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Level measurement - differential pressure level transmitter

Jan 18, 2019

Level measurement - differential pressure level transmitter 

Differential pressure level transmitter overview

A differential pressure type level transmitter is a meter that uses the pressure generated by a liquid column to measure the height of the liquid level. After the liquid level changes, the pressure received by the diaphragm at the high pressure side flange is as a result of the change, the pressure difference calculated by the transmitter will also change, and there is a linear relationship between them. Normally, the high pressure side (H side) and the low pressure side (L side) cannot be reversed. Generally, the H side is installed at a low position of the device, and the L side is installed at a high place of the device. The flange surface of the equipment should be cleaned before installation, and the gasket should not be deflected to prevent the diaphragm from being punctured or deformed to cause distortion of the transmitter.

Level measurement - differential pressure level transmitter

According to the measurement range, the transmitter can be divided into general pressure transmitter (0.001MPa ~ 35MPa) and differential pressure transmitter (0 ~ 1.5kPa), three kinds of negative pressure transmitter, from the perspective of accuracy, because we The national standard accuracy is 0.5%. In recent years, it can be divided into high-precision pressure transmitters (0.1% or 0.2% or 0.075%) and general pressure transmitters (0.5%). If the liquid phase density changes greatly, it is not appropriate to use a differential pressure level transmitter.

First, The structure and working principle of the differential pressure level transmitter

Double flange differential pressure transmitter structure: The main components are sensor module, electronic component housing, capillary tube, high and low pressure side flange and diaphragm. The sensor module includes an oil-filled sensor system (isolator, oil-filled system and sensor) and sensor electronics.

The working principle of the differential pressure level transmitter ( dp sensor ): divides a space sensitive component (multi-purpose capsule) into two chambers, and when the pressure is introduced into the two chambers respectively, the sensor generates displacement (or displacement) under the joint pressure of both sides. The trend is proportional to the pressure difference between the two chambers (differential pressure), which translates this displacement into a standard signal (4-20mADC signal) output that reflects the magnitude of the differential pressure, capillary, pressure tube, fill The function of the liquid is to transfer the received pressure to the inside of the transmitter for calculation.

The result measured by the differential pressure transmitter is the pressure difference, ie ΔP = ρg Δh. The barrel groove is generally cylindrical and the cylindrical cross-sectional area S is constant. When the temperature changes, the volume of the medium expands or shrinks, and the actual liquid level also rises or falls, but the pressure detected by the transmitter remains constant. If the actual liquid level needs to be displayed, it can also be solved by introducing medium temperature and pressure compensation in the DCS system.

Level measurement - differential pressure level transmitter

Second, The type and application of differential pressure level transmitter

The differential pressure transmitter has a common differential pressure transmitter and a differential pressure transmitter. According to the external structure, it can be divided into: single flange type differential pressure level transmitter, double flange type differential pressure level gauge, balanced container type. Differential pressure level gauge.

1. Single-flange differential pressure level transmitter: 

Single-flange liquid level transmitter can measure the liquid level and density of various open containers. There are two types of flat flanges and plug-in flanges. Install on the flange of the pipe or container. Since the isolating diaphragm is directly in contact with the liquid medium, it is not necessary to take out the pressure guiding tube on the positive pressure side, so that the liquid level, pressure and density of the medium such as high temperature, high viscosity, easy crystallization, easy precipitation and strong corrosion can be measured.

The difference with the double flange differential pressure level transmitter:

(1), in principle: all are differential pressure transmitter + flange pressure;

(2) From the structure: the single flange liquid level transmitter is only one flange membrane box connected to the high pressure side, the low pressure side is open to the atmosphere, the double flange liquid level transmitter is the high and low pressure side respectively through the oil filling The capillary is connected to two flanged bellows (there is also a capillary tube, the single flange transmitter is mounted below, and one capillary is taken from the low pressure side of the single flange transmitter to the upper flange bellows);

(3) From the engineering application: only the fixed-density liquid level can be measured. The single-flange transmitter can only be used for the liquid level of the atmospheric equipment that is imaginary with the atmosphere, while the double-flange transmitter It can be used to measure the liquid level of the closed equipment tank;

(4) In terms of economy, the double flange is definitely more expensive than the single flange. After all, there is one flange box and capillary.

