US20140202399A1

US20140202399A1 - Dual end plate subcooling zone for a feedwater heater

Info

Publication number: US20140202399A1
Application number: US14/057,290
Authority: US
Inventors: Ranga Nadig
Original assignee: Maarky Thermal Systems Inc
Current assignee: Maarky Thermal Systems Inc
Priority date: 2013-01-21
Filing date: 2013-10-18
Publication date: 2014-07-24
Also published as: CN104919263A; WO2014113109A1

Abstract

Steam power plants utilize high pressure and low pressure feedwater heaters to increase overall system efficiency. Both types of feedwater heaters generally include a subcooling zone that is separate from a condensing zone. The viability of the separation of the zones is critical to the efficiency of the overall system, specifically with regard to the inefficiencies inherent in the leakage of steam from the condensing zone into the subcooling zone. An improvement in the viability of that zone separation for such feedwater heaters is taught by the use of dual end plates for the subcooling zone to create a triple layer of separation between the condensing and subcooling zones to prevent unwanted and inefficient steam leakage.

Description

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No. 61/754,754 filed Jan. 21, 2013.

FIELD OF THE INVENTION

The instant invention relates to steam power plants and an improvement in the construction and use of feedwater heaters, both high pressure and low pressure feedwater heaters, having subcooling and condensing zones.

BACKGROUND OF THE INVENTION

In a steam power plant, feedwater heaters are used to gradually increase the temperature of the feedwater to the saturation temperature at boiler operating conditions. Preheating the feedwater improves the thermodynamic efficiency of the system, reduces the plant operating costs, and minimizes thermal shock to the boiler metal. A steam power plant may be equipped with a number of feedwater heaters.
The energy used to heat the feedwater is usually derived from steam extracted from the steam turbine. Since the steam that would be used to perform expansion work in the turbine (and therefore generate power) is not utilized for that purpose, the extraction steam must be carefully optimized for maximum power plant thermal efficiency
Feedwater heaters can be open or closed heat exchangers. In an open feedwater heater, the extraction steam is directly allowed to mix with the feedwater heater thereby heating it.
A closed feedwater heater is typically a shell and tube heat exchanger wherein the feedwater passes through the tubes and is heated by turbine extraction steam flowing inside the shell on the outside of the tubes. Examples of such closed systems are shown in U.S. Pat. No. 2,412,573 to Fraser and U.S. Pat. No. 6,095,238 to Kawano. Additionally, steam power plant improvements have been numerous over the years, as shown by U.S. Pat. No. 2,729,430 to Sieder and U.S. Pat. No. 2,812,164 to Thompson.
In a steam power plant, the feedwater heaters located upstream of the boiler feed pump are termed as high pressure feedwater heaters and those located downstream of the boiler feed pump are referred to as low pressure feedwater heaters.
In high pressure feedwater heaters, the turbine extraction steam has a sizeable amount of superheat. Therefore, feedwater in high pressure feedwater is typically heated in three stages in three separate compartments: a desuperheating zone; a condensing zone; and a subcooling zone. Initial heating of the feedwater heater is carried out in the subcooling zone by subcooling the condensed turbine extraction steam. The secondary heating of the feedwater heater is carried out in the condensing zone from the condensing turbine extraction steam. The final heating of the heating of the feedwater is carried out in the desuperheating zone by the superheat in the turbine extraction steam.
In low pressure heaters, the turbine exhaust steam has a lower amount of superheat. Therefore, feedwater in low pressure feedwater is typically heated in two stages in two separate compartments: a condensing zone; and a subcooling zone. Initial heating of the feedwater heater is carried out in the subcooling zone by the subcooling the condensed turbine extraction steam. The secondary and final heating of the feedwater heater occurs in the condensing zone from the condensing turbine extraction steam.

