过程装备与控制工程专业英语阅读材料翻译

1、四种类型的阀门回转阀毫无疑问，回转阀是人类构想的第一种截断流体的器具。它满足人类使用超过两千年，由于它的结构简单，还会将继续使用。使用的时候通常是通过旋转塞子打开或者关闭四分之一，如图5.13所示。通过旋转使得塞子与阀体的相对位置发生改变，形成互补，达到各自打开和闭合的目的。这种阀门源自早期罗马工人的的智慧，并由有兴趣的工人及工程师流传下来。将锥形塞紧密塞于锥形壳或壳里中做成锥形阀使之保持紧密的想法，但不会太紧密，是其发明者的一个突破。对于敞开的铅制金属管，老练的管子工会环状滚压，折叠成近似圆弧状，并用其将管道纵向封住。回转阀的设计主要是凭经验，而不是靠理论确定。塞子的坡口角度一般是10

2、76;，如果角度过小，会导致塞子和阀体卡住或者难以开闭。闭锁式自由阀虽然，相对于已经使用两千多年的回转阀，闭锁式自由阀算是新东西。但实际上，它是在钳的基础上改进而成的。它最初是由我们所知道的历史上两件大事相关联，螺纹车床的发明（17951800年）及特里维西克、瓦特的蒸汽机应用。直到1768年，由于瓦特发明的循环蒸汽机要求锅炉要变得要求更高，普通的旋转阀已经很快不能满足快速增长的蒸汽压强的要求（相对现在的标准说来只能算接近大气压）。 闭锁式自由阀的外观上存在不同，有的采取球形截流阀及环绕封闭的形式，有的却采取水平或直线形式。这样的截止阀连结点，可以使得主管道流动方向进行九十度的改变，或者导致锅

3、炉的竖向积压，或作为一个可控给水止回阀。其中活门并不是安装在中轴上（由其中轴控制）以进行单向流动。只有外螺纹螺旋阀，是面与面相贴紧的，各部分的几何位置使得它们之间产生固定挤压。楔式闸阀由詹姆斯在1839年发明，它的重要性超过了水管栓旋塞（来源已经不明），历史记载楔式闸阀是由于供水系统阀门的不足启发了内史密斯，希望他能发明出更可靠的东西来代替。这些流体控制设备有着共同的特点，但是，由于都是依靠楔作用而达到面与面贴近的效果，所以会影响到密封问题，这种阀门一个面是平面，另一个面为圆锥状或圆周面，都经过了一定时效处理，在依照详细设计及所需材质合理改进的。下面仅对楔式闸阀进行介绍。如图5.14简图所示的

4、阀门，我们发现两个活门同时关闭，这使得防渗漏有双重保险，这正是这种詹姆斯发明的阀的优点。达到双重封闭需要满足如下条件：（a）两个活门同时关闭并且是平行横切的。（b）由轴提供的轴向力必须能够克服活门受到的表面压力（压强及冲力）以及活门所受阻力。（c）活门必须完整，完全封闭等等。双重阻挡面不是这种类型的阀门绝对必要的，反而，只要能阻挡面能防止流体六国，就可以算严格意义上的“活门”了。无论是何种旋式截止阀或是平行闸阀（弹簧片类型），采用的都是单面关闭形式双密封的活门是相当理想的，但在以下设计条件得到满足时，才可以用单活门来阻挡流体流动。根据安装条件的不同，阀门有时也须详细检测，并与大口径的阀门进行工

5、作对比测试。安全泄压阀安全泄压阀可以防止压力容器内的压力超出限定的压力范围，例如蒸汽锅炉或接受器，而其他压力容器则较少要求使用这种保护。安全和泄压有着同一个意义，尽管“安全”普遍是说阀门防止压力容器爆炸，造成生命危险，而后者则是指压力容器内装非膨胀性物体，消除偶尔的爆发，防止过压，进而防止爆炸。比如，为了防止不必要的阀门过度的压力水管会被称为一个安全阀。忽略它们的类别，这些阀门的设计原则基本是相同的，这些阀门必须独立运作（尽管在某些设计中，在阀门处的测试操作杆有时必须脱离其基座，以便测定效率或者清除污垢），总之，阀门必须独立操作，在超压时要及时打开，在排气后回到正常压力并及时关闭，但基于具体的

