Application and selection of pressure and differential pressure transmitters

1. Overview

Among all kinds of instruments, transmitters are the most widely used and common, and transmitters are roughly divided into pressure transmitters and differential pressure transmitters. Transmitters are often used to measure pressure, differential pressure, vacuum, liquid level, flow and density. There are two-wire and four-wire transmitters, especially two-wire transmitters; there are intelligent and non-intelligent transmitters, and there are more and more intelligent transmitters; there are pneumatic and electric transmitters, and most of them are electric transmitters. ; In addition, according to the application, there are intrinsically safe (intrinsically safe) and explosion-proof types; according to the application conditions, the main types of transmitters are as follows:
※ Low (micro) pressure / low (micro) 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 resistance/pressure, differential pressure transmitter;
※ Easy to crystallize / pressure, differential pressure transmitter.
The selection of transmitters is usually considered based on installation conditions, environmental conditions, instrument performance, economy and use of media. 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.

2. Introduction of pressure/differential pressure transmitter

Pressure transmitters and differential pressure transmitters measure pressure and differential pressure (the difference between two pressures) by name, but they can indirectly measure many quantities. Such as pressure transmitter, in addition to measuring pressure, it can also measure the liquid level in the equipment. 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 for the lower measurement limit and one for the upper measurement limit, and the liquid level can be measured by subtracting their output signals. When the differential pressure transmitter is generally 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 0 absolute pressure to more than 100 MPa (general situation).
In addition to measuring the differential pressure value of the two measured pressures, the differential pressure transmitter can also measure the flow of the medium fluid with various throttling elements, and can directly measure the liquid level of the pressure container and the liquid level of the atmospheric pressure container. As well as pressure and negative pressure.
2.1 Production
From the structure of the pressure/differential pressure transmitter, there are ordinary type and isolation type. The ordinary pressure/differential pressure transmitter has one measuring bellows, which directly senses the pressure or differential pressure of the measured medium; the isolated measuring bellows receives the pressure of a stable liquid (usually silicone oil). The stabilizing liquid is sealed between the two diaphragms, the diaphragm that directly receives the measured pressure is the outer diaphragm, and the diaphragm of the original ordinary diaphragm is the inner diaphragm. When the outer diaphragm receives the pressure signal Through the transmission of silicone oil, the pressure of the outer diaphragm is transmitted to the ordinary diaphragm, so that the pressure felt by the outer diaphragm can be measured.
The isolated pressure/differential pressure transmitter is mainly designed and used for the special medium to be measured. If the measured medium leaves the device, it will crystallize, while the use of the ordinary pressure/differential pressure transmitter needs to take out the medium, which will cause crystallization. The pressure guiding tube bellows chamber is blocked so that it cannot work normally, so the isolation type must be selected. Isolation transmitters are usually made into flanged installation, that is, the opening on the device under test makes the sensor diaphragm part of the device wall after the transmitter is installed, so that it will not take out the medium to be measured, and generally will not cause Crystallization and clogging.
When the measured medium requires a high crystallization temperature, the structure of protruding the diaphragm can be selected, so that the sensing diaphragm can be inserted into the equipment, so that the temperature of the sensed medium will not decrease, so the measurement is guaranteed. That is, the plug-in flange transmitter is selected.
The isolation type transmitter is divided into remote transmission type and integrated type. The remote transmission type means that the outer capsule and the measuring capsule are connected by a reinforced capillary, generally the capillary is 3~5 meters, so that the outer capsule is installed on the equipment, and the inner capsule and the transmitter can be installed on the installation bracket for easy maintenance. The other form is that the outer membrane box and the transmitter are integrated and directly installed on the equipment by the flange. For the isolation type 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 inner thread boss is welded on the device under test, the transmitter can be directly Screwed onto the device, installation is very easy.
The isolation type pressure/differential pressure transmitter is complicated to manufacture and has high material requirements, so its price is usually 3~4 times that of the ordinary type.
2.2 Selection principle
