Instrument installation and maintenance

First, the role of the instrument; Instrument is the instrument will be measured (known value or information) can be directly observed values ​​or information, and can also be used to control or pass the information to other equipment tools.

Second, the instrument can be divided into two categories according to his work area.
(A), electrical instruments; such as voltage and current meters, power meters, power factor meters, energy meters, frequency meters.
(B), thermal instrumentation; such as, temperature measurement instruments, pressure side connected instrument, flow measurement instrument, middle measurement instrument, a variety of material content measurement instruments, various analytical instruments, various control instruments.

Third, the following brief introduction to the thermal instrument: Thermometer according to different measurement methods can be divided into two categories:
(1) Non-electricity measurement, (ie, methods that do not use electrical signals for measurement, such as pressure gauges, thermometers, etc.). (2) Non-electricity electrical measurement, (ie, the way the physical state is measured by converting the sensor into an electrical signal. Examples include pressure digital displays, temperature indicators, etc.)

Fourth, the electrical measuring instrument can be divided into two parts according to different functions (except integrated instrument). The second part of the position detection instrument (ie, a watch, sensor) is the display instrument (ie, the secondary meter, measurement display instrument).
(A), let us first introduce the detection instrument, sensor: sensor is to feel the change in the measured environment while the change is converted into (functional, linear or non-linear) power output to the display instrument.
1. There are many kinds of sensors and there are many classification methods. Here we divide the sensor into two types according to the working principle of the sensor and the signal transmission method:
(1) Piezoelectric active sensors: They can generate currents or voltages that vary according to the change being measured, such as thermocouples and shaft vibrations.
(2) Physical quantity passive sensor: It can change its own physical quantity (resistance, capacitance magnetic field) according to the measured change, pressure, differential pressure transmitter and remote pressure gauge.
2. Below, we recognize the thermocouple and shaft vibration.
(1) Thermocouples consist of two different, special materials, metal conductors that are combined into a circuit. After the two conductors are combined, they are connected between the junction (called the hot end) and the terminal (called the cold junction). When there is a temperature difference, an induced potential will be generated on the two conductors, and the magnitude of the potential will change in direct proportion to the change in the temperature of the temperature sensing point, so it is called a thermoelectric potential.
(2) The composition of the thermocouple: even wire, insulating sleeve, protective tube, terminal block, junction box.
(3) Thermocouples are divided into armored thermocouples and assembled thermocouples depending on the environment and method of use. Their main difference is that the squid is installed in a metal inverted pipe with insulation. He can't dismantle it, but his airtightness is good and the shock resistance is good. Assembling the thermocouple is to put the coupler into the protective tube after putting it on the insulating sleeve. His benefits are: easy assembly and disassembly, can be changed for easy maintenance, and his measuring range is even larger than that of the mounting (4). Selection: (1) Select the thermocouple according to the work and the environment and temperature to be measured. (2) According to the measured environment to select the material of the wire and the material of the protection tube, the following two lists we can refer to: Table 1, the thermocouple measurement range thermocouple type code indexing number of long-term range °C short time range °C
Platinum crucible 30-platinum crucible 6 WRR B 0-1600 1800
Platinum Rhodium 10-Platinum WRP S 0-1400 1600
Nickel-Chromium-Nickel Silicon WRN K 0-1200 1300
Nickel-Chromium-Copper Nickel WRE E 0-800 900
Table 2. Tolerance range of thermocouple protection tube Protection tube material Long-term use temperature Short-term use temperature This parameter must be installed vertically without strong oxidation. Steel Jade 1600 1800
High aluminum 1300 1600
Cr25Ti (stainless steel) 1000 1100
1Cr18Ni9Ti stainless steel -200-+800 900
Carbon Steel 20# -100—+500 600
(3) According to the external environment to select the structure and installation methods, for example, is it necessary to waterproof, splash, explosion (according to the environment to select the explosion rating), there are even the fixed way, is the valve connection or thread Connected or no connection. The choice of even diameter is based on the vibration of the environment and the degree of oxidation of the couple, as well as the thermal inertia of the couple. Even the accuracy level should be selected according to operating conditions. Note that the use of Kai thermocouples in areas with large vibrations should extend the service life. For example, the circulating water pipes of the steam turbines of the CHP are now all gone. We must pay attention later.
Thermal Inert Thermocouple Protection Tube Diameter Protection Tube Material Time Constant (sec)
¢16 non-metallic protective tube 90-180
Metal 90-180
¢20 non-metallic protective tube is less than 180
Metal is less than 180
¢25 Non-metallic protective tube is less than 300

(4) The choice of even length: if it is on the pipe vertical installation even insert depth (we call him L) to protect the pipe or connector size + the radius of the pipe, if you want to tilt the installation of 45 degrees, its insertion The depth should be the diameter of the straight connector and the outside diameter of the pipe being 0.7. That is, the temperature point of the thermocouple should be inserted into the center of the tested pipe. The temperature of the side temperature sensor: 1) The temperature of the thermocouple. It is the device hot junction, 2) The thermal resistance is the center of its resistance rod, the length of the resistance rod (the platinum resistance is 30-80mm, the copper resistance is 64mm, and the bimetal thermometer is 50mm from the front end). If conditions do not allow, the insertion depth of the couple should be 8-10 times the even diameter. When the length of the protection tube of the bimetal thermometer is not more than 300mm, the insertion depth is not less than 70mm, and if it is greater than 300mm, the insertion depth thereof is not less than 100mm. The total length of the couple (we call him Large L) must ensure that the junction box temperature does not exceed 100°C.