6、压力情况，阀门往往不能及时关闭。安全和泄压阀可以分为四种基本类型（如果包括已经逐步淘汰的弹簧阀的话则可以分为五类），近年来有许多由国外引进的新型阀门，我们对这些阀门不做讨论。这四种类型是：（1） 杠杆型（2） 重力型（3） 弹簧型（4） 扭力杆型Reading Material 25Primary Sealing ComponentsThe primary sealing components，designated as the seal seat and seal head，characterize entire design configuration in a mechanical en

7、d face sealThe seal head generally，although not always，rotates with the shaftIn most designs the seal seat is in a stationary position，but sometimes the seal seat is in a rotating position，rotating with the shaft，and the seal head is stationary The geometry of the primary seal components is governed

8、 by a multitude of factorsBoth types of primary seal components have distinct design configurationsThe design geometry of the rotary seal head is usually the result of the degree of dynamic balance to be utilized to control the hydraulic force activitiesThe secondary seal components greatly influenc

9、e the shape of the glandLast，but not least，the selection of the pusher springs is another influential factor in determining the configuration of the seal head ring The geometry of the seal seat ring is determined primarily by the environmental control methods chosento be discussed in detai11Design o

10、f the Seal Head The seal head is the predominant constituent of any mechanical end face sealAs Fig515 and Fig516 indicate，the seal head represents a unit consisting of several parts required to provide the interface functionThe seal head must be designed to achieve optimal flexibility，adequate loadi

11、ng in conjunction with suitable pressure balance，uniformity in the circumferential distribution of the automatic pushing forces，supplied by compatible spring action，and a guarantee of proper positive drive facilities for the seal head ring when subject to rotation with the shaft The secondary seal c

12、omponents play a significant role in head ring geometry and are next to the most decisive factor of hydraulic fluid balancing2Design of the Seal SeatThe mating partner of the primary seal head，the seal seat，can be designed to be equipped with either one or two seal facesThe second seal face is usual

13、ly machined to be used after the initial front face has been worn beyond repairThe seal seat generally functions in a stationary position The seat ring design is governed primarily by the configuration of the gland，which satisfies one or the other of several diversified environmental control methods

14、，such as flushing，quenching，cooling，or a combination of severalThe second significant factor determining the shape of the seal seat ring is the selection of the secondary seal components·These can be of many types as，for example，a cup ring or a Vring or even metallic gaskets of the flexotallic

15、type The factors governing the design of primary seal seats by environmental control methods are discussed in detail in references Seal seat design must incorporate simple components，simplicity of installation，ease of replacement and maintenance，and a secondary sealing component that permits flexibi

16、lity of the seat ring，providing optimal seal effectivenessA pressedin design is not favorable，although this method is found in a variety of design configurations In seal design in which the seat rotates with the shaft，the seat arrangement with regard to the shaft can either be rigid or elastomers in

17、 the form of highly elastic 0-rings with drive pin attachment for positive driveThey provide some flexibility，which allows compensation for irregularities in the shaft motionSome of the conventional drive methods are pressfit，pins，set screws，dents，and many others For rotating primary seat rings a fi

18、xed attachment to the shaft is sometimes preferred to assure positive drive action Seal seats frequently consist of brittle or fragile materials，such as carbon，ceramics, and the likeSuch materials are very sensitive and，therefore，susceptible to stress. particularly tensile stressesThis must be consi

19、dered when seal seats are attached solidly to the rotating shaftIf 0-rings are used，the elastomer must be chosen from a group of materials that provide optimal resiliency and freedom from swellingSwelling O-rings develop tensile stresses in the ring cavity against the mating ring wall，leading finall

20、y to destruction of the seat ringAccurate information on the swelling characteristics of elastomer O-rings is mandatory if failures are to be avoided When using standard gland ring plates without builtin environmental control devices great care must be taken in attaching the gland ring to the housin

21、gIt is important that the gland ring be properly aligned with the housing to secure appropriate location of the seal seat ring in relation to the shaftThe use of adequate gland pilots provides a means for achieving reliable gland centeringGland pilots can be designed in many different ways，particula

22、rlysince they do not create any problem With the solution of alignment of the pump gland ring plates to the housing，other problems in seal assembly are not critical，except for uneven bolting in fastening As is discussed in a later section in connection with seal balancing，perfect alignment of the se

23、al seat with the seal head on the shaft should be the aim of assemblyThe pilot centering device of the gland ring plates facilitates this requirement；it also represents the simplest way at minimum cost It is common experience that the elastomeric O-ring as a secondary sealing component for the stati

