Improve the accuracy of differential pressure transmitter measurement accuracy

Abstract:

The differential pressure transmitter is designed with proven transmission technology and single-chip computer 

technology and sensor digital conversion technology. It has a wide range of applications in the field of measurement

and control in industrial fields. This paper briefly introduces the basic structure and working principle of the differential 

pressure transmitterand proposes some measures to improve the measurement accuracy of the differential

 pressure transmitter from the thermal control installation and commissioning of the differential pressure transmitter

 used in thermal power plants. method.



Introduction:

Differential pressure transmitters are used in many fields such as petroleum, chemical, steel, power, environmental 

protection and light industry. The data required for various industrial processes such as pressure, differential pressure,

 liquid level and flow rate can be measured in real time. In general, the differential pressure transmitter is made of a 

material that resists harsh environmental conditions and strong corrosive media. This is the scope of application of 

differential pressure transmitters. Below we will briefly introduce the working principle of the differential pressure

 transmitter, and find out the method to improve the measurement accuracy of the differential pressure transmitter

 according to its working principle.

Understand its structure to improve the working principle of differential pressure transmittersduring thermal control

 installation and unit commissioning. The figure below shows the working principle of a differential pressure transmitter

 that is often used in industrial fields.


1. Working principle of differential pressure transmitter:

The working principle of the differential pressure transmitter can be well explained by taking various common differential pressure transmitters as an example. The following is a working principle of a differential pressure transmitter that is often used in industrial sites:It can be seen that the working module of this type of differential pressure transmitter includes 

a sensor, a signal conversion circuit, a 16-bit single chip microcomputer, a current control circuit, a zero adjustment

button, a communication interface and a display module.

The specific operation process is as follows: on the sensor at one end of the differential pressure transmitter, the pressure

 or differential pressure will change the capacitance value in the sensor, and the changed capacitance value is converted 

into a frequency signal by the digital conversion of the signal conversion circuit and the 16-bit single chip microcomputer. Transfer to the microprocessor. The microprocessor converts the frequency signal into a current control signal through operation, and the current control signal is transmitted to the current control circuit, and the current control circuit converts

the current control signal into an analog current output of 4-20 mA while the microprocessor (16-bit single chip microcomputer) Also responsible for handling communication, display and other module interactions.

The differential pressure transmitter has two parts: measuring and amplifying. When measuring, the pressure will isolate the diaphragm and the filling silicone oil will be transmitted to the measuring diaphragm. The deformation variable is proportional to the pressure in the range of error tolerance. . Measuring the displacement of the deformation of the diaphragm causes the capacitance value of the capacitor formed by the central electrode and the fixed electrode to change, and the amount of capacitance change is converted into a current signal by the conversion of the capacitance-current circuit.

The differential pressure transmitter compares the current signal generated by the capacitance change with the zeroing signal generated by the zeroing and zeroing circuits and the feedback signal generated by the feedback circuit to calculate their difference and input the difference to the differential pressure difference. Another important part of the pressure transmitter amplifier. The difference processed by the amplifier is the output signal value of the differential differential pressure transmitter. The output signal is displayed in digital form on the transmitter's display module.


2. Method for improving the accuracy of differential pressure transmitter measurement during thermal control installationand unit commissioning.


2.1 Method for improving the accuracy of differential pressure transmitter measurement during thermal control installation.

The error generated during the thermal control installation of the differential pressure transmitter is the main source of error

 in the differential transmitteruse, as shown in the specific position of the differential pressure transmitter thermal control installation and the differential pressure transmitter during thermal control installation. slope. The change of the thermal 

control installation position of the differential pressure transmitter will not only cause the change of the output value, but 

also the change of the height of the measurement point of the differential pressure transmitter and the thermal control installation point and the differential pressure transmission during the thermal control installation process. A change in

 the tilt angle of the device produces an additional error.

In combination with the overall design requirements of the thermal power plant site, the differential pressure transmitter

 is generally not allowed to be installed near the measurement point at the use site, but by using different performances

 to a relatively concentrated place. Such a processing method. On the one hand, it is beneficial to the management and maintenance of the differential pressure transmitter of the thermal power plant. However, due to the different height 

difference between the resettlement point and the measurement point, additional error will be generated, which seriously affects the accuracy of the differential pressure transmitter measurement. The magnitude of this additional error can be calculated by the following formula.

