EP0617778B1 - Fossil-fuelled continuous steam generator - Google Patents

Abstract

In continuous steam generators with a fossil-fuel burner (11) with a vertical flue (1) of substantially vertical pipes (2, 3), their inlet ends are connected to an inlet manifold (9) and their outlet ends to an outlet manifold (12). According to the invention, a pressure equalising pipe (25) branches off from each pipe (2) at the same height H which is connected to a pressure equalising vessel (4) and the height H is such that, on overheating of the single pipe (2), between this inlet manifold (9) and the branch of the pressure equalising pipe (25) in relation to the average heating of all the pipes (2), the mass flow through this single pipe (2) increases.

Description

The invention relates to a once-through steam generator with burners for fossil fuels with a vertical gas flue from essentially vertically arranged pipes which are connected with their inlet ends to an inlet collector and with their outlet ends to an outlet collector. Such a steam generator is known from document US-A-3 308 792.

The invention also relates to continuous flow steam generators of this type which have a funnel arranged at their lower end, which has at least four walls made of pipes welded to one another in a gastight manner and inlet and outlet collectors for these pipes.

In the case of once-through steam generators fired with fossil fuels with vertically tube-shaped combustion chamber walls, the tubes at the outlet of the combustion chamber walls often have large temperature differences, since different amounts of heat are transferred to the individual tubes of the parallel tube system. The causes of the different amounts of heat transferred are due to the different heat flow density profile - e.g. less heat is transferred in the corners of the combustion chamber than in the vicinity of the burners - and in the differences in the heated pipe lengths, especially in the funnel area, for continuous steam generators designed for coal firing.

To reduce these temperature differences at the pipe ends, a solution with throttle orifices and a pressure compensation collector is known from a publication in VGB Kraftwerkstechnik 64, number 4, pages 298 and 299. After this, the individual pipes are provided with throttling orifices at the inlet in order to adapt the water / steam throughput of individual pipes to the heating and length differences. Disadvantages of this solution are that the orifice plates at the pipe entry can only be designed for a single operating case and that changing soiling of the combustion chamber walls can, however, result in a disproportionate temperature deviation of individual pipes.

It has also been shown that the throttle diaphragms can become blocked, so that too little water is supplied to the pipes affected thereby.

The pressure compensation collector is arranged in the wet steam area - i.e. at a point where all pipes are still at the same temperature but have wet steam of different steam contents - at the point where an average steam content of 80% is reached at 35% of the boiler load. The entire evaporator mass flow is passed through pressure compensation collectors, so that a mixture of the wet steam emerging from the individual tubes of the parallel tube system is forced.

In this known pressure equalization collector, the incoming wet steam can therefore be segregated in such a way that individual outgoing pipes preferably receive water and others preferably steam. The result is that, even with uniform heating of the tube walls above the pressure compensation collector, a strongly different heating of the steam and thus different tube wall temperatures and the resulting thermal stresses occur, which can lead to tube tears.

The invention has for its object to design the pipe walls of the vertical throttle cable so that despite the unavoidable different heating of individual pipes, the steam temperatures at the outlet of all pipes are almost the same and that malfunctions, such as may occur due to clogging of throttle orifices at the pipe inlet, are avoided .

das heißt, daß der gesamte Druckabfall (Δp_Ges) des betrachteten Rohrabschnittes bei einer Mehrbeheizung (ΔQ̇) abnehmen muß, wenn man den Durchsatz (Ṁ) konstant hält.According to the invention, this object is achieved for continuous-flow steam generators of the type mentioned at the outset in that a pressure compensation vessel is arranged on the outside of the combustion chamber walls at an altitude which ensures that a multi-heated tube has a greater throughput than a parallel tube with medium heating. This is generally the case when the geodetic pressure drop of a pipe with medium heating is a multiple of its friction pressure drop. The pressure drops mentioned relate to the part of the evaporator tubes which is located between the collector located at the inlet to the evaporator and the branch located downstream to the pressure compensation vessel. The condition for a mass flow increase in a more heated pipe is:

this means that the total pressure drop (Δp _Ges ) of the pipe section in question must decrease in the case of multiple heating (ΔQ̇) if the throughput (Ṁ) is kept constant.

