Pressure transmitter application and selection

Pressure transmitter application and selection:

I. Overview

In all kinds of meters, the application of the transmitter is the most extensive and common, and the transmitter is roughly divided into pressure.

Transmitters and differential pressure transmitters. Transmitters are often used to measure pressure, differential pressure, vacuum, liquid level,

Flow and density. Transmitter has two-wire and four-wire system, especially two-wire transmitters;

There are intelligent and non-intelligent points, smart transmitters gradually; pneumatic and electric points, electric change

Most of the feeders; In addition, according to the application of intrinsically safe (intrinsically safe) and flameproof type; According to application conditions, the main types of transmitter are as follows: low (micro) pressure / low (micro) differential pressure Transmitters; medium / medium differential pressure transmitters; high pressure easy crystallization / pressure, differential pressure transmitters. Selection normally considered by the transmitter according to installation conditions and other aspects of the medium, the environmental conditions, the performance of the instrument, and that the economy. There are two kinds of direct measurement and indirect measurement in practical applications;

Use with a measurement process which, like process control and chain apparatus. Common transmitter ordinary pressure transmitters, differential pressure transmitter, the transmitter Portland single, double transmitter Portland, Portland plug-like transmitter.

The amount of titanium, pressure / differential pressure transmitter and the pressure transmitter introduces single differential pressure transmitter is measured in terms of the name (the difference between the two pressure) and the pressure differential, but can indirectly measure but many . For example, pressure transmitters, in addition to measuring pressure, can also measure the level in the device. When measuring the liquid level in an atmospheric pressure vessel, a pressure transmitter is required. When measuring the level of a pressurized vessel, consider using two pressure / differential pressure transmitters, one measuring the lower limit, one measuring the upper limit, and their output signals.

The pressure rating of the transmitter must match the application . From the economic point of view, the material of the outer film box and the inserting part is more important, so we must choose the right one, but the connection of the blue can reduce the material requirements, such as the selection of carbon steel, chrome plating, etc. This will save a lot of money. Isolated pressure transmitters are the best choice for threaded connections, which saves money on installation and convenience. For ordinary pressure and differential pressure transmitters selection type, also taking into account the corrosion problems of the measured medium, but the medium temperature can not be considered for use, as is the common type of the impulse to the table, long-term operating temperature It is normal temperature, but the use of general-purpose maintenance than the isolated type. The first is the insulation problem. When the temperature is below zero, the pressure guiding tube will freeze, and the transmitter will not work or even be damaged. This will increase the heating and incubators and other devices. From the economic point of view, the selection of the transmitter, only the crystallization medium may be ordinary transmitter, but also for low pressure and crystallization medium may also be added to the indirect measurement of the purge medium (as long as the process allows liquid or gas purge ), the application of common type transmitter is requiring maintenance personnel to conduct more regular checks, including a variety of pressure tube leakage, purge media is normal, good insulation, etc., as long as the maintenance is good, a lot of use of ordinary type transmitter disposable Investment will save a lot. Maintenance should pay attention to the combination of hardware maintenance and soft maintenance. The selection of measuring range from the transmitter, the transmitters typically have a range of adjustable range, the measuring range to be used is preferably set at 1/4 to 3/4 segment of its range, so the accuracy will have Guaranteed, it is more important for the micro differential pressure transmitter. In practice some applications (liquid level measurement) requires the migration of the transmitter measurement range, the measurement range is calculated and migration are migrated based on current mounting position, there is migration of the migration Positive and negative points. Currently, smart transmitters and quite, which is characterized by high precision, large tuning range, and very easy to adjust and good stability, selection should be more consideration. According to design specifications, design engineering selection, whether the use of pneumatic or electric transmitter transmitter, its own strengths, should be considered and analyzed according to the specific conditions of the device. The following points can be referred to for selection: ※ Concentrated operation level and response speed; ※ Whether it is compatible with operation of DCS meter; ※ Economical, reliability and use and maintenance; ※ Safety (explosion-proof, power failure, gas source failure Etc.) ※ Environmental conditions and transmission distance. In general, the following conditions appropriate to use pneumatic transmitter: ※ from the transmitter to the display unit adjusts the distance between the short, normally not more than 150 m is more appropriate; ※ process medium material is flammable and Relatively high humidity occasions; ※ requires less investment in instruments, the response speed requirements are not fast occasions; ※ generally small and medium-sized enterprises require easy maintenance, economic and reliable; in large-scale devices mainly electric meters, some field control loop does not require the introduction of The central control room operates centrally.

