In the process of industrial production, the instrument plays an important role in detecting, displaying, recording or controlling the process parameters. The detection of the production process is the basic means to understand and control the industrial production. Only by accurately understanding and controlling the whole process at any time can we ensure the smooth production process and produce qualified products with high productivity and small consumption. Therefore, although the appearance is not impressive, it is the most indispensable! This paper introduces the selection of automation instrument, temperature instrument, pressure instrument, flow instrument and level instrument. The content is very comprehensive!

one

General principles of automatic instrument selection

The general principles for selection of test instruments (components) and control valves are as follows:

1. Process conditions

Temperature, pressure, flow rate, viscosity, corrosiveness, toxicity, pulsation and other factors in the process are the main conditions to determine the selection of instruments, which are related to the rationality of instrument selection, the service life of instruments, and the fire prevention, explosion-proof, security and other issues in the workshop.

2. Operational importance

The importance of the parameters of each detection point in operation is the basis for the selection of the functions of the instrument, such as indication, recording, accumulation, alarm, control and remote control. Generally speaking, the variables that have little influence on the process but need to be monitored frequently can be selected as indicator type; For the important variables that need to know the change trend frequently, record type should be selected; Some variables that have great influence on the process and need to be monitored at any time should be controlled; For the variables related to material balance and power consumption and requiring measurement or economic accounting, it is appropriate to set up an integral calculation; For some variables that may affect production or safety, alarm should be set.

3. Economy and unity

The selection of instruments also depends on the scale of investment. On the premise of meeting the requirements of process and automatic control, necessary economic accounting should be carried out to obtain the appropriate performance / price ratio.

In order to facilitate the maintenance and management of the instrument, attention should be paid to the unity of the instrument when selecting the type. Try to choose the same series, the same specification and the same manufacturer's products.

4. Use and supply of instruments

The selected instruments should be mature products with reliable performance proved by field use; At the same time, it should be noted that the selected instruments should be in sufficient supply, which will not affect the construction progress of the project.

two

Selection of temperature instrument

<1> General principles

1 unit and scale (scale)

The scale (scale) unit of temperature instrument is the temperature in centigrade (℃).

2. Insertion length of detection element

The selection of insertion length should be based on the principle that the detection (measuring) element is inserted into the representative position where the temperature change of the measured medium is sensitive. But in general, in order to facilitate the exchange, the first to second gear length is often selected for the whole device.

When installing on flue, furnace and masonry equipment with thermal insulation material, it should be selected according to the actual needs.

The material of protective sleeve for detecting (measuring) element shall not be lower than that of equipment or pipeline. If the protective sleeve of the finalized product is too thin or not corrosion-resistant (such as armored thermocouple), additional protective sleeve shall be added.

The temperature instrument, temperature switch, temperature detecting (measuring) element and transmitter with local electric contact installed in flammable and explosive places shall be explosion-proof type.

<2> Selection of local temperature instrument

1 accuracy level

General industrial thermometer: select grade 1.5 or grade 1.

Thermometer for precision measurement and laboratory use: grade 0.5 or 0.25 should be selected.

2 measurement range

The maximum measurement value is not more than 90% of the upper limit of the measuring range of the instrument, and the normal measurement value is about 1 / 2 of the upper limit of the measuring range of the instrument.

The measured value of the pressure thermometer should be between 1 / 2 and 3 / 4 of the upper limit of the measuring range of the instrument.

3 bimetal thermometer

When meeting the requirements of measurement range, working pressure and accuracy, it should be preferred.

The case diameter is generally selected φ In the place with poor lighting conditions, high position and long observation distance, it should be selected φ 150mm。

The connection mode between the instrument shell and the protection pipe should generally be universal type, or axial or radial type according to the principle of convenient observation.

4 pressure thermometer

It is suitable for local or local display with low temperature below - 80 ℃, no close observation, vibration and low accuracy requirement.

5 glass thermometer

It is only used for special occasions with high measurement accuracy, small vibration, no mechanical damage and convenient observation. However, it is not suitable to use glass mercury thermometer because of mercury harm.

6 base instrument

For local or local installation of measurement, control (regulation) instruments, base type temperature instruments should be selected.

7 temperature switch

It is suitable for temperature measurement where contact signal output is required.

<3> Selection of centralized temperature instrument

1 detection element

(1) According to the temperature measurement range, select the thermocouple, thermistor or thermistor with the corresponding index.

(2) Thermocouples are suitable for general applications. Thermal resistance is suitable for vibration free applications. The thermistor is suitable for the situation where the measuring speed is required to be fast.

(3) According to the response speed requirements of the measuring object, the following time constant detection (measuring) elements can be selected:

Thermocouple: 600s, 100s and 20s;

Thermal resistance: 90 ~ 180s, 30 ~ 90s, 10 ~ 30s and < 10s;

Thermistor: < 1s.

(4) According to the environmental conditions, the junction box shall be selected according to the following principles:

Common type: places with good conditions;

Splash proof, waterproof: wet or open place;

Flameproof: inflammable and explosive places;

Socket type: only suitable for special occasions.