For internally finned tubes, the friction pressure drop (ΔP _R ) is according to Q. Zheng, W. Köhler, W. Kastner and K. Riedle, "Pressure loss in smooth and internally finned evaporator tubes, heat and mass transfer 26", pp. 323 - 330, Springer Verlag 1991 to determine, while the geodetic pressure drop (ΔP _G ) according to Z. Rouhani "Modified correlation for void-fraction and two-phase pressure drop", AE-RTV-841, 1969 is to be determined. In contrast, the acceleration pressure drop (ΔP _B ) is of minor importance and can be neglected in this calculation.

Gleichung (2) sagt noch nichts über das Ausmaß des Massenstromanstiegs aus. Erwünscht wäre ein Anstieg, der die Mehrbeheizung gerade vollständig kompensiert. In diesem Fall würde auch im Rohr mit stärkerer Beheizung die gleiche Aufheizspanne, d.h. die gleiche Enthalpie-Erhöhung wie in den Rohren mit mittlerer Beheizung vorliegen, was zu einer sehr starken Verminderung der beschriebenen Temperaturdifferenz bis auf Null führen würde. Die Bedingung hierfür lautet:

According to the invention, however, the mass flow in a tube with multiple heating should not remain constant, but should increase (ΔṀ> 0). This is the case in a parallel pipe system if equation (1) is fulfilled. This applies to the multi-heated pipe $$\frac{\text{Δ}\dot{\text{M}}}{\text{Δ}\dot{\text{Q}}}\text{> 0}$$

Equation (2) says nothing about the extent of the mass flow increase. An increase would be desirable that just completely compensates for the additional heating. In this case, the same heating span, ie the same enthalpy increase, would also be present in the pipe with stronger heating as in the pipes with medium heating, which would lead to a very strong reduction in the described temperature difference to zero. The condition for this is:

The index Ref refers to a reference pipe that has the mean throughput Ṁ and the mean heat absorption Q̇.

In practice, it will not always be possible to meet the condition listed in equation (3). The height of the pressure compensation vessel, i.e. the connection of the pressure compensation vessel into the parallel pipe system of the vertically arranged pipes with at least part of their length internally finned, is therefore selected so that one of the following conditions applies: $$\frac{\text{Δ}\dot{\text{M}}}{\text{Δ}\dot{\text{Q}}}\text{> 0}$$

Since in this fluidic design all parallel pipes have different throughputs with different heating, but have approximately the same steam content (with wet steam) or temperatures (with superheated steam), a throughput of the entire mass flow through the pressure compensation collector is not necessary. A throughput of the entire mass flow through the pressure compensation collector would even be disadvantageous because there would again be a risk of segregation of the water-steam mixture. Therefore, only one pressure compensation vessel is provided, through which only part of the total wet steam flow flows. This resulting partial flow not only brings about an equalization of the flow distribution and a flow distribution adapted to the heating profile in the parallel pipes between the inlet header and the outgoing pressure compensation pipes to the pressure compensation vessel, but it also leads to an additional mass flow through the pressure compensation pipes, so that in the pipes between the Pressure equalization pipes and the downstream outlet collector have an almost even flow distribution. There is no risk of the wet steam separating into water and steam, so that all pipes at the upper end of the pipe walls have approximately the same temperature and damage due to thermal stresses cannot occur.

• Figur 1 einen Längsschnitt eines Durchlaufdampferzeugers in vereinfachter Darstellung und
• Figur 2 ein einzelnes Rohr aus einem vertikal berohrten Teil des Durchlaufdampferzeugers mit einem Anschluß dieses Rohres an ein Druckausgleichsgefäß.

An embodiment of the invention is explained in more detail with reference to a drawing. It shows

• 1 shows a longitudinal section of a once-through steam generator in a simplified representation and
• Figure 2 shows a single tube from a vertically tube-sectioned part of the continuous steam generator with a connection of this tube to a pressure compensation vessel.

