Knowledge || Differential pressure transmitters for level measurement

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A differential pressure transmitter is a type of thermal measurement instrument that is very common in the field of industrial production. It is primarily used to measure the pressure of liquid media, as well as various physical parameters such as liquid level, level gauges, level transmitters, capacitive level gauges, capacitive level transmitters, level switches, ultrasonic level switches, and flow rates. Currently, in industrial automation systems, differential pressure transmitters are being used more and more frequently. However, as general-purpose measuring instruments, they often experience operational failures. When problems arise during production, it is necessary to address them promptly; failure to do so will inevitably affect the smooth progress of production to some extent, and in severe cases, such failures can even pose a risk to human safety. This article provides a brief overview of the use of differential pressure transmitters in level measurement, based on the extensive experience accumulated by Runzhong Instrument Technology Co., Ltd. I. Working principle of differential pressure transmitters Differential pressure transmitters are commonly used to measure the liquid level in sealed containers, utilizing the pressure difference generated by the gravity of the liquid itself to determine the level of the liquid within the container. The high-pressure side measuring tube (located at the top of the diagram) remains filled with water due to steam condensation, thus maintaining a constant pressure, while the low-pressure side measuring tube (located at the bottom of the diagram) forms a communicating vessel with the container, and its pressure changes linearly as the liquid level in the container changes. Let △P be the differential pressure signal received by the transmitter, P0 be the pressure inside the container, P+ be the pressure on the positive side of the transmitter, and P- be the pressure on the negative side of the transmitter ； ρ is the density of the liquid inside the container ； g is the acceleration due to gravity ； h1 is the height from the process zero point to the pressure tapping point at the upper part of the container ； h2 is the level of the process fluid in the container ； h is the height from the transmitter to the zero level of the process liquid level. Thus, we have: P+ = P0 + ρgh1 + ρgh2; P- = P0 + ρgh1 + ρgh2. ΔP = 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 its maximum value to ΔP = 0. By adjusting the transmitter, the output current changes from 4 mA to 20 mA. II. Setting of the transmitter’s zero point: When a differential pressure transmitter is used to measure liquid levels, setting the zero point is a very important step. When the high-pressure (H) and low-pressure (L) sides of the transmitter are connected in the same manner as the high-pressure and low-pressure sides of the local measuring tube, the high-pressure pressure guide tube remains filled with water at all times. The pressure measured at the high-pressure end of the transmitter is P + kPa, while the pressure measured on the low-pressure side of the transmitter, which is connected to the low-pressure pressure guide tube, is P – kPa. Thus, the actual differential pressure measured by the transmitter is (P +) – (P –) kPa. When the liquid level in the container is at its lowest, the differential pressure is at its highest value; this corresponds to the LRV set inside the transmitter, which is the zero point of the transmitter. At this point, the transmitter outputs a current of 4 mA. When the liquid level in the container is at its highest, the differential pressure is 0; this corresponds to the URV set inside the transmitter, which is the full scale of the transmitter. At this point, the transmitter outputs a current of 20 mA. When the high-pressure (H) and low-pressure (L) sides of the transmitter are connected to the high-pressure and low-pressure sides of the local measuring cylinder in the opposite manner, modifications must be made to the internal settings of the transmitter: specifically, the LRV of the transmitter should be set to (P–P+) kPa (where this difference is negative). In other words, regardless of how the transmitter is connected to the pressure transduction tubes, the full scale of the transmitter corresponds to the full water level in the measuring container, and the differential pressure remains 0; thus, the full scale URV of the transmitter is 0 kPa, with an output current of 20 mA. When the high-pressure side of the transmitter is connected to the high-pressure side of the pressure guide tube, the transmitter’s zero point LRV 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 guide tube, the transmitter’s zero point LRV is set to the negative of the maximum differential pressure value. III. Installation requirements for differential pressure transmitters: To achieve accurate level measurement, in addition to properly selecting and calibrating the differential pressure transmitters, it is also necessary to ensure that the installation of the entire system meets the required standards. The reading displayed by the transmitter sometimes does not reflect the actual parameters of the medium being measured, as errors can arise from the measurement system itself. The system installation requirements include: the location of the pressure tapping ports, the proper routing of the connection cables, and the installation location of the transmitter, among others. Firstly, the pressure tap should be located in an area where the fluid flow is steady and free of vortices, and the process conditions should ensure that the desired process parameters can be measured. For example, when using a differential pressure transmitter to measure the water level in a boiler’s drum, the actual distribution of the water level along the axial and radial directions of the drum is different; generally, along the axial direction, the water level is higher in the middle and lower on the sides ； Radially, the side with denser downcomers is higher. In a certain power plant, the level transmitter for the turbine condenser had its measurement point located near the inlet of the condensate pump; as a result, when the condensate pump was in