Thread Content
In industrial production processes, especially in industries such as chemicals and petroleum refining, pressure is one of the important operational parameters. Measurements of pressure and vacuum are encountered frequently, including high pressures that are much higher than atmospheric pressure, ultra-high pressures, as well as vacuums that are much lower than atmospheric pressure. If the pressure is not within the required range, it will not only affect production efficiency and reduce product quality, but sometimes it can also lead to serious production accidents. Furthermore, the significance of pressure measurement is not limited to itself; the measurement of some other parameters, such as level and flow rate, is often carried out by measuring pressure or differential pressure. In other words, by determining the pressure or differential pressure, it is possible to determine the level or flow rate. I. The concept of pressure Pressure is a force that acts vertically and evenly on a unit area. It is expressed as follows: P = F/S, where P represents pressure, in Pa ； F: Vertical force, N ； S: Area under stress, in cm2. Pressure can also be expressed as the equivalent height of a liquid column, that is: P = F/S = S•ρ•h/S = ρ•h. Here, h is the height of the liquid column, in cm ； ρ: Density of the liquid, in g/cm2. The units of pressure include the following: 1. Physical atmosphere (also known as standard atmosphere). Internationally, it is defined as the gravitational pressure of an air column at sea level at a latitude of 45°, with a temperature of 0°C and an area of 1 cm2; this value corresponds to one physical atmosphere, or one standard atmosphere. 2. Engineering atmosphere: One engineering atmosphere corresponds to a uniform, vertical pressure of 1 kg per square centimeter of area. 3. Pascal (Pa) – Pascal is a unit in the International System of Units (SI). 1 Pa is the pressure resulting from a force of 1 N acting vertically and evenly on an area of 1 m2. 4. Millimeters of water column and millimeters of mercury are two pressure units commonly used to represent low pressures. It is equivalent to the gravitational pressure exerted by 1 mm of water or mercury vertically acting on a base area. The condition is that the acceleration due to gravity g=980. 665 cm/s2, at a temperature of 0°C (mercury) or 4°C (water). In pressure measurement, there are often distinctions between gauge pressure, absolute pressure, negative pressure, and vacuum level. The relationship among them is as follows: Absolute pressure: Pressure measured with the absolute pressure zero point as a reference, i.e., pressure above that zero point. Positive pressure: Pressure that is higher than atmospheric pressure, measured with atmospheric pressure as the reference. Negative pressure (vacuum): A pressure that is below atmospheric pressure, measured with atmospheric pressure as the reference. Differential pressure: The difference between two pressures. Gauge pressure: Pressure that is greater than or less than atmospheric pressure, measured with atmospheric pressure as a reference. II. Pressure measuring instruments 1. Elastic pressure gauges Elastic pressure gauges are measuring instruments that utilize various types of elastic elements; they work on the principle that, under the pressure of the medium being measured, these elastic elements undergo elastic deformation as a result of being compressed. This instrument features advantages such as a simple structure, reliable operation, clear readings, durability, low cost, a wide measurement range, and sufficient accuracy. By adding additional devices such as recording mechanisms, electrical conversion units, control elements, etc., it is possible to achieve pressure recording, remote transmission, signal alarms, automatic control, and more. Elastic pressure gauges can be used to measure pressures in the range of several hundred pascals to several thousand megapascals, which is why they are the most widely used type of pressure measuring instrument in industry. (1) Elastic elements are simple and reliable pressure-sensitive components. It is not only the pressure-sensing element of elastic pressure gauges, but it is also often used as a basic component in pneumatic unit combination instruments. Commonly used elastic elements include helical springs, diaphragms, bellows, and corrugated tubes; their structures are shown in the figure: ① Helical spring type elastic element. The pressure measurement range of helical spring type elastic elements is quite wide, allowing them to measure pressures as high as 1000 MPa. A single-loop spring tube is a metal tube bent into an arc, with a cross-section that is flat circular or oval in shape, as shown in Figure (a). To increase the displacement of the free end, a multi-turn spring tube can be made, as shown in Figure (b). ② Thin-film elastic elements