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What Is Differential Pressure Transmitter?
Dec 11, 2018


What is differential pressure transmitter?



ⅰ.Overview

Among the various types of instruments, the transmitter is the most widely used and widely used, and the transmitter is roughly divided into pressure.

Transmitter and differential pressure transmitter. Transmitters are commonly used to measure pressure, differential pressure, vacuum, liquid level,

Flow and density, etc. The transmitter has two-wire and four-wire systems, and two-wire transmitters are especially popular;

There are intelligent and non-intelligent points, and there are more and more intelligent transmitters; there are pneumatic and electric parts, and electric

In most cases, there are intrinsically safe (intrinsically safe) and explosion-proof types according to the application; according to the application conditions, the main types of transmitters are as follows: low (micro) pressure / low (micro) differential pressure Transmitter; medium voltage / medium differential pressure transmitter; high pressure easy to crystallize / pressure, differential pressure transmitter. Transmitter selection is usually based on installation conditions, environmental conditions, instrument performance, economy, and media usage. In practical applications, it is divided into direct measurement and indirect measurement;

Its uses include process measurement, process control and device interlocking. Common transmitters include ordinary pressure transmitters, differential pressure transmitters, single-lamp transmitters, dual-lamp transmitters, and plug-in flange transmitters.

What is differential pressure transmitter?

ⅱ.The pressure / differential pressure transmitter introduced pressure transmitter and differential pressure transmitter from the name of the measurement is the pressure and differential pressure (the difference between the two pressures), but the amount of indirect measurement can be a lot. For example, a pressure transmitter can measure the pressure in the device in addition to the pressure. When measuring the liquid level in an atmospheric pressure vessel, a pressure transmitter is required. When measuring the liquid level of a pressurized container, two pressure/differential pressure transmitters can be considered, that is, one lower limit of measurement, one upper limit of measurement, and their output signals.

The pressure rating of the transmitter must match the application. From an economic point of view, the outer membrane box and the insert part of the material are more important, to choose the right, but the connection of the flange can reduce the material requirements, such as the choice of carbon steel, chrome, etc., which will save a lot of money. 

The isolated pressure transmitter is good for threaded connection, which saves money and is easy to install. For the selection of common pressure and differential pressure transmitters, the corrosiveness of the measured medium should also be considered, but the medium temperature used can be ignored, because the common type is pressurized into the meter and the long-term working temperature is normal temperature. 

However, the normal type of maintenance is larger than the isolation type. The first is the insulation problem. When the temperature is below zero, the pressure guiding tube will freeze. The transmitter will not work or even be damaged. This will increase the heat tracing and incubator. From an economic point of view, when the transmitter is selected, only the crystallization medium can be a common type of transmitter, and for the low pressure and easy crystallization medium, the purge medium can also be added for indirect measurement (as long as the process allows the use of purge liquid or gas) The application of ordinary transmitters requires maintenance personnel to carry out regular inspections, including whether the various pressure guiding tubes are leaking, whether the cleaning medium is normal, whether the insulation is good, etc. 

As long as the maintenance is good, a large amount of ordinary transmitters are used for one-time investment. It will save a lot. Pay attention to the combination of hardware maintenance and soft maintenance during maintenance. From the selection of the measuring range of the transmitter, the general transmitter has a range adjustable range, and the range of the range that Zui will use is set in the range of 1/4~3/4 of its range, so the accuracy will be Guaranteed, it is even more important for differential pressure transmitters. 

In practice, some applications (level measurement) need to migrate the measurement range of the transmitter, calculate the measurement range and migration according to the current installation location, and migrate with positive and negative migration. At present, the smart transmitter has been quite similar. It is characterized by high precision, large adjustable range, and very convenient adjustment and good stability. It should be considered when selecting the type. According to the design specifications, in the selection of engineering design, whether pneumatic or electric transmitters are used, because of their special features, should be comprehensively considered and analyzed according to the specific conditions of the device. 

What is differential pressure transmitter?

