MX2012008402A

MX2012008402A - Controlling variables in boiler pressure vessels.

Info

Publication number: MX2012008402A
Application number: MX2012008402A
Authority: MX
Inventors: Donald W Bairley; Wesley P Ii Bauver; Glenn T Selby; Ian J Perrin; Francois Droux; Christoph Ruchti; Falk Ruecker
Original assignee: Alstom Technology Ltd
Priority date: 2010-01-20
Filing date: 2010-12-08
Publication date: 2012-10-09
Also published as: EP2526338A2; US20110174240A1; CN109028009A; EP2526338B1; WO2011090576A3; WO2011090576A2; CN102859276A

Abstract

A method of controlling stress in a boiler pressure vessel comprises limiting the diameter of a drum (10) of the boiler pressure vessel and preheating at least a portion of the wall (12) of the drum (10). Limiting the diameter of the drum (10) allows pressure in the drum (10) to be increased for a given mechanical stress. Furthermore, preheating the wall (12) of the drum (10) reduces peak thermally induced stresses in a material from which the drum (10) is fabricated.

Description

CONTROL VARIABLES IN BOILER PRESSURE CONTAINERS TECHNICAL FIELD The present application is generally directed to systems and methods for controlling variables in boiler vessels under pressure. More particularly, the present application is directed to systems and methods for reducing stresses in the walls of boiler vessels under pressure.

BACKGROUND A pressure boiler vessel (hereinafter "boiler") is a closed vessel comprising a shell or shell and containing a liquid that can be heated under controlled conditions, using a fuel or hot gases. The housing is a drum (hereinafter "drum" or "boiler drum") that is defined by one or more walls. Chemical energy contained in the fuel is converted into thermal energy, which heats the liquid in the boiler and causes it to evaporate. The mixture of liquid and vapor enters the drum. The walls of the drum are designed to withstand pressures exerted by the evaporated liquid. The evaporated liquid can be taken from the drum and used to provide work or as a source of heat.

Starting with a boiler that is initially at ambient conditions often causes rapid temperature changes experienced through the walls of the drum. These changes in temperature can generate stress or thermal stress inside the walls. These efforts can cause cracking and growth in the wall material. In some cases, these efforts can also cause cracking and growth in a layer of magnetite that forms inside the walls of the drums that contain water.

Both in boilers of natural circulation and boiler of assisted circulation in which water is heated and evaporates in steam, the drum is a steam drum used to separate steam from water. In boilers that operate at high pressures and / or have large drum diameters, the wall thickness is higher (compared to boilers that operate at lower pressure and / or have smaller drum diameters) to maintain acceptable pressure stress levels. Increased wall thickness results in increased thermal stress within the walls. High stresses inside the walls of the drums also occur at different sites or penetrations that extend through the wall. Typical penetrations include nozzles and the like. Since the penetrations are points of weakness in the walls of the drum, the maximum operating pressure of the boiler is effectively restricted due to limitations imposed by the European Standard code (EN = European Norm) at maximum boiler stress intervals (and more particularly in boiler drums). The stress interval also limits the number of quick starts to which the boiler can be subjected as well as the total number of starts over the life of the boiler.

Thick-walled boiler drums, in general, are heated only on their interior surfaces, resulting in temporary and non-uniform wall temperatures, particularly during the start-up period. As wall thickness increases, so does the temperature gradient across the wall. The induced thermal stress increases for a given proportion of internal temperature change as the wall thickness of the drum increases. Over time, the wall is heated to a uniform temperature, thus eliminating this type of thermal stress. The pressure stress dominates then. These stresses due to thermal gradients and internal pressure, when repeatedly applied and removed, can cause cracking and growth in the component material. The need to limit efforts to avoid these cracks can effectively limit the speed of temperature change in the drum. By limiting the rate of temperature change, the operational flexibility (for example, maximum achievable pressures) of the boiler decreases. This flexibility is convenient to provide quick starts to respond to changes in energy demand.