can be further classified into diaphragms and bellows, etc., depending on their structure. Its pressure measurement range is lower than that of the Bourdon tube type. Figure (c) shows a diaphragm-type elastic element, which is an elastic diaphragm made of metallic or non-metallic materials (available in flat and corrugated forms) that can deform under pressure. Sometimes, two metal diaphragms can also be welded together along their perimeters to form a thin-walled box filled with a liquid (such as silicone oil), which is called a diaphragm box, as shown in figure (d). ③ Bellows-type elastic element: A bellows-type elastic element is a thin-walled metal cylinder with corrugations around its perimeter, as shown in Figure 3–5(e). Such elastic elements are easy to deform and can undergo large displacements; they are commonly used for measuring micro- and low pressures (generally not exceeding 1 Mpa). (2) Bourdon tube pressure gauge ① Structure: ② Working principle: When the pressure to be measured is applied, the cross-section of the bourdon tube tends to expand from oval to circular, causing the entire tube to straighten slightly (the angle of rotation for a typical pressure gauge is 5–20°), which in turn forces the free end to expand upward to the right. The pressure value is displayed on the dial through gear transmission. 2. Electrical pressure gauge: An electrical pressure gauge is a device that can convert pressure into electrical signals for transmission and display. This type of gauge has a wide measurement range, capable of measuring pressures from 7×10-5 Pa to 5×102 MPa, with an allowable error as low as 0. 2%. Since signals can be transmitted over long distances, automatic pressure control and alarm functions can be implemented in industrial production processes, and it can also be used in conjunction with industrial control computers. An electrical pressure gauge generally consists of a pressure sensor, a measurement circuit, and a signal processing unit. Common signal processing devices include indicators, recorders, controllers, microprocessors, etc. The function of a pressure sensor is to detect pressure signals and convert them into electrical signals for output. When the electrical signals generated can be further transformed into standard signals, such a pressure sensor is also referred to as a pressure transmitter. A standard signal is one whose form and numerical range of a physical quantity comply with international standards. For example, direct current of 4–20 mA, air pressure of 0. 02 one 0. 1 Mpa is the currently standard signal. (1) Hall plate pressure sensor: The Hall plate pressure sensor is based on the Hall effect; it uses a Hall element to convert the displacement of an elastic element caused by pressure into a Hall voltage, thereby enabling the measurement of pressure. A Hall element is a thin sheet made of a semiconductor material such as germanium, as shown in the figure: A constant magnetic field with intensity B is applied in the Z-axis direction of the Hall element, while an external electric field is applied in the Y-axis direction (by connecting it to a DC regulated power supply). A constant current then flows in the Y-axis direction. As electrons move through the Hall element, they are deflected due to electromagnetic forces, causing an accumulation of electrons on one end face of the element and a surplus of positive charges on the other end face. As a result, a potential difference is created in the X-direction of the Hall element; this potential difference is known as the Hall voltage. This physical phenomenon is referred to as the “Hall effect”. The magnitude of the Hall voltage is related to factors such as the semiconductor material, the current flowing through it (commonly referred to as the control current), the magnetic flux density, and the geometric dimensions of the Hall element. It can be expressed by the following formula: UH = RHBI, where UH represents the Hall voltage ； RH: Hall constant, which is related to the material of the Hall element and its geometry ； B: Magnetic flux density ； I: Current passing through. From the above equation, it can be seen that the Hall potential is proportional to the magnetic flux density and the current. Increasing the B and I values can increase the Hall potential UH. But both have certain limits: generally, I is between 3 and 20 mA, B is around a few thousand gauss, and the resulting Hall voltage UH is on the order of several tens of millivolts. If a Hall element is selected and the current is kept constant, then in a non-uniform magnetic field, the magnetic flux density experienced by the Hall element varies depending on its position; this results in a Hall voltage that is proportional to the displacement, thereby achieving a linear conversion between displacement and voltage. (2) Strain gauge pressure sensor A strain gauge pressure