Double flange differential pressure transmitter in chemical oil production
Due to its special nature, the double flange differential pressure transmitter is different from other single flange differential transmitters. The double flange differential pressure transmitter is used for continuous level measurement and boundary measurement of towers, tanks, tanks, etc. in continuous or intermittent 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. The following is a simple and easy-to-understand language to analyze some typical applications of double flange differential pressure transmitters in chemical petroleum production.
First, double flange differential pressure transmitter in chemical oil selection attention point
1 Determine the model of the double flange differential pressure transmitter
The model identification 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), process flange type, capillary connection, and LCD. Model parameters such as the header are selected. 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 screen will not be displayed.
2 Determine 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 MDSDP150 series double flange differential pressure transmitter may not be able to adapt to the measuring medium temperature during use, resulting in the meter not being able to accurately measure. .
Second, the transmitter installation notes
(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) The installation of double flange differential pressure transmitter should consider the limitation of capillary length.
(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 the extrusion between the flange gasket and the bellows should not occur, so as 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 environment temperature. The phase of the phase results in 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 instrument protection requirements.
Third, the transmitter 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.
The double flange differential pressure transmitter measures the level of the closed container. If the density of the measured medium is ρ1, the density of the capillary filling tank of the double flange differential pressure transmitter is ρ2 (usually ρ2<ρ1), at which time the pressures of the positive and negative pressure chambers are respectively
The pressure difference between the positive and negative pressure chambers is
P1-p2= h1ρ2g+Hρ1g- h2ρ2g
That is, Δp= Hρ1g-(h2-h1)ρ2g(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;
H1 is the height of the positive pressure chamber pressure valve to the transmitter;
H2 is the height of the negative pressure chamber pressure valve to the transmitter.
Comparing equation (4-4) with equation (4-1), it is known that this is the differential pressure reduction (h2-h1)ρ2g term, that is, when H=0 is, Δp=-(h2- H1) ρ2g, compared with no migration, is equivalent to a pressure in the negative pressure chamber, the fixed value is (h2-h1) ρ2g. 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 I0=4mA; when H=Hmax, Δp=Δpmax, then the transmitter outputs I0=20mA. However, when there is migration, according to Equation (4-4) shows that 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=Hmax, the input of the transmitter is less than Δpmax Its output must be less than 20mA. 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 differential pressure (h2- h1). The effect of ρ2g is such that when H = 0, the output of the transmitter still returns to 4 mA, and when H = Hmax, the output of the transmitter can be 20 mA. According to the actual experience of the project, the zero point migration method can achieve this purpose, that is, the lower limit of the measurement of the transmitter can be adjusted by a standard instrument such as a button on the meter or a HART communicator to offset the fixed differential pressure (h2-h1) ρ2g. effect. 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.
Fourth, the transmitter common faults and treatment methods
4.1 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.
4.2 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 drift differential transmitter zero drift.
Five, the 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 low maintenance.
Six, double flange differential pressure transmitter is also suitable for the following conditions
(1) Higher tanks and towers
Such as distillation towers, fermenters and other high equipment, the upper and lower pressure spacing is relatively large, such as more than 6m, the ERS system can effectively eliminate the measurement error of long capillary, shorten the response time and improve the overall measurement performance.
(2) The differential pressure is large and the static pressure is small. The ERS calculates the differential pressure by measuring the pressure. To ensure the accuracy of the differential pressure signal, the ratio of static pressure SP to differential pressure DP cannot exceed the minimum accuracy requirement of the transmitter. (SP+DP): DP<100:1, if it is better within 30:1.
(3) Large ambient temperature difference or need to be accompanied by heat ERS does not require a pressure guiding tube or capillary, which fundamentally eliminates the influence of ambient temperature on the measurement.
(3) Installation of more complicated working conditions ERS only need to fix two transmitters separately, then connect the communication cable, which is easy to install and easy to maintain. The ERS manufacturer offers three standard length communication cables: 15m, 30m, 45m. The selection should be based on the actual installation location. Three ranges are required to be supplied to the manufacturer when ordering: the main meter needs to provide differential pressure and pressure range, and the auxiliary meter only needs to provide pressure range. The pressure range of the two meters is recommended to be uniform (differential pressure plus static pressure), so that the high and low pressure sides of the field are interchanged.