2. Double flange type differential pressure level transmitter:

The double flange differential pressure level transmitter is a meter that measures the difference between the high and low pressure sides of the transmitter, and outputs standard signals to systems such as DCS or PLC. It differs from the general pressure transmitter in that it has two pressure ports. The differential pressure transmitter is generally divided into a positive pressure terminal and a negative pressure terminal. Under normal circumstances, the pressure of the positive pressure terminal of the differential pressure transmitter should be greater than negative. The pressure end can be measured.

The double-flange level transmitter is measured using a capillary flange transmitter, which is equivalent to extending the isolating diaphragm in the measuring component of the transmitter to the opening of the device, effectively eliminating viscous, corrosive or existing The effect of severe phase change media on measurement.

Level measurement - differential pressure level transmitter

3. Balanced container type differential pressure level transmitter:

The balanced container type differential pressure level transmitter is mainly composed of a balance container (also called a water level differential pressure conversion), a pressure signal conduit and a differential pressure transmitter. The balance container is used to measure the pressure in the steam water line. The balance container is used to prevent the liquid column in the pressure tube from fluctuating with the sudden change of pressure, thereby reducing the error when measuring the water level.

The balance container is divided into a single chamber balance container and a double chamber balance container. The single-chamber balance container itself does not have self-compensation. The double chamber balance container is a water level measuring device with a clever structure and a certain self-compensation ability. There is a circular funnel structure above the reference cup to divide the entire double chamber balance container into upper and lower parts. In order to distinguish it from the single chamber balance container, it is called a double chamber balance container.

Single room:

In the single-chamber balance container, the water surface height L is constant. When the water surface is to be increased, the water overflows into the closed container through the steam side communication pipe; when it is lowered, it is replenished by the steam condensed water. Therefore, when the water density in the balance container is constant, the positive pressure tube pressure is constant, and the negative pressure tube is in communication with the closed container, and the change in the output pressure reflects the change in the water level in the container.

Nowadays, the single-chamber balance container is used more. It writes the relationship between pressure and density, the relationship between temperature and density, etc. into the DCS system. After the temperature and pressure change, the signal is transmitted by the pressure transmitter and the temperature transmitter. Into the DCS system, the system automatically calculates the water level change. Before the measurement, the plug of the balance container should be disassembled according to the nature of the measured medium, and cold water or other liquid should be poured as the isolation liquid. Single-chamber balance vessels are typically used to measure the level of the drum in a large boiler or a vessel with a large pressure change.

Double room:

The double-chamber balance container is composed of inner and outer two-layer chambers, and the outer chamber of the balancer is connected with the steam of the boiler drum and filled with condensed water; the inner layer chamber passes through the lower side of the balance container and the water of the boiler drum Connected, using the principle of the connected, so the water level of the inner chamber varies with the water level of the drum. The double-layered container of such a structure ensures that the water temperatures of the outer chambers are substantially equal, thereby reducing measurement errors due to temperature differences. When the water surface is higher than the upper end of the balancer, the water flows into the boiler drum through the pressure guiding tube, so that the water level of the outer chamber remains unchanged. The inner layer chamber is connected to the boiler drum water level via the lower side of the balancer, and the water level varies with the water level of the steam drum. If the pressure and temperature parameters of the steam are constant, the output signal of the differential pressure transmitter is only related to the water level of the boiler drum.

The mechanical part of the double-chamber balance container compensates for the influence of pressure on the water level measurement. It is accurate under zero water level and rated working condition. When deviating from zero water level or rated pressure, over-compensation and under-compensation often occur. Therefore, the double-chamber balance container is generally used to measure the steam level of a small boiler or a container whose pressure does not change much.