An additional restriction on boiler drums, in compliance with the requirements of the EN code is the limitation and intervals of efforts to avoid cracking or cracking of magnetite. To avoid cracking or magnetite cracking, the difference between the highest compressive stress and the highest tensile stress, should not exceed 600 mega pascais (MPa). This stress range is illustrated in Figure 1, which shows a typical stress history for a steam drum during a boiler start-up. The thermal stress that occurs early in the start-up process is shown to decrease as the temperature of the drum wall becomes more uniform as steady-state operating conditions approach. As it approaches steady state conditions, the stress due to internal pressure dominates the thermal stress. For a given drum diameter, the positive circumferential or tangential stress (traction) can be reduced by increasing the thickness of the drum wall, but this increases the negative stress due to temperatures across the wall at start-up and limits the speed of the drum number. starts COMPENDIUM In accordance with the aspects illustrated herein, a method for controlling stresses in a boiler pressure vessel is provided. This method comprises limiting the diameter of a drum of the boiler pressure vessel and preheating at least a portion of the wall of the drum. Limiting the diameter of the drum allows the drum pressure to increase for a given mechanical stress. In addition, the Preheating the drum wall reduces thermally induced peak stresses on a material from which the drum is manufactured.

In accordance with other aspects illustrated herein, a method for operating a boiler pressure vessel is provided. This method comprises applying local heating to a portion of the boiler pressure vessel prior to a boiler pressure vessel startup operation, during an operation of the boiler pressure vessel and / or during a shut-off operation of the pressure vessel of the boiler. boiler. By applying local heating to the boiler pressure vessel, thermally induced stresses are reduced in the boiler pressure vessel.

In accordance with other aspects illustrated herein, a method for controlling variables in a boiler pressure vessel is provided. This method comprises providing a steam drum of a boiler; controlling mechanical stresses on a wall of the steam drum by limiting the diameter of the steam drum; and controlling thermal stress on the wall of the steam drum by heating at least a portion of the steam drum. The heating of the portion of the steam drum is effected by preheating penetrations in the steam drum and / or an area surrounding a penetration in the steam drum during at least one of a starting period and a period of shutdown of the container. at boiler pressure.

The features described above and others are exemplified by the following Figures and Detailed Description.

BRIEF DESCRIPTION OF THE DRAWINGS Now with reference to the Figures, which are exemplary modalities, and where similar elements are numbered in a similar way: Figure 1 is a graphical representation of a typical stress history for a steam drum; Figure 2 is a schematic representation of a vertical section of a steam drum of a boiler; Y Figure 3 is a perspective view of a vertical section of a steam drum of a boiler.

DETAILED DESCRIPTION Now with reference to Figure 2, an exemplary embodiment of a steam drum of a boiler is generally shown at 10 and is referred to below as "drum 10" or "steam drum 10". The drum 10 may be of a natural circulation boiler, an assisted circulation boiler, or any other type of boiler. The drum 10 is of an elongated cylindrical shape and has a wall 12 which is penetrated by nozzles 14 receiving a high temperature steam / liquid mixture and discharging this mixture in an annular space 16 between a drum or deflector 18 coating and an inner surface 15 of the wall 12. The wall 12 also has an outer surface 17. The nozzles 14 may extend beyond the inner surface 15 of the wall (Figure 2) or may end on the inner surface 15 (Figure 3) . A liquid 26 such as water, for example, is collected at the bottom of the drum 10. One or more separation units 24 are located outside the volume circumscribed by the deflector 18. Steam of the vapor / liquid mixture 34 and the evaporation of the Water 26 passes through a drying assembly 32 and is withdrawn through an outlet 30. The configuration of Figure 2 is not limited to that illustrated, as other configurations are possible.

Before operation of the boiler, particularly at start-up from ambient conditions, the nozzles 14 and the areas 15a of the inner surface 15 of the wall 12 surrounding the nozzles 14, are affected by the vapor / liquid mixture 34. Transient of temperature (for example, the movement of heat from one area to another) through the materials of the nozzles 14 and the wall 12, produce thermal stresses.