The main basis for the selection of pressure/differential pressure transmitters: the properties of the measured medium shall prevail, and the reference to save money, facilitate installation and maintenance. If the measured medium is of high viscosity, easy to crystallize and strong corrosion, the isolation type transmitter must be selected.
When selecting the type, the corrosion of the measured fluid medium to the metal of the diaphragm box must be considered, and the material of the diaphragm box must be selected, otherwise the outer diaphragm will be corroded and damaged in a short time after use, and the flange will also be corroded, causing equipment or equipment to be damaged. Personal accident, so the choice of membrane box material is very critical. The membrane box of the transmitter is made of ordinary stainless steel, 304 stainless steel, 316/316L stainless steel, tantalum and so on.
The temperature of the medium to be measured should be considered when selecting the type. If the temperature is high, reaching 200℃~400℃, the high temperature type should be selected, otherwise the silicone oil will vaporize and expand, making the measurement inaccurate.
When selecting the model, the working pressure level of the equipment should be considered, and the pressure level of the transmitter must be consistent with the application. From an economic point of view, the material of the outer membrane box and the insert part is more important, and it should be selected appropriately, but the material requirements can be reduced by connecting the flange, such as carbon steel, chrome plating, etc., which will save a lot of money.
The isolation type pressure transmitter is best to use the threaded connection form, which not only saves money, but also facilitates installation.
For the selection of ordinary pressure and differential pressure transmitters, the corrosiveness of the measured medium should also be considered, but the temperature of the medium used can be ignored, because the ordinary type is pressured into the meter, and the temperature during long-term operation is Normal temperature, but the maintenance of the ordinary type is larger than that of the isolation type. The first is the problem of heat preservation. When the temperature is below zero, the pressure guiding pipe will freeze, and the transmitter will not work or even be damaged. This requires additional devices such as heat tracing and incubators.
From an economic point of view, when selecting a transmitter, as long as the medium is not easy to crystallize, ordinary transmitters can be used, and for low-pressure easy-to-crystallize media, purging medium can also be added for indirect measurement (as long as the process allows the use of purging liquid or The application of ordinary transmitters requires maintenance personnel to conduct regular inspections, including whether various pressure guiding pipes are leaking, whether the purging medium is normal, whether the insulation is good, etc. As long as the maintenance is good, a large number of ordinary transmitters are used once Sex investment will save a lot. When maintaining, pay attention to the combination of hardware maintenance and soft maintenance.
In terms of the selection of the measuring range of the transmitter, generally the transmitter has a certain adjustable range. It is best to set the range used in the range of 1/4~3/4 of its range, so that the accuracy will be improved. Guarantee is even more important for differential pressure transmitters. In practice, in some applications (liquid level measurement), the measurement range of the transmitter needs to be migrated. The measurement range and migration amount are calculated according to the on-site installation location for migration. The migration can be divided into positive migration and negative migration.
At present, smart transmitters are quite popular. They are characterized by high precision, large adjustable range, very convenient adjustment and good stability. More consideration should be given when selecting models.
According to the design specification, in the engineering design selection, whether to use a pneumatic transmitter or an electric transmitter should be comprehensively considered and analyzed according to the specific conditions of the device because of its own advantages. The following points are available for reference when selecting models:
※ The degree of centralized operation and the speed of response;
※ Whether it is compatible with DCS computer operation;
※Economy, reliability and use and maintenance;
※ Safety (explosion-proof, power failure, air source failure, etc.);
※ Environmental conditions and transmission distance.
Generally speaking, the following conditions are suitable for selecting a pneumatic transmitter:
※ The distance from the transmitter to the display adjustment unit is short, usually no more than 150 meters;
※ The process materials are flammable and explosive media and occasions with high relative humidity;
※ For occasions requiring less investment in instruments and unpleasant response speed;
※ Generally, small and medium-sized enterprises require easy maintenance, economical and reliable;
※ In the large-scale installations mainly based on electric instruments, some on-site adjustment circuits do not require the introduction of the central control room for centralized operation.
The following conditions are suitable for the selection of electric transmitter:
※ The distance between the transmitter and the display adjustment unit is more than 150 meters;
※ Large enterprises require central control with highly centralized management;
＊ ※ Set up objects controlled and managed by DCS computer;