Applications and Selection of Pressure and Differential Pressure Transmitters

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Among various types of instruments, transmitters are the most widely used. Transmitters are generally divided into pressure transmitters and differential pressure transmitters. Transmitters are commonly used to measure pressure, differential pressure, vacuum, level, flow rate, and density, among other things. Transmitters come in two-wire and four-wire types, with two-wire transmitters being particularly common ； There are intelligent and non-intelligent types, with an increasing number of intelligent transmitters ； There are pneumatic and electric types, with electric transmitters being more common ； Additionally, depending on the application, they are divided into intrinsically safe types and flameproof types ； Based on the application conditions, the main types of transmitters are as follows: ※ Low (micro) pressure/low (micro) differential pressure transmitters ； 　　※ Medium pressure/medium differential pressure transmitter ； 　　※ High-pressure/high-differential pressure transmitter ； 　　※ Gauge/Vacuum/Negative Pressure Differential Pressure Transmitter ； 　　※ High temperature/pressure, differential pressure transmitters ； 　　※ Corrosion-resistant/pressure, differential pressure transmitters ； 　　※ Easy-to-crystallize/pressure, differential pressure transmitters. 　　 The selection of a transmitter is usually based on factors such as installation conditions, environmental conditions, instrument performance, cost-effectiveness, and the medium to be handled. In practical applications, it is divided into direct measurement and indirect measurement ； Its applications include process measurement, process control, and device interlocking. Common transmitters include ordinary pressure transmitters, differential pressure transmitters, single-flange transmitters, double-flange transmitters, and insert-type flange transmitters. 　　 　　2. Introduction to Pressure/Differential Pressure Transmitters 　　　Although pressure transmitters and differential pressure transmitters, as suggested by their names, are used to measure pressure and differential pressure (the difference between two pressures), they can indirectly measure many other quantities as well. For example, a pressure transmitter can not only measure pressure but also the liquid level inside a device. To measure the liquid level in a pressure-rated container, only a pressure transmitter is required. When measuring the liquid level in a pressurized container, it is possible to use two pressure/differential pressure transmitters – one to measure the lower limit and another to measure the upper limit. By performing a subtraction operation on their output signals, the liquid level can be determined; in this case, differential pressure transmitters are generally used. It can also be used to measure the density of the medium when the liquid level and pressure values inside the container remain constant. The measurement range of pressure transmitters can be very wide, starting from 0 absolute pressure and going up to over 100 megapascals (under normal conditions). 　　 In addition to measuring the differential pressure between two pressures, a differential pressure transmitter can also be used in conjunction with various throttling elements to measure the flow rate of fluid media. It can directly measure the liquid level in pressurized containers as well as in containers at atmospheric pressure, as well as pressure and negative pressure. 　　2.1 Construction 　　 Based on the structure of the pressure/differential pressure transmitter, there are standard types and isolated types. The measuring diaphragm of a standard pressure/differential pressure transmitter is single, and it directly senses the pressure or differential pressure of the medium being measured ； The isolated type of measuring diaphragm box is subjected to the pressure of a stable fluid (usually silicone oil), which is sealed between two diaphragms. The diaphragm that directly experiences the pressure being measured is the outer diaphragm, while the diaphragm in a conventional diaphragm box is the inner diaphragm. When a pressure signal is applied to the outer diaphragm, this pressure is transmitted intact to the conventional diaphragm box via the silicone oil, thereby allowing the pressure felt by the outer diaphragm to be measured. 　　 Isolated pressure/differential pressure transmitters are designed and used specifically for handling special types of media to be measured. If the medium in question crystallizes after leaving the device, and a conventional pressure/differential pressure transmitter is used, the removal of that medium can cause blockage in the diaphragm chamber, preventing the transmitter from functioning properly; therefore, an isolated type must be used. Isolated transmitters are usually designed for flange mounting, meaning that an opening is created in the device being monitored so that, after the transmitter is installed, its sensing diaphragm becomes part of the device’s wall. This prevents the transmitter from coming into contact with the medium being measured, and it also reduces the risk of crystallization and blockages. 　　 When the medium to be measured requires a high crystallization temperature, a structure with a protruding diaphragm can be used; this allows the sensing diaphragm to be inserted inside the device, so that the temperature of the medium being sensed does not drop. This ensures accurate measurements, which is why an insert-type temperature transmitter is chosen. 　　 