SUMMARY OF THE INVENTION

The problem to be solved is as follows: In both the high pressure and low pressure feedwater heater, the extraction steam in the condensing zone has to be prevented from entering the subcooling zone. If the extraction steam enters the subcooling zone, then condensate in the subcooling will be heated instead of subcooled and, therefore, the entire function of the condensate subcooling will be nullified. In the prevailing feedwater heater designs the subcooling zone is isolated from the condensing zone by maintaining the water level above the entrance to the subcooling zone and employing an end plate at the end of the subcooling zone to separate the condensing zone from subcooling zone. The end plate is usually 2″-3″ thick and the tubeholes through the end plate are drilled to a tight tolerance. When the steam enters the tight spaces between the tube outer diameter and the end plate tube hole, it condenses and forms a water seal that prevents ingress of steam into the subcooling zone.
Improper tube hole drilling tolerances, extended usage, normal wear and tear, or a combination thereof, can widening the gap between the outer diameter of the tube and the tube hole. In such scenarios, steam enters from the condensing zone, heating the condensate and compromising the performance of the subcooling zone, the entire heater, and the entire steam power plant. With each passing year the problem escalates until the decrease in efficiency is unsustainable. Eventually, the heater must be replaced.
It is an object of the instant invention to prevent steam ingress into the subcooling zone through the end plate tubeholes since that is a major factor affecting the performance of feedwater heaters.
According to the present invention, the ingress of steam into the subcooling zone can be avoided by using two end plates between the subcooling zone and condensing zone with a water seal in between for additional protection. According to the present invention, a feedwater heater is equipped with a subcooling zone that uses two end plates instead of one, and the tube holes in the end plates are drilled to tight tolerances.
Additionally, a semi-circular plate is welded to two end plates. A flat plate is welded to the top of the end plates thereby creating an enclosure between the two end plates. Holes are drilled into the top plate connecting the two end plates to admit condensate into the enclosure, and holes are drilled at the bottom of the circular plate to drain the condensate. In this aspect of the invention a water dam is created with a minor flow of condensate through the enclosure. The water dam with a minor flow constitutes a water seal.
The dual end plate with a water seal in between offers advantages of a triple layer of separation between the condensing and the subcooling zone. The present day technology has a single layer of separation.
According to the present invention, the first layer of separation is provided by the condensate collected in the annular space between the tube outer diameter and the tube hole in the outer end plate. The annular space between the tube outer diameter and the tube hole in the outer end plates is filled with condensate at condensing zone steam saturation temperature. The absence of a heat sink creates less of an incentive for the condensing zone steam to enter the annular space between the tube hole and the outer diameter of the tube in the outer end plate.
The second layer of separation, according to the present invention, is derived from the water dam between the outer and inner end plate. The enclosure between the inner and outer end plate is filled with condensate at or slightly below the condensing zone saturation temperature. Any steam from the condensing zone that might leak through the annular space between the tube outer diameter and the tube hole in the outer end plate is condensed by the water dam.
The third layer of separation, according to the present invention, is created by the condensate occupying the annular space between the tube outer diameter and the tube hole in the inner end plate. Any steam from condensing zone that might leak through the annular space between the tube outer diameter and the tube hole in the outer end plate that due to some reason was not condensed by the condensate in the enclosure between the inner and outer end plate is condensed by condensate occupying the annular space between the tube outer diameter and the tube hole in the inner end plate.
The triple barrier design, pursuant to this invention, consisting of the dual end plate with a annular condensate dam in between eliminates the ingress of steam into the subcooling zone. The performance of subcooling zone is preserved and the life of the feedwater heater is prolonged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. is a schematic of a steam power plant system.

FIG. 2 is a cutaway side view of a closed system high pressure feedwater heater.

FIG. 2A indicates the temperature changes of steam and feedwater inherent in high pressure feedwater heaters.

FIG. 3 is a cutaway side view of a closed system low pressure feedwater heater.

FIG. 3A indicates the temperature changes of steam and feedwater inherent in low pressure feedwater heaters.

FIG. 4 is a detailed view of the subcooling zone of a prior art feedwater heater.

FIG. 5 is a cross sectional view of the subcooling zone of FIG. 4 as viewed from the condensing zone.

FIG. 6 shows a cutaway view of the prior art single end plate design for the subcooling zone of a standard low pressure feedwater heater.

FIG. 7 is a detailed perspective view of the dual end plate subsystem of the present invention.

