Issues regarding the measurement range and calibration of differential pressure level measurement
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Hello everyone, I have quite a few questions to ask, which I will divide into categories: Regarding the measurement of the level in the heat exchanger, our plant uses differential pressure level gauges. The high-pressure side is filled with steam, while the low-pressure side contains water. In the DCS system, 4 MA is set as the full-level value, and 20 MA as the zero-level value. My questions are: 1. How was this differential pressure gauge calibrated during installation, and how was its range adjusted? Is it to apply pressure on the low-pressure side up to the maximum pressure, and then set it to 4MA? In the manual, I saw that the installation specification states that water should be used on the high-pressure side and steam on the low-pressure side. Here, however, steam is used on the high-pressure side and water on the low-pressure side. Is this type of installation acceptable? 2. Regarding the calibration of the drum water level transmitter, we use digital pressure gauges for this purpose. Is it necessary to set the digital pressure gauge to the MM water column mode for calibration? Is the range setting for the drum water level the same as the setting method for liquid level gauges such as those in the high-pressure heaters?
The installation of the differential pressure transmitter for measuring the drum liquid level is the opposite in the chemical industry; therefore, the gas phase is connected to the positive pressure chamber in the drum. But it doesn’t make any difference; it’s just positive or negative transfer. The current cannot be set. It is always 4--20mA corresponding to 0--100% or 100--0%.
This post was last edited by 1111111 on 2018-5-9 at 15:07. 1. The method for taking pressure readings from your boiler drums is shown in the diagram. 2. As can be seen from the diagram, the differential pressure ΔP = P+ – P- = ρ1gL – ρgH; by transforming this equation, we obtain an equation for measuring the liquid level H, namely H = Lρ1/ρ – ΔP/(ρg). For measuring the liquid level in low- and medium-pressure boilers, since the densities ρ and ρ1 are considered constant values, the measurement equation simplifies to H = b – kΔP. 3. Since b and k are positive constants, it follows from this equation that the greater the differential pressure ΔP, the lower the liquid level H; conversely, the smaller the differential pressure ΔP, the higher the liquid level H. 4. This method of taking pressure readings using a differential pressure gauge is often referred to as the “positive connection with reverse measurement” method, meaning that the differential pressure gauge is always subjected to pressure in the positive direction.
This post was last edited by 1111111 on 2018-5-9 at 13:15. 5. Migration – those are all outdated concepts; they’re completely obsolete. :lol Nowadays, almost all smart transmitters are used in boiler drums. As for smart transmitters, first, they allow for the direct setting of the differential pressure value corresponding to 4mA and that corresponding to 20mA; once these two values are set, everything is fine, and there’s no need to worry about things like offset adjustments. Second, they automatically calculate the percentage of the operating range based on the relationship between the differential pressure value and the current value. Some of these instruments even allow for conversion to metric units as well
The last edit to this post was made by 1111111 on 2018-5-9 at 13:13. 6. For the calibration of differential pressure gauges, calibration in kPa is sufficient. The standard pressure unit in metrology, mH2O, has a precise physical definition; the value of ‘m’ in this unit is not equivalent to the amount of water in meters present in your boiler. 7. There’s no need to calibrate the liquid level value on the boiler drum by using calculation equations to determine values such as ρ, ρ1, and g. Those who engage in such calculations are nothing but fools and idiots; it’s like trying to catch fish by climbing a tree :lol. Such pointless calculations are far less accurate than simply taking a measurement —— Bring a pot of water to a boil, record the reading of the differential pressure gauge at the operating range – it’s accurate and simple. When H=0, record the reading of the differential pressure gauge; use this value as the corresponding value for 4mA or 20mA. When H=X meters, that is, at the upper limit of the operating range, record the reading of the differential pressure gauge; use this value as the corresponding value for 20mA or 4mA
Everyone, can we understand it this way? In this configuration, the pressure generated by the liquid column on the high-pressure side remains constant, while the pressure on the low-pressure side increases as the water level rises. As a result, the pressure difference between the high-pressure and low-pressure sides decreases, which indicates an increase in the water level. During debugging, the high-voltage and low-voltage sides are separated, both sides are filled with water, the differential pressure value is set to 20 mA, 4 mA is set to 0, and finally the range is configured in the DCS.