys tem of the steam generator, and by connecting a valve pipe to each of the aforesaid tube lines in the neighborhood and upstream of the respective check valve. When a valve in one of the pipes is opened the pressure in the respective tube line is reduced to a subcritical pressure and the check valve at the outlet of the tube line to which the pipe is connected closes. The result is that wet steam flows through the tube line whereby salts deposited therein are dissolved and carried out of the steam generator by the wet steam and removed from the circuit of the steam power plant of which the steam generator forms a part.

The 'amount of operating medium needed for washing out the individual tube lines of the tubular heating sys tem of a forced flow steam generator may be quite great and it is important that the heat and energy loss caused by removing the washing fluid from the system is held as small as possible by recovering at least a portion of the heat and pressure energy contained in the operating me- Tnis 1S 7 3,127,747 Patented Apr. 7, 1964 dium which has been used for washing out. The washingout operation should not disturb the operation of the steam generator and the turbines receiving operating steam therefrom, i.e., the temperature and amount of live steam leaving the steam generator should not be affected by the washingcut operation.

Lt is an object of the invention to provide a system for Washing out the heating tubes of a forced flow steam generator which tubes are arranged in parallel relation with respect to the flow of the operating fluid therethrough, without substantially affecting the quality of the produced steam and the continuity of steam production.

A further object of the invention is to provide a system for recovering at least a portion of the thermal energy contained in the part of the operating medium which has been used for washing out a tube line forming part of the tubular heating system of a forced flow steam generator, by transferring heat from the washing fluid to the feedwater of the steam generator. In a modification of the system the washing fluid is separated into a liquid phase and into a steam phase, the latter being introduced into the steam part of the cycle of the power plant of which the steam generator forms a part or used for indirectly heating a feedwater preheater. The liquid phase is preferably used for indirectly preheating feedwater.

A further object of the present invention resides in the provision of apparatus for recovering at least a portion of the thermal energy contained in the part of the operating medium which has been used for washing out a tube line forming part of the tubular heating system of a forced flow steam generator, particularly of a steam generator operating at critical or supercritical pressure whereby the washing fluid is conducted through heat exchangers interposed in the flow of the operating fluid through the power plant of which the steam generator forms a part, for transferring heat from the washing fluid to the operating fluid and/ or washing fluid is returned to the operating fluid in the part of the cycle of the steam power plant outside of the steam generator for utilizing the heat and pressure energy contained in the washing fluid. In a modification of the system according to the invention the washing fluid which has been used for Washing out a tube line of the steam generator is passed through a separator wherein the liquid phase of the washing fluid is separated from the steam phase; the soobtained liquid phase is conducted through one or more heat exchangers for transferring heat to the feedwater of the steam generator and the steam phase is conducted to a turbine forming part of the steam power plant which includes the steam generator, for converting thermal energy contained in the steam phase to mechanical power. Alternatively, the steam phase may be used for heating feedwater which is under high pressure and the liquid phase may be used for transferring heat to feedwater which is under low pressure.

In plants wherein the Washing-out operation is not continuous so that there are periods of time when no washingout fluid is available for feedwater heating, the missing heat may be derived from steam tapped from a turbine forming part of the power plant which includes the steam generator. In this case it is preferred to effect the switch from Washing-out fluid as a heating medium to tapped steam as a heating medium by automatically actuating valves in response to the pressure in the pipe line which normally conducts the washing fluid to the heat exchanger or heat exchangers.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, and additional objects and advantages thereof will best be understood from the following description of an embodiment thereof when read in connection with the accompanying drawing, the single figure of which is a diagrammatic Layout of a steam power plant according to the invention.

Referring more particularly to the drawing, numeral 1 designates a condenser from which condensate is pumped by a low pressure feed pump 2 through low pressure preheaters 3 and 3a to a container 4. A high pressure teed pump 5 draws water from the container 4 and pumps it through high pressure preheaters 6 and 6a to a forced flow steam generator 7. The steam generated therein is superheated in a superheater 8 wherefrom the steam is conducted to a first turbine 9 and partly expanded. The steam leaving the turbine 9 enters a second turbine it) the exhaust of which is conducted into the condenser 1.

The steam generator 7 includes a heating tube system consisting of .a plurality of tube lines connected in parallel relation with respect to the flow of the operating medium, four tube lines 11 being shown. The invention, however, is not restricted to steam generators having this number of parallel tube lines. At the end of each tube line 11 a one-way or check valve 12 is provided which permits flow of operating medium in one direction only, namely out of the tube lines 11. The outlets of the valves 12 are connected to a collector 13 from which the operating medium is conducted through a pipe 14 into heating tubes 15. Water is converted into steam in the tubes 11, Le. upstream of the valves 12, and it is in these tubes where salt contained in the feedwater may be deposited. The tubes 15 contain only steam when the plant is in normal operation and no salt is deposited therein. Close to and upstream of the valves 12 a pipe provided with a valve 16 is connected to each of the tubes 11. These pipes are connected to a pipe 17 through. which the fluid leaving the individual tubes 11 through the valves 16 is conducted to the high pressure feedwater preheater 6a which is of the indirect heat exchanger type. The operating medium received in the heater 6a from the pipe 17 leaves the heater 6a through a pipe 18 and is conducted to the low pressure 'feedwater preheater 3a wherefrom the operating medium is conducted either through a pipe line 39 as heating agent to the feedwater heater 3 or is blown down through a pipe 37. The pipe line 18 is connected to the container 4 by a pipe 23 in which a valve 33 is inserted which will be described later.

