In industrial process production, such as petrochemical, chemical, electric power and other industries, often to the production process of storage container equipment material level detection, in order to ensure the continuous supply of materials in each link of production or economic accounting; To know whether the level is in the specified range, in order to make the production process normal, to ensure product quality and safety in production.

    The object position refers to the interface position formed by density difference between the liquid, powdery solid or insoluble two liquids in storage containers or industrial production equipment. Generally, the liquid level, material level, interface collectively referred to as the level. The instrument for measuring object level is called object level detection instrument. Due to the wide variety of objects to be measured, the detection conditions and environments are also very different, so there are a variety of object level detection methods to meet the measurement requirements of different production processes. According to the working principle classification, object level measuring instrument has direct reading type, differential pressure type, buoyancy type, electrical type, ultrasonic type, radar, radioactive and so on.

一、 Design and selection

    HG/T20507-2014 Automatic instrument selection design specification for liquid level and interface measurement should choose differential pressure instrument. When the requirements are not met, capacitive, radio frequency admittance, radar, static pressure, nuclear radiation, external measurement and other instruments can be used.For continuous liquid level measurement, differential pressure transmitter should be selected. Selection requirements:

1.  For the liquid level measurement of crystalline liquid, viscous liquid, easy gasification liquid, corrosive liquid, liquid containing suspended matter should choose flat flange differential pressure transmitter.

2.  In the detection of the liquid level of high crystallization liquid, high viscosity liquid, gel liquid, precipitation liquid, it is appropriate to choose the insertion flange differential pressure converter.

3.  Used to measure corrosive liquid, viscous liquid, crystalline liquid, molten liquid, precipitation liquid level, in the measurement accuracy is not high, can be used to blow cleaning method, with differential pressure transmitter for measurement.

4.  When measuring the interface, can choose the differential pressure transmitter, the upper liquid level should always be higher than the upper pressure inlet.

5.  The differential pressure transmitter for measuring liquid level should be equipped with a migration mechanism, and the positive and negative migration should be determined when the instrument range is selected.

6.  When the liquid density changes obviously under normal working conditions, it is not suitable to use differential pressure transmitter for liquid level and interface measurement.

    Therefore, differential pressure transmitter is widely used in industrial process control level gauge, simple structure, easy to install and maintain, easy to calibrate.

二、Principle and characteristics

    Differential pressure type level gauge is the use of the container of liquid level change, the static pressure generated by the liquid column also change the principle and work. Because pressure is the force applied per unit area

p=F/S       F: Force (N)   S: area (m2).   And because F = hSρg     p=F/S

ρ : liquid density (kg/m3 )    h: liquid column height (m)       g: gravity acceleration (m/s2)    P=ρghs/s=ρgh（N/m2）

    From the formula above, the pressure is equal to the product of the height of the liquid column, the density of the liquid and the acceleration of gravity. The density of a liquid is constant at a given temperature, so the pressure can also be expressed in terms of the height of the liquid column.

Characteristics of differential pressure transmitter:

(1) The detection element occupies almost no space in the container, so it only needs to open one or two holes on the wall of the container;

(2) The detection element is only one or two pressure tube, simple structure, convenient installation, easy operation and maintenance, reliable work;

(3) The use of flange differential pressure transmitter can solve the problem of liquid level measurement of high viscosity, easy solidification, easy crystallization, corrosive, containing suspended matter medium;

(4) The differential pressure transmitter has strong versatility, which can be used to measure liquid level, and can also be used to measure pressure and flow parameters.

三、Differential pressure transmitter type

◆  Double flanged differential pressure transmitter

    If the measuring medium contains impurities, crystallization, condensation or easy self-clustering of the measured medium, the selection of ordinary differential pressure transmitter will cause the blockage of the connecting pipeline, at this time, the need to use a double flange differential pressure transmitter. The flange is directly connected with the flange on the container as the measuring head of the sensitive element (metal membrane box) through the capillary and the measurement chamber of the transmitter. Silicone oil is filled in the closed system composed of membrane box, capillary and measuring chamber as the pressure transmission medium, and the capillary is protected by metal snakeskin tube.

◆  Electronic remote differential pressure transmitter

   Electronic differential pressure transmitter is a new best practice technology specially designed for high tank and distillation tower applications. Electronic differential pressure directly eliminates excess capillary and oil filling, and speeds up time response.The positive pressure side and negative pressure side of electronic remote differential pressure transmitter are two pressure transmitters respectively. The two pressure signals are subtracted in the main transmitter. The main transmitter has synchronization function and the pressure transmitter has higher accuracy.The system consists of a main sensor on the high voltage side and a secondary sensor on the low voltage side. The signal is connected and transmitted through a communication cable. The main sensor calculates the differential pressure and outputs 4-20 mA analog or digital signals. Key features are high performance, high accuracy, and high reliability compared to the long capillary performance of diaphragm sealing systems, especially suitable for high storage tanks.

四、differential pressure type level gauge installation and matters needing attention

◆ The installation height of the meter should not be higher than that of the lower interface of the liquid level pressure taking interface of the measured container. The installation position is easy to maintain and observe， And close to the position of the pressure taking part.

◆ If the double flange differential pressure transmitter is selected, the transmitter installation position is only limited by the length of the capillary, the bending radius of the capillary should be greater than 50mm, and the protection and insulation measures should be taken for the capillary.

◆ The differential pressure level gauge should be installed vertically, and the positive and negative pressure chambers should be properly connected with the process vessel.

◆ If the measured medium is low boiling point medium (such as liquid ammonia, liquid oxygen, etc.), low boiling point medium is easy to vaporize in the environment. In order to output signal and stability of indicating value, the measuring pressure guide tube should not be too short, and the installation position of the level meter should be higher than the pressure taking interface under the liquid level of the measured container.

◆ In order to reduce the influence of environmental temperature difference, the high and low pressure side of the capillary bundle together, and fixed to prevent wind and vibration, etc. (the super long part of the capillary should be rolled together and fixed).

◆ If the transmitter works in a state of negative pressure, if it exceeds its allowable negative pressure value, the filling liquid will vaporize, the isolation diaphragm will be bulging, resulting in inaccurate measurement, and even cause instrument damage. Flange transmitter must be installed under two flanges, only in this way, to avoid negative pressure, to ensure correct measurement.

◆ When the measurement condition is high temperature and high vacuum, high temperature and high vacuum (silicone oil) should be used for diaphragm sealing liquid in the selection of double flange differential pressure level meter.

◆ The high precision of electronic remote differential pressure transmitter is based on measuring pressure, not on working condition differential pressure. Each pressure transmitter shall not be in the state of overpressure. The operating pressure and the measured differential pressure must be within the allowed measuring range at the same time before the electronic remote differential pressure level system can be used.