(5) In general, the threaded connection mode can be selected, and the flange connection mode shall be selected for the following occasions:

Installation on equipment, lining pipes and non-ferrous metal pipes;

Crystallization, scarring, plugging and strong corrosive medium;

Flammable, explosive and highly toxic media.

(6) Thermocouple and thermal resistance used in special occasions:

When the temperature is higher than 870 ℃ and the hydrogen content is more than 5%, the tungsten rhenium thermocouple or gas blowing thermocouple is selected;

The surface or armored thermocouple and thermal resistance shall be selected for the temperature of equipment, pipe outer wall and swivel surface;

The medium containing hard solid particles is wear-resistant thermocouple;

When multi-point temperature measurement is required in the same detection (measurement) element protection sleeve, multi-point (Branch) thermocouple is selected;

In order to save special protective tube materials (such as tantalum), improve the response speed or require the detection (measuring) element bending installation, armored thermocouple can be selected.

2 transmitter

The measurement or control system matched with the receiving standard signal display instrument is transmitter.

Under the condition of meeting the design requirements, it is recommended to select the transmitter with integration of measurement and transmission.

3 display instrument

(1) General indicator should be selected for single point display, digital indicator should be selected for multi-point display, and general recorder should be selected for historical data.

(2) Indicator or recorder with contact signal output should be selected for signal alarm system.

(3) Medium sized recorder (such as 30 point recorder) should be selected for multi-point recording.

4. Selection of accessory equipment

(1) When multiple points share one display instrument, reliable switch should be selected.

(2) Thermocouple is used to measure the temperature below 1600 ℃. When the change of cold end temperature makes the measurement system unable to meet the accuracy requirements, and the matching display instrument has no cold end temperature automatic compensation function, the cold end temperature automatic compensator should be selected.

(3) Compensating conductor

a. According to the number of thermocouples, index number and service environment conditions, compensation wires or cables meeting the requirements shall be selected.

b. Different levels of compensation wires or cables shall be selected according to the ambient temperature

- 20 ～ + 100 ℃ select ordinary grade;

Heat resistant grade is selected from - 40 ℃ to + 250 ℃.

c. Shielding compensation wires or cables should be used in places with power-off heating or strong current or magnetic field.

d. The cross-sectional area of the compensation conductor shall be determined according to the reciprocating resistance value of its laying length and the external resistance allowed to be input by the supporting display instrument, transmitter or computer interface.

three

Selection of pressure instrument

<1> Selection of pressure gauge

1. Select according to the use environment and the nature of the measuring medium

(1) In the atmosphere corrosive, dust more and easy to spray liquid and other harsh environment, it is appropriate to choose closed all plastic pressure gauge.

(2) Acid resistant pressure gauge, ammonia pressure gauge or stainless steel diaphragm pressure gauge shall be selected for dilute nitric acid, acetic acid, ammonia and other general corrosive media.

(3) Thin hydrochloric acid, hydrochloric acid gas, heavy oil and similar media with strong corrosivity, solid particles, viscous liquid, etc. should choose diaphragm pressure gauge or diaphragm pressure gauge. The material of the diaphragm or diaphragm must be selected according to the characteristics of the measuring medium.

(4) Crystallization, scab and high viscosity medium should choose diaphragm pressure gauge.

(5) In the case of strong mechanical vibration, shock resistant pressure gauge or marine pressure gauge should be selected.

(6) In flammable and explosive occasions, if electric contact signal is needed, explosion-proof electric contact pressure gauge shall be selected.

(7) Special pressure gauges shall be used for the following measuring media:

Gas ammonia, liquid ammonia: ammonia pressure gauge, vacuum gauge, pressure vacuum gauge;

Oxygen: oxygen pressure gauge;

Hydrogen: hydrogen pressure gauge;

Chlorine: chlorine resistant pressure gauge, pressure vacuum gauge;

Acetylene: acetylene pressure gauge;

Hydrogen sulfide: sulfur resistant pressure gauge;

Lye: alkali resistant pressure gauge, pressure vacuum gauge.

2 selection of accuracy level

(1) The pressure gauge, diaphragm case pressure gauge and diaphragm pressure gauge for general measurement should be grade 1.5 or 2.5.

(2) Pressure gauges for precision measurement and calibration should be grade 0.4, grade 0.25 or grade 0.16.

3. Selection of external dimensions

(1) Pressure gauges installed on pipes and equipment with nominal diameter of φ 100 mm or φ 150mm。

(2) The nominal diameter of the pressure gauge installed on the instrument pneumatic pipeline and its auxiliary equipment is φ 60mm。

(3) The nominal diameter of the pressure gauge installed in low illumination, high position and difficult to observe indication is φ 200 mm or φ 250mm。

4 selection of measurement range

(1) When measuring the stable pressure, the normal operating pressure should be 2 / 3 ~ 1 / 3 of the upper limit of the measuring range of the instrument.