The valves 16 are periodically opened, one after another, so that there is either no or only one valve 16 open at a time. Opening of one of the valves 16 causes a pressure drop in the respective tube line 11 below the critical pressure so that the operating fluid passes as wet steam through the open valve 16 into the pipe 17 Presence of wet steam in the tube line 11 causes solution of salts which have previously been deposited in the tube line. The wet steam or washing fluid also carries along such impurities as are not soluble at the prevailing pressures and temperatures of the fluid in the tube line The wet steam which has acted as a washing fluid in the tube line 11, which is connected by the open valve 36 to the pipe line 17, consecutively transfers heat in the preheaters 6a and 3a to the feedwater. A part of the washing fluid is reintroduced through pipe 23 into the feedwater in the container 4.

By transferring heat from the operating fluid which has been used as washing fluid, in the preheaters 6a and 3a to the feedW-ater land by direct return of a portion of the washing fluid to the teedwater, a considerable portion of the heat energy is recovered which would otherwise be lost.

The high pressure feedwater preheater 6a is selectively connected to the pipe line 17 or to a pipe 31 which is connected to the turbine 9 and includes a valve 30. A valve 32 is provided in the pipe line 18. The valves 30, 32 and 33 are operated in response to the pressure prevailing in the pipe 31 which pressure acts on a pressure sensitive device 34 causing opening of the valves 30, 32. and 'closing'of the valve 33 when the pressure in the pipe 31 drops below a predetermined value. If the pressure in the pipe 31 rises above the predetermined value, the valves 30 and 32 are closedand the valve 33 is opened. The second turbine 10 is connected to the low pressure feedwater heater 3a by a bleeder conduit 35 in which a valve 36 is interposed. Blowdown water is removed from the preheater 3a through the pipe 37 provided with a valve 38. The pipe 39 is connected to the pipe 37 upstream of the valve 38. The pipe 39 is provided with a valve 40 and conducts blowdown water from the preheater 3a through the feedwater preheater 3 which is upstream of the preheater 3a, into the condensate flowing from the condenser 1 to the feed pump 2. The valves 36, 38 and 49 are controlled in response to the pressure prevailing in the bleeder conduit 35 downstream of the valve 36 by means of a pressure sensitive device 41 which causes opening of the valves 36 and 4t) and closing of the valve 33 upon dropping of the pressure in the conduit 35 below a predetermined value, and vice versa.

The illustrated arrangement is preferably used in plants where cyclic operation of the valves 16 is not possible so that there are periods of time when no washing fluid is available as heating agent for the feedwater preheaters. As long as washing fluid is available due to opening of one of the valves 16 the high pressure preheater 6a and the low pressure preheater 3a receive Washing fluid through the pipes 17 and 18 upon opening of the valves 32 and 38. If the washing operation is stopped, all valves 16 are closed and the pressure in the pipes 17 and 31 drops. This causes the pressure responsive device 34 to open the valves 30 and 33 and to close the valve 32. Partly expanded steam tapped from the first turbine 9 now flows through the valve 36 and the pi es 31 and 17 to the high pressure preheater 6a and from the latter through the pipes 18 and 23 and through the valve 33 into the container 4. Closing of the valve 32 causes dropping of the pressure in the pipe 18 downstream of the valve 32 and in the conduit 35. Under these conditions the pressure responsive device 41 opens the valves 35 and 40 and closes the valve 38. The low pressure preheater 3a which under these conditions does not receive any Washing fluid is now heated by steam tapped from the tubine 10. This steam flows through the conduit 35 and the pipe 39 back into the circuit of the operating fluid in the steam power plant upstream of the low pressure feed pump 2.

Although the present invention is particularly suited for steam generators operating at critical or supercritical pressure and described in this combination, the invention may also be advantageously used in combination with forced flow steam generators operating at subcritical pressures, particularly if it is desired to omit a water separator interposed in the main stream of the operating medium.

I claim:

In a stream power plant in which operating fluid moves in a circuit including a forced flow steam generator operating at least at the critical pressure of water, a prime mover connected for steam flow to said steam generator for converting thermal energy of the steam to mechanical power, means connected to said prime mover for receiving steam therefrom and condensing the steam, and means connected to said condensing means for receiving condensate therefrom and returning the condensate as feedwater to said steam generator, said last mentioned means including feedwater preheating means, said steam generator including a heating tube system comprising a plurality of tube lines arranged in parallel relation with respect to the flow of operating fluid therethrough, a check valve placed at the outlet of each of said tube lines permitting flow of operating fluid out of said tube lines and preventing backing up of operating medium into said tube lines, a valved pipe connected to each of said tube lines upstream of the respective check valve for relieving operating medium from the individual tube lines by I. opening the respective valved pipe for Washing out the respective tube line, a Washing fluid conduit connected to all of said valved pipes for receiving the Washing fluid relieved by said valved pipes, said washing fluid conduit 6 responsive means connected to said pipe downstream of said valve and to said valve for opening said valve upon a pressure drop in said pipe below a predetermined pressure and for closing said valve upon an increase of the being connected to at least one of said feedwater pre- 5 pressure in said pipe above said predetermined pressure.

heating means for conducting Washing fiuid thereto and transferring at least a part of the thermal energy contained in the Washing fluid to the operating fluid moving in the circuit, a pipe interconnecting said washing fluid conduit and said prime mover for tapping steam from the latter, a valve interposed in said pipe, and pressure References Cited in the file of this patent UNITED STATES PATENTS 1,942,861 Huster Jan. 9, 1934 10 2,842,103 Profos July 8, 1958 2,900,792 Buri Aug. 25, 1959

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