What are the differences between the selection of pressure transmitters for nuclear power plants and those for conventional applications?

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This article introduces the types of pressure transmitters commonly used in nuclear power plants. Based on the technical requirements for such transmitters in nuclear power plants, it provides selection methods and precautions, offering a reference for the selection and design of pressure transmitters in these facilities. Pressure transmitters: yunrun.com.cn/product/list_84.html In nuclear power plants, process monitoring instruments serve as the interface between the entire instrument control system and the industrial process; the quality of these instruments directly affects the performance of the instrument control system. Among various types of process monitoring instruments, pressure transmitters are the most widely used and common; they are primarily employed to measure pressure, flow rate, and liquid level in process systems. Safety-grade pressure transmitters provide the essential signals for the protection systems in nuclear power plants as well as for post-accident monitoring purposes, serving to ensure safety and facilitate monitoring after an accident occurs. At the same time, the availability of pressure transmitters required for severe accident management under severe accident conditions is also relevant to whether the severe accident management strategies outlined in the guidelines can be effectively implemented in practice. Therefore, selecting pressure transmitters correctly and appropriately is crucial for ensuring the safe and reliable operation of nuclear power plants. Types of pressure transmitters in nuclear power plants: There are many types of pressure transmitters, which can be classified into the following categories based on different classification criteria. 1. Based on the type of pressure being measured, transmitters can be classified into pressure transmitters, differential pressure transmitters, and gauge pressure transmitters. 2. Based on the measurement principle, the pressure transmitters commonly used in nuclear power plants can be classified into: capacitive transmitters, varistor-type pressure transmitters, inductive displacement pressure transmitters, monocrystalline silicon pressure transmitters, and force-balanced transmitters. 3. Based on whether they measure the medium pressure directly, pressure transmitters can be divided into basic transmitters and flange-mounted transmitters with remote capillaries. 4. Based on the functions they perform, nuclear power plant pressure transmitters can be classified as safety-related pressure transmitters and non-safety-related pressure transmitters, depending on whether they carry out safety functions. Safety-related pressure transmitters must undergo quality assessments in accordance with specific certification requirements to ensure that they can perform their functions correctly when needed. Design principles for pressure transmitters in nuclear power plants. The selection and design of pressure transmitters in nuclear power plants are based on the following principles: 1. Meeting the requirements of relevant design, manufacturing, inspection, and quality assessment standards and specifications for pressure transmitters in nuclear power plants ； 2. Meet the measurement requirements of the process system for this type of instrument ； 3. Meet the specific requirements for this type of instrument as stipulated in the guidelines for nuclear power plants regarding the implementation of safety functions, safety analysis, and accident management ； 4. Facilitates the installation and maintenance of instruments ； 5. Consider increasing the local production rate of equipment; where technical requirements are met, opt for domestically produced products whenever possible ； 6. To meet the requirements of subsequent upgrades to power plant equipment, minimize the variety of instruments to facilitate maintenance. Selection of pressure transmitters for nuclear power plants: The selection of pressure transmitters and differential pressure transmitters in nuclear power plants is typically determined based on factors such as the functions performed by these devices, the type of process medium being measured, the measurement range, characteristics, installation conditions, environmental conditions, instrument performance, and cost considerations. These transmitters must not only meet requirements for high reliability, stability, and precision but also satisfy the safety design requirements of protection systems. Additionally, they need to comply with the design requirements of fully digital control systems, while also taking into account the operational and maintenance needs of the power plant. When selecting pressure transmitters for nuclear power plants, the following factors should be taken into consideration: 1. Safety classification. The safe operation of a nuclear power plant depends primarily on the reliability of the equipment that carries out safety functions. Electrical equipment that performs safety functions such as emergency shutdown, emergency core cooling, heat removal, containment isolation, containment heat dissipation, and prevention of the release of radioactive materials into the environment is referred to as Class 1E electrical equipment. Especially for nuclear-grade differential pressure transmitters/pressure transmitters installed within nuclear power plant containment, the working conditions are extremely harsh, as they operate in an irradiated environment and are required to maintain their functionality and performance under seismic and accident conditions such as LOCA accidents. To ensure that the transmitter can perform its specified safety functions under the most severe environmental conditions during normal operation or in accident scenarios at nuclear power plants throughout its service life, transmitter products that have passed quality assessment must be used. For some instruments that need to transmit signals to post-incident monitoring systems and protection systems, qualified instruments should be selected in accordance with the requirements for K1, K2, and K3 class instruments specified in RCC-E (with some of these requirements potentially being met equivalently through IEEE-323/344 standards). While taking into account the safety criteria for power plants, due to limited domestic manufacturing capabilities, for some K1-class instruments, products from foreign companies that have the qualifications to supply nuclear equipment and possess extensive experience in such applications are considered. For pressure transmitters that must be used in severe accidents, quality assessment of such transmitters in accordance with technical requirements or a credible analysis is necessary to prove that they can withstand the environmental conditions of severe accidents and remain functional during and after an accident. 