24、onary seal seat represents an elegant possibility to compensate for rotational irregularities of the seal head on the shaft3Springs for Face LoadingSteady contact between the rubbing faces of the primary seal rings to ensure proper seal performance is accomplished by using elastic springs，which perm

25、it a steady automatic pushing actionThis activity is further assisted by the hydraulic fluid pressure provided by the pumpSystem fluid pressure and spring selection must be closely balanced against each otherA wide range of springs is available and in practical use 密封分类 物件的分类，无论是技术的还是非技术的，都是为了确定分类，是

26、为了分析他们问题更容易。因此密封可以分为两大类，静态的和动态的。 静态密封由三个密封件组成，它们包括垫片密封、密封胶密封、和直接接触密封。 动态密封可以被细分为两个基本的密封，一种是针对轴的密封，另一种是针对往复密封。在数量上，这两种密封占了工业的绝大多数，而主要的定做密封的设备需要进行特殊考虑，动密封的分类需要用商标来确定设备的不同分类。这些商标必须使用，这是因为没有其他任何来精确区分设备。 根据旋转轴密封和径向密封。界面密封提出了工业密封式这个大家族，主要是密封件和旋转轴之间有着接触。 间隙密封包括了四个不相同种类的家族。与旋转轴成比例的部件。密封元件允许部分泄露来控制流体在外力控制下可以

27、通过间隙。 间隙密封的功能是在被密封的流体上产生一个压降，同时允许在自由运动部件中存在相对运动。间隙密封在机器内部与环境之间产生一个压力差，不同于见面密封，移动的部件之间没有主动接触。将摩擦降低到一个最小值作为一种控制方式来限制流体，当然，必须允许稍微的流动。间隙密封的例子是黏胶密封，速度密封和轴封。铁磁流体是一个例外，磁铁介质充满间隙，在一个磁场或多或少的帮助下，使得磁介质约束在间隙内。建立一个密封机械部件不必要，因为间隙密封没有滑动接触，因此运动部件之间的摩擦或磨损全部被消除。作为轴向密封的设备的机械端面密封。与垫片密封相比，端面密封属于机械密封，使用独特的不同的密封原则，机械密封第一次被

28、大规模使用在汽车工业中，用于发动机冷却液和给水系统。现在使用的国家众多也证明了对一种工业的重要性。在化工、石油化工、公共事业、机关事业，随着密封以及密封配件技术和用于密封配件的结构材料不断地改进，机械密封体现出很大的价值。除了转速的要求提高以及的温度和压力的要求不断增长，现代密封设计者要不断扩大视野和知识。机械端面密封的原理机械密封目前的技术水平已经发展到如下的程度，从十的负五次方托到五千磅每平方英寸的高真空度都能处理。新型材料，尤其是金属波纹管，使得机械密封的使用范围到达以前度不能达到的允许范围内。周转速度的旋转达到50000RPM成为可能机械密封很复杂，包括一系列单一组件的设计，主要通过两

29、个带有贴片的密封环来防止泄露达到密封。重要密封环连在轴上并且随它一起转动，另一个密封环沿着固定环的密封面摩擦，因此这两个密封环相当于轴承一样工作，而且受摩擦力磨损，任何流体泄露时都流过这个表面。因为作用在轴线方向的力使得摩擦接触的限制存在。轴向推力可能是机械力也可能是水压力。在很多设计中两者有相同作用。推力的建立是保持在周部件的连续接触并形成界面。稳定的接触防止了或最小化了摩擦区域的泄露。在固体接触不断摩擦作用产生的热和磨损的存在下。热量会积累并最终导致摩擦区域的破坏。为了防止这种情况，应用具有双重性的润滑剂，首先将摩擦接触产生的热带走进而减少热量的积累，其次，介质上覆盖一个润滑剂微小薄膜从而

30、减少摩擦同时建立紧密的密封。润滑剂流体可以是泵系统流体，也可以是其他流体，可以被输送并与任何其他液体相协调。很薄的润滑膜是我们机械断面密封面密封产生良好密封性的关键，但作用依然是个谜。对一个机械断面来说，轴的可靠性分析是不可能的。这因为每一个机械密封都是在一个纯经验主义基础上设计发展而来的，任何新的密封设计都必须以经验为主进行测试，因为对密封特性而言没有可靠理论基础只能对最后表现进行预测。Reading Material 26Introduction to Process Control ()4. The Control Loop Control in one form or another