The following points can be used for selection: 

※ centralized operation and response speed; 

※ whether it is compatible with DCS computer operation; 

※ economy, reliability and maintenance; 

※ safety (explosion-proof, power failure, gas source failure Etc); 

※ environmental conditions and transmission distance. Generally speaking, the following conditions are suitable for the selection of pneumatic transmitter: 

※ The distance from the transmitter to the display adjustment unit is short, usually not more than 150 meters; 

※The process material is flammable and explosive medium and relative When the humidity is very large; ※There is a requirement for less investment in the instrument and the response speed is not fast; 

※ Generally, small and medium-sized enterprises require easy maintenance and economical reliability; in large devices mainly based on electric instruments, some on-site regulating circuits are not required to be introduced into the center. The control room is operated centrally.

The following conditions are preferred for the selection of electric actuators:

The distance between the transmitter and the display adjustment unit is more than 150 meters; the large enterprise requires central control with high centralized management; the object with DCS computer for control and management; the requirement for fast response, complex information processing and computation. In fact, in modern production facilities, they are mixed and selected for their respective characteristics.

ⅲ. The selection of pressure transmitter From a physics point of view, the pressure on any object includes the atmospheric pressure and the pressure of the measured medium (generally referred to as gauge pressure). The sum of the pressures of the two parts acting on the object to be measured is called the absolute pressure. P absolutely = P table + Instrument for measuring absolute pressure at atmospheric pressure is called absolute pressure gauge. For ordinary industrial pressure gauges, the gauge pressure value is measured, that is, the pressure difference between absolute pressure and atmospheric pressure. When the absolute pressure is greater than the atmospheric pressure value, the measured gauge pressure value is called positive gauge pressure; when the absolute pressure is less than the atmospheric pressure value, the measured gauge pressure value is negative value, which is called negative gauge pressure, that is, vacuum degree. The meter that measures the degree of vacuum is called a vacuum gauge.

(1) In order to ensure the accuracy of pressure measurement, the minimum pressure measurement value should be higher than 1/3 of the measurement range of the pressure gauge;

(2) For applications requiring high distance measurement or high measurement accuracy, pressure sensors or pressure transmitters should be used;

(3) When the measurement accuracy requirement is not high, a resistive or inductive, Hall effect remote transmission pressure transmitter can be selected;

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

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

What is differential pressure transmitter?

ⅳ. The selection of differential pressure transmitter

  The differential pressure transmitter is selected according to the following points:

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

(2) The environment faced by the measuring instrument, such as the petrochemical industrial environment, has the presence of a hot (toxic) and explosive atmosphere, and has a high ambient temperature;

(3) Physicochemical properties and state of the measured medium, such as strong acid, strong alkali, viscous, easy to solidify crystallization and vaporization;

(4) Changes in operating conditions, such as changes in medium temperature, pressure, and concentration. Sometimes it is necessary to take into account changes in the concentration and density of the gas and liquid phases from the start of the drive to the normal production of the parameters;

(5) The structure, shape and size of the container to be tested, equipment accessories in the container and various inlet and outlet pipe ports must be considered, such as towers, solution tanks, reactors, boiler steam drums, vertical tanks, spherical tanks, etc.;

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

(7) There must be a unified consideration in the selection of engineering instruments, and it is required to reduce the number of specifications as much as possible and reduce spare parts for profit management;

Actual process situation:

1 Consider which type of device the measured object belongs to. For tanks and tanks, the volume of the tank is small, the measurement range is not too large, the tank volume is large, and the measurement range may be large;

2 To see the physical and chemical properties and cleanliness of the medium, the conventional differential pressure transmitter and the float type liquid level transmitter are preferred, and the material of the contact medium part is also selected;

3 For some suspensions, foams and other media, single-lange differential pressure transmitters are available. Some insertable double-blue differential pressure transmitters that are easy to precipitate and easy to crystallize;

4 The level of the high-viscosity medium and the liquid level of the high-pressure equipment, because the equipment can not be opened, can be measured by the radiation level gauge;

5 In addition to measurement methods and technical problems, there are also investment problems with instruments. In summary, the selection of the transmitter is technically feasible, economically reasonable, and convenient to manage.