A continuous steam generator according to Figure 1 with a vertical throttle cable 1 consists of tube walls, which are gas-tightly welded together in the lower part from tubes 2 arranged vertically and next to one another, and which consist of tubes 3 arranged vertically and next to one another in the upper part, which are also gas-tightly welded to one another . The tubes, which are welded to one another in a gastight manner, form a gas-tight tube wall, for example in a tube-web-tube construction or in a fin tube construction.

The vertical throttle cable 1 has a funnel 10 at its lower end for receiving ash, the surrounding walls of which are also formed by the tube walls. In the lower part of the vertical throttle cable 1, main burners 11 for fossil fuel are attached.

The tubes 2 are connected with their inlet ends to an inlet header 9 and at a height H, measured from the central axis of the inlet header 9, go directly into the inlet ends of the tubes 3 with their outlet ends. The tubes 3 are connected with their outlet ends to an outlet header 12.

The outlet headers 12 are connected by connecting lines 13 to a separator 14 to which an outlet line 15 and a connecting line 16 are connected.

The connecting line 16 leads to an inlet header 17 of a superheater heating surface 18, the pipe outlet ends of which are connected to a superheater outlet header 19. In addition, an intermediate superheater heating surface 21 with an inlet header 20 and an outlet header 22 and an economizer heating surface 6 with an inlet header 5 and an outlet header 7 are arranged within the vertical throttle cable 1. The outlet header 7 is connected to the inlet header 9 by a connecting line 8.

FIG. 2 shows a single tube 2, which at point H, at which a pressure compensation tube 25 branches off, merges with its outlet end directly into the inlet end of tube 3. The pressure compensation tube 25 is connected to a pressure compensation vessel 4, which is located outside the vertical throttle cable 1. A pressure compensation tube 25 branches off from each tube 2 of the tube walls.

A feed pump, not shown, conveys water into the inlet collector 5 and from there into the economizer heating surface 6, in which the water is preheated. The water then flows through the connecting line 8 and the inlet header 9 into the tubes 2 of the tube walls of the vertical gas flue 1, in which it largely evaporates. The remaining evaporation and the first part of the overheating takes place in the tubes 3 of the tube walls of the vertical throttle cable 1.

The separator 14 is only in operation during the start-up process, that is, as long as not all water evaporates in the pipe walls due to insufficient heat input.

The entering water-steam mixture is then separated in the separator 14. The separated water is led through the drain line 15, for example, to an expansion device (not shown), the separated steam flows through the connecting line 16 to the superheater heating surface 18.

The steam expanded in the high-pressure part of the steam turbine is reheated in the reheater heating surface 21.

The mass flow density in the vertically arranged pipes 2 and 3 is chosen so that the geodetic pressure drop in the pipes is substantially greater than the friction pressure drop. The result of this is that a pipe receives a higher throughput in the case of multiple heating and the effect of the multiple heating with regard to the outlet temperature is largely compensated for. With very long vertical evaporator tubes, e.g. used in continuous steam generators in single-pass design, despite a low mass flow density of 1000 kg / m²s and less, based on 100% load, the frictional pressure drop in the pipes of the upper part of the vertical throttle cable, i.e. in pipes 3, increases due to the large steam volumes strong. The drop in frictional pressure in relation to the geodetic drop in pressure can be so great that the throughput decreases due to a multi-heated pipe compared to the parallel pipes and this leads to undesirably high steam temperatures at the pipe end.

The arrangement of the pressure compensation vessel 4 now causes the pipes 2 to be uncoupled from the pipes 3 with regard to the pressure drop. All tubes 2, which flow from bottom to top and are connected in parallel in terms of flow, have the same pressure drop between the inlet header 9 and the pressure compensation vessel 4. At this pressure drop, the proportion of the geodetic pressure drop is a multiple of the proportion of the friction pressure drop, so that the advantage of Increasing the throughput when heating individual pipes is very effective. This is particularly important in the lower part of the vertical throttle cable 1, in which the different heating in the area of the funnel and the main burner is particularly pronounced.

