WO1998028231A1

WO1998028231A1 - FEEDWATER pH CONTROL FOR STEAM GENERATORS

Info

Publication number: WO1998028231A1
Application number: PCT/US1997/021877
Authority: WO
Inventors: Frank Gabrielli
Original assignee: Combustion Engineering, Inc.
Priority date: 1996-12-23
Filing date: 1997-12-01
Publication date: 1998-07-02
Also published as: AU7872598A

Abstract

The feedwater of a steam generator (10) and particularly the feedwater to a heat recovery steam generator (10) is injected with non-volatile chemicals such as trisodium phosphate to maintain a desired pH. The use of the non-volatile chemicals is possible because the system includes the recycle of blowdown from the steam drums (28, 48, 54) to the feedwater line at a point downstream from the condensate pump (20) and downstream from where feedwater (18) is extracted for attemperation.

Description

Feedwater pH Control for Steam Generators

Background of the Invention

The present invention relates to the control of the water composition in a steam generator and most particularly to the control of the feedwater pH in a heat recovery steam generator or similar cycles by the addition and maintenance of a strong alkali concentration of trisodium phosphate, sodium hydroxide or similar treatment chemicals. The failure of tubes in steam generator components, especially economizers, may be caused by a variety of factors. One factor is the erosion and corrosion of the internal surfaces of the tubes. The mechanism of this type of attack involves a cyclic phenomenon of dissolution and reformation of the protective magnetite layer on the tubes. This is greatly influenced by pH, temperature, local velocity and turbulence as well as oxygen levels. A pH of 8 or below has been found to promote the highest dissolution of the carbon steel material.

The common method for pH control of the feedwater is by the use of volatile amines such as ammonia. Volatile amines are used since they volatilize with the steam as a vapor, thus offering several advantages. They do not affect the dissolved solids inventory in the boiler water and thus pose no demands for blowdown to reduce solids. They recirculate with the steam and condensate thus minimizing chemical usage and cost. Also, they do not restrict the use of feedwater for attemperation purposes. However, in some situations, the inclusion of amines or other volatiles are restricted or prohibited by the particular end use of the steam.

Summary of the Invention

The present invention involves the injection of a non-volatile chemical such as trisodium phosphate to the feedwater of a steam generator system at a level to maintain a desired pH. The use of the non-volatile is possible because the steam generator system is equipped to include the recycle of blowdown from the steam drums to the feedwater line downstream from where feedwater is extracted for the attemperation of superheated steam. The invention is particularly applicable to heat recovery steam generators where blowdown from a higher pressure steam drum can be recycled to the lower pressure feedwater at a point downstream of the feedwater or condensate pump.

Brief Description of the Drawings

Figure 1 is a graph showing the effect of pH on the thinning of tube walls due to erosion corrosion.

Figure 2 is a flow diagram of a heat recovery steam generator system incorporating the present invention.

Description of the Preferred Embodiment

The present invention relates to the reduction of the effect of feedwater pH on the erosion/corrosion of the tubes in a steam generator and particularly the economizer tubes. Figure 1 is an example of the effect that pH can have on tube thinning for a particular set of conditions. This same general effect is also applicable at other conditions. As can be seen, the tube wall thinning is most evident at a pH range below about 8. Above a pH of 8, the rate of wall thinning drops off dramatically. An object of the present invention is to maintain the feedwater in the higher pH range to minimize the erosion/corrosion with the same non-volatile treatment chemicals used for boilerwater pH control. Since the present invention is particularly applicable to heat recovery steam generators, the detailed description will be directed to such a steam generator. As an example, gas turbines are widely used to provide standby or peaking electric power. However, the thermal efficiency is low because of the high exit gas temperature. The thermal energy remaining in the exhaust gas can be recovered in a heat recovery steam generator to produce steam such as for the production of additional electricity using a steam turbine. The combined output of electricity from the gas turbine and the steam turbine may be 30 to 50 percent greater than that obtained from the gas turbine alone with no additional fuel input. Of course, the use of heat recovery steam generators with gas turbines is only one example of where heat recovery steam generators may be used.

Referring to Figure 2, a simplified fiow diagram of a horizontal gas flow heat recovery steam generator 1 0 is illustrated. At the left end is the inlet 1 2 which would be connected to the exhaust of the gas turbine (not shown) or other hot gas source. The hot gases flow through the heat recovery steam generator 1 0 contacting the heat exchange surface as will be explained and exits at 1 4 into the flue gas stack 1 6.