Isolated transmitters are divided into remote-type and integrated types. In the remote-type configuration, a reinforced capillary tube connects the outer diaphragm box to the measuring diaphragm box; the length of such a capillary tube is usually 3 to 5 meters. In this way, the outer diaphragm box is mounted on the equipment, while the inner diaphragm box and the transmitter can be installed on mounting brackets that facilitate maintenance ； Another form is an outer membrane box integrated with the transmitter, which is directly mounted on the equipment by flange. For isolated pressure transmitters, they can also be designed with a threaded connection; that is, the outer diaphragm or outer elastic element can be located in front of the mounting threads. By welding internal threaded protrusions onto the device to be measured, the transmitter can be directly screwed onto the device, making installation very convenient. 　　 Isolated pressure/differential pressure transmitters are complex to manufacture and require high-quality materials, so their price is usually 3 to 4 times that of ordinary types. 　　2.2 Selection Principles 　　 The selection of pressure/differential pressure transmitters is primarily based on the properties of the medium to be measured, with considerations for cost savings as well as ease of installation and maintenance. If the medium being measured is highly viscous, prone to crystallization, or highly corrosive, an isolated transmitter must be used. 　　 When selecting a suitable model, it is necessary to take into account the corrosive effect of the fluid being measured on the metal of the diaphragm box; the material of the diaphragm box must be chosen carefully. Otherwise, the outer diaphragm will be corroded after a short period of use, and the gaskets will also be damaged, leading to equipment failures or safety incidents. Therefore, the selection of the diaphragm box material is extremely important. The diaphragm materials for transmitters include ordinary stainless steel, 304 stainless steel, 316/316L stainless steel, and tantalum. 　　 When selecting a model, the temperature of the medium to be measured must be taken into account. If the temperature is high, ranging from 200°C to 400°C, a high-temperature version should be chosen; otherwise, the silicone oil will vaporize and expand, leading to inaccurate measurements. 　　 When selecting a device, the operating pressure rating of the equipment must be taken into consideration; the pressure rating of the transmitter must be suitable for the application. From an economic perspective, the material of the outer membrane box and the insertion parts is quite important; it is necessary to choose the appropriate material. However, the use of coupling flanges can reduce the material requirements, as materials such as carbon steel or chromium-plated steel can be used, which helps to save a lot of money. 　　 For isolated pressure transmitters, a threaded connection is the best choice, as it saves costs and facilitates installation. 　　 When selecting ordinary pressure and differential pressure transmitters, the corrosivity of the medium being measured also needs to be taken into account. However, the temperature of the medium is not a factor to consider, as in the case of ordinary transmitters, the pressure is introduced inside the gauge, and the operating temperature remains at room temperature over time. Nevertheless, ordinary transmitters require more maintenance compared to isolated types. First is the issue of heat retention; when the temperature is below zero, the pressure transmission pipes freeze, preventing the transmitters from functioning or even causing damage. This necessitates the use of devices such as heating elements and insulation enclosures. 　　 From an economic perspective, when selecting transmitters, ordinary-type transmitters can be used for media that do not tend to crystallize. For low-pressure media that are prone to crystallization, indirect measurement can be achieved by using a purge medium (provided that the process allows the use of purge liquid or gas). The use of ordinary-type transmitters requires maintenance personnel to conduct regular inspections, including checking for leaks in various pressure conduits, ensuring that the purge medium is functioning properly, and verifying that the insulation is in good condition. With proper maintenance, the upfront cost of using ordinary-type transmitters can be significantly reduced. When maintaining, it is important to combine hardware maintenance with software maintenance. 　　 In terms of the measurement range of the transmitter selected, most transmitters have a certain adjustable range; it is best to set the operating range within 1/4 to 3/4 of that range, as this ensures better accuracy, which is especially important for differential pressure transmitters. In practice, there are some application scenarios (such as level measurement) that require the adjustment of the transmitter’s measurement range. The measurement range and the amount of adjustment are determined based on the installation location; such adjustments can be either positive or negative. 　　 Currently, smart transmitters are quite widespread. They feature high precision, a wide adjustable range, easy adjustment, and good stability, factors that should be taken into consideration when selecting them. 　　 According to design specifications, when selecting a transmitter for engineering design, whether to use a pneumatic transmitter or an electric transmitter should be determined through comprehensive consideration and analysis based on the specific conditions of the installation, as each type has its own advantages. The following points can be used as a reference when making a selection: ※ Degree of centralized operation and response speed ； 　　 ※ Is it coordinated with DCS computer operations? ； 　　 ※ Economics, reliability, and operation/maintenance ； 　　 ※ Safety (explosion protection, power outages, gas supply failures, etc.) ； 　　 ※ Environmental conditions and transmission distance. 　　 