What is the principle behind measuring the drum liquid level?

For measuring the liquid level in a steam drum, differential pressure transmitters and two-chamber balance vessels are generally used to achieve greater accuracy. The balance vessel helps to equalize the pressures in the positive and negative chambers, thereby eliminating the effects caused by the coexistence of gas and liquid. The transmitter has a measurement range that includes 100% negative drift, and it operates on the principle of differential pressure.

Generally, two or more units are used simultaneously.

Some power plants use capacitive level gauges, which can also achieve good results.

Based on principle, they can be divided into differential pressure type and communicating vessel type. The power plant has regulations requiring the installation of 2 sets of level gauges based on different principles. Generally, mica type, electrical contact, and transmitter instruments are all installed.

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Differential pressure measurement is based on the principle of measuring static pressure. The electrode type is based on the principle of measuring resistance. The float type is based on the principle of buoyancy.

Differential pressure types are based on the weight of the liquid column, while there are also buoyancy types. Capacitive. Ultrasonic waves, and so on

【Boilers】are common power devices in industrial production processes. To ensure their proper operation, it is necessary to maintain the water level in the boiler’s drum at a certain level. If the water level is too low, there is less water in the drum while the demand for steam is high; coupled with the fast rate of water vaporization, this leads to a rapid decrease in the amount of water in the drum. If this is not controlled in time, all the water in the drum will vaporize, resulting in damage to the boiler or even an explosion； Excessively high water levels can affect the separation of steam and water inside the drum, causing moisture to be carried in the steam, which in turn can damage subsequent production equipment. Therefore, to maintain the drum water level at the specified value, it is necessary to ensure that the boiler’s water supply volume is equal to the steam output volume. 【Boiler drum level measurement】 includes measurement methods using a balance vessel in combination with a differential pressure transmitter, methods using a balance vessel together with a double-diaphragm level gauge, electric contact level measurement methods, capacitive measurement methods, etc. Usually, the principle of 【using a balance vessel in combination with a differential pressure transmitter】 is employed for this purpose. Posts on the forum regarding the measurement principle of balance containers can be found at http://bbs.hcbbs.com/thread-43631-1-5.html. There is also a [capacitive level gauge]; its main features are: ① compact structure, small size, and easy installation. ②Polytetrafluoroethylene probes can be used as sensors, capable of withstanding highly corrosive liquids such as acids and alkalis. ③The temperature of the medium under test can reach up to 200°C, and the pressure can exceed 32 MPa. ④Fully sealed with no moving parts, it offers accurate measurements, good stability, is maintenance-free, and has a long service life. 【Working Principle】 The principle of capacitance-based level measurement is that the probe wire and the conductive liquid form a capacitor, where the metal core of the probe wire serves as one pole of the capacitor, and the conductive liquid acts as the other pole. In between are high-stability PPR or polyvinyl fluoride materials; that is, the insulating outer layer of the probe wire functions as the dielectric between the two poles. As the liquid level changes, the area of the probe wire covered by the liquid changes, which in turn alters the relative areas of the two capacitor poles and results in a change in capacitance. Using the formula for a concentric cylindrical capacitor, it is possible to determine the relationship between the liquid level and capacitance. The capacitance of a capacitor is influenced by three factors: ① the distance between the electrode surfaces, ② the size of each electrode surface, and ③ the dielectric constant of the material between the electrode surfaces. Since Co, ε, and D/d are constant values, it follows that C = KH, meaning that the capacitance is directly proportional to the height to which the liquid covers the probe (i.e., the relative area of the capacitor plates).

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Level detection methods: ①Direct reading type ②Static pressure type ③Buoyancy type ④Mechanical contact type ⑤Electrical