FIG. 8 is a cross sectional view of the dual end plate subsystem of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein like or similar references indicate like or similar elements throughout the several views, FIG. 1 is a block diagram of a steam power plant system 1000 creating superheated steam to power steam turbine 11 driven electric generator 12 to produce electrical energy. The low pressure steam exiting the steam turbine is condensed in a steam surface condenser 13 using cold water from a cooling tower, lake, river or sea that enters on inlet 130. Alternatively, the steam can be condensed in an air cooled condenser where the cold ambient air is used to condense the low pressure steam exiting the steam turbine 11. The steam condensed in the water or air cooled condenser 13 is pumped by a condensate pump 14 into a low pressure feedwater heater 15. The condensate entering the low pressure feedwater heater 15 through line 152 is termed feedwater. In the low pressure feedwater heater 15 the feedwater is heated by the steam extracted from the steam turbine 11 that enters at inlet 150. The extraction steam condensed in the low pressure feedwater heater 15 is discharged through outlet 151 into the steam surface condenser 13. The heated feedwater flows through line 155 into a deaerator 16 wherein the feedwater is deaerated using the steam extracted from the steam turbine 11 that enters the deaerator at inlet 160. The heated feedwater from deaerator 16 is pumped by boiler feed pump 17 into a high pressure feedwater heater 18 at inlet 182 wherein the feedwater is further heated by the turbine extraction steam that enters the heater inlet 180. The heated feedwater goes out through outlet 183 and enters the boiler 10. The extraction steam condensed in the high pressure feedwater heater 18 is discharged through outlet 181 into a deaerator 16 through inlet 161. Depending on the size, a power plant can have a multitude of low and high pressure feedwater heaters.
The closed, high pressure feedwater heater 18 of FIG. 1 is shown in greater detail in outline in FIG. 2. High pressure feedwater heater 18 has three zones: desuperheating zone 187; condensing zone 189; and subcooling zone 188. In certain instances the high pressure feedwater heater 18 may be equipped with just a condensing zone and a subcooling zone. The high pressure feedwater heater 18 is equipped with a number of tubes that carry the feedwater. A representative tube 190 is shown in outline running through each of the zones. The feedwater enters tubeside of the heater through inlet 182, travels through the tubes, and gets heated. The heated feedwater leaves the high pressure feedwater heater 18 through outlet 183 and flows into the boiler 10, as shown in FIG. 1. Turbine extraction steam enters high pressure feedwater heater 18 at inlet 180 and condensate exits said closed high pressure feedwater heater 18 at outlet 181 and back to deaerator 16 also as shown in FIG. 1. In the subcooling zone 188, the feedwater is heated by the subcooling condensate. In condensing zone 189, the heat from the condensing extraction steam heats the feedwater. In the desuperheating zone 187, the feedwater is further heated by the superheat in the extraction steam. Steam in the condensing zone is separated from the condensate exiting the subcooling zone by seal ring 191 and by end plate 185. Inlet 184 allows only condensate into the subcooling zone 188.
FIG. 2A illustrates the relative temperatures, plotted on the vertical axis of the graph, for steam and feedwater in subcooling, condensing and desuperheating zone in a high pressure feedwater heater. In FIG. 2A, from left to right, the temperature of the extraction steam decreases in the desuperheating zone to a value close to the saturation temperature. In the condensing zone, the temperature of the extraction steam remains constant at the saturation temperature. In the condensing zone the extraction steam condenses on the tubes involving a phase change. In the subcooling zone the condensed extraction steam is subcooled to a temperature slightly higher than the feedwater inlet temperature. The flow of the feedwater inside the tubes is countercurrent to the of the flow of extraction steam and resulting condensate outside the tubes. The temperature of the feedwater, as shown in FIG. 2A, increase as it flows through the tubes in the subcooling, condensing and desuperheating zone.
Shown in FIG. 3, for comparison purposes to high pressure feedwater heater 18, is low pressure feedwater heater 15. A high or low pressure feedwater heater equipped with a subcooling zone will benefit from the present invention. Low pressure feedwater heater 15 has two zones: condensing zone 157 and subcooling zone 158. A representative tube 159 is shown in outline running through each of the two zones. Feedwater enters the tube at inlet 155 travels through the subcooling and condensing zone and exits at outlet 156. Turbine extraction steam enters low pressure feedwater heater 15 at inlet 150 and condenses in the condensing zone 157. The condensed extraction steam enters the subcooling zone 158 at inlet 154 and gets subcooled as it loses its heat to the feedwater travelling inside the tubes. The subcooled condensate exits the low pressure feedwater heater at outlet 151.
Analogously to FIG. 2A, FIG. 3A shows relative temperatures, plotted on the vertical axis of the graph, for steam and feedwater temperatures in a low pressure feedwater heater. In FIG. 3A, in the condensing zone, the temperature of the extraction steam remains constant at the saturation temperature as the extraction steam condenses. The temperature remains constant during phase change. In the subcooling zone, the temperature of the condensed turbine extraction steam decreases as the condensate loses its heat the feedwater travelling inside the tubes. The flow of feedwater is counter current to the flow of condensed extraction steam in the subcooling zone. The temperature of the feed water steadily increases as it travels through the subcooling and condensing zone.
