Why is a balance vessel required when using a differential pressure level gauge to measure the liquid level in a boiler drum? ? Principle?

I. Working principle of the single-chamber equilibrium vessel: As shown in the figure below, the principle behind measuring the water level in a single-chamber equilibrium vessel is very simple. A pipe is drawn from the steam-side sampling port of the steam drum to the equilibrium vessel (also known as the condensation chamber; it is a vessel with a large surface area and without any insulation). The saturated steam entering the equilibrium vessel continuously condenses into water due to heat exchange with the surrounding environment. Excess water flows back to the steam drum through the sampling pipe, thanks to the principle of overflow, thereby maintaining a constant water level inside the equilibrium vessel. Therefore, the positive pressure head of the differential pressure transmitter remains constant due to the constant water column in the balancing vessel, while the negative pressure head changes with the level of water in the drum. By measuring the differential pressure between the positive and negative circuits, and then using the formula P=ρ*g*h, it is possible to easily determine the actual water level in the drum. It’s best if you view it in conjunction with the diagram. II. Working Principle of the Two-Chamber Balanced Container As shown in the figure below, the structure of a two-chamber balanced container is more complex than that of a single-chamber balanced container; it consists of components such as a condensation chamber, a reference cup, an overflow chamber, and a communicating vessel. The saturated water vapor coming from the drum condenses in the condenser chamber to form water, which then flows into the reference cup. The function of the reference cup is to collect the condensed water from the condenser chamber and to direct the pressure generated by this condensed water out of the container, to the positive pressure side of the differential pressure transmitter. The volume of the reference cup is limited; once it is filled with condensate, the excess flows over into the overflow chamber. This chamber collects the condensate that overflows from the reference cup and directs it into the boiler downcomer. As this fluid moves, it heats and stores heat within the entire container, ensuring that its temperature matches that in the steam drum. A communicating vessel transmits the pressure generated by the dynamic water level in the drum to the negative pressure side of the transmitter, and compares it with the (reference) pressure on the positive pressure side in order to determine the water level in the drum. III. Comparison of operating characteristics between single-chamber and double-chamber equilibrium vessels: In a single-chamber equilibrium vessel, the temperature difference across the reference liquid column is large, as is the difference in density; as a result, the error is relatively high. However, this error can be reduced through methods such as temperature compensation ； The water in the reference water column of the two-chamber balanced vessel is in constant flow, has a relatively high temperature, is not much different from the temperature of the steam drum, and has a similar density as well. Moreover, it serves to compensate for the water level within a certain range of pressure and temperature, which results in small errors. However, when the drum pressure drops suddenly, the saturated water in the two-chamber balance vessel will vaporize, thereby causing a change in the reference liquid column itself and leading to measurement errors! Exacerbates the false water level. In a single-chamber equilibrium vessel, since the temperature of the reference liquid column is not sufficient, there is no need to consider the series of effects resulting from a sudden drop in the pressure of the steam drum. Moreover, modern DCS systems generally come equipped with fairly sophisticated temperature compensation mechanisms; therefore, new units such as those produced by Shaodian use single-chamber balance vessels to measure the water level in the steam drum.

Well, by storing condensate water, the transmitter diaphragm and silicone oil are separated from high-temperature steam or water; the expansion of the silicone oil inside the diaphragm box due to heat can cause significant measurement errors.