Abstract: This paper describes the experience of installation, commissioning, maintenance, fault handling, etc. of the dual flange differential pressure transmitter and its advantages as a level transmitter.
The double flange differential pressure transmitter is a widely used liquid level measuring instrument in petroleum, chemical and petrochemical production processes. It is characterized by simple structure, high precision, good linearity, easy installation and maintenance, and easy assembly into a control system. Continuous level measurement and boundary measurement of tanks, tanks, tanks, etc. for continuous or batch production processes. The double flange differential pressure transmitter is mainly used to measure the measured medium with corrosive, high viscosity, easy to crystallize, and low freezing point liquid level.
1 Introduction to the equipment
The standard dual flange differential pressure transmitter includes the following components: flange, capillary, mounting bracket, measuring head
The flanges are used to connect the instrument to the process equipment, while the flange bellows section is used for pressure measurement. Flanges are sized and pressure graded, and their dimensions and flange dimensions should match the process equipment.
The capillary tube is used to connect the flange to the measuring head, and the tube is filled with silicone oil to provide pressure transmission.
1.3 Mounting bracket
The mounting bracket is a standard part that can be mounted to the field insulation protection box or mounting bracket.
1.4 Measuring Head
Electrical connection, circuit board and optional LCD head measurement unit.
(1) Electrical connection. It provides 1/2NPT standard electrical interface for connection of other signals such as 4~20mA. Most of the headers are connected to the left and right branches, which is convenient for position adjustment during actual installation. (2) Circuit board. The circuit board is located inside the measuring head and is connected to the measuring head by a cable. It can be removed according to the needs of the work. (3) Liquid crystal display head. It can be used to display real-time measured values on site. It can display different unit indications according to actual needs (using HART Communicator for internal parameter modification), which is convenient for numerical comparison with process local level gauge during on-site debugging.
2 Selection considerations
2.1 Determine the model of the pressure transmitter
The type determination of the pressure transmitter refers to the type confirmation of the measuring head. The confirmation process should be based on the measured pressure range, output mode (4 to 20 mA with HART protocol or Profibus, etc.), process flange type, capillary connection type, Whether to select the model with parameters such as LCD head. Whether the choice of LCD head is based on the ambient temperature of the transmitter, if the ambient temperature is too low (below 25 °C), it is not recommended to have a liquid crystal meter, because the ambient temperature is too low according to the actual application experience on site. The LCD will not be displayed.
2.2 Determining the diaphragm seal assembly model
The model definition of the diaphragm seal assembly refers to the type confirmation of the flange and the capillary. During the confirmation process, the model selection should be made according to the parameters of the diaphragm seal type, the seal position, the capillary filling liquid, and the capillary connection length. The capillary filling liquid should be selected according to the temperature of the measuring medium. If the filling liquid is improperly selected, the double flange differential pressure transmitter may not be able to adapt to the temperature of the measuring medium during use, and the meter cannot be accurately measured.
3 Installation Precautions
(1) The installation position of the double flange differential pressure transmitter should be easy to maintain, easy to observe, and close to the pressure taking parts. (2) Double flange differential pressure transmitter installation should consider the capillary length limit. (3) The bending radius of the capillary during the installation of the double flange differential pressure transmitter should be greater than 50mm. (4) During the installation process of the double flange differential pressure transmitter, it should be noted that there should be no extrusion between the flange gasket and the bellows to avoid affecting the normal measurement and transmission of pressure. (5) The capillary of the double flange differential pressure transmitter should be protected and fixed. The suspended part should be fixed with 40 (or 50) angle steel to prevent capillary sloshing caused by external environment such as strong winds from affecting pressure measurement and transmission. (6) The capillary of the double flange differential pressure transmitter should be insulated. In order to meet the temperature requirements of the measuring medium during the selection process, the capillary filling liquid is divided into low temperature filling liquid and high temperature filling liquid. Therefore, certain thermal insulation measures must be taken for the capillary to prevent the capillary filling liquid from being affected by the change of the external environmental temperature. The phase of the phase causes a pressure measurement deviation. (7) If the double flange differential pressure transmitter is installed outdoors, it should consider the supporting instrument insulation protection box, and install the transmitter measurement meter into the thermal insulation protection box to ensure the use temperature of the measuring meter circuit board and the liquid crystal display. Require and meet the instrument protection requirements.
4 Debugging method
When using a differential pressure transmitter to measure the liquid level, generally, the relationship between the differential pressure Δp and the liquid level height H is as follows.