Prior art FIG. 4 shows in detail the cross section detail of the subcooling zone 158 of a low pressure feedwater heater 15 in which the tubesheet 152 is shown with a representative tube 159 running there through. The end plate 200 forms the single, non-welded, barrier between subcooling zone 158 and condensing zone 157. The longitudinal baffle 201, semi-circular subcooling zone shroud 202 and the seal ring 153 together form the welded boundaries between said two zones. The welded boundaries constitute a permanent leak proof barrier between the condensing and subcooling zone. Condensate level 299 is shown by the sinusoidal broken line at the bottom of the condensing zone 157 and it is maintained well above the inlet 154 to the subcooling zone thereby ensuring that the steam from the condensing zone does not enter the subcooling zone.
Prior art FIG. 5 is a sectional view of end plate 200 as shown from the right (or condensing zone 157) side showing tube holes 203 through which tubes, such as representative tube 159, run. The subcooling zone inlet 154 is shown in perspective in front of condensate outlet 151. The welded boundaries between the condensing zone 157 (from which the view is taken) and said subcooling zone 158 are the longitudinal baffle 201 on the top, the semicircular longitudinal baffle 202 and the seal ring 153 (not shown). The end plate 200 containing the tube holes 203 and tubes running through the tube holes is the only non-welded boundary separating the subcooling zone 158 from the condensing zone (from which the view is taken).
Prior art FIG. 6 illustrates prior art for a subcooling zone 158. The longitudinal baffle 201 and the longitudinal shroud 202 along with the seal ring 153 (not shown) form the welded barrier between the condensing zone and subcooling zone. A single end plate 200, consisting of a multitude of tubes, such as representative tube 159, running though a multitude of tube holes 203 forms the single non-welded barrier between the condensing zone 157 (not shown) and subcooling zone 158. The tube holes 203 in the end plate 200 are drilled to a diameter which is slightly above that of the tubes so as to permit sliding the tubes through the tube hole 203 in end plate 200. While it is anticipated that the water collected in the annular space between the tube outer diameter and the tube hole in the end plate 200 will form a permanent seal for the entire life of the heater, operational stresses using the prior art system prevent this from occurring. This is due, in part, to the fact that feedwater heaters are required to operate twenty-four hours a day for twenty-five to thirty years.
FIGS. 7 and 8 show in detail the present invention over the prior art of FIG. 6, that is, the use of end plate system 2000 comprised of dual end plates 200 and 300 (with a water seal in between) together forming the triple barrier between subcooling zone 158 and condensing zone 157 (from which the view is taken) in a feedwater heater with a subcooling zone 158. In FIG. 7 for a feedwater heater with subcooling zone, the end plate system 2000 of the present invention is shown in part, with the inner end plate 200, an outer end plate 300. A semicircular plate 302 is welded to the inner end plate 200 and outer end plate 300. A short horizontal plate 301 (not shown in its entirety) is welded to the top of the inner end plate 200 and the top of outer end plate 300 and the semicircular plate 302 is likewise welded to such end plates to form a water chamber between. The lips of the semicircular plate 302 are extended above the short horizontal plate 201 and bars are welded to the top of the inner end plate 200 and outer end plate 300 to form a water dam on top of the short horizontal plate 301. Small holes (not shown) are drilled in the short horizontal plate 301 to allow condensate to enter the water chamber between the end plates 200 and 300, having tube holes 203 and 303, respectively. As shown in FIG. 8, drain holes (representative of which is shown as drain 305) is placed at the bottom of the semicircular plate 302 to drain the condensate. The intent is to keep the water chamber between the end plates flooded at all times and create a small water flow and avoid stagnation of condensate.
According to the present invention, the ingress of steam into a subcooling zone, which has been one of the main reasons for degrading of performance of feedwater heaters worldwide, is eliminated by using a triple barrier design consisting of an inner end plate 200, an outer end plate 300 and a water seal in between.
The outer end plate 300, with tightly drilled tube holes constitutes the first barrier. Steam condenses in the annular space between the outer diameter of the tubes and the tube hole. Condensate accumulated in the small annular gap prevents the entry of any additional steam. Due to normal wear and tear, extended usage or minor errors in end plate tube hole drilling, the annular gap between the tube outer diameter and the end plate tube hole could enlarge over time and steam from condensing zone could breach the first barrier.
In such an event, the ingressing steam would come in contact with the second barrier, comprising condensate collected in the annular space between the inner and outer end plates, and condense.
The inlet holes on the longitudinal baffle 201 on top and the drain 305 located at the bottom of the semi-circular cylinder 302 create a minor flow of condensate and prevent stagnation in the water chamber between inner and outer end plate.
If, due to some unforeseen reason, steam from the condensing breaches the first and second barrier it is prevented from entering the subcooling zone by the third barrier comprising the inner end plate 200. The condensate in the annular gap between the tube outer diameter and the inner end plate 200 tube holes 203 prevents the steam from the condensing zone from entering the subcooling zone.
In this way, pursuant to this invention, the dual end plate with an annular condensate trough in between prevents the ingress of steam into the subcooling zone. The performance of subcooling zone is secured and the life of the feedwater is heater is prolonged.
Although specific arrangements of components have been described herein, other suitable arrangements and components may be used as indicated with similar results in the viability of the seal between the subcooling and condensing zones of feedwater heaters, including, but not limited to, utilizing a plurality of such end plates to provide more than one water seal between said subcooling and condensing zones.
Other modifications of the present invention will occur to those skilled in the art on reading the instant disclosure. Those modifications are intended to be covered within the scope of this invention such as, without limitation, the use of a plurality of plates and seals created thereby.