Accordingly, the nozzles 14 and the areas 15a surrounding the nozzles, i.e. the wall of the drum 12 and particularly on the inner surface 15, are subjected to stress from the vapor / liquid mixture with high temperature 34. Mechanical stresses such as circumferential or tangential stress in the wall 12 of the drum 10 are also encountered as a result of pressure.

Mechanical stress in the wall 12 is a function of various process variables, i.e., the radius of the drum 10, the thickness of the wall 12, and the internal pressure of the drum 10. This can be described by the equation: am = f (PR / t) where: < 7m is the circumferential or tangential stress of the drum; P is the internal pressure; R is the drum radius; Y t is the wall thickness of the drum.

For a certain internal pressure and stress, reducing the diameter or radius of the drum results in reducing the thickness of the wall 12 of the drum 10.

An approach to accommodate mechanical stress that is applicable to both natural circulation boilers and assisted circulation boilers with steam production greater than 50 kilograms per second (kg / s) to allow operation at higher pressures, which is convenient due to the efficiency of the resulting upper cycle, is to limit the thickness of the wall 12 of the drum 10. The thickness of the wall 12 is limited by using a steam drum with a relatively small diameter, for example, a steam drum having an internal diameter of about 1, 000 millimeters (mm) and about 1,775 mm. When the diameter of the drum 10 is reduced and the thickness of the wall 12 is limited to a value that is consistent with drums having inner diameters greater than about 1.775 mm, the vapor for P may be increased for a given circumferential or tangential stress . Typical wall thicknesses may be in the range from about 70 mm to about 150 mm.

Thermal stresses within the wall 12 of the drum 10 also occur at the nozzles 14 or other penetrations through the wall 12 to the inner surface 15 as well as on the inner surfaces 15a proximate the nozzles 14. With reference to Figure 3, a localized high stress interval area is shown at 20. This high localized stress interval area 20 is located on the inner surface 15 near the area in which the nozzle 14 penetrates the wall. The stress in this localized high stress interval area 20 is at least twice the stress in any other area in the rest of the drum.

It has been found that applying local heating at least to portions of the drum 10 in a controlled manner can reduce the temperature transients and thermal stresses inside the drum 10.

One approach to applying local heating to accommodate thermal stress is to preheat the nozzles 14 and the area 15a adjacent thereto (e.g., the interior surface area 15a of the wall 12 in the area of the nozzle 14) before starting or starting the the boiler, when the drum 10 is at ambient pressure conditions. In one embodiment, local heating can be applied to the outer surface 17 of the drum 10 close to the area in which the nozzle 14 enters the drum 10 (e.g., area 17a). This will reduce the thermally induced peak stresses in a material from which the wall 12 of the drum 10 is manufactured that would otherwise limit the number of starts of ambient conditions or even avoid the use of drum type boilers over certain Pressure intervals due to EN code limits of stress intervals. The local preheating of the nozzles 14 and / or the wall 12 can be used as an alternative to or in conjunction with limiting the diameter of the drum 10.

It will also be appreciated that the approach is not limited to being performed at boiler start-up, since nozzles 14 and wall 12 can be heated during a shut-down operation. In doing so, the speed at which the heat dissipates from the nozzles 14 and the wall 12 will be reduced, thereby reducing the thermally induced stresses in the material of the nozzles 14 and the wall 12.

In addition to reducing thermally induced stresses when using local heating, it is contemplated that the local heat uses much less energy than that required to heat the entire drum 10 (e.g., the entire interior surface 15) and the fluid 26 it contains, from this way reducing operational costs. Without any kind of on-site preheating feature, the number of cold starts can potentially be limited to an absolute maximum in the specification (eg, 300) compared to an essentially unlimited number of cold starts with preheat.

The maximum possible thermal stress for a certain rise in ramp in temperature (transient temperature) is also a function of various process variables and varies approximately as the square of the thickness of the wall. Reduced thickness will result in reduced thermal stress for the same temperature change rate. This is described by the equation: < * i = (rt2) where at is the thermal stress; Tr is the speed of temperature change; Y t is the wall thickness of the drum.