sensor is constructed based on the principle of resistive strain. Resistive strain gauges are divided into two types: metal strain gauges (metal wires or metal foils) and semiconductor strain gauges. The measured pressure causes strain in the strain gauge. When a strain gauge experiences compressive strain, its resistance decreases ； When a strain gauge experiences tensile strain, its resistance increases. The change in the resistance value of the strain gauge results in a corresponding millivolt-level voltage output through a bridge circuit; this voltage is then displayed using a millivoltmeter or other recording instrument to indicate the pressure being measured, thereby forming a strain gauge-type pressure gauge. Strain gauges r1 and r2, together with two fixed resistors r3 and r4, form a bridge circuit, as shown in the figure. Changes in the resistance values of r1 and r2 cause the bridge to lose balance, resulting in an unbalanced voltage ΔU that serves as the output signal of the sensor. When a DC regulated power supply with a maximum voltage of 10V is used for the bridge, a maximum output value of ΔU of 5mV can be obtained. The pressure to be measured by the sensor can reach 25 Mpa. Since the natural frequency of the sensor is above 25,000 Hz, it exhibits good dynamic performance. Suitable for pressure measurement in rapidly changing conditions. The nonlinearity and hysteresis errors of the sensor are less than 1% of the rated pressure. (3) Piezoresistive pressure sensor: A piezoresistive pressure sensor is constructed by utilizing the piezoresistive effect of single-crystal silicon. Its working principle is as follows: A single-crystal silicon wafer is used as the elastic element; through integrated circuit techniques, equivalent resistors are diffused in specific directions within the single-crystal silicon membrane, and these resistors are connected in a bridge configuration. The single-crystal silicon wafer is then placed inside the sensor chamber. When the pressure changes, the single crystal silicon undergoes strain, causing the strain resistor located directly on it to exhibit a change that is proportional to the measured pressure; a corresponding voltage output signal is then generated by the bridge circuit. Piezoresistive pressure sensors feature high precision, reliable operation, high frequency response, low hysteresis, small size, light weight, and a simple structure. They can operate in harsh environmental conditions and facilitate digital display. Piezoresistive pressure sensors can not only be used to measure pressure; with slight modifications, they can also be used to measure parameters such as differential pressure, altitude, velocity, and acceleration. (4) Torque-balanced pressure transmitter: The torque-balanced pressure transmitter is a typical self-balancing measuring instrument. It utilizes the principle of negative feedback to overcome the adverse effects of component materials and manufacturing processes, thereby enabling the instrument to possess advantages such as high measurement accuracy (usually at 0.5 grade), stable and reliable operation, good linearity, and a small range of insensitivity. The DDZ—Ⅲ type electric torque-balanced pressure transmitter will be used as an example below. The DDZ—Ⅲ series is powered by 24V DC, outputs 4–20mA (DC), operates on a two-wire system, and is intrinsically safe and explosion-proof. Through derivation, the relationship between the output current and the measured pressure can be obtained: I0=Kpθ, where K is the conversion ratio constant; once the structure and electromagnetic properties of the transmitter are determined, K becomes a constant value. The above equation shows that, once the angle θ of the vector mechanism is determined, the output current I0 of the transmitter is proportional to the input pressure p. As shown in the figure above, by adjusting the range-setting screw 5, the angle θ of the vector mechanism can be changed, thereby enabling a continuous adjustment of the transmission ratio between the two levers; this in turn allows for the adjustment of the transmitter’s range. Typically, the vector angle θ can be adjusted between 4° and 15°; as a result, tgθ changes by about a factor of 4, and accordingly the range can also change by a factor of 4. Adjusting the tension of spring 12 allows for the adjustment of the zero point. If the pressure-sensing elastic element of the above-mentioned pressure transmitter is slightly modified, it can be used to continuously measure differential pressure or absolute pressure, with the working principle remaining essentially the same. (5) Capacitive pressure transmitters: Introduced to the market in the early 1970s by the United States, these are open-loop measuring instruments that offer a number of advantages such as simple structure, high overload capacity, good reliability, high measurement