In the upper part of the vertical throttle cable 1, in which the pipes 3 are located, both the heating and their irregularities are less than in the lower part of the gas cable 1. The pressure compensation vessel 4 now causes a partial flow of through a part of the pressure compensation tubes 25 the tubes 2 flows to the pressure compensation vessel 4 and a partial stream flows from the pressure compensation vessel 4 to the tubes 3 through another part of the pressure compensation tubes 25. As a result, despite the uneven flow through the tubes 2, even flow through the tubes 3 is achieved with very different heating.

This effect occurs particularly clearly according to the invention if the pressure compensation vessel is switched on to the parallel pipe system at such a height that, with 100% load and a heating of a% in a single pipe, the mass flow through this single pipe either by at least depending on the other boundary conditions 0.25. a% or 0.50. a% or 0.75. a% increases.

The cooling of the pipes 2 and 3 is improved and thus the pipe wall temperature is reduced if the pipes have a multi-thread ribs on their inside. This is particularly true in the areas of high heat radiation, e.g. in the area of the burner 11, required. The ribs forming the multi-start thread expediently extend over more than 50% of the length of the tubes 2.

Compared to arrangements with known pressure equalization collectors, there is the possibility that the mass flow density in the solution according to the invention with a pressure equalization vessel and with internally finned tubes in the area of the flame space is less than 1000 kg / m²s at full load due to the good heat transfer properties of internally finned tubes.

Claims (6)

1. Once-through flow steam generator comprising burners (11) for fossil fuels, having a vertical gas flue (1) comprising essentially vertically arranged tubes (2, 3) whose inlet ends are connected to an inlet header (9) and whose outlet ends are connected to an outlet header (12),
   characterised in that

   - from each tube, above the burners (11) and at the same height H, a pressure equalisation tube (25) branches off which is connected to a pressure equalisation vessel (4) and, as a result, through part of the pressure equalisation tubes (25), a branch stream flows from the tubes (2) to the pressure equalisation vessel (4), and through another part of the pressure equalisation tubes (25) a branch stream flows from the pressure equalisation vessel 4 to the tubes (3), and in that

   - the height H is selected such that in the event of an individual tube (2) receiving additional heating between the inlet header (9) and the branching-off point of the pressure equalisation tube (25), compared to the mean value of the heating of all the tubes (2), the mass flow through this individual tube increases.
2. Once-through flow steam generator according to Claim 1,
   characterised in that the tubes (2), over more than 50% of their length, internally have ribs which form a multiple thread.
3. Once-through flow steam generator according to Claim 1 or 2,
   characterised in that the tubes (2, 3) of the gas flue (1) are welded to one another in a gastight manner.
4. Once-through flow steam generator according to one of Claims 1 to 3, characterised in that the height H is selected such that at a nominal load and with an individual tube receiving a% of additional heating between the inlet header (9) and the branching-off point of the pressure equalisation tube (25), compared to the mean value of the heating of all the tubes (2) which corresponds to 100%, the calculated mass flow through this individual tube (2) increases by at least 0.25 · a%.
5. Once-through flow steam generator according to one of Claims 1 to 3, characterised in that the height H is selected such that at a nominal load and with an individual tube (2) receiving a% of additional heating between the inlet header (9) and the branching-off point of the pressure equalisation tube (25), compared to the mean value of the heating of all the tubes (2) which corresponds to 100%, the calculated mass flow through this individual tube (2) increases by at least 0.50 · a%.
6. Once-through flow steam generator according to one of Claims 1 to 3, characterised in that the height H is selected such that at a nominal load and with an individual tube (2) receiving a% of additional heating between the inlet header (9) and the branching-off point of the pressure equalisation tube (25), compared to the mean value of the heating of all the tubes (2) which correspond to 100%, the calculated mass flow through this individual tube (2) increases by at least 0.75 · a%.

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