Classification and selection of pressure measuring instruments in petrochemical industry
First, classify and sort out the pressure instruments. According to their purposes and usage habits, the pressure measuring instruments are divided into two categories: on-site pressure gauges and remote pressure gauges. Then, they are further classified in detail according to their measurement principles.
Field pressure gauges can be divided into Bourdon tube pressure gauges, bellows pressure gauges, diaphragm pressure gauges, vacuum gauges, absolute pressure gauges, etc. Bourdon tube pressure instruments use elastic elements as measuring elements. When measuring, elastic deformation occurs under the action of pressure. The size of the deformation has a certain linear relationship with the applied pressure. The measured pressure is indicated by a pointer or other display device through the transmission amplification mechanism (movement).
Remote pressure gauge can be divided into general pressure transmitter, differential pressure transmitter, pressure switch, etc. According to the requirements of process media, pressure switches, pressure switches with diaphragm seals, differential pressure switches, etc. can be selected for the pressure detection instruments to alarm, and the output contacts are mostly SPDT. Special attention shall be paid to the relationship between the normal value/set value and the dead zone of the pressure switch when selecting the pressure switch. When possible, the pressure switch shall not be selected, but the pressure transmitter shall be used. The core element of the pressure transmitter is the piezoelectric pressure sensor, and the piezoelectric effect is the main working principle of the piezoelectric sensor. It can feel (or respond to) the specified measured pressure force and convert it into available signals (usually electrical signals) for output according to certain rules. When the output is a specified standard signal (e.g. 4~20mA DC current, 1~5V DC voltage), it becomes a pressure transmitter. The processed electrical signals can be transmitted to the process control system for data acquisition, data processing, recording and printing, etc.
There are also some special pressure measuring instruments, such as electric contact pressure gauge, liquid column pressure gauge, air bag pressure gauge, high-pressure pressure gauge (pressure transmitter), etc.
As the working conditions and environment of petrochemical industry are relatively complex, designers should be familiar with various types of pressure instruments and their measuring principles, and at the same time, carefully analyze the actual working conditions, so as to select the most reasonable pressure instruments, contribute their professional strength to petrochemical production, promote the improvement of specific production efficiency and improve the guarantee of production safety.
The selection of measuring range of pressure gauge is generally consistent with the standard series of finalized products. Refer to the manufacturer's samples or relevant national standards and specifications in the process of specific project design. Generally, when measuring the stable pressure, the normal operating pressure is 1/3~2/3 of the full range. Under the condition of measuring pulse pressure or possibly exceeding the range for a short time, over-voltage protection device is required. When the gas pressure is greater than or equal to 2.5 MPa or the liquid pressure is greater than or equal to 6 MPa, a pressure gauge with a pressure relief device housing is required. Generally, the accuracy of the pressure gauge, capsule pressure gauge and diaphragm pressure gauge used for measurement is 1.6. The accuracy of the pressure gauge for precision measurement and calibration is 0.5, 0.2 or even 0.1. Capacitive or vibration frequency measuring elements are selected as the pressure transmitter. Two wire smart transmitter outputs 4-20 mA DC with HART communication protocol. The measurement accuracy is mostly ± 0.1%.