Generally speaking, the following conditions indicate that it is appropriate to use a pneumatic transmitter: ※ The distance between the transmitter and the display/control unit is short; usually, it is better not to exceed 150 meters ； 　　 ※ Process materials are flammable and explosive substances, as well as in environments with high relative humidity ； 　　 ※ Suitable for applications where low investment in instruments is required and fast response times are not necessary ； 　　 ※ Generally, small and medium-sized enterprises require ease of maintenance and cost-effectiveness ； 　　 ※ In large installations dominated by electric instruments, some on-site control loops do not require centralized operation from a control room. 　　 The following conditions indicate that it is advisable to use an electric transmitter: ※ The distance between the transmitter and the display/control unit is more than 150 meters ； 　　 ※ Large enterprises require a centrally controlled system with a high degree of centralization ； 　　 ※ Objects equipped with DCS computers for control and management ； 　　 ※ Suitable for applications that require fast response times and complex information processing and calculations. 　　 In practice, in modern production facilities, they are combined and selected based on their respective characteristics. 　　 　　3. Selection of pressure transmitters 　　 From a physical perspective, the pressure acting on any object consists of two components: atmospheric pressure and the pressure of the medium being measured (commonly referred to as gauge pressure). The sum of these two pressures acting on the object under test is called absolute pressure. 　　 P_abs = P_gauge + Atmospheric pressure. A gauge used to measure absolute pressure is called an absolute pressure gauge. Ordinary industrial pressure gauges measure gauge pressure, which is the difference between absolute pressure and atmospheric pressure. When the absolute pressure is greater than the atmospheric pressure, the gauge pressure measured is positive, and it is referred to as positive gauge pressure ； When the absolute pressure is lower than the atmospheric pressure, the gauge pressure measured is negative; this is known as negative gauge pressure, or vacuum level. A device used to measure vacuum level is called a vacuum gauge. 　　 ⑴ To ensure the accuracy of pressure measurement, the minimum pressure reading should be above 1/3 of the gauge’s measurement range ； 　　 ⑵ For applications that require remote measurement or high measurement accuracy, pressure sensors or pressure transmitters should be used ； 　　 ⑶ When high measurement accuracy is not required, resistive or inductive, Hall-effect remote pressure transmitters can be chosen ； 　　 ⑷ Pneumatic base-type pressure indicating regulators are suitable for on-site pressure indication and regulation ； 　　 ⑸ Pressure transmitters and pressure switches should be selected appropriately in accordance with the explosion-proof requirements of the installation location. 　　 　　4. Selection of differential pressure transmitters The selection of differential pressure transmitters is based on the following factors: (1) Measurement range, required accuracy, and measurement functions ； 　　⑵ The environment in which measuring instruments operate, such as the industrial environments in the petrochemical industry, features the presence of hot (toxic) and explosive atmospheres, as well as high ambient temperatures ； 　　⑶ The physicochemical properties and state of the medium under test, such as conditions involving strong acids, strong bases, viscosity, tendency to solidify or crystallize, and vaporization ； 　　⑷ Changes in operating conditions, such as variations in medium temperature, pressure, and concentration. Sometimes it is also necessary to take into account the changes in the concentrations and densities of the gas and liquid phases from the moment driving starts until the parameters reach normal production levels ； 　　⑸ The structure, shape, and dimensions of the container under test, as well as the equipment and accessories inside it and various inlet and outlet pipes, must all be taken into consideration, such as towers, solution tanks, reactors, boiler drums, vertical tanks, spherical tanks, etc ； 　　⑹ Other requirements, such as environmental protection and hygiene requirements ； 　　⑺ The selection of engineering instruments should be based on unified criteria, with an effort to minimize the number of different models and specifications as well as the amount of spare parts required, to facilitate management ； 　　⑻ Actual process conditions: 　　 ① Consider which category of equipment the object under test belongs to. Such as tanks and vessels: tanks have a smaller volume, so the range of measurement is not very large, while vessels have a larger volume, resulting in a potentially larger measurement range ； 　　 ② Depending on the physical and chemical properties of the medium as well as its level of cleanliness, conventional differential pressure transmitters and float-type level transmitters are the preferred choices; in addition, it is necessary to select the material for the parts that come into contact with the medium ； 　　 ③ For some media such as suspended solids and foam, a single-element Bourdon tube differential pressure transmitter can be used. For those that are prone to precipitation and crystallization, use plug-in dual-element differential pressure transmitters ； 　　 ④ For the level of highly viscous media as well as that of high-pressure equipment, where it is not possible to make openings in the equipment, radio level gauges can be used for measurement ； 　　 ⑤ In addition to issues with measurement methods and technology, there is also the issue of investment in instruments. 　　In summary, the selection of a transmitter should be technically feasible, economically reasonable, and convenient to manage.