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原文标题： The steam and water circuits
2009年 4 月 26 日
蒸汽和水循环1
1. 蒸汽的形成和使用
在传统的火电厂中，用来产生蒸汽的热量可能通过燃烧化石燃料获得或可能通过燃气轮机的排气得到。在核电厂这热量可能通过核燃料的放射性衰变获得。在这一章中，我们应研究锅炉和回热锅炉水及蒸汽的循环，以及汽轮机和那些把冷凝蒸汽回收到锅炉的对象。
在本文中提到的那些水在冷凝管内流动的单循环对象，被称为炉或燃烧室。在组合循环对象中管道来自余热锅炉的一部分。在这两种情况下，应用热形成热对流去加热在管中的水，使其达到一个容器中,称为汽包，这是水汽分离的地方。在一些设计的对象中，自然循环过程被强制循环增强，水被给到蒸汽回路中而不是所允许的对流循环。
这本书主要研究有汽包的对象，但还有一种值得一提的对象，在这对象中水由液体变成蒸汽是不使用汽包这个汽水分离器的。这样的单循环锅炉给水和蒸汽控制的原理和本书中提到的具有明显的不同之处，超出了本书的范畴。
1
Power-plant Control and Instrumentation - The Control of Boilers and HRSG Systems
图2.1是锅炉汽包的示意图。在这里，蒸汽是在暴露在由燃烧形成的辐射热的冷凝管中形成的。当然，余热锅炉的电厂没有辐射能是可能的（因为燃烧过程发生在燃气轮机本身），热燃气轮机排气通过混合对流和传导的方式给蒸馏管加热。在这种类型的电厂中有两个或两个以上的蒸汽/水回路（见图2.6）是很平常的，各有其自己汽包，在这种电厂中每个回路的描述如下。
蒸汽离开汽包，进入管道，在管道中通过燃气获得更多的热去加热蒸汽，使蒸汽在流向涡轮机之前达到过热温度。图中的这部分对象中的过热器是由单一的管束组成，但在许多情况下多个阶段的过热器悬浮在气流中，每个从废气中吸收额外的热。锅炉（而非余热锅炉），其中一些管束暴露在由燃烧产生的辐射热中，并因此被称为辐射过热。另一些情况下，在对流阶段，是避开辐射能，但从燃烧产生的热气体中对流吸热。
在废气离开过热器后，进入第三套管束（称为省煤器），在水进入汽包前，几乎所有的余热被提取用来给水预热。
最后尾气中的余热被用来加热空气，加热后的空气在燃烧过程中被用来输送燃料。（此空气预热器是不会显示在图，因为它是空气和天然气的对象，在下一章中讨论）
在图中显示的主要运动项目机械是给水泵，主要用来给系统供水并提供风扇
空气燃烧所需的燃料。（在大多数电厂中这些每一个是重复冗余的）在联合循环电厂中的燃烧空气扇和燃料点火系统被涡轮机的排气取代。
图2.1只显示了与锅炉有关的重大项目。在发电站，蒸汽通过汽轮机后，将液化成水，因此必须使用热交换提取蒸汽中最后残余的热量，充分凝结成液体。在冷凝水返回锅炉之前除去水中的气体。
其余电厂蒸汽/水循环周期的主要项目组成部分现在将简要说明并解释他们的运转过程。
2. 蒸汽轮机
在电厂通过使用汽轮机，把在锅炉中产生的蒸汽的能量，先转换成动能，然后机械旋转和最后转换成电能。蒸汽做功离开汽轮机后进入凝汽器，在凝汽器中蒸汽完全冷凝成水，对冷凝水进行进一步的处理后流向给水泵。在以下的段落中，我们将讲解这一进程（除了转换电能的发电机）。
在汽轮机中，蒸汽是通过喷嘴冲击相继的叶片，其中静叶固定在外壳上，动叶固定在
转轴上（图2.2）。
这样，有热能的蒸汽通过喷嘴后，首先转换成动能，再冲击在动叶上，转变成机械能。当蒸汽离开一组动叶后，进入静叶，通过作用在静叶上的反作用力又冲向另一组动叶，继续做功。由于蒸汽是以这样的方式穿越汽轮机的，在穿越过程中，蒸汽不断的膨胀并牺牲一些热能。目前作用在叶片上使轴旋转的作用力是由每个叶片上的平均作用力组成的。由于每个阶段作用在环上的能量有蒸汽提供，所以作用在下个环上的作用力要比前面环上的小。因此，为了确保在每一个阶段都给轴提供恒定的作用力，只需把后面的叶片做的比前面的更长些。这使汽轮机拥有典型的锥式形状。蒸汽进入汽轮机时叶轮直径最小，离开时叶轮直径最大。在这本书控制图中展示的汽轮机是锥形状的，这通常是汽轮机的象征（图2.3）。
汽轮机可以包括一个或多个阶段，在电厂蒸汽离开高压缸或中压缸后，进入锅炉中的管束进行再热，这锅炉中的管束称之为再热器。再热后的蒸汽离开锅炉，进入汽轮机的最后一个阶段-低压缸。进入低压缸的蒸汽热量比高压缸时少，所以这部分的汽轮机叶片最长。