As indicated, the heat recovery steam generator 1 0 contains a series of banks of heat exchange surfaces. Starting from the cold end, the feedwater or condensate 1 8 from the condensate pump 20 flows into the low pressure economizer section 22 and then into the low pressure boiler section 24. The steam or steam/water mixture flows from the low pressure boiler section 24 through line 26 into the low pressure steam drum 28. In the particular arrangement shown, the steam from the drum 28 is fed at 30 to the deaerator 32 for feedwater heating. The recycle condensate from the steam turbine is also fed to the deaerator 32 as will be explained hereinafter. The deaerated condensate 34 from the deaerator 32 is ' then recycled through the system by the condensate pump 20. The steam from the low pressure drum 28 can also be used for other plant processes.

The liquid 36 from the low pressure drum 28 is fed by pump 38 into the intermediate pressure economizer 40 and by the pump 42 into the high pressure economizer 44. The effluent from the intermediate pressure economizer 40 flows to the intermediate pressure boiler bank 46 and then to the intermediate pressure steam drum 48. The separated intermediate pressure steam 50 from the steam drum 48 may be used for a variety of purposes such as process steam or injection into the gas turbine combustor to reduce NO_x emissions. It could also be used for additional power recovery in the lower pressure stages of the steam turbine.

The effluent from the high pressure economizer 44 flows to the high pressure boiler bank 52 and then to the high pressure steam drum 54. The separated high pressure steam 56 from the drum 54 then flows through the superheater 58 and the superheated steam 60 goes to the steam turbine 62. The superheated steam may be attemperated or desuperheated at 64 as required. The attemperation spray will be further explained hereinafter. The expended steam 66 from the turbine 62 goes to the condenser 68 and through pump 70 to the deaerator 32.

In the present invention, phosphate chemistry is utilized for pH control. Initially, trisodium phosphate is injected at 72 into the feedwater at the condensate pump discharge. Sufficient phosphate is injected to raise the pH above 8.5. This normally requires about 1 ppm of trisodium phosphate in the feedwater. It is even preferable to raise the pH higher than 8.5 but this depends on a number of factors such as the boiler pressure for a specific plant.

The use of the dissolved solids such as trisodium phosphate for feedwater pH control in the present invention is made possible because of the blowdown cascade and recycle feature from one or more higher pressure drums to the lower pressure feedwater line downstream of the condensate pump discharge. This results in very little phosphate content in the stream effluents and conserves the phosphate, i.e., the phosphate is continuously recycled with only small amounts being required for makeup of any lost such as through whatever minor blowdowns to waste may be required. The blowdown scheme of the present invention is shown in Figure 2 where the blowdown 74 from the high pressure steam drum 54 is fed into the intermediate pressure steam drum 48 and the blowdown 76 is recycled at 78 to the low pressure feedwater line 1 8. Any small amount of blowdown to waste that may be required to reduce accumulations of other solids is through line 80. The following table lists one specific example of the flow rates, pressures and amounts of trisodium phosphate at various key points in the process:

The indicated blowdown rates given in the example in the table (streams 74 and 78) are based on maintaining 1 00 ppm and 200 ppm of trisodium phosphate in the high and intermediate pressure drums 54 and 48, respectively. This will result in about 1 3 ppb sodium in the intermediate pressure steam 50 and about 80 ppb in the high pressure steam 60. These steam sodium values can be altered by changing the blowdown rates. Another significant feature of the present invention comes into play when there is a need for attemperation or desuperheating of the superheated steam 60. In most steam generator systems, feedwater is used for this purpose. However, in the present invention where the feedwater contains dissolved solids in the form of the trisodium phosphate and where it is important to maintain low levels of solids in the steam, feedwater cannot be used for this purpose. Therefore, the attemperation water 64 is derived from some selected source with a low solids content external to the circulating steam generator water. In the present invention, the pH can be automatically controlled if desired. A pH monitor 82 is connected into the feedwater line 1 8 and the output is used to control the trisodium phosphate feed at 84 as needed to replace any loses or otherwise correct the pH.

Claims

Claims:

1 . A method of operating a steam generator system including a high pressure steam drum and a lower pressure feedwater supply comprising the steps of: (a) adding a phosphate pH control material as a dissolved solid to said feedwater to obtain a desired pH;

(b) recycling water containing said dissolved solid from said high pressure steam drum to said lower pressure feedwater supply thereby recycling said dissolved solid through said steam generator system.

2. A method as recited in claim 1 wherein said phosphate pH control material is trisodium phosphate.

3. A method as recited in claim 1 wherein said steam generator system further includes an intermediate pressure steam drum and wherein said step of recycling said water from said high pressure steam drum includes recycling through said intermediate pressure steam drum.