(2) When measuring the pulsating pressure (such as the pressure at the outlet of pumps, compressors and fans), the normal operating pressure should be 1 / 2 ~ 1 / 3 of the upper limit of the measuring range of the instrument.

(3) When measuring the high and medium pressure (more than 4MPa), the normal operating pressure should not exceed 1 / 2 of the upper limit of the measuring range of the instrument.

5 units and scale (scale)

(1) All pressure instruments use legal measurement units. They are: PA, kPa and MPa.

(2) For foreign design projects and imported instruments, international general standards or corresponding national standards can be adopted.

<2> Selection of transmitter and sensor

(1) When transmitting with standard signal (4 ~ 20mA), transmitter should be selected.

(2) Pneumatic transmitter or explosion-proof electric transmitter shall be selected for inflammable and explosive occasions.

(3) Flange type transmitter shall be selected for crystallization, scabbing, plugging, viscous and corrosive media. Materials in direct contact with the medium must be selected according to the characteristics of the medium.

(4) When the environment is good and the measurement accuracy and reliability are not high, resistance type, inductance type remote pressure gauge or hall pressure transmitter can be selected.

(5) When measuring micro pressure (less than 500pa), micro differential pressure transmitter can be selected.

<3> Selection of installation accessories

(1) When measuring water vapor and medium with temperature higher than 60 ℃, spiral or U-bend pipe should be selected.

(2) When measuring liquefiable gas, if the pressure point is higher than the instrument, the separator should be selected.

(3) When measuring the gas containing dust, the dust collector should be selected.

(4) When measuring fluctuating pressure, damper or buffer should be selected.

(5) When the ambient temperature is close to or lower than the freezing point or freezing point of the measuring medium, insulation or heat tracing measures should be taken.

(6) Instrument protection (temperature) box shall be selected for the following occasions.

Pressure switches and transmitters for outdoor installation.

The pressure switch and transmitter installed in the workshop with serious atmospheric corrosion, dust and other harmful substances.

four

Selection of flow meter

<1> General principles

1 scale selection

The instrument scale should meet the requirements of the instrument scale module. When the scale reading is not an integer, it can also be selected according to the integer for the convenience of reading conversion.

(1) Square root scale range

The maximum flow rate shall not exceed 95% of full scale;

The normal flow rate is 70% - 85% of full scale;

The minimum flow rate shall not be less than 30% of full scale.

(2) Linear scale range

The maximum flow rate shall not exceed 90% of full scale;

The normal flow rate is 50% - 70% of full scale;

The minimum flow rate shall not be less than 10% of full scale.

2 instrument accuracy

The flowmeter used for energy measurement shall comply with the provisions of general rules for the allocations and management of energy measuring instruments in enterprises (Trial).

(1) It is used to measure the settlement of fuel in and out of the plant, ± 0.1％；

(2) It is used to measure the technical and economic analysis of workshop team and process, ± 0.5％～2％；

(3) For industrial and civil water measurement, ± 2.5％；

(4) For steam metering including superheated steam and saturated steam, ± 2.5％；

(5) It is used for the measurement of natural gas, gas and domestic gas, ± 2.0％；

(6) It is used to measure oil for key energy consuming equipment and process control, ± 1.5％；

(7) Measurement of other energetic working fluids (such as compressed air, oxygen, nitrogen, hydrogen, water, etc.) for process control, ± 2％。

3 flow unit

The volume flow is m3 / h, L / h;

The mass flow rate is kg / h, t / h;

The volume flow of gas under standard state is Nm3 / h (0 ℃, 0.1013mpa).

<2> Selection of flow measuring instruments for general fluid, liquid and steam

1 differential pressure flowmeter

(1) Throttling device

① Standard throttling device

Standard throttling devices (standard orifice plate and standard nozzle) should be selected for general fluid flow measurement. The selection of standard throttling device must comply with the provisions of gb2624-8l or international standard ISO 5167-1980. If there are new national standards, the new regulations shall be implemented.

② Non standard throttling device

Venturi tube can be used if the following conditions are met:

Accurate measurement under low pressure loss is required;

The measured medium is clean gas and liquid;

The inner diameter of the pipe is in the range of 100-800mm;

The fluid pressure is within 1.0MPa.

If the following conditions are met, double orifice plate can be selected:

The measured medium is clean gas and liquid;

The Reynolds number is greater than or equal to 3000 and less than or equal to 300000.

If the following conditions are met, 1 / 4 round nozzle can be selected;

The measured medium is clean gas and liquid;

The Reynolds number is more than 200 and less than 100000.

Round hole plate can be selected if the following conditions are met:

Dirty media (such as blast furnace gas, mud, etc.) in which the measured media may produce sediment before and after the orifice plate;

There must be horizontal or inclined pipes.

③ Selection of pressure taking mode

It should be considered that the whole project should adopt a unified pressure taking method as far as possible.

Generally, corner connection or flange connection is adopted.

According to the conditions of use and measurement