蒸汽离开汽轮机的低压缸时，几乎用完了所有从锅炉中得到的热量，因此当蒸汽进入冷凝器后完全转换为水，冷凝水可以作为循环水在利用。冷凝器由换热器组成，换热器中循环流着冷水。图2.4显示了简化的完整回路。
输送给冷凝器进行热交换的冷凝水虽然是在闭合回路中，却是流动的。或者，也可能是来自河流或大海它然后返回。在后一种情况下，由于从冷凝器中吸收热量，在排放时必须小心，避免在河流或大海的排放口附近照成热污染。
在闭合回路中，热量通过冷却塔释放到空气中。这样，空气可以通过自然对流或风扇辅助流过冷却塔对水冷却。空气冷却可以避免大量水蒸汽进入大自然，是可取的。因为如果大量水汽排到自然环境后，会影响附近环境的降水，并可能在道路上结冰。
3．凝结水和给水系统
在电厂蒸汽和谁系统形成一个闭环，当水离开冷凝器后，反馈到给水泵，供水给锅炉循环在利用。然而，一些电厂的其他项目也参与者循环，因为水离开冷凝器时温度比较低且含有气体，所以必须去除水中的气体。
在系统刚启动时空气进入给水系统（各种容器是空的），在正常运行时，水中的空气会在工作在低于大气压的循环系统中释放出来，如冷凝器、抽油泵、低压加热器。泄露可能发生在法兰或密封泵的旋转轴处等这些领域。水中夹带着空气有两方面原因：一方面是冷水比热水能夹带更多的氧气和其他可溶解的气体，另一方面低压的循环环节必然是低温的。
锅炉或余热锅炉的给水系统中的剩余氧气是极不可取的，因为它会造成锅炉管道的腐蚀（特别是在焊接，冷挤压部分和表面不连续的地方），大大减少设备的使用寿命。因此必须注意要把水中的氧气去除。
去除溶解氧主要在几个方面，其中过程中最重要的部分是除氧器（见图2.4），位于冷
凝器抽气泵和锅炉给水泵之间。
4 除氧器
除氧器通过使水沸腾和搅拌来去除溶解在水中的气体，这过程称为'脱' 。一种类型的除氧器显示在图2.5。在这，水进入除氧器上方时是和向上的蒸汽混合的，蒸汽是从锅炉或汽轮机中引出的，给水形成伞状水膜，与由下而上的加热蒸汽进行混合式传热，溶解在水中的气体被解析出来。蒸汽给除氧器加压使从水中析出的溶氧及其他气体排到大气中。
要想达到最小腐蚀程度，水中氧的浓度保持低于0.005ppm或更少，虽然除氧器提供了一个有效的方法去除大部分的夹带气体，但它不能使水中的氧浓度低于0.007。因此在水中加入化学净化剂，移除最后的微量氧。
5 化学计量
挥发性氧清除剂，如肼（N2H4）和钠亚硫酸盐（Na2SOs）已用于除氧（虽然肼现在被怀疑致癌物质）。无论它们是什么形式，在一个集中的形式中都会加入化学脱硫，而且不断或定期的冲洗喷射管以防止管道阻塞是很必要的。类似的，将冷却水排入下水道或某专用容器的过程，常用于不断或定期将一部分水从锅炉中排除，同时，为了确保锅炉水保持正确的浓度，会在这个过程加入自动或手动的化学采样。
从控制和仪器仪表的角度看，上述化学品计量业务高度专业化，因此设备通常扮演着水处理工厂的一部分的角色。控制系统（通常为基础的可编程逻辑控制系统（ PLC ）将产生的数据和报警信号，用于连接主要电厂的计算机控制系统，通常称为分散控制系统（ DCS ）。
在水已经被除氧和处理后，水传输到给水泵，通过给水泵把水传输到高压锅炉。
The steam and water circuits
1. Steam generation and use
In a conventional thermal power plant, the heat used for steam generation may be obtained by burning a fossil fuel, or it may be derived from the exhaust of a gas turbine. In a nuclear plant the heat may be derived from the radioactive decay of a nuclear fuel. In this chapter we shall be examining the water and steam circuits of boilers and HRSGs, as well as the steam turbines and the plant that returns the condensed steam to the boiler.