accuracy, small size, light weight, and ease of use. They have now become the most popular type of pressure and differential pressure transmitters, with their output signal being a standard 4–20mA (DC) current signal. A capacitive pressure transmitter first converts pressure changes into changes in capacitance, and then performs the measurement. The schematic diagram of the capacitive differential pressure transmitter is shown on the left: the outer sides of the symmetrically shaped stainless steel bases are processed into annular corrugated grooves, and a corrugated diaphragm is welded to them. The structure of the capacitive differential pressure transmitter enables effective protection of the measuring diaphragm. When the differential pressure becomes too high and exceeds the allowable measurement range, the measuring diaphragm presses smoothly against the concave glass surface, which prevents it from being damaged. It also has good recovery characteristics after being overloaded, thereby **improving its overload tolerance. Compared to the torque-balancing type, the capacitive type lacks a lever mechanism, resulting in a compact size, good sealing and vibration resistance, as well as improved measurement accuracy that can reach 0. Level 2. 3. Intelligent pressure transmitters: Intelligent pressure or differential pressure transmitters are smart sensing instruments that are created by adding a microprocessor circuit to conventional pressure or differential pressure sensors. For example, by combining a temperature-compensated capacitive sensor with a microprocessor, an accuracy of 0 can be achieved. A pressure or differential pressure transmitter of grade 1, with a range of 100:1. The time constant can be adjusted between 0 and 36 seconds. Using a handheld communicator, it is possible to set the operating parameters of field transmitters within a range of 1500 meters, adjust their measurement ranges, and send information data to the transmitters. Smart transmitters are characterized by their ability to enable remote communication. Using a handheld communicator, it is possible to select and calibrate various operating parameters for the field transmitter ； It has high precision and is easy to use and maintain. By developing various programs, the transmitter is equipped with multiple functions such as self-correction, self-compensation, self-diagnosis, and error condition alarm. Thus, the accuracy of the transmitter is improved, and the adjustment, calibration, and maintenance processes are simplified. It enables direct communication between the transmitter, the computer, and the control system. 4. Selection and installation of pressure gauges (1) Selection of pressure gauges ① Selection of instrument type: The choice of instrument type must meet the requirements of the production process. For example, whether remote transmission, automatic recording, or alarms are required ； Do the physicochemical properties of the medium being tested (such as corrosivity, temperature, viscosity, level of contamination, flammability, and explosiveness) impose any special requirements on the measuring instruments? ； Whether on-site environmental conditions (such as high temperatures, electromagnetic fields, vibrations, and installation conditions at the site) impose any special requirements on the type of instrument, etc. ②Determination of the instrument’s measurement range: The measurement range of an instrument refers to the range within which it can measure the quantity being measured with the specified accuracy. It is determined based on the magnitude of the parameters that need to be measured during operation. When measuring pressure, in order to extend the service life of the instrument and prevent the elastic components from being damaged due to excessive stress, the upper limit of the pressure gauge should be higher than the maximum pressure that may occur during production. According to the \"Technical Regulations for Chemical Process Automation Control Design\": when measuring stable pressure, the maximum operating pressure should not exceed 2/3 of the upper measurement limit ； When measuring pulsating pressure, the maximum operating pressure should not exceed 1/2 of the upper measurement limit ； When measuring the maximum pressure, the maximum operating pressure should not exceed 3/5 of the upper measurement limit. To ensure the accuracy of the measurement values, the pressure value being measured should not be too close to the lower limit of the instrument; in other words, the range of the instrument should not be set too large. It is generally advisable that the minimum value of the pressure to be measured be no less than 1/3 of the instrument’s full scale. After calculating the upper and lower limits of the instrument based on the maximum and minimum values of the parameters being measured, rounding should be performed