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.
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.
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.
Referring to the drawings wherein like or similar references indicate like or similar elements throughout the several views,
The closed, high pressure feedwater heater 18 of
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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.
This application claims the benefit of U.S. Provisional Application No. 61/754,754 filed Jan. 21, 2013.
Number | Date | Country | |
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61754754 | Jan 2013 | US |