31、is an essential part of any chemical engineering operation. In all processes there arises the necessity of keeping flows, pressures, temperatures, compositions, etc., within certain limits for reasons of safety or specification. Such control is most often accomplished simply by measuring the variabl

32、e it is required to control (the controlled variable), comparing this measurement with value at which it is desired to maintain the controlled variable (the desired value or set point) and adjusting some further variable (the manipulated variable) which has a direct effect on the controlled variable

33、 until the desired value is obtained.In order to design such a system to operate not only automatically but efficiently, it is necessary to obtain both the steady-state and dynamic (unsteady state) relationships between the particular variables involved. It can be seen that automatic operation is hi

34、ghly desirable, as manual control would necessitate continuous monitoring of the controlled variable by a human operator. The efficiency of observation of the operator would inevitably fall off with time. Furthermore, fluctuations in the controlled variable may be too rapid and frequent for manual a

35、djustment to suffice. The second section of the mechanism (the controlled) produces an output which is a function of the magnitude of. This is fed to a control valve in the steam line, so that the valve closes when increases and vise versa. This system as shown may be used to counteract fluctuations

36、 in temperature due to extraneous causes such as variations in water flowrate or upstream temperature-termed load changes. It may also be employed to change the water temperature at Y to a mew value by adjustment of the desired value. 5. Block DiagramA control system may be more simply represented i

37、n the form of a block diagram (Fig.6.3). This shows how information flows around the control loop and the function of each constituent section. Each component is represented by a block which denotes the relationship between the variable entering the block and the variable leaving. The symbols used i

38、n Fig.6.3 are widely employed by control engineers although considerable variation does occur in the literature. The control loop is generally made up of five essential parts, i.e. (a) the process, (b)the measuring element, (c) the comparator, (d) the controller, and (e) the final control element. C

39、omparison of Fig. 6.2 and 6.3 shows that the final control element is the control valve on the steam line. The manipulated variable (M v) is the steam flow rate or the flow of heat to the water. The load (U) enters the loop at this point as changes in load will affect the heat entering the system. T

40、he input to the process is the sum effect of both U and Mv. the process, in this case, is simply the movement of any change in temperature from X to Y. The controlled variable (C) is the temperature of the water at Y.It can be seen that the control loop is appropriate as information passes around a

41、closed loop of components. This form of control is called closed-loop variable or feedback (referring to the feed-back of information from the controlled variable to the comparator). The control loop as shown in Figs. 6.2 and 6.3 could equally well consist of electronic or pneumatic components or a

42、mixture of both. The choice of which to use is dictated by consideration of cost, accuracy and safety. Although pneumatic mechanisms were used almost universally for many years, electronic installations are now rapidly gaining in popularity. 6. The Open-loop Control Another type of control is occasi

43、onally employed which does not require the feed-back of information concerning the controlled variable. This is termed feed-forward, predictive or open-loop control. A possible arrangement is shown in Fig.6.4. It is assumed that the inlet water temperature remains constant. The heat input to the wat

44、er is adjusted directly by measurement of the water flow rate. This method has the advantage of anticipating the effect on of variations in water flow rate and that will not have to change form its desired value before corrective action can be taken (as with the feedback arrangement ). The difficult

45、y is that in order to design such a predictive system it is necessary to determine first how the temperature at Y will respond to changes in both water and steam folw rates. This becomes a considerable problem with more complex systems. 过程控制简介（II）4. 控制系统某种形式的控制是化工过程中不可缺少的一部分，应用在所有需要保持工质流动，压力，温度，混合等安

46、全限制及其他详细限制的过程。这样的控制通常是由测量到的变化因素决定（控制变量，变量是与想要保持的控制变量（理想值或设定点）以及进一步的校准变量（操作变量）相比较的，操作变量直接影响控制变量，使控制变量达到理想值。为获得自动而有效的控制系统，有必要得到稳定与变化间的动态关系。总所周知，自动化控制是极其需要的，同样，人工控制在连续监视控制中也很必要，这样说来，效率就不可避免地被拉低了。此外，控制变量的变化过于迅速和频繁使得人工控制难以适应要求。图6.2为一个简单的控制回路，在Y的水温()由热电偶测取，由热电偶反馈到控制装置。控制装置分为两个部分（通常装配于同一单元）。第一部分装置（比较器）的测量值