The starting of a boiler that initially is at ambient conditions, results in rapid temperature changes in the drum 10 as well as in other components of the drum 10 (for example, nozzles 14 and the like). These temperature changes can generate thermal stress within these components. These stresses can cause cracking and growth on the material from which the component is manufactured and in some cases in a layer of magnetite that is formed on the inner surface 15 of these drums 10 containing water 26. Preheating at least portions of the drum 10 or other components of the pressure vessel in a controlled manner, can reduce the rate of temperature change, thereby reducing the thermal stresses within the component. The preheating of the drum 10 can be effected by heating with electrical resistance or other readily available means.

Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes and equivalents may be made may be substituted for their elements without departing from the scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential scope. Therefore, it is intended that the invention not be limited to the particular embodiments illustrated in the foregoing description, but that the invention include all modalities that fall within the scope of the appended claims.

Claims

1 . A method for controlling stress in a boiler pressure vessel, the method is characterized in that it comprises: limiting the thickness of the walls by reducing the diameter of a drum of the boiler pressure vessel; and preheating at least a portion of the drum wall, in order to reduce thermally induced peak forces in a material from which the drum is made.

2. The method according to claim 1, characterized in that the preheating of at least a portion of the wall includes preheating an inner surface of the drum.

3. The method according to any of claims 1 and 2, characterized in that limiting the diameter of the drum comprises using a drum having an internal diameter of less than about 1.775 mm.

4. The method according to any of claims 1 and 2, characterized in that preheating at least a portion of the wall of the drum comprises locally preheating penetrations in the wall of the drum.

5. The method according to claim 4, characterized in that the locally preheating penetrations in the wall of the drum, comprises heating nozzles that extend inside the wall of the drum.

6. The method according to claim 4, characterized in that it also comprises locally preheating areas of the drum wall adjacent to the penetrations.

7. The method according to any of claims 1, 2 and 4, characterized in that the preheating of at least a portion of the wall of the drum is carried out at least one of the starting of pressure vessel of the boiler and during an operation of the pressure vessel of the boiler.

8. The method according to any of claims 1, 2 and 4, characterized in that the preheating of at least a portion of the wall of the drum is carried out during a shutdown of the pressure vessel of the boiler.

9. A method for operating a pressure vessel of a boiler, the method is characterized in that it comprises: applying local heating to a portion of the pressure vessel of the boiler for at least one of before a boiler pressure vessel booster operation , during an operation of the pressure vessel of the boiler, and during a shutdown operation of the boiler pressure vessel; where to apply local heating to the pressure vessel of the boiler, reduces thermally induced stresses in the boiler pressure vessel.

10. The method according to claim 9, characterized in that locally applying heating to a portion of the pressure vessel of the boiler includes preheating an inner surface of the drum.

The method according to any of claims 9 and 10, characterized in that local heating applied to the wall portion of the pressure vessel of the boiler, comprises at least one of: heating a penetration that extends within a surface of a drum of the boiler pressure vessel; and heating an area surrounding the penetration that extends into the surface of the drum.

12. In addition, it comprises limiting a diameter of a drum of pressure vessel of the boiler, where limiting the thickness of the walls has reduced the diameter of the drum to reduce mechanical stresses in pressure vessel. of the boiler.

13. A method for controlling variables in a pressure vessel of the boiler, the method is characterized in that it comprises: providing a steam drum of a boiler; controlling mechanical stress in a wall of the steam drum by limiting the diameter of the steam drum to reduce the thickness of the walls of the drum; and controlling thermal stress on the wall of the steam drum by heating at least a portion of the steam drum; and wherein the heating of the portion of the steam drum is effected by preheating at least one of the penetrations in the steam drum and an area surrounding a penetration to the steam drum during at least one of a starting period and a period shut-off of the pressure vessel of the boiler.

14. The method according to claim 13, characterized in that preheating at least a portion of the wall includes preheating an inner surface of the drum.

15. The method according to any of claims 13 and 14, characterized in that the preheating of at least a portion of the wall of the drum, comprises locally preheating penetrations in the wall of the drum.