Application and selection of pressure transmitters and differential pressure transmitters

1 Overview

Among various types of instruments, transmitters are widely used. Transmitters are generally divided into pressure transmitters and differential pressure transmitters. Transmitters are commonly used to measure pressure, differential pressure, vacuum, liquid level, flow and density, etc. Transmitters can be divided into two-wire and four-wire systems, with two-wire transmitters being the most common. They can be divided into intelligent and non-intelligent transmitters, with more and more intelligent transmitters being used. They can be divided into pneumatic and electric, with electric transmitters being the majority. ;In addition, according to the application, there are intrinsically safe type and explosion-proof type; the main types of transmitters according to the application conditions are as follows:

• Low (micro) pressure/low differential pressure transmitter;

• Medium pressure/medium differential pressure transmitter;

• High pressure/high differential pressure transmitter;

• Absolute pressure/vacuum/negative pressure Differential pressure transmitter;

• High temperature/pressure, differential pressure transmitter;

• Corrosion-resistant/pressure, differential pressure transmitter;

• Easy to crystallize/pressure, differential pressure transmitter.

The selection of transmitter is usually based on the installation conditions, environmental conditions, instrument performance, economy and application medium. 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 flange transmitters, double flange transmitters, plug-in flange transmitters, etc.

In noun terms, pressure transmitters and differential pressure transmitters measure pressure and the difference between two pressures, but they indirectly measure many parameters. For example, a pressure transmitter, in addition to measuring pressure, can also measure the liquid level in the device. When measuring liquid level in a normal pressure vessel, a pressure transformer is required. When measuring the liquid level of a pressure vessel, two pressure transmitters can be used, namely one for lower limit measurement and one for upper limit measurement. Their output signals can be subtracted to measure the liquid level. Generally, a differential pressure transmitter is used. It can also be used to measure the density of the medium when the liquid level and pressure in the container remain unchanged. The measurement range of the pressure transmitter can be very wide, starting from absolute pressure 0 to 100MPa (generally).

2 Introduction to pressure/differential pressure transmitter

In addition to measuring the differential pressure value of two measured pressures, the differential pressure transmitter can also be used with various throttling components To measure flow, you can directly measure the liquid level of the pressure vessel and the liquid level of the normal pressure vessel as well as the pressure and negative pressure.

2.1 Production

Pressure and differential pressure transmitters are divided into ordinary types and isolated types based on their production structures. The ordinary type of measuring membrane box has one, which directly senses the pressure and differential pressure of the measured medium; the isolated type measuring membrane box receives the pressure of a stabilizing fluid (usually silicone oil), and this stabilizing fluid is It is sealed between two diaphragms, and the diaphragm that receives the measured pressure is the outer diaphragm. The diaphragm of the original ordinary diaphragm boxIt is the inner diaphragm. When the outer diaphragm receives the pressure signal, the pressure of the outer diaphragm is transmitted intact to the ordinary diaphragm box through the transmission of silicone oil, and the pressure felt by the outer diaphragm is measured.

Isolated transmitters are mainly used for special measured media. For example, the measured medium will crystallize after leaving the device. However, when using an ordinary transmitter, the medium needs to be taken out, which will cause the pressure tube to The membrane box chamber is blocked and cannot work properly, so an isolation type must be selected. The isolation type is usually installed in a flange type, that is, a flange is added to the opening of the device under test so that after the transmitter is installed, its sensing diaphragm becomes part of the device wall, so that it will not take out the measured medium and generally will not cause crystal blockage. . When the measured medium requires a higher crystallization temperature, a structure with a protruding diaphragm can be used. In this way, the sensing diaphragm can be inserted into the interior of the device, so that the temperature of the sensed medium will not decrease. In this way, the measurement is guaranteed. That is, choose a plug-in flange transmitter.

Isolated transmitters include remote transmission type and integrated type. The remote transmission type means that the outer diaphragm box and the measuring diaphragm box are connected with a reinforced capillary tube. Generally, the capillary tube is 3~5 meters. In this way, the outer diaphragm box is installed on the equipment, and the inner diaphragm box and transmitter can be installed on a bracket for easy maintenance. ; Another form is that the outer diaphragm box and the transmitter are integrated and directly installed on the equipment through the flange. For the isolated pressure transmitter, it can also be made into a threaded connection type, that is, the outer diaphragm box or the outer elastic element can be in front of the installation thread. As long as the internal thread boss is welded to the device under test, the transmitter can be directly connected Screws onto the device and is very easy to install.

The production of the isolated pressure/differential pressure transmitter is complicated and the material requirements are high, so its price is usually 3 times that of the ordinary type.

2.2 Selection Principles

The main basis for selecting pressure/differential pressure transmitters is: based on the properties of the measured medium to save money and facilitate installation and maintenance. For reference. If the measured medium is of high viscosity, prone to crystallization and strong corrosion, an isolation transmitter must be selected. When selecting the model, the corrosion of the diaphragm metal by the medium must be considered. The material of the diaphragm must be selected. Otherwise, the outer diaphragm will be corroded in a short time after use, and the flange will also be corroded, causing equipment and personal accidents. , so material selection is very important. The membrane box material of the transmitter includes ordinary stainless steel, 304 stainless steel, 316L stainless steel, tantalum membrane box material, etc. When selecting a model, the temperature of the medium to be measured should be considered. If the temperature is high, generally 200°C to 400°C, a high-temperature type should be selected. Otherwise, the silicone oil will vaporize and expand, making the measurement inaccurate.

When selecting, the working pressure level of the equipment should be considered. The pressure level of the transmitter must be consistent with the application situation. From an economic point of view, the material of the outer diaphragm box and the insertion part is more suitable, but the connecting flange can be made of carbon steel or chrome plating, which will save a lot of money.