Level measurement - differential pressure level transmitter

Third, Instrument anti-corrosion measures

Instrument anti-corrosion measures:

In the petroleum and chemical industries, a large amount of corrosive substances are produced in the production process of enterprises. The instrumentation used in the process is highly corrosive, which greatly reduces the service life of the instrument.

In the actual production process, the existence of corrosion causes the instrument failure rate to occur frequently. Enterprises often ensure the normal operation of production equipment by changing the instrument frequently, thus increasing the operation and maintenance costs of the enterprise. The anti-corrosion methods widely used in the chemical industry include: isolation method, anti-corrosion material method, development of anti-corrosion sensor, installation of anti-corrosion valve, etc., among which the isolation method is the most commonly used.

Gas isolation method: The principle of gas isolation is that the pressure guiding tube between the detection point and the instrument detecting component is filled with air, so that the detected pressure medium is directly in contact with the instrument, and the protection instrument is not damaged by the high pressure measured medium. Gas isolation is also called gas injection protection and is used when measuring pressure or low pressure using a pressure transmitter. The pressure change at the detection point is sensed by the air in the pressure guiding tube into the instrument transmitter, and the result is detected by the meter sensitive component.

Liquid isolation method: When measuring hydrogen chloride, nitrous oxide, chlorine and other media, it is filled with perfluorotributylamine or other isolation liquid in the isolation tank to isolate the corrosive medium from the metal parts of the instrumentation. There are some drawbacks in the liquid isolation method. For example, when the liquid seal is added, a liquid seal medium will appear, and the measured medium may chemically react with the liquid seal medium, thereby causing new corrosion problems and reducing the isolation effect. At the same time, the price of the spacer liquid in the market is generally expensive, the filling method is troublesome, and the anti-corrosion effect is not good. In practical applications, the liquid isolation method is not widely used.

Diaphragm isolation: Due to its high lubrication, high temperature resistance, weathering resistance, corrosion resistance, non-adhesion, low tension and low friction coefficient, PTFE is resistant to most chemical solvents and pharmaceuticals. Therefore, the anti-corrosion isolating diaphragm of the chemical instrument is composed of polytetrafluoroethylene. The specific application method is to adhere or spray on the bellows of the pressure transmitter or the spring tube of the pressure transmitter with a polyperfluoroethylene propylene or a polytetrafluoroethylene membrane, so that the corrosive medium and the sensing element are isolated to achieve the purpose of anticorrosion. .

The relative density, viscosity, vapor pressure, boiling point, freezing point, flash point, properties and use of each spacer are different. It is very important to select the spacer according to the physical properties and chemical properties of the measured medium. Choosing an inappropriate spacer fluid will lightly contaminate the measured medium, and in turn will cause property damage and casualties caused by accidents.

Excluding the chemical reaction between the medium and the spacer, the same medium will be very different in the isolation liquid selected in the south and north of China. For example, the temperature in the north can drop below minus 15 degrees, and the lowest temperature in the south is less than 0 degrees. In colder areas, an isolation fluid with antifreeze capability should be selected, preferably based on the lowest temperature in the region over the years, with additional margin. When the differential pressure transmitter in the cold region is installed, the pressure guiding tube should be as short as possible, and the insulating layer should be insulated as needed.

For the easy-to-frozen instrument, it is necessary to add steam to accompany the heat or electric heating, but to ensure the amount and pressure of the hot steam, the electric heating should consider explosion-proof in the flammable and explosive area. In addition to this, it is necessary to strengthen the inspection to ensure that the operation status of each instrument can be counted.

Under the influence of high temperature medium, the evaporation or evaporation of the isolation liquid will cause distortion of the differential pressure value of the transmitter. In addition, the isolation liquid will solidify or crystallize under the influence of the low temperature medium, and the pressure cannot be normally transmitted to the transmitter. So choosing the right spacer is especially important. The table below shows the nature and use of commonly used spacers.