The principle of a two-chamber equilibrium vessel

A dual-chamber balanced container is a cleverly designed water level measurement device for steam drums that possesses certain self-compensation capabilities. Its main structure is shown in Figure 1. Above the reference cup, there is a circular funnel-shaped structure that divides the entire two-chamber balance vessel into upper and lower sections; it is called a two-chamber balance vessel to distinguish it from a single-chamber balance vessel. For ease of explanation, the various main components are named accordingly based on their functional characteristics: condenser chamber, reference cup, overflow chamber, and communicating vessel. Additionally, the water level measurement device in the dual-chamber balance vessel is referred to simply as the vessel in this text. 3.2. Condenser: Under ideal conditions, the saturated water vapor coming from the steam drum releases its latent heat of vaporization as it passes through this section, resulting in saturated condensate that is supplied to the reference tank and subsequent stages for use. 　　3.3. Reference cup: Its function is to collect the condensate water from the condenser chamber, and to discharge the pressure generated by this condensate water out of the container, directing it to the positive pressure side of the differential pressure measuring instrument – the differential pressure transmitter (hereafter referred to as the transmitter). The volume of the reference cup is limited; once it is filled with condensate, the excess flows over into the overflow chamber. Since the height of the mouth of the reference cup is fixed, it is called a reference cup. 　　3.4. Overflow Chamber: The overflow chamber occupies most of the space within the container. Its main function is to collect the condensed water that overflows from the reference cup, and to discharge this condensed water into the boiler’s downcomer. As this water flows, it heats and stores heat throughout the container, ensuring that the temperature there is consistent with that in the steam drum. Under normal conditions, due to the dynamic action of the fluid in the boiler downcomer, there is essentially no water accumulation in the overflow chamber, or only a small amount. 　　3.5. Communicating vessels: An inverted T-shaped communicating vessel, with one end of its horizontal portion connected to the steam drum and the other end connected to the negative pressure side of the transmitter. Undoubtedly, its main function is to transmit the pressure generated by the dynamic water level in the drum to the negative pressure side of the transmitter, and to compare it with the (reference) pressure on the positive pressure side in order to determine the water level in the drum. It is designed in an inverted T shape to ensure that the fluid in the communicating vessel has a certain degree of fluidity, preventing it from freezing in the pipelines between the steam drums during winter. The temperature of the fluid inside the communicating vessel is likely to differ from that in the drum, resulting in a liquid level that is different from that in the drum; however, due to the self-balancing effect of the fluid, this has no impact on the measurement of the drum’s liquid level. 　　3.6. Calculation of differential pressure As explained earlier, the temperature of the fluid in the condenser chamber, the reference cup, and the pressure guide tubes located at the bottom of these components is equal to the temperature of the fluid in the steam drum; that is, γw = γ’w and γs = γ’s. Therefore, it is not difficult to obtain the differential pressure output by the container. This article takes the double-chamber balance vessel with a measurement range of ±300 mm, used in the DG670-13.73-8A type boilers manufactured by Dongfang Boiler Factory, as an example (as shown in Figure 1). As can be seen from Figure 1, the pressure output on the positive-pressure side of the container equals the total static pressure above the horizontal level of the reference cup opening, plus the condensate pressure in the vertical section between the reference cup opening and the horizontal axis of the L-shaped pressure guide tube, plus the pressure generated by the fluid in the vertical pipe section outside the container, located between the horizontal axis of the L-shaped pressure guide tube and the horizontal axis of the communicating vessel. Obviously, the pressure in the last part is such that, since the medium there is at rest and located far from the container, its density should be the density at the ambient temperature. Therefore, P = PJ × 320 × γw × (580 – 320) × γc. Here, P is the pressure at the positive pressure side of the container; γw is the density of the medium inside the container (γw = γ`w); γc is the density of water at the ambient temperature. PJ represents the total static pressure above the reference level of the cup mouth. The pressure on the negative pressure side equals the total static pressure above the horizontal level of the reference cup mouth, plus the pressure generated by saturated water vapor between that horizontal level and the vapor-water interface in the drum, plus the pressure generated by saturated water between the vapor-water interface and the horizontal axis of the connector. That is, P– = PJ × (580 – hw) × γshw × γw. Here, P– is the pressure at the negative pressure side of the container; hw is the vertical distance from the vapor-water interface to the centerline of the horizontal tube in the connector; γs is the density of saturated water vapor in the drum. Thus, the differential pressure ΔP = P – P– = 320 × γw – 260 × γc – (580 – hw) × γs – hw × γw. In other words, ΔP = 260 × γc + 320 × γw – 580 × γs – (γw – γs) × hw. It should be noted that γc, the density of water at the ambient temperature mentioned in equation (1), generally changes with the seasons, and such changes can affect the accuracy of measuring the water level in the drum. For the vessel in this example, when the ambient temperature rises from 25°C to 50°C, the effect of the change in density on the differential pressure is –2.3 mm of water column; after compensation by the compensation system, the impact on the final drum water level will be between 2.3 and 5.5 mm. Under normal circumstances, such errors can be ignored; in other words, the temperature here can be considered constant. However, to minimize errors, the temperature here must be determined appropriately. The temperature can be determined following this principle: choose the higher value rather than the lower