Correct selection and application of differential pressure transmitter in measuring liquid level

Differential pressure transmitter is a kind of thermal measuring instrument that is very common in the field of industrial production. It is mainly used to measure the physical parameters of liquid medium such as pressure, liquid level and flow. At present, the application scope of differential pressure transmitter in industrial automation production equipment is becoming more and more extensive, but as a general-purpose measuring instrument, it often fails in operation. If there is a problem in production, we need to solve it in time. If it cannot be handled quickly, it will definitely affect the normal production to a certain extent. Some serious failures may even endanger personal safety. This article is a brief discussion of the application of differential pressure transmitter in liquid level measurement by Runzhong Instrument Technology Co., Ltd. through long-term experience.
 
　1.  Working principle of differential pressure transmitter
　　Differential pressure transmitter is usually used to measure the liquid level in a closed container, and the pressure difference generated by the liquid's own gravity is used to measure the liquid level in the container (as shown in Figure 1). The high-pressure side measuring tube (located above the figure) is always filled with water due to steam condensation, keeping the pressure constant, while the low-pressure side measuring tube (located below the figure) forms a connector with the container, and its pressure changes linearly with the change of the liquid level in the container.
　　Assume △P is the differential pressure signal received by the transmitter, P0 is the internal pressure of the container, P+ is the pressure on the positive pressure side of the transmitter, and P- is the pressure on the negative pressure side of the transmitter; ρ is the density of the liquid in the container; g is the acceleration of gravity; h1 is the height from the process zero point to the pressure port on the upper part of the container; h2 is the process liquid level of the container; h is the height from the transmitter to the process liquid level zero point. Then:
　　P+=P0+ρgh1+ρgh
　　P-=P0+ρgh2+ρgh
　　△P=P+-P-=ρgh1-ρgh2
　　When the liquid level changes from h2=0 to h2=h1, the differential pressure measured by the differential pressure transmitter changes from the maximum value to ΔP=0. By setting the transmitter, the output current changes from 4mA to 20mA.
 
