A single flange level transmitter is an on-site transmitter that is mounted directly on a pipe or vessel. The single-flange liquid level transmitter can accurately measure the liquid level and density of various containers. It can be divided into two types: flat flange and plug-in flange, because the isolation 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 it is suitable for the measurement of medium with high temperature, high viscosity, easy crystallization, easy precipitation and strong corrosion.
At present, there are many methods for measuring the liquid level in a closed container. Common measuring instruments include a magnetic flap level gauge, a double flange differential pressure transmitter, a single flange differential pressure transmitter, a differential pressure transmitter, and a radar liquid. Bit count and so on.
The magnetic flap level gauge is easy to lose magnetism in some environments, which causes the liquid level meter to malfunction. The radar level gauge has high precision, but the investment is large. At the same time, due to the large number of signal interference sources in the initial stage of operation, the measurement has a great influence on the measurement. The false signal needs to be frequently removed; the differential pressure transmitter has lower cost, high measurement accuracy and less interference source, so it is more common to measure the liquid level in the closed container by the differential pressure principle.
The single-flange transmitter is divided into two types: plug-in type and flat flange. The measurement principle and installation method of the two types of transmitters are basically the same, but the different process conditions have a great influence on the measurement data of the transmitter. Take the steam condensate flashing barrel of the salt industry as an example (see Figure 1), A is the steam condensate inlet, B is the flash steam outlet, and C is the single flange transmitter installation position. D is the steam condensate drain pipe, and point E is the pressure point of the negative pressure side of the single flange transmitter.
In the process of instrument selection. We tend to ignore the influence of water flow on the pressure. If point A is perpendicular to the barrel tangent, the water in the barrel will not form a vortex, but the water column falling from point A will affect the measurement of point C pressure, resulting in single flange transmission. The measurement data on the positive pressure side is unstable; if the A point is not perpendicular to the barrel tangent, the water in the barrel will form a vortex. No matter where the C point is, the measurement data on the positive pressure side of the single flange transmitter will be distorted. Considering the flashing efficiency of steam condensate, the inlet method of point A parallel to the tangential direction of the barrel is generally used. Such a vortex will inevitably form in the barrel, and the selection of the transmitter is crucial.
At the beginning of the selection, our company did not notice the influence of water flow on the pressure. The plug-in single-flange transmitter used was often low in liquid level after being put into use. Due to the inaccuracy of the level gauge, the liquid level was automatically adjusted. The regulating valve can not be used normally. Only the operator observes the glass tube liquid level gauge on the spot to adjust the liquid level in the barrel. The amount of T is not increased, the liquid level control is not good, the flashing efficiency is low, and the production is caused. A big impact.
At first, it was suspected that it was a single-flange transmitter. After the test, it was found that the transmitter was in the laboratory. After careful observation of the on-site process piping. It was found that the measured value of the transmitter was suddenly large and small, and it was analyzed that the water flow caused distortion of the positive pressure measurement data of the transmitter.
As shown in Figure 2, point A is the condensate inlet pipe, point B is the installation position of the plug-in single flange transmitter. The inlet pipe enters the water along the tangential direction of the barrel, and a vortex is formed in the barrel. The positive pressure diaphragm of the plug-in single-flange transmitter is in the vortex flow. The impact of the water flow causes the pressure at the positive pressure measurement point of the transmitter to be unstable and the measurement data to be distorted. Since the steam condensate bucket belongs to the pressure vessel and cannot be modified at will, the only way to solve the problem is to replace the transmitter.
As shown in Figure 3, points A and B are the same as in Figure 2, where C is a static water area between the tank wall and the pressure point. The presence of this static water isolates the vortex and flat flange level in the tank. The transmitter is positively pressing the diaphragm so that the pressure measured by the transmitter at point B is the water pressure in the barrel that is not affected by the vortex flow. After confirming the good plan, through the manufacturer, the plug-in single-flange liquid level transmitter is all returned to the factory to be converted into a flat flange liquid level transmitter. Through the transformation, the data of the condensed water balance barrel liquid level data is eliminated. At present, the instrument runs smoothly.
Whether it is a plug-in single-flange liquid level transmitter or a flat-flange liquid level transmitter, it is a differential pressure transmitter. The principle of measuring the liquid level is calculated by the formula P=pgh. The pressure at the position, then negatively shift the transmitter's range and calculate the height of the liquid level in the container by differential pressure. Therefore, only if the pressure measurement is accurate, the liquid level may be correct. The advantage of the plug-in single-flange liquid level transmitter is that it is not easy to block the pipe in the medium with high viscosity and large particle size, but it still appears to be weak in the environment where the fluid impact is serious. In summary, in the process of instrument selection, we must not only consider the price, brand, material and other issues of the instrument, but also consider whether the working conditions in the area where the instrument is located can make it stable and reliable.