Calculations regarding the migration of double-flange level gauges in sulfuric acid tanks
Thread Content
It turns out that no migration has been carried out for the sulfuric acid tank before; now, a problem has arisen during this process. The flange height of the sulfuric acid tank is 3.8 meters. Initially, the migration was set to -38 kPa to 0, but this gave incorrect results. Analyzing the reasons, with a density of 1.6 for sulfuric acid, should the migration range be set to -37 KPa to +23.8 KPa? The idea is as follows: when the tank is empty, the negative pressure measured is -37 KPa; when the sulfuric acid reaches the upper flange, the pressure should be 60.8 kPa. Based on the data available on site, the zero point of the transmitter should be set at -37 KPa, and the range should be set at 60.8 – 37 = 23.8 KPa. I’m not sure if this setting is correct. Thank you!
This post was last edited by zxg.wylton on 2017-5-17 11:16. The migration methods outlined in the manual are for reference only. Personally, I suggest that this is the correct way (absolutely right): 1. The flange should be installed properly, with the high-pressure side on the lower side, and it’s important to ensure that the PTFE gasket does not come into contact with the diaphragm. It’s best to secure it in place after installation. 2. When the tank is empty, check the pressure level; then adjust L based on this pressure to achieve an output of 4 mA. When the tank is full, adjust U to achieve an output of 20 mA. Track, verify, and modify at runtime.
That can’t be right. Performing a migration essentially means rewriting the values for 4mA and 20mA liquid levels; many beginners get confused by this term of \"migration\". It’s actually very simple. First, determine the height of the transmitter’s measurement point: for a double-flange level gauge, how high is the upper flange (that is, the upper pressure-taking point, which corresponds to the actual maximum range)? The lower flange (which, if it is at the same height as the pressure tapping point of the transmitter, can also be considered the zero point of the actual range) – we will discuss that later. Additionally, the common migration issue on storage tanks occurs because the pressure tapping point is above the zero level, which confuses many beginners. The conditions you mentioned are incomplete; let’s assume a scenario. Suppose a sulfuric acid storage tank with a density of 1.6. The height of the upper and lower flanges relative to the tank bottom is 2m and 0.5m, respectively. On the DCS, the range is 0~2m. So, in other words, the actual range of the transmitter is 0.5~2m. When the liquid level is 0, 4 mA should be outputted; however, the pressure at this point should be -0.5 m of pressure, so as to ensure that a current value of 0.5 mA is outputted when the liquid level is 0.5 m. Similarly, at 2m, the output is 20mA, but the actual pressure should be 1.5m. Then just do the calculation; take 475 and make the change. The pressure at 4mA is -0.5*1.6*9.8, and the pressure at 20mA is 1.5*1.6*9.8. Then everything will be fine; there’s no need to worry about positive or negative transfer – it’s not an exam, so no need for such nonsense
In practice, the liquid level in this tank does not reach the upper flange edge; therefore, the measurement range needs to be calculated. When draining the liquid, the tank can end up empty, so the zero point can be determined – the differential pressure at that time is -37 KPA. The main factor to consider is the measurement range, with a flange spacing of 3.8 meters
With the highest liquid level at the upper flange and the lowest liquid level at the lower flange, here’s a formula for you to use. Let the height of the upper and lower flanges be H, and let the density of sulfuric acid be ρ. Once the level gauge is installed and the tank is empty, the pressure value P read by the transmitter is obtained (this value represents the migration amount). In this case, you need to set the gauge to the range of -p to Hρg - p. If there is a medium in your tank, then you need to know the density of that medium in your capillaries. Generally, the density of silicone oil at low temperatures is 0.94. Assuming the density of the silicone oil is ρ1, then the amount of migration will be Hρ1g
This post was last edited by liuyuqian on 2017-5-17 at 17:40. When the tank is empty, a negative pressure of -37 KPa is measured – this is the amount by which the transmitter needs to be adjusted. You’re correct; for double-flange level gauges (with dual capillaries), the adjustment amount must be determined on-site based on actual measurements taken when the tank is empty, as this adjustment amount is affected by the installation height. The range is the differential pressure between your two flanges. Just do the calculation. The formula is =Hρg, where H is the distance between the two flanges. The original poster has the wrong understanding.
Original poster, just say you haven’t used a manual controller – why say such nonsense? The range is 60.8 kPa; set it accordingly, then close the double flanges and arrange for parallel condensation. Of course, it’s important to ensure safe operation, and after that adjust the zero point – just press a button and that’s it.
After the calculation, I adjusted the zero point and range – it’s all set now. Thank you everyone
The double-flange displacement depends only on the density of the silicone oil, while the range depends only on the distance between the flanges