　2.  Installation requirements of differential pressure transmitters
　　To accurately measure the liquid level, in addition to correctly selecting and calibrating the differential pressure transmitter, it is also necessary to pay attention to the installation of the entire system to meet the requirements. The indication of the transmitter sometimes cannot reflect the actual parameters of the measured medium because the measurement system itself will produce errors. System installation requirements include: the opening position of the pressure tapping port, the reasonable laying of the connecting conduit and the installation position of the transmitter.
　　First of all, the pressure tapping port should be in an area where the fluid flows smoothly and there is no eddy current, and the process should be able to ensure that the selected process parameters are measured. For example, when using a differential pressure transmitter to measure the water level of the boiler drum, the distribution of the actual water level of the drum in the axial and radial directions of the drum is different. Generally, in the axial direction, the water level in the middle is high and the water level on both sides is low; along the radial direction, the side with denser downcomers is higher. The liquid level transmitter of the condenser of the turbine of a power plant has a measurement point close to the inlet of the condensate pump. When the condensate pump is running, the water surface at the point of the measurement point sinks, resulting in a significantly low indication of the transmitter. Later, the position of the measuring tube was moved to a place far away from the inlet of the condensate pump. The condenser water level transmitter is consistent with the actual water level indication on site, ensuring the safe and stable operation of the condenser.
　　Secondly, when measuring liquid level, the differential pressure value measured by the differential pressure transmitter is relatively small, generally between a few kPa and 100 kPa, so the entire measurement system has a great influence on the measurement accuracy. As shown in Figure 1, when installing the pressure pipe, the horizontal section of the pressure pipe should have a certain slope, and the slope should be as large as possible to avoid the accumulation of liquid inside the pressure pipe, resulting in inaccurate measurement. When the transmitter range is very small, it will cause fluctuations in the transmitter output. In addition, when the transmitter is put into operation, the bubbles in the liquid column of the pressure pipe should be emptied as much as possible. These accumulated gases will affect the accuracy of the measurement. It is also possible to consider installing an exhaust device at the highest point of the upper bend of the pressure pipe.
　
　3. Setting the zero position of the transmitter When
　　the differential pressure transmitter measures the liquid level, the setting of the zero position is a very important link. When the high pressure (H) side and low pressure (L) side of the transmitter are connected to the high pressure side and low pressure side of the local measuring tube, the high pressure side pressure pipe is always filled with water, the pressure measured at the high pressure end of the transmitter is P+kPa, and the low pressure side of the transmitter is connected to the low pressure side pressure pipe, and the measured pressure is P-kPa, then the actual differential pressure measured by the transmitter is (P+-P-) kPa. When the container liquid level is the lowest, the differential pressure value is the largest, corresponding to the LRV set inside the transmitter, that is, the zero position of the transmitter, at this time the transmitter output current is 4mA, when the container liquid level is the highest, the differential pressure value is 0, corresponding to the URV set inside the transmitter, that is, the full scale of the transmitter, at this time the transmitter output current is 20mA.
　　When the high pressure (H) side and low pressure (L) side of the transmitter are connected oppositely to the high pressure side and low pressure side of the local measuring tube, the internal settings of the transmitter need to be modified: that is, the LRV of the transmitter is set to (P--P+) kPa (this difference is a negative number), that is, no matter how the transmitter is connected to the pressure pipe, the full scale of the transmitter corresponds to the full water level of the measuring container, and the differential pressure is always 0, that is, the full scale URV of the transmitter is 0kPa, and the output current is 20mA. When the high pressure side of the transmitter is connected to the high pressure side of the pressure pipe, the zero position LRV of the transmitter is set to the maximum differential pressure value. When the high pressure side of the transmitter is connected to the low pressure side of the pressure pipe, the zero position LRV of the transmitter is set to the negative of the maximum differential pressure value.
　
4. Migration of the zero position of the transmitter
　　When the differential pressure transmitter measures the liquid level, if the positive and negative pressure chambers of the differential pressure transmitter are on the same horizontal plane as the pressure taking point of the container, no migration is required. In actual applications, due to the consideration of the installation location of the equipment and the convenience of maintenance, the transmitter may not be on the same horizontal plane as the pressure point; for example, if the measured medium is a highly corrosive or heavy viscosity liquid, the medium cannot be directly introduced into the transmitter. An isolation liquid tank must be installed to transmit the pressure signal with isolation liquid to prevent the transmitter from being corroded. At this time, the influence of the liquid column of the medium and isolation liquid on the measured value of the transmitter should be considered. When the installation position of the transmitter is often not on the same horizontal plane as the lowest liquid level, in order to correctly indicate the height of the liquid level, the differential pressure transmitter must do some technical processing, namely migration. Migration is divided into no migration, negative migration and positive migration.
　　The so-called "migration" of the transmitter is to move the measuring range of the transmitter while the range remains unchanged. Usually, moving the measurement starting point below the reference point "0" is called negative migration; moving the measurement starting point above the reference point "0" is called positive migration. Take a 30kPa range differential pressure transmitter as an example. When there is no migration, the measurement range is 0-30kPa, when the positive migration is 100%, the measurement range is 30-60kPa, when the negative migration is 100%, the measurement range is -30-0kPa, and when the negative migration is 50%, the measurement range is
　　-15-+15kPa.
　　In actual operation, first determine the range of the differential pressure transmitter. After calibration, use the migration screw to adjust the measurement starting point or full-scale output to the corresponding position or use the hand operator to directly input the migration amount. For example: if you need to measure a differential pressure of -30-0kPa, the range is 30kPa. When calibrating the transmitter, pressurize the negative pressure chamber by 30kPa, and adjust the zero knob of the differential pressure transmitter to make its output 4mA; then, do not pressurize the negative pressure chamber, adjust the range knob of the differential pressure transmitter until the output is 20mA. If you use a hand operator, set the LRV of the transmitter to -30kPa and the URV to 0kPa. The measuring range of
　　the differential pressure transmitter is equal to the sum of the range and the migration amount, that is, the measuring range = range + migration amount. Therefore, the essence of positive and negative migration is to change the upper and lower limits of the differential pressure transmitter range, while the size of the range remains unchanged. According to the principle of the differential pressure transmitter measuring the positive and negative migration of the liquid level, in practical applications, the liquid level measurement method can be improved accordingly according to the use conditions of the instrument, the process conditions of the production equipment and the surrounding environment.
　　
The capacitive transmitter produced by Runzhong Instrument Technology Co., Ltd. has high measurement accuracy and simple debugging. The possibility of the transmitter itself failing is extremely small. After working for a period of time, if the indication deviation is large or even cannot be indicated normally, the main reasons are blockage or leakage of the high and low pressure side pressure pipes or three-valve group, change in migration volume, zero drift, loss of isolation liquid, etc., or the transmitter works in a negative pressure environment, the transmitter measurement circuit is not well sealed, and external gas enters the measurement pipeline. When the output display value of the differential pressure transmitter appears to be too large or too small, the pressure pipe can be disconnected from the three-valve assembly, the negative pressure side can be connected to the atmosphere, and the differential pressure signal can be sent to the positive pressure side of the transmitter with a hand pump to observe whether the output of the transmitter is correct, that is, the output current is between 4 and 20mA, and the error does not exceed 0.8mA. If the transmitter output is correct, it can be determined that the pressure pipeline is blocked or leaking.