Temperature Measurement

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   May 16, 2005
• At one time, operators controlled
  processes manually, it required the
  technician to open and close valves in
  piping lineups manually, which has not
  adopt to industrial produce
• Now an instrument or computer can
  control the opening, closing, and
  positioning of valves; start and stop
  equipment, measure process variables,
  and respond automatically
• In a control center a single process
  technician to monitor and control large,
  complex process networks

计                过
算                程
    打印机          器
    Temperature Measurement
• Temperature is a important parameter in
  the process industry, common thermometer
  cannot apply
• Local temperature indicators usually contain
  a bimetallic strip that differentially expands
  with increasing temperature, causing a
  deflection that is correlated with
• The most common temperature-measuring
  devices used in the chemical-processing
  industry are thermoelectric: resistive
  temperature detectors(RTDs) and
      Temperature Measurement
• RTD composed of a small
  platinum(铂) or nickel(镍)
  wire encased in a rugged
  metal tube, the electrical
  resistance in the wire
  changes with temperature,
  electronic circuit is formed
  and directed to a indicator
• Thermocouples are
  composed of two different
  types of metal, J-type and
  K-type thermocouples are
  the most common
 Temperature Measurement

Temperature   transmitter   controller

              valve         transducer
       Pressure Measurement
• Pressure is an important variable
• Pressure is measured in psi(pounds pre
  square inch), pa(N/m2)
• There are three commonly used pressure
  scales: gauge, absolute, and vacuum
• There are three basic types of manometer:
  U-tube, well manometer(单管压力计),
• Primary pressure elements designed to
  sense changes in pressure and convert it to
  mechanical motion, such as bellows
  pressure and bourndon tube
Pressure Measurement
         Pressure Measurement
• A pressure transmitter uses a pressure
  element to sense pressure and sends a signal
  to a controller or recorder
• Pressure control loop includes a pressure
  transmitter, controller, transducer, and control
      Fluid Flow Measurement
• Continuous chemical
  reactions require
  precise measurements
  to ensure that all of the
  reactant are combined
  in the proper
  proportions to form the
  final products
• Flow transmitters use
  differential pressure to
  measure flow rate, such
  as orifice plates, venturi
  tubes, or flow nozzles
      Fluid Flow Measurement
• There are two types of
  positive displacement
  meters: nutating disc and
  oval gear
• A rotameter is composed
  of a tapered tube, scale,
  ball or float, and inlet and
     Fluid Flow Measurement
• Weir flow-measuring devices, when a weir is
  placed into a process stream,it creates a
  restriction that forces the level to build
• Flume is a narrow, sloping pass that funnels
  flow, which are used for higher flows
• Flow control loop is like pressure or
  temperature control loop
         Level Measurement
• Level measurement is also important
• Such as vessel tap, the lowermost tap
  represents zero level, and the uppermost
  tap is 100%
• Lever-measurement devices can be
  classified as direct or indirect
• Direct type is as follows
• Indirect type is a pressure-sensitive
  instrument that measures hydrostatic
  pressure in open vessels and converts it to
  a level indication
• A bubbler system is a level-measurement
  device that forces air through a tube to
  measure the liquid’s resistance
Level Measurement
  Basic Elements of a Control Loop
 • Instruments include a transmitter coupled
   with a sensing device or primary element,a
   controller, a transducer, and a control valve
 • The basic elements of a control loop are
 • Measurement device----primary elements
   and sensors
Flow       Orifice plate,flow nozzle, DP cell
           DP cell
Level      Float, displacer, DP cell  DP cell
Pressure Helix, spiral, bellows,      DP cell
           bourdon tube, DP cell
tempera Capillary tubing, thermal Thermocou
ture       and resistance bulb        ple, RTD
 Basic Elements of a Control Loop
• Transmitter----a device designed to convert
  a measurement into a signal, this signal will
  be transmitted to another instrument
• Controller----a device designed to compare
  a signal with a set point and transmit a
  signal to final control element
• Transducer----a device designed to convert
  an sir signal to an electric signal to a
  pneumatic signal, sometimes referred to as
  an I to P or as a converter
• Final control element----the part of a
  control loop (control valve, damper,
  governor for speed control) that actually
  makes the change to the process the final
  control element is governed by a controller
  Basic Elements of a Control Loop
• Controllers are mounted between 400(closed
  loop) and 1000(open loop) feet from the
• The signal is proportional
• Standard output signals are 4~20mA, or 1~5V
• Pneumatic transmitters require a 20~30 psig
  air supply in order to run the standard 3~15
  psig output
• Smart transmitters have an internal diagnostic
  system that warns the operator if a problem is
  about to occur,advantageous features are
  speed, reliability, internal diagnostics, strong
  digital signal, remote calibration
 Basic Elements of a Control Loop
• Controllers and controller modes
• Controller is to receive a signal from a
  transmitter, compare this signal with a set
  point, and adjust the process to stay within
  the range of the set point
• There are three basic designs: pneumatic,
  electronic, and computer
• Compare with pneumatic, electronic mode
  are the reduction of lag time, low
  installation expense, ease of installation, of
  course computer mode more advance
• Controllers can be operated in manual,
  automatic, remote control
 Basic Elements of a Control Loop
• The proportional band describes the scaling
  factor used to take a controller from
• Controller modes are designated as
  proportional(P), proportional plus
  integral(PI), proportional derivative(PD),
• Proportional control is used to provide
  gain(增益,放大倍数),PI works best where
  large changes occur slowly, PD works best
  when frequent small changes are required,
  PID is applied where massive rapid load
  changes occur, it reduces swinging
      Final Control Elements
• Final control elements typically are
  automatic valves,but motors or other
  electric devices can be used.
• Automates valves open or close to regulate
  the process
• An actuator is the device that automates a
  valve, it come in three basic designs:
  pneumatic, electric, and hydraulic
• Pneumatic actuators convert air pressure to
  mechanical , which is the most common
• Electrically operated actuators convert
  electricity to mechanical
• Hydraulically operated actuators convert
  liquid pressure to mechanical
      Final Control Elements
• The most common type of automatic valve
  is a globe valve because of its versatile,
  on/off or throttling feature
• An interlock is a device designed to prevent
  damage to equipment and personnel by
  stopping or preventing the start of certain
• There are two types of interlocks: software
  and hardwire, hardwire interlocks cannot be
  bypassed, they must be satisfied before the
  process they are part of can take place
Virtual Instrument
Virtual Instrument
       Virtual Instrument
The hardware is consisted of sensor,
  computer and Data Acquisition Card
  etc. The software is write by
现场信号,        信号调理(
传感器执行        放大、滤波
机构           、隔离、激

             数据采集板            计算机数据
             、卡               采集控制软
             数据采集盒              件
Virtual Instrument
Virtual Instrument
Virtual Instrument

关定时器 T0      报警处理

数据采集         通道显示

数字滤波         参数计算

标度变换         控制量输出

       Industrial Control System

 CPU           内   模拟量输入通道          测量变送
存储器                模拟量输出通道          执行机构
               部                           控
人—机接口              数字量输入通道          电气开关
               总                           象

系统支持板              数字量输出通道          电气开关

       计算机系统                 生产过程
   Industrial Control System
综合信息管理级   管理       网间
          微机       链接


          工程师    操作员     监控      网间
          操作站    操作站     计算机     链接

分散过程控制级     现场     PLC    智能       其他
           工作站    控制器     调节器      装置

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