The following conditions are suitable for selecting a motorized transmitter:

※ The distance between the transmitter and the display adjustment unit exceeds 150 meters; ※ The large-scale enterprise requires central control of highly centralized management; ※ The object to be controlled and managed by the DCS computer is set ; ※ The response speed is fast, and the information processing and calculation are complicated Occasions. In fact, in the modern production facilities, their respective characteristics are exerted and mixed and selected .

Third, selection of the pressure transmitter from the physics point of view, an object subject to any pressure comprises measuring the atmospheric pressure and the pressure medium (generally referred to as gauge pressure) in two parts. These two partial pressures acting on the object referred to the sum of the absolute pressure. Table P = P + absolute atmospheres absolute pressure measuring instrument called Absolute pressure. For common industrial pressure gauges, the gauge pressure is measured , which is the pressure difference between absolute pressure and atmospheric pressure. When the absolute pressure is greater than the atmospheric pressure , the measured gauge pressure is a positive value, which is referred to as a positive gauge pressure. When the absolute pressure is less than the atmospheric pressure value, the measured gauge pressure is a negative value, which is called a negative gauge pressure, that is, a vacuum degree. Measuring the degree of vacuum in said vacuum instrument table.

â‘´ To ensure that the pressure measurement accuracy, the measured value should be higher than the minimum pressure measurement range 1/3 gauge;

(2) Pressure sensors or pressure transmitters should be used for applications requiring long distance measurement or high measurement accuracy ;

⑶ less precision in the measurement, selectable resistive or inductive, Hall effect pressure transmitter pass away;

(4) The pneumatic base type pressure indicating regulator is suitable for local pressure indication adjustment;

(5) The pressure transmitter and pressure switch should be reasonably selected according to the explosion-proof requirements of the installation site.

Fourth, the differential pressure transmitter selection

Differential pressure transmitters are selected according to the following points:

(1) Measurement range, required accuracy and measurement function;

(2 ) The environment facing the measuring instrument, such as the petrochemical industrial environment, has the potential for hot (toxic) and explosion hazardous atmospheres, high ambient temperatures, etc.;

(3) The physical and chemical properties and state of the measured medium, such as strong acid, strong alkali, viscous, easy solidification, crystallization, and vaporization;

(4) Changes in operating conditions, such as changes in medium temperature, pressure, and concentration. Sometimes taking into account the parameters to reach while driving from normal production variations gas, and liquid-phase concentration and density;

⑸ measured object container structure, shape, size, equipment and various accessories in the container feed inlet nozzle outlet must be considered, such as the tower, the solution tank, the reactor, the boiler drum, Storage Tank, ball cans and the like;

(6) Other requirements, such as environmental protection and health requirements;

⑺ engineering instrument selection must be unified to consider requesting the reduction varieties as much as possible, reduce spare parts, to facilitate management;

Actual process conditions:

1 Consider what kind of equipment the measured object belongs to. For tanks and tanks, the volume of the tank is small, the scope of measurement is not too large, the volume of the tank is large, and the range of measurement may be large;

2 depends on the physical and chemical properties of the medium and the degree of cleanliness, the preferred conventional differential pressure transmitter and buoyancy level transmitter, but also to contact the media part of the material selection;

3 Single-branch differential pressure transmitters are available for some suspensions, foams and other media. Some easily precipitated, by inserting double Portland differential pressure transmitter easy to crystallize;

4 For the liquid level of high viscosity medium and the liquid level of high pressure equipment, since the equipment can't open the hole, you can use the radiation level meter to measure;

5 In addition to measurement methods and technical issues, there are instrumentation investment issues. In summary, the selection of transmitters is technically feasible, economically reasonable, and convenient in management .

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