This is a general "no migration" situation. When H = 0, the pressure acting on the positive and negative pressure chambers is equal. However, in practical applications, the relationship between H and Δp is not so simple. For example, as shown in Figure 1, a dual flange differential pressure transmitter measures the level of the closed container. If the density of the measured medium is ρ 1 , the density of the capillary filling tank of the double flange differential pressure transmitter is ρ 2 (usually ρ 2 >ρ 1 ), at which time the pressures of the positive and negative pressure chambers are respectively
p 1=h1 ρ 2 g+Hρ 1 g+p 0 (4-2)
p 2=h2 ρ 2 g+ p 0 (4-3)
The pressure difference between the positive and negative pressure chambers is
p 1-p2 = h 1 ρ 2 g+Hρ 1 g- h 2 ρ 2g
That is, Δp= Hρ 1 g-(h 2 - h 1)ρ2 g(4-4)
Where Δp is the differential pressure between the positive and negative pressure chambers of the transmitter; H is the height of the measured liquid level; h 1 is the height of the positive pressure chamber pressure valve to the transmitter; h 2 is the pressure of the negative pressure chamber The height of the valve to the transmitter.
Comparing equation (4-4) with equation (4-1), it is known that this is the differential pressure reduction (h2 - h 1) ρ2 g term, that is, when H = 0, Δp = -( h 2 - h 1) ρ2 g, compared to no migration, equivalent to a pressure in the negative pressure chamber, the fixed value is (h2 - h 1) ρ2 g. It is assumed that the DDZ-III differential pressure transmitter is used, and its output range is 4 to 20 mA.
In the absence of migration, when H=0, Δp=0, this is the output of the transmitter I 0 =4mA; when H=H max , Δp=Δp max , then the transmitter outputs I 0 =20mA. But there is During migration, according to formula (4-4), due to the existence of a fixed differential pressure, when H=0, the input of the transmitter is less than 0, and its output must be less than 4 mA; when H=H max, the transmission The input to the device is less than Δp max and its output must be less than 20 mA. In order to make the output of the meter correctly reflect the value of the level, that is, the zero and full scale of the level can correspond to the upper and lower limits of the transmitter output, we must try to offset the fixed pressure difference (h 2 - h 1) The effect of ρ2 g is such that when H = 0, the output of the transmitter still returns to 4 mA, and when H = H max , the output of the transmitter can be 20 mA.
According to the actual engineering experience, this method can be achieved by zero-point migration method, that is, the lower limit of the measurement of the transmitter is adjusted by a standard instrument such as a HART Communicator to offset the effect of the fixed differential pressure (h 2 - h 1) ρ2 g.
Here, the role of standard instruments such as the HART Communicator is to change the zero point of the transmitter. Both migration and zeroing make the transmitter's starting value correspond to the measured value, except that the zero adjustment is usually small and the zero migration is larger.
5 Common faults and treatment methods
5.1 Instrument indication fluctuations
(1) Double flanged differential pressure transmitter capillary filling tank liquid selection error, filling tank liquid use temperature can not meet the equipment operating temperature. (2) The double flanged differential pressure transmitter capillary is not fixed, causing the capillary to swing freely. (3) The double flanged differential pressure transmitter capillary is not insulated, and it is greatly affected by the ambient temperature. (4) The damping time setting of the double flange differential pressure transmitter is too small.
5.2 Instrument indication is larger (smaller than local) level gauge
(1) Under the double flange differential pressure transmitter (upper), the flange and the flange diaphragm face are squeezed during the fixing process, causing the diaphragm to be forced under the free state, and the flange is removed and replaced. The piece is refastened. (2) The actual density of the measured medium is larger (small) than the original design density. (3) Double flange differential pressure transmitter positive (negative) pressure chamber pressure valve has foreign matter or frozen block. (4) Double flange differential pressure transmitter zero drift.
6 Advantages of double flange differential pressure transmitter
(1) It can measure the level of corrosive, high viscosity, easy to crystallize, low freezing point characteristic medium. (2) It has high sensitivity and stable signal processing characteristics, and can cope with various process conditions. (3) Repeatability ensures the reliability and accuracy of the liquid level measurement even under the most difficult conditions. (4) Simple commissioning and small maintenance.
This paper introduces the application of double flange differential pressure transmitter in chemical plant, and analyzes it from the aspects of instrument selection, debugging and fault treatment. It is expected to help the chemical plant to choose the double flange differential pressure transmitter. Position, so that it meets the requirements of accurate and stable measurement of the level, and can maximize the safety, economy and reasonable production.