47、（B）与期望值（R）比较，得出差值()，即=R-B。第二部分装置（控制器）输出一个动作，反馈到蒸汽管道的调节阀，参数升高时关闭，反之打开。该系统可用于消除由于外部因素，如流量变化或者逆流温度负荷变化等引起的温度波动。它也可用来改变Y的水温到达一个期望值。5. 结构简图图6.3为更加简明的控制系统结构框图，表明数据信息在控制回路中的流动情况，以及每个组成部分的功能。各个组成部分由方框表示，表明了输入变量和输出变量间的关系。工程文献上的表示方法不一，但图6.3中的表示符号是广泛使用的。控制回路分为不可缺少的五个部分，也就是：（a）过程，（b）测量元件，（c）比较器，（d）控制器，(e)最终控制器。

48、通过图6.2和6.3的比较可见6.3中最终控制器是指6.2中的管道控制阀。操纵量（Mv）指的是蒸汽流量以及热流量，（u）处负荷的改变会影响到进入系统的热量。输入过程由（Mv）和（u）共同影响。如此说来，任何温度一点X到Y过程温度变化都轻易地影响到过程。其中可控变量（c）是Y中水的温度。可见，当数据消息在闭环设备系统中传递时，控制回路才是合理的。这种控制方式成为循环变量或循环反馈（指的是从可控变量到比较器间的反馈变量）。在图6.2及6.3中所示的控制回路可以由电子装置或者气动装置或者两者混合装置组成，使用何种装置要求考虑到成本，精确度，以及安全性。虽然多年来普遍使用的是气动装置，但现在电子装置也

49、开始迅速普及。6. 开环控制 开环控制是一种不常用的控制方式，就控制变量而言，它不需要反馈信息。这称为前馈控制、超前控制或者开环控制。图6.4所示为一可行开环控制系统，它假定进口水温为定值，供热量由流量计直接调整。这种方法的优点是预期流量变化影响到的数值，这样就可以保持定值，在系统动作之前不需要从它的理想值变动（反馈控制也一样）。设计这样一个控制系统的问题就在于要首先设计一个预测系统，预测当水和蒸气流量变化时，Y中的温度会如何变化，这成为设计更加复杂的控制系统的相当大的难题。Reading Material 27Control Strategies1Feedback ControlThe co

50、ntrol scheme shown in Fig68(a)is referred to as feedback control，also called a feedback control loopThis technique was first applied to control of an industrial process by James Watt about 200 years agoThe application consisted of maintaining constant speed of a steam engine under variable load；this

51、 was a regulatory control applicationIn this scheme the controlled variable is obtained and fed back to the controller SO that it can make a decisionOne must understand the working principles of feedback control to recognize its advantages and disadvantages：the heat exchanger control loop shown in F

52、ig68(a)is presented to foster this understanding If the inlet process temperature increases，thus creating a disturbance，its effect must propagate through the entire heat exchanger before the outlet temperature changesOnce the outlet temperature changes，the signal from the transmitter to the controll

53、er also changesIt is then that the controller becomes aware that it must compensate for the disturbance by changing the steam flowThe controller then signals the valve to close its opening and thus decrease the steam flowFig68(b)shows graphically the effect of the disurbance and the action of the co

54、ntroller. It is interesting to note that at first the outlet temperature increases，because of the increase in inlet temperature，but it then decreases even below set point and continues to oscillate around set point until the temperature finally stabilizesThis oscillatory response shows that the oper

55、ation of a feedback control system is essentially a trial-and-error operationThat is，when the controller notices that the outlet temperature has increased above the set point，it signals the valve to close，but the closure is more than requiredTherefore，the outlet temperature is brought down below the

56、 set pointNoticing this，the controller signals the valve to open again somewhat to bring the temperature back upThis trial and error continued until the temperature reached and stayed at set point The advantage of feedback control is that it is a very simple technique，as shown in Fig68(a)，that compe

57、nsates for all disturbancesAny disturbance will affect the controlled variable，and once this variable deviates from set point，the controller will change its output to return it to set pointThe feedback control loop does not know，nor does it care，which disturbance enters the processIt tries only to maintain the controlled variable at set point and in so doing compensates for a11 disturbancesThe disadvantage of feedback control is that it can compensate for a disturbance only after the controlled variable has deviated from set pointThat is，the distur