Claims

What is claimed is:

1. A dual end plate apparatus for use in a feedwater heater comprising:

an outer end plate separating a condensing zone from a water seal having a plurality of tubes passing through holes in said end plate; and

an inner end plate separating a subcooling zone from a water seal having a plurality of tubes passing through holes in said end plate, whereby said water seal separates said inner end plate and outer end plate.

2. The apparatus of claim 1 further comprising:

a semicircular plate welded to said inner end plate and to said outer end plate; and

a horizontal plate welded to said inner end plate and to said outer end plate, whereby said semicircular plate and said horizontal plate form an enclosure between said inner end plate and said outer end plate.

3. The apparatus of claim 2 further comprising holes in said horizontal plate in order to permit condensate to flow into and collect in said enclosure.

4. The apparatus of claim 2 further comprising a drain at the bottom of said semicircular plate in order to drain condensate collected in said enclosure.

5. A dual end plate apparatus for use in a low-pressure feedwater heater comprising:

6. The apparatus of claim 5 further comprising:

7. The apparatus of claim 6 further comprising holes in said horizontal plate in order to permit condensate to flow into and collect in said enclosure.

8. The apparatus of claim 6 further comprising a drain at the bottom of said semicircular plate in order to drain condensate collected in said enclosure.

9. A dual end plate apparatus for use in a high-pressure feedwater heater comprising:

10. The apparatus of claim 9 further comprising:

11. The apparatus of claim 10 further comprising holes in said horizontal plate in order to permit condensate to flow into and collect in said enclosure.

12. The apparatus of claim 10 further comprising a drain at the bottom of said semicircular plate in order to drain condensate collected in said enclosure.

13. A multiple end plate apparatus for use in a feedwater heater comprising:

an outer end plate separating a condensing zone from a water seal having a plurality of tubes passing through holes in said outer end plate;

at least one additional plate forming a boundary of a water seal having a plurality of tubes passing through holes in said plate; and

an inner end plate separating a subcooling zone from a water seal having a plurality of tubes passing through holes in said inner end plate; whereby said multiple plates form a plurality of water seals separating said condensing zone from said subcooling zone.

14. The apparatus of claim 13 further comprising:

a semicircular plate welded to said inner end plate, to each of said additional plates, and to said outer end plate; and

a horizontal plate welded to said inner end plate, to each of said additional plates, and to said outer end plate, whereby said semicircular plate and said horizontal plate form an enclosure between said inner end plate and said outer end plate.

15. The apparatus of claim 14 further comprising holes in said horizontal plate in order to permit condensate to flow into and collect in said enclosure.

16. The apparatus of claim 14 further comprising a drain at the bottom of said semicircular plate in order to drain condensate collected in said enclosure.