in accordance with the regulations or standards set by the **competent authority. Therefore, when selecting the scale limits for instruments, it is also necessary to use the values specified in the relevant regulations or standards (which can generally be found in the corresponding product catalogs). ③Selection of instrument accuracy class: The accuracy of an instrument is determined based on the maximum allowable measurement error in the manufacturing process. For example: the outlet pressure range of a reciprocating compressor is 25–28 MPa, and the measurement error must not exceed 1 MPa. The process requires on-site monitoring as well as alarm functions for high and low limits. Try to select a pressure gauge correctly, indicating the model, accuracy, and measurement range. Solution: Due to the large pressure fluctuations at the outlet of reciprocating compressors, the upper limit value for the instrument is set to P1 = Pmax × 2 = 28 × 2 = 56 MPa. Based on on-site observations and the need for high and low limit alarms, a Y-150 type electric contact pressure gauge is selected, with a measurement range of 0–60 MPa. Since 25/60 > 1/3, the minimum value of the measured pressure is not lower than 1/3 of the full scale, which is acceptable. Furthermore, based on the requirements for measurement error, the allowable error can be calculated as 1/60×100% = 1. 67%, so the accuracy level is 1. The instrument at level 5 fully meets the error requirements. It can now be determined that the selected pressure gauge is the Y-150 type electric contact pressure gauge, with a measurement range of 0 to 60 MPa. The accuracy grade is 1. Level 5. (2) Installation of the pressure gauge The correct installation of the pressure gauge directly affects the accuracy of the measurement results as well as its service life. ①Selection of pressure measurement points: The selected pressure measurement points should be able to reflect the true magnitude of the pressure being measured. To this end, the following points must be noted: a It should be selected in the section of the pipeline where the medium flows in a straight line, and not at pipe bends, forks, dead corners, or other areas prone to vortex formation. b When measuring the pressure of the flowing medium, the pressure measurement point should be perpendicular to the direction of flow. The inner end surface of the pressure measurement tube should be flush with the inner wall at the connection point to the production equipment; there should be no protrusions or burrs. When measuring liquid pressure, the pressure measurement point should be located at the lower part of the pipe to prevent gas from accumulating in the pressure conduit; when measuring gas pressure, the pressure measurement point should be located at the upper part of the pipe to prevent liquid from accumulating in the pressure conduit. ②Layout of pressure conduits: The diameter of tube A should be appropriate, with an inner diameter generally ranging from 6 to 10 mm. Its length should be as short as possible, with a maximum length of 50 m, in order to reduce delays in pressure indication. When the pressure guiding pipe is installed horizontally, it should have a slope of 1:10 to 1:20. To facilitate the discharge of the liquid (or gas) accumulated within it. c When the medium is prone to condensation or freezing, insulation and heating pipelines must be installed. A shut-off valve should be installed between the pressure tapping point and the pressure gauge, for use when servicing the pressure gauge. ③Installation of the pressure gauge: The pressure gauge should be installed in a location where it is easy to observe and maintain. b The installation location should be chosen to minimize the effects of vibration and high temperatures. When measuring steam pressure, a condensate tube should be installed to prevent high-temperature steam from coming into direct contact with the pressure-sensing element, as shown in Figure (a) below ； For pressure measurement in corrosive media, an isolation tank filled with a neutral medium should be used. Figure (b) shows the two situations where the density ρ2 of the medium to be measured is greater than or less than the density ρ1 of the isolation fluid. At the connection points of the c-pressure gauge, appropriate materials should be selected as sealing gaskets, depending on the level of pressure to be measured and the properties of the medium, in order to prevent leaks. When the pressure being measured is low and the pressure gauge is not at the same height as the pressure sampling point, the measurement error resulting from this height difference should be corrected using Δp=±Hρg. For safety reasons, in addition to choosing a pressure gauge with vent holes for measuring high pressures, its casing should be installed facing the wall or in a location where no one passes by, to prevent accidents.