2. Type and measurement range of pressure transmitters: Depending on the requirements of the process parameters and the type of pressure to be measured, appropriate pressure transmitters, differential pressure transmitters, or gauge pressure transmitters should be selected. When choosing a pressure transmitter, it is necessary to take into account the operating pressure rating of the equipment as well as that of the transmitter itself. The range of the selected transmitter falls within the transmitter’s measurement range ； The range of the selected transmitter falls within the actual usable range ratio of the transmitter ； The range of the selected transmitter should be as close as possible to the transmitter’s maximum range. When measuring stable pressure, the range of a conventional pressure transmitter is usually chosen to be close to or slightly greater than 1.5 times the normal pressure value ； When measuring pulsating pressure, a pressure transmitter is generally selected with a value that is close to or slightly greater than twice the normal pressure level, taking into account the transmitter’s overload capacity beyond its rated range ； When measuring the pump outlet pressure, the maximum range is generally chosen to be close to the maximum pressure value at the pump outlet ； When measuring high-pressure values, the maximum range selected should generally be more than 1.7 times the highest measured pressure value. The measurement range must lie within the transmitter’s specified range; it must not be below the transmitter’s minimum range. Especially in applications where differential pressure is used to measure level, it is important to ensure that the range of the differential pressure transmitter after calibration does not exceed its maximum range (URL), otherwise accurate measurements cannot be obtained. When selecting a differential pressure transmitter, its ability to withstand static pressure and the impact of static pressure on measurement accuracy should be taken into consideration. For situations where the medium being measured is viscous, prone to solidification, prone to crystallization, or operates at high temperatures, a remote-flange transmitter can be chosen. For the continuous measurement of the containment sump level used for post-accident monitoring, a remote-flange transmitter with a single-sided capillary was also selected. When selecting the capillary length for remote pressure transmitters and remote differential pressure transmitters, it is necessary to take into account both the convenience of installation and maintenance, as well as the adverse effects that an overly long capillary can have on the measurement process, such as output variations caused by temperature changes and slower response times. 3. Select the appropriate material: Based on the physical and chemical properties of the medium to be measured, choose the materials for the liquid-contacting parts (process interfaces) and non-liquid-contacting parts, as well as the diaphragm of the transmitter. In the parts that come into contact with the medium, the material used for the transmitter must meet the requirements of the RCC series standards; generally, austenitic ultra-low carbon stainless steel is required for such applications. Commonly used materials include 304L or 316L (according to ASTM standards). For systems exposed to seawater, corrosion-resistant Hastelloy C should be chosen. At the same time, the requirements for nuclear cleanliness must be taken into account; different cleanliness standards are applied depending on whether it is the primary circuit, the media injected into the primary circuit, the secondary circuit fluids, or the waste liquid storage tanks. In the actual selection process, due to the differences in the structure of the transmitters themselves, some special considerations are required. For example, if the discharge mechanism of the transmitter is a needle valve, using 316L material for the sealing ball can be detrimental to the sealing surface after multiple discharge operations; in such cases, it is advisable to use a sealing ball made of 316 material. 4. Requirements for operation and maintenance: When selecting pressure transmitters and differential pressure transmitters, it is necessary to take into account the requirements of the control system; instruments with higher precision should be chosen based on the desired control performance. For the instruments used in the thermal balance calculations of the test instrument system (KME), pressure transmitters and differential pressure transmitters with a precision of (0.025%) and superior performance were selected to meet the precision requirements. Second, the convenience of maintenance must be taken into account. Taking radiation protection zones into account, instruments should be placed in areas accessible to personnel, mounted on instrument racks for easy maintenance and operation, to meet the requirements for regular testing and instrument calibration. 5. Other considerations: In the selection and design of pressure transmitters for nuclear power plants, it is also necessary to take into account the requirements specified in the safety guidelines regarding the functional accuracy, response time, accuracy after an accident, and consistency of special measurement points. Furthermore, if the environment in which the instrument is located contains a flammable and explosive atmosphere, explosion protection must be considered. Some instruments, although classified as non-safety grade in the electrical equipment classification list, have requirements to perform their functions in earthquake situations, or the radiation level in their installation locations is such that a certain dose of radiation is present. To ensure the measurement accuracy and stability of pressure transmitters over the long term, when selecting such transmitters, it is common to opt for those that have undergone radiation aging tests as well as seismic resistance tests. Selection of Pressure Transmitters for Nuclear Power Plants yunrun.com.cn/tech/1916.html This article describes the types of pressure transmitters commonly used in nuclear power plants as well as their design principles. It summarizes the issues that need to be considered in the selection and design of pressure transmitters, along with special considerations, and can serve as a reference for selecting such transmitters in nuclear power projects. Authors: Liu Li, Wu Ping, Guo Lin, Run Guiyin, China Nuclear Power Engineering Corporation