operation, the water level at that location dropped, causing the transmitter to show significantly lower values. Later, the position of the measurement tube was moved to a location further away from the condensate pump inlet, and the condenser level transmitter then showed values consistent with the actual local water level, thereby ensuring the safe and stable operation of the condenser. Secondly, when measuring the liquid level, the differential pressure value measured by the differential pressure transmitter is relatively small, generally ranging from a few kPa to 100 kPa; therefore, the entire measurement system has a significant impact on the accuracy of the measurement. As shown in Figure 1, when installing the pressure guide tube, its horizontal section should have a certain slope, and this slope should be as large as possible to prevent liquid from accumulating inside the tube, which could lead to inaccurate measurements. In cases where the transmitter’s range is very small, this can cause fluctuations in the transmitter’s output. Furthermore, when putting the transmitter into operation, efforts should be made to remove bubbles from within the liquid column in the pressure guide tube, as these trapped gases can affect the accuracy of the measurements. It is also possible to consider installing an exhaust device at the highest point of the bend in the pressure guide tube. IV. Zero offset adjustment of the transmitter: When a differential pressure transmitter is used to measure liquid level, if the positive and negative pressure chambers of the transmitter are at the same level as the pressure sampling point in the container, no zero offset adjustment is required. In practical applications, due to considerations such as the installation location of the equipment and ease of maintenance, transmitters do not necessarily have to be at the same level as the pressure sampling point ； For example, when the medium to be measured is a highly corrosive or highly viscous liquid, it is not possible to introduce this medium directly into the transmitter; therefore, an isolation fluid tank must be installed, using the isolation fluid to transmit pressure signals and thereby prevent the transmitter from being corroded. At this point, it is necessary to consider the influence of the liquid columns of the medium and the isolation fluid on the transmitter’s measurement values. When the installation location of the transmitter is not usually on the same level as the lowest liquid level, in order to accurately indicate the liquid level height, the differential pressure transmitter must undergo certain technical adjustments, namely offsetting. Migration is divided into no migration, negative migration, and positive migration. The so-called “migration” of a transmitter refers to moving its measurement range while keeping the range constant. Moving the measurement starting point below the reference point “0” is generally referred to as negative migration ； Moving the measurement starting point above the reference point “0” is called positive migration. Taking a differential pressure transmitter with a range of 30 kPa as an example, the measurement range is 0–30 kPa when there is no offset. When the offset is 100% positive, the measurement range becomes 30–60 kPa; when the offset is 100% negative, it is –30–0 kPa. A 50% negative offset results in a measurement range of –15–+15 kPa. During actual operation, first determine the range of the differential pressure transmitter; after calibration, use the transfer screw to adjust the measurement start point or full-scale output to the appropriate position, or directly enter the transfer value using the manual operator. For example: if a differential pressure of -30 to 0 kPa needs to be measured, the range is 30 kPa. When calibrating the transmitter, 30 kPa of pressure is applied to the negative pressure chamber, and the zero adjustment knob of the differential pressure transmitter is adjusted so that its output is 4 mA ； Thereafter, without applying pressure to the negative pressure chamber, adjust the range knob of the differential pressure transmitter until the output is 20mA; if using a handheld controller, set the LRV of the transmitter to -30kPa and the URV to 0kPa. The measurement range of a differential pressure transmitter equals the sum of the span and the offset, that is, measurement range = span range + offset. Therefore, the essence of positive and negative migration is to change the upper and lower limits of the range of the differential pressure transmitter, while the size of the range remains unchanged. Based on the principle of differential pressure transmitters in measuring positive and negative level drift, in practical applications, the method of level measurement can be improved accordingly according to the operating conditions of the instrument, the process parameters of the production facility, and the surrounding environment. The capacitive transmitters produced by Runzhong Instrument Technology Co., Ltd. feature high measurement accuracy and simple calibration; the possibility of failures occurring in these transmitters is extremely low. After operating for a period of time, if there is a significant deviation in the readings or even the transmitter fails to function properly, the possible causes include blockages or leaks in the pressure conduits on the high- and low-pressure sides, changes in migration amounts, zero-point drift, loss of isolation fluid, etc. Another possibility is that the transmitter is operating in a negative pressure environment, with poor sealing in its measurement circuit, allowing external air to enter the measurement pipeline. When the output value of the differential pressure transmitter is too high or too low, the pressure guiding tube can be disconnected from the three-valve assembly to allow the negative pressure side to be in contact with atmospheric pressure. A hand pump can then be used to send the differential pressure signal to the positive pressure side of the transmitter, in order to check whether the transmitter’s output is correct – that is, whether the output current lies within the range of 4–20 mA with an error of no more than 0.8 mA. If the transmitter’s output is correct, it can be concluded that there is a blockage or leakage in the pressure guiding pipeline. (Baidu Wenku)