(5), even the installation: (1) choose to install the location, choose to install the location is to avoid the high temperature area and the danger zone, to choose to install and repair the place. When installing on the pipeline, in addition to the occasional temperature point to be inserted into the center of the pipe, attention should also be paid to the liquid pipe. When measuring the temperature of the horizontal liquid pipe, it is necessary to take points downwards to avoid the presence of gas. When measuring high temperature and high pressure gases and liquids (such as steam pipelines and blast furnace furnace checkpoints), also add a protective tube of the same material as the pipeline, with a wall thickness of 2 mm or more (reference pressure). If the diameter of the pipe to be measured is small, the installation is to reverse the direction of the medium flow to the tilt. If it is measured in other objects, such as a furnace or a flue, its insertion depth should not be less than 8-10 times the diameter of its protective tube.
(2) The sealing must be done when installing the thermocouple. The threaded connection must be added with a hardened copper pad. The valve blue must be installed to disturb the asbestos rope, and the refractory soil should be added at high temperature. The winding length is the protection tube. 1/3 of the length.
(3) The connection of thermocouple, thermocouple and display meter connection is to use compensation wire, compensation wire is the cold junction of the thermocouple to guide the way the constant temperature or temperature fluctuation range is small, in fact, the cold end is extended, The thermoelectric performance of the compensation wire is 0-100°C, and the characteristics of the material used for the thermocouple are the same. Note: The temperature of the compensating wire and the junction box can not be too high at normal temperature, and the measured value is the most accurate. From April 10 to April 18, 2005, the temperature of the steam collecting box on the combined heat and power boiler is It stayed around 28 degrees Celsius until the afternoon of 19th. After a rainy day, the temperature of the thermocouple junction box slowly dropped from 54 degrees Celsius to 26 degrees Celsius, which was the signal transmitted to the main control room. Reduced to normal value. Since the data measured in the low temperature winter season has been very stable, we judge that the temperature of the junction box of the thermocouple cannot be too high and it is the most accurate in a normal temperature environment. The thermoelectric potential we mentioned earlier is generated by the temperature difference between the cold junction and the hot junction of the thermocouple. Now the cold junction and the hot junction of the thermocouple are far apart and the temperature of the cold junction is unknown. In the normal measurement, the temperature of the cold junction is at zero degrees Celsius. The measured values ​​of the cold and hot ends are actual values, so we need to eliminate the error due to the influence of the cold junction temperature. There are many ways to eliminate this error. We usually use the cold-end compensation method (ie, internal compensation). The principle is to use a temperature-sensing element to measure the actual temperature at the cold junction, and then add the measured values ​​at the hot and cold ends. The result at this time is actual. Measurements. In other words, the display value is artificially modified. There are temperature measurement elements in the display table and the PLC, and there are parameters for setting the cold junction compensation.
(6) The working principle of the shaft vibration: The shaft vibration sensor is also called the magnetoelectric sensor. It is composed of a special coil and a permanent magnetic core. The sensor is fixed on the measured object. After the magnet hand shakes, the magnetic wire cuts the coil and generates. Micro potential, this signal is transmitted to the display table and it gets the vibration value. Shaft vibration is packaged in one package. The installation location is determined by the equipment. It must be tightly fixed during installation to avoid errors during measurement.
3, we know about passive sensors, thermal resistance and pressure and differential pressure sensors. (1) Thermal resistance: The temperature is measured by the characteristics that the temperature sensing element changes with the temperature and the resistance changes. There are many kinds of thermal resistance, we usually use: WZP series, and WZC series, W, said temperature meter, Z said thermal resistance, P, said platinum resistance, C, said copper resistance. The table below shows the different operating ranges of different thermal resistances at different temperatures.
Category Model Index No. Measuring Range °C 0°C Resistance (Ω)
Platinum thermal resistance WZP Pt10 — 200—+ (glass, ceramic framework)
-200-+420 (mica skeleton) 10
Pt100 100
Copper thermal resistance WZC Cu50 —50—+100 50
Cu100 100
(2) The temperature sensing element and the thermocouple of the thermal resistance are different. The platinum thermal resistance is composed of a platinum wire with a diameter of Φ0.03-Φ0.05 mm interfering with the skeletons of different materials. The copper resistance is made up of an insulated wire with a diameter of φ0.11 mm and is double wound on a plastic drum frame, which is then led to the junction box by wires. Because the temperature sensing element of the thermal resistor is too thin, it cannot be loaded with vibration. For example, the thermal resistance of circulating water pipes in the cogeneration project has now been broken. The thermal resistance element is placed in the center of the pipe during installation.
(3) The installation of thermal resistance is basically the same as that of the galvanic couple. Only different wiring methods, thermal resistance body has two-wire and three-wire system: In fact, the thermal resistance temperature sensor has only two terminals, the three-wire thermal resistance is just on a terminal and a wire lead to the junction box . The external connection of the thermal resistor is three-wire system and four-wire system: the three-wire system is used to subtract line resistance and four-wire system. Its function is also to eliminate the line resistance, but the method is different and more accurate. The wire of the thermal resistance should use a copper wire with a diameter of 1.5mm2 or more, to avoid the influence of excessive resistance on the measurement.

(4) This figure shows the thermal resistance wiring principle: 250uA constant current is applied between A and b. At this time (see three-wire system), voltage will be generated between B and b due to the presence of line resistance, and the total voltage bA will be used. Subtracting this voltage by a factor of two (that is, subtracting the line resistance of A), the exact value is obtained. The four-wire RTD is more accurate. There is a 250uA constant current source between A and B. A and B are the voltage signals of the resistor. This signal has no current and is equivalent to directly measuring the voltage across the resistor. Get the exact value.