Float level controller manufacturers guide how to install the meter accurately
1. Background of this article The application range of the float-type liquid level controller
is very wide, and it is widely used in both industrial production, agricultural production, and civil fields. It has many advantages such as simple structure, good stability, and convenient maintenance. It is also widely used in power plant production. It is a limit alarm device for detecting and controlling liquid level changes. The basic principle is also a float-type liquid level gauge. Similarly, it uses Archimedes's buoyancy principle. When the floating ball moves up and down due to buoyancy, the reed switch in the terminal box is affected by the arm end rod to perform "NC" (normally closed) contacts and "NO" ( (Normally open) contacts are interchanged to output a switch signal of the liquid level, and then drive other interlocking actions. The float level controller can achieve a very stable and reliable level measurement control effect, so it is commonly used in many thermal power plants to control the liquid level of water, oil and other media in open or pressure-containing containers. In general, most installation methods are installed in an ideal state (pure water at 4 ° C), thus ignoring the fact that although the liquid level is the same, the difference in the density of different media on the float in the float level controller This difference in buoyancy causes an error in the alarm value. This is also the main content of this article.
This article explains that when installing the float level controller, the influence of the physical characteristics of the measuring medium on the height of the float must be fully taken into account. Based on the principles of physics and mechanics, use the method of reference to calculate the oil or other The liquid level controller is installed on the medium container and the deviation value under ideal conditions (4 ° C pure water) is installed, thereby greatly reducing the error generated by the old method when installing the float ball liquid level controller.
When the liquid levels are the same, according to the formula F = ρgV, where ρ is the density of the medium, g is the acceleration of gravity, and V is the volume of the medium in the container. It can be known from this that the greater the density of the medium, the greater the buoyancy F (physically, it is generally assumed that the density of pure water is the largest at 4 ° C), and vice versa. As shown in Figure 1, two float level controllers in the same container are installed at the same height. When the container filled with pure water reaches the alarm position H (assuming that H is the alarm height set by the working conditions of the medium), when the alarm signal is issued, the container filled with oil or other media has no alarm, but the The liquid level has indeed reached the alarm line.
Second, what is the solution to the phenomenon in Figure 1? The reason is that the height h of the floating ball immersed in the medium is different due to the difference in the density of the medium. In Figure 1, the float of the float level controller installed on the pure water container is driven by the buoyancy to push the connecting rod to a distance of l1, and the switch triggers an alarm. Due to the low density of the liquid medium, the float of the liquid level controller is immersed in a certain height h in the medium, and it has not yet reached the position where the linkage alarm is triggered. Only when the liquid level in the container containing oil or other medium rises h, the liquid level controller will alarm. Obviously, at this time, the liquid is already higher than the height of the liquid in the water container. Therefore, although the liquid levels of the two containers are the same, the results obtained are different due to the influence of the density of the medium. When installing the float level controller, if the density of the measured medium is not taken into account, the alarm signal value will cause an error h. How to avoid this error, so that when the liquid level is the same, two float level controllers will alarm at the same time?
After adjusting the height of the float ball level controller mounted on the oil or other medium container down to h height, we can get the effect of simultaneously alerting two different media when the liquid level reaches the same height.
From the above two figures, it can be concluded that the main reason for the error value h is because the two media have different densities. If you want to get the alarm signal of ***, you must overcome this error, but how do you get the error value of h? Still Install according to the installation method in Figure 1. We can imagine that when the floats of the two containers reach the alarm position, assuming that the media density in the two containers is ρ1 and ρ2 respectively, and the two floats are stressed respectively. It is F1 and F2, and the height of the liquid surface is H1 and H2, respectively. According to the principle of mechanics, the buoyancy given by the two media at the moment of the alarm is the same, that is, F1 = F2 (as shown in Figure 3).
According to the formula: The float of the water container float level controller is subjected to buoyancy F1 = ρ1gV1, or the float of other media controllers is subjected to buoyancy F2 = ρ2gV2, that is: ρ1gV1 = ρ2gV2. × Bottom area of the container × H2 and because: H2 = H1 + h, when substituted into the above formula: h = (ρ1 × H1-ρ2 × H1) / ρ2 (1) According to the two definitions of medium specific gravity: medium specific gravity ∫ = medium weight G / media volume V and the ratio of the medium density to the pure water density at 4 ℃. The weight of the medium G = the density of the medium ρ × g × V. The calculation is: ρ2 / ρ1 = ρ2 × g, which is: ρ1 = 1 / g Substituting into equation (1), we get: h = (H1 / g-ρ2 × H1) / ρ2 (2) From equation (2), we can see that g is a constant and H1 is a given medium working condition. Fixed value, as long as we know the density ρ2 of the measured medium, we can find the error value h.
After obtaining h, when installing the float level controller, we can avoid the error value h by aligning the trigger line of the rising limit and falling limit on the controller with a place h lower than the set value of the working condition. Of course, measurement errors cannot be avoided, and the reference object we use, pure water at 4 ° C, is also ideally dense, but in fact, the floating ball will be slightly immersed in it. The above method is only Greatly reduces measurement errors.
Third, summing up the quality is a hundred-year plan. After using this method to install the float ball level controller, the accuracy of the measurement has been maintained at a high level, which ensures the safe and stable operation of the unit and reduces the number of misoperations caused by the inaccurate alarm signal measurement. The company's thermal control construction office has obtained a good reputation for quality.