  It indicates the density of the transmission medium used in the impulse tube: g indicates the acceleration of gravity 

at the location of the thermal power plant; and indicates the absolute value of the height difference between the installation point of the differential pressure transmitter and the measurement point. It is not difficult to calculate the additional error caused by the air transmission medium of the IM height difference is 12.12. ap If the transmission medium is not air but

 water, the additional error is as high as 9.8pa when the height difference is lm. Considering the general thermal power 

plant use The differential pressure transmitter is a 0.2-stage differential pressure transmitter, which will seriously reduce 

the measurement accuracy of the differential pressure transmitter when the influence of the additional error on the measured value is greater than 0.1Fs. Therefore, if the air transmission medium is used in a thermal power plant, it is necessary to re-zero the transmitter for a range of less than 12.12pa. If the differential transmitter measures the liquid flow rate, when using 

the differential transmitter, we should open a suitable pressure port at the appropriate position on the side of the flow pipe.

In this way, on the one hand, the problem of the precipitation of dross which often occurs in the process pipe can be eliminated; on the other hand, it is convenient to install the differential transmitter to be placed beside or below the pressure tap to reduce the bubble inside the bubble discharge pipe. The error caused by overflowing the pipe. In the case of differential transmitter measurements, the location of the pressure tap that we open in the process piping should be at the top of the process piping or on the side of the process piping.

 In this case, we should install the differential transmitter beside the process pipe or above the process pipe, so that the accumulated liquid can be easily flowed in the pipe to avoid mixing of the liquid and the gas to be measured. Measurement error. When we use the differential transmitter to measure the steam flow, we must choose to open the pressure port on the side of the process pipe when selecting the position of the pressure tap. The purpose of this is to conveniently install the differential transmitter in the pressure. Below the mouth, the vapor that has accumulated in the condensation can flow into the pressure guiding tube by the action of gravity, ensuring the accuracy of measuring the steam.

 Since the differential pressure transmitter measures the industrial field data by measuring the deformation displacement of the diaphragm through the center; to sense the pressure values at both ends. At the same time, the central measuring module of the differential pressure transmitter is filled with silicone oil. In the process of thermal control installation of the differential pressure transmitter, the filling silicone oil cannot be pressured on the measuring diaphragm, which causes the measurement error to affect the measurement accuracy of the differential pressure transmitter.

According to the long-term accumulation experience in the installation process of the differential pressure transmitter, it can be known that when the differential pressure transmitter is installed in a direction perpendicular to the measuring diaphragm, the filling of the silicone oil will measure the film. The sheet generates pressure, which causes deformation of the measuring diaphragm, causing measurement errors and thus reducing the measurement accuracy of the differential pressure transmitter; if the differential pressure transmitter is installed while tilting in a direction horizontal to the measuring diaphragm The filling of the silicone oil generally does not cause pressure on the measuring diaphragm, nor does it cause deformation of the measuring diaphragm, and has no influence on the measurement accuracy of the differential pressure transmitter.


2.2 Method for improving the measurement accuracy of differential pressure transmitter during the single commissioning process:

  When performing on-site single-unit debugging of the differential pressure transmitter used in thermal power plants,

it is the top priority of the work to correct the problem that the measurement accuracy is not high due to the change 

of the installation position of the differential pressure transmitter used in the thermal power plant. The effects on the measurement of differential pressure transmitters during thermal control installation are unilateral and linear. We can

 debug and correct the differential pressure transmitter based on this remarkable feature. The specific method is: the

 test port of the high and low ends of the differential pressure transmitter is connected to the atmosphere, and the factors

 such as air flow are not affected by the on-site debugging. Use the field communicator or use the zero adjustment device provided with the differential pressure transmitter to adjust the zero output of the differential pressure transmitter and read

 the display value of the zero output of the transmitter on the display or the computer.

  If the displayed value is within the maximum error tolerance of the differential pressure transmitter, the unit commissioning

 is ended; if the displayed value is outside the allowable range of the maximum error of the differential pressure transmitter,

 the commissioning must be repeated until the displayed value is at the transmitter. Within the maximum error range, the metering performance requirements of the differential pressure transmitter are met.


 Conclusion:

 Differential pressure transmitters can measure important parameters such as flow, pressure, and liquid in production operations such as petroleum, chemical, steel, electric power, environmental protection, and light industry. In view of the 

wide application of differential pressure transmitters in the field of industrial field monitoring, it is of great practical significance to improve the measurement accuracy of differential transformers.

 An important way to improve the measurement accuracy of differential pressure transmitters in thermal power plants is to reduce errors in thermal control installation and unit commissioning. The error generated during the thermal control installation and the single-module debugging process of the differential pressure transmitter is embodied in the specific position of the thermal control installation of the differential pressure transmitter and the inclination angle of the differential pressure transmitter during the thermal control installation. From these two aspects, it is proposed that the targeted method is the only way to improve the measurement accuracy of the differential pressure transmitter.