one, taking into account factors such as the local climate and winter heating requirements. For example, if the ambient temperature here typically varies between 0 and 50°C throughout the year, with an average of 25°C, then the temperature here can be set to 35°C. This is because the rate of change of water density increases as the temperature rises; choosing a higher temperature will result in the environmental temperature changes having a lesser impact on the entire system. For the container in this example, when the temperature rises from 0°C to 25°C, the change in temperature has an effect of only about 1 mm on the final result of the measurement system; whereas an increase in the ambient temperature from 25°C to 50°C results in an effect ranging from 2.3 to 5.5 mm. Therefore, the temperature should be set high rather than low. 　4. Operating characteristics of dual-chamber balance vessels The operating characteristics of these vessels are very important for the drum water level measurement and compensation systems; understanding them helps users to apply them effectively and master the relevant techniques. The density of saturated water and saturated water vapor at various pressures can be found in the \"Density Tables for Saturated Water and Saturated Water Vapor\". By substituting the drum water levels of 0, ±50, ±100 mm, etc., into equation (1), a series of differential pressures output by the container can be obtained, as shown in Table 1 below, \"Reference Table for the Inherent Compensation Characteristics of Dual-Chamber Balanced Containers\". Table 1 shows the operating characteristics of the two-chamber equilibrium vessel. As can be seen from Table 1, the differential pressure output by the container at each water level changes differently as pressure varies (due to changes in the densities of saturated vapor and water). First, note the differential pressure corresponding to the 0 water level; its variation pattern is significantly different from that of other water levels, as it fluctuates only within a narrow range. Since the design pressure of this container is 13.73 MPa, its fluctuation range is smaller below 14.5 MPa, remaining within only ±5 mm of water column. In other words, when the water level in the drum is at 0 level, regardless of how the pressure changes, even in the absence of a compensation system, the impact on the measurement of 0 level is minimal or essentially none. Regarding other water levels, the differential pressure corresponding to a drum water level that is closer to the 0 level is less affected by pressure changes, whereas it is more affected in the opposite case. Therefore, a two-chamber equilibrium vessel is a drum water level measuring device with a certain self-compensation capability. This capability is primarily reflected in the fact that as the water level in the drum approaches the 0-level, the differential pressure it outputs is less affected by pressure changes; in other words, its impact on the measurement of the drum’s water level is reduced. Undoubtedly, the characteristics of a container are determined by its own structure, which is why they are also referred to as inherent compensation characteristics. In Table 1, 0 MPa corresponds to two rows of differential pressure values, and the reason for this will be explained later. The reason for this characteristic of dual-chamber balance vessels lies in the fact that they are designed and manufactured with a special structure that minimizes the impact of changes in the density of steam and water on the pressure difference associated with normal operating water levels. Nevertheless, it cannot fully meet the production needs, and compensation still remains necessary. 5. Compensation System 5.1. Basic Knowledge and Concepts It can be seen from the characteristics of containers that dual-chamber balanced containers cannot fully meet the requirements of production. The reason for this is due to changes in the medium density. Therefore, certain measures must be taken to further eliminate the impact of density changes on drum water level measurement. This measure used to eliminate the effects of density variations is called compensation. The water level in the drum is accurately determined through compensation. There are generally two methods for compensating the drum water level measurement: pressure compensation and temperature compensation. The compensation effect is the same regardless of which method is used. However, there is a slight difference between them: temperature compensation can start at 0°C, whereas pressure compensation can only start at 100°C. This is because temperature can correspond one-to-one with both the saturated density and the unsaturated density at temperatures below 100°C, whereas pressure can only correspond one-to-one with the saturated density; that is, the lowest pressure of 0 MPa corresponds only to the saturated density at 100°C. Therefore, the compensation starting points corresponding to each of the compensation systems formed by these two methods are different, that is, the ranges of the differential pressure transmitters vary. This is why there are two differential pressure values corresponding to 0 MPa in Table 1 ； The previous row corresponds to temperature compensation, while the next row corresponds to pressure compensation. Obviously, temperature compensation can also start at 100°C

This post was last edited by Cool breeze brings news_Z3722 on 2022-6-3 19:27. Why is + connected to transmitter L and - connected to transmitter H? I don’t understand. Isn’t it + corresponding to H and – corresponding to L?

The H and L markings on the transmitter denote not only high and low, but also Heighten and Lower. The output magnitude of the transmitter is proportional to the H side and inversely proportional to the L side. If simply the high-pressure signal is connected to the H side and the low-pressure signal to the L side, the transmitter’s output will actually decrease as the liquid level rises. In fact, in all differential pressure level measurements, the signal from the lower level is connected to the H side, while the signal from the higher level is connected to the L side. If the vapor pressure guiding tube is filled with condensate, filled with isolation fluid, or when measuring the level or density, large signals will be generated at point L, as well as migration issues will arise. .

Can it be understood that the side where the dominant pressure difference changes is connected to +, and the other side is connected to –?

There are also capacitive level gauges now.