In the type of plant being considered in this book, the water is contained in tubes lining the walls of a chamber which, in the case of a simple-cycle plant, is called the furnace or combustion chamber. In a combined-cycle plant the tubes form part of the HRSG. In either case, the application of the heat causes convection currents to form in the water contained in the tubes, causing it to rise up to a vessel called the drum, in which the steam is separated from the water. In some designs of plant the process of natural circulation is augmented by forced circulation, the water being pumped through the evaporative circuit rather than allowed to circulate by convection.
This book concentrates on plant where a drum is provided, but it is worth mentioning another type of plant where water passes from the liquid to the vapour stage without the use of such a separation vessel. Such 'once-through' boilers require feed-water and steam-temperature control philosophies that differ quite
significantly from those described here, and they are outside the scope of this book.
Figure 2.1 shows a drum boiler in schematic form. Here, the steam generation occurs in banks of tubes that are exposed to the radiant heat of combustion. Of course, with HRSG plant no radiant energy is available (since the combustion process occurs within the gas turbine itself) and the heat of the gas-turbine exhaust is transferred to the evaporator tubes by a mixture of convection and conduction. In this type of plant it is common to have two or more steam/water circuits (see Figure 2.6), each with its own steam drum, and in such plant each of these circuits is as described below.
The steam leaves the drum and enters a bank of tubes where more heat is
taken from the gases and added to the steam, superheating it before it is fed to the turbine. In the diagram this part of the plant, the superheater, comprises a single bank of tubes but in many cases multiple stages of superheater tubes are suspended in the gas stream, each abstracting additional heat from the exhaust gases. In boilers (rather than HRSGs), some of these tube banks are exposed to the radiant heat of combustion and are therefore referred to as the radiant superheater. Others, the convection stages, are shielded from the radiant energy but extract heat from the hot gases of combustion.
After the flue gases have left the superheater they pass over a third set of tubes (called the economiser), where almost all of their remaining heat is extracted to prewarm the water before it enters the drum.
Finally the last of the heat in the gases is used to warm the air that is to be used in the process of burning the fuel. (This air heater is not shown in the diagram since it is part of the air and gas plant which is discussed in the next chapter.)
The major moving items of machinery shown in the diagram are the feed pump, which delivers water to the system, and the fan which provides the air needed for combustion of the fuel (in most plants each of these is duplicated). In a combined-cycle plant the place of the combustion-air fan and the fuel firing system is taken by the gas turbine exhaust.
Figure 2.1 shows only the major items associated with the boiler. In a power-generation station, the steam passes to a turbine after which it has to be
condensed back to water, which necessitates the use of a heat exchanger to extract the last remaining vestiges of heat from the fluid and fully condense it into a liquid. Then, entrained air and gas has to be removed from the condensed fluid before it is returned to the boiler.
The major remaining plant items forming part of the steam/water cycle will now be briefly described and their operations explained.
2. The steam turbine
In plants using a turbine, the energy in the steam generated by the boiler is first converted to kinetic energy, then to mechanical rotation and finally to electrical energy. On leaving the turbine the fluid is fed to a condenser which completes the conversion back to water, which is then passed to further stages of processing before being fed to the feed pumps. In the following paragraphs, we shall examine this process (with the exception of the conversion to electrical energy in the alternator).
In the turbine, the steam is fed via nozzles onto successive rows of blades, of which alternate rows are fixed to the machine casing with the intermediate rows attached to a shaft (Figure 2.2). In this way the heat energy in the steam is converted first to kinetic energy as it enters the machine through nozzles, and then this kinetic energy is converted to mechanical work as it impinges onto the rotating blades. Further work is done by the reaction of the steam leaving these blades when it encounters another set of fixed blades, which in turn redirect it onto yet another set of rotating blades. As the steam travels through the machine in this way it continually expands, giving up some of its energy at each ring of blades. The moment of rotation applied to the shaft at any one ring of blades is the multiple of the force applied to the blades and mean distance of the force. Since each stage of rings abstracts energy from the steam, the force applied at the subsequent stage is less than it was at the preceding ring and, therefore, to ensure
that a constant moment is applied to the shaft at each stage, the length of the blades in all rings after the first is made longer than that of the preceding ring. This gives the turbine its characteristic tapering shape. The steam enters the machine at the set of blades with the smallest diameter and leaves it after the set of blades with the largest diameter. On the control diagrams presented in this book, this is indicated by the usual symbol for a turbine, a rhomboidal shape (Figure 2.3).
