仪表培训教材

仪表培训教材１目录第一章绪论···························································3第一节化工测量仪表···············································3第二节化工仪表的主要品质·········································3第二章压力测量仪表···················································4第一节压力测量的基本概念·········································4第二节压力测量仪表的分类·········································6第三节压力表的安装、使用和维护···································7第三章流量测量仪表···················································8第一节流量测量的基本概念·········································8第二节流量仪表的分类·············································9第三节差压式流量计···············································9第四节差压变送器·················································12第五节漩涡流量计·················································16第六节质量流量计·················································18第七节阿牛巴流量计···············································20第八节电磁流量计·················································21第九节面积流量计·················································24第四章物位测量仪表···················································26第一节物位测量仪表的基本概念·····································26第二节玻璃液位计·················································27第三节浮力式液位计···············································28第四节差压式液位计···············································29第五节用法兰式差压式液位变送器测量液位···························30第六节放射性液位计···············································30第五章温度测量仪表···················································32第一节温度测量的基本概念·········································32第二节温标·······················································32第三节测温仪表的分类·············································33第四节热电阻温度计···············································34第五节热电偶温度计···············································36第六节双金属温度计···············································37第七节温度变送器·················································38第八节测温元件的安装·············································39第六章气动调节阀·····················································40第一节气动执行机构···············································41第二节气动调节阀·················································44２第三节气动执行器的特点···········································48第四节阀门定位器和薄膜调节阀的其他辅助装置·······················51第七章控制系统简介···················································53第一节概述·······················································53第二节工艺控制系统···············································53第三节停车和联锁·················································54第四节控制方案简介···············································54第八章分析仪表简介···················································64第一节概述·······················································64第二节PH分析仪··················································65第三节电导率分析仪···············································68第四节CO/CO2分析仪···············································69第五节可燃及有毒气体报警器·······································69３第一章绪论第一节化工测量仪表应用DCS进行过程控制时，需要测量和控制各有关的工艺参数，需要选用各类检测仪表将工艺生产参数转换成DCS系统能识别的并能代表被测量的信号，最后通过执行机构调节工艺参数的变化。检测仪表包括各种参数的测量和变送仪表，仪表所测参数的不同，可分成压力测量仪表，流量测量仪表，物位测量仪表，温度测量仪表，成分分析仪表等等。被测量的参数直接或间接转换成DCS系统可接受的电模拟信号或脉冲信号。仪表测量信号：电动仪表为4-20mADC；气动仪表为0.02-0.1Mpa第二节化工仪表的主要品质指标任何一种产品都要有它的质量标准，仪表也不能例外，来衡量仪表质量好坏的标准，通常叫做品质指标。在化工生产中，操作人员住往都是根据仪表显示的数值进行工艺操作的，但仪表指示的数值是不是真实的数值呢？实际上，从被测工艺参数的原始变化，直到最后稳定时的读数，是经过各种不同的仪表元件的转换和传递。由于这些元件的性能和制造精度等的影响，在一般情况下，仪表读数和真实的数值，往往是不一样的，因此，仪表读数和真实数值之间就存在著一个差值，我们把这个差值叫做测量误差。可是，其实数值往往是很难测量出来的，因此，人们就把与真实数比较更接近，更可靠的标准仪表的读数当作真实数值。把用于工业测量的仪表和标准仪表对同一参数进行比较，其读数的差值就叫做仪表的测量误差，误差数值越小，便说明仪表指示值和真实值越近，仪表测量的结果越准确，仪表的品质指标(或质量)就越好。化工仪表在保证可靠的前提下，主要的品质指标有如下几种。一．基本误差和仪表的准确度基本误差是指在规定的使用条件下，仪表的最大测量误差与它的测量范围的百分比。例如，有一台压力表，测量范围为0—10.0Mpa，它的最大测量误差(或绝对误差)为0.15Mpa，那么它的基本误差：0.15／10X100％＝1.5％。从仪表的基本误差可以看出仪表的精度等级，上例的压力表，其精度级就是1.5级，或称1.5级表。为了便于表示精度等级，习惯上就是基本误差去掉(％)号的数值。仪表的精度级以规定的符号形式标志在仪表的标尺上，如“0.5”“1.0”等。目前，我国生产的仪表精度级有0.005；0.02；0.05；0.1；0.2；0.35；0.5；1.0；1.5；2.5；4.0；等。现场用的电动仪表精度大多为0.5级，而气动变送器多为1.0级。二．仪表的变差在外界条件不变的情况下，同一台仪表对同一台仪表进行正、反行程(即以最小到最大，再由最大到最小)测量时，其所得到的仪表指示值是不相等的，对同一点所测得的正、４反行程的两读数之差就是该点的变差(也可称回差)。仪表变差不应越过仪表最大基本允许误差的绝对值。变差小，表示仪表的恒定度好，工作可靠。例如；有一台0—1.0MpaG，1.6级的压力表，在校验0.5MpaG这一刻度点时，压力作正行程校验，仪表指针读数是0.505MpaG，压力作反行程校验，仪表指针读数是0.495MpaG，其变差为：0.505-0.495=0.01MpaG，现已知此表的精度为1.6级，量程为0-1.0MpaG，则其最大基本允许误差为1.0~1.5％；0.015MpaG。因此，这仪表的变量小于0.015MpaG，符合变差规定的要求。三．仪表的灵敏度仪表的灵敏度是指仪表的灵敏程度。例如有两台压力表，都通入同样微小的压力信号，其中一个表的指示针不动，另一个表的指针动作，也就是说后一台表要比前一台表灵敏。从而可知使仪表指针开始动作的信号越小，仪表的灵敏度就越高。四．仪表的稳定性仪表的稳定性是指仪表的稳定程度。在同样的条件下，对同一被测量值经仪表多次测量，各次测量结果相差越小，仪表的稳定性就越好。第二章压力测量仪表在化工生产过程中，压力不仅影响生产过程的平衡关系和反应速率，而且又影响系统物料平衡的重要参数，在工业生产过程中，有的需要比大气压力高很多的高压，如高压聚乙烯要在150MPAG的高压下进行聚合，而有的则需要在比大气压力低很多的负压下进行。如炼油厂的减压蒸馏就是这样。PTA化工厂的高压蒸汽压力为8.0MPAG，氧气进料压力约为9.0MPAG，压力测量如此广泛，操作人员应当严格遵守各种压力测量仪表的使用规则，加强日常维护，任何疏忽和大意。都有可能招致巨大的危害和损失，达不到优质，高产，低消耗和安全生产的目的。第一节压力测量的基本概念一．压力的定义在工业生产中，通常所称的压力，就是指均匀而垂直地作用在单位面积上的力，它的大小是由受力面积和垂直作用力的大小决定的。用数学式表示为：P＝F/S式中P为压力F为垂直作用力S为受力面积二．压力的单位在工程技术中，我国采用的是国际单位制（SI）。计算压力的单位是帕（Pa），1Pa就是1牛顿（N）的力垂直而均匀地作用在1平方米（M2）面积上所产生的压力，其表示为N/m2(牛顿/平方米)，压力单位除采用帕外，还可以采用千帕，和兆帕，它们之间的换算关系为：1MPA＝103KPA＝106PA５由于多年的习惯，工程上仍多采用工程大气压。为了便于使用中的互相换算，现将几种常用的测压单位之间的换算关系列表于2-1。表2-1压力单位工程大气压Kg/cm2毫米汞柱mmHg毫米水柱mmH2O物理大气压a