Turbines may consist of one or more stages, and in plant which uses reheating the steam exiting the high-pressure or intermediate stage of the machine (the HP or IP stage, respectively) is returned to the boiler for additional heat to be added to it in a bank of tubes called the reheater. The steam leaving this stage of the boiler enters the final stage of the machine, the low pressure (I,P) stage. Because the energy available in the steam is now much less than it was at the HP stage, this
part of the turbine is characterized by extremely long blades.
By the time it leaves the final stage of the turbine, the steam has exhausted almost all of the energy that was added to it in the steam generator, and it is therefore passed to a condenser where it is finally cooled to convert it back to water which can be re-used in the cycle. The condenser comprises a heat exchanger through which cold water is circulated. A simplified representation of the complete circuit is shown in Figure 2.4.
The cooling water that is pumped through the condenser to abstract heat from the condensate may itself be flowing though a closed circuit. Alternatively, it may be drawn from a river or the sea to which it is then returned. In the latter cases, because of the heat received from the condenser, care must be taken to avoid undesirable heating of the river or sea in the vicinity of the discharge (or outfall).
In a closed circuit, the heat is released to the atmosphere in a cooling tower. Within these, the air that is used for cooling the water may circulate through the tower by natural convection, or it may be fan-assisted. It is usually desirable to minimise the formation of a plume since, as well as being very visible, such plumes can cause disturbance to the nearby environment by falling as a fine rain and possibly freezing on roads.
3. The condensate and feed-water system
Inside the plant, the steam and water system forms a closed loop, with the
water leaving the condenser being fed back to the feed pumps for reuse in the boiler. However, certain other items of plant now become involved, because the water leaving the condenser is cold and contains entrained air which must be removed.
Air becomes entrained in the water system at start-up (when the various vessels are initially empty), and it will appear during normal operation when it leaks in at those parts of the cycle which operate below atmospheric pressure, such as the condenser, extraction pumps and low-pressure feed heaters. Leakage can occur in these areas at flanges and at the sealing glands of the rotating shafts of pumps. Air entrainment is aided by two facts: one is that cold water can hold greater amounts of oxygen (and other dissolved gases) than can warm water; and the other is that the low-pressure parts of the cycle must necessarily correspond with the low-temperature phases.
The presence of residual oxygen in the feed-water supply of a boiler or HRSG is highly undesirable, because it will cause corrosion of the boiler pipework (particularly at welds, cold-worked sections and surface discontinuities), greatly reducing the serviceable life-span of the plant. For this reason great attention must be paid to its removal.
Removal of dissolved oxygen is performed in several ways, and an important contributor to this process is the deaerator which is shown in Figure 2.4, located between the condenser extraction pump and the boiler feed-water pump.
4. The deaerator
The deaerator removes dissolved gases by vigorously boiling the water and agitating it, a process referred to as 'stripping'. One type of deaerator is shown in Figure 2.5. In this, the water entering at the top is mixed with steam which is rising upwards. The steam, taken directly from the boiler or from an extraction point on the turbine, heats a stack of metal trays and as the water cascades down past these it mixes with the steam and becomes agitated, releasing the entrained gases. The steam pressurises the deaerator and its contents so that the dissolved gases are vented to the atmosphere.
Minimising corrosion requires the feed-water oxygen concentration to be maintained below 0.005 ppm or less and although the deaerator provides an effective method of removing the bulk of entrained gases it cannot reduce the concentration below about 0.007 ppm. For this reason, scavenging chemicals are added to remove the last traces of oxygen.
5. Chemical dosing
Volatile oxygen scavengers such as hydrazine (N2H4) and sodium sulphite (Na2SOs) have been used for oxygen removal (although hydrazine is now suspected of being carcinogenic). Whatever their form, the chemical scavengers are added in a concentrated form and it is necessary to flush the injection pipes continually or on a periodic basis to prevent plugging. Similarly, blow down, a process of bleeding water to drains or a special vessel, is used to continually or
periodically remove a portion of the water from the boiler, with automatic or manual chemical sampling being used to ensure that the correct concentration is maintained in the boiler water.
From a control and instrumentation viewpoint, the above chemical dosing operations are highly specialised and are therefore usually performed by equipment that is supplied as part of a water-treatment plant package. The control system (often based on a programmable-logic control system (PLC)) will generate data and alarm signals for connection to the main plant computer-control system (frequently referred to as the distributed control system (DCS).)
After the water has been deaerated and treated, it is fed to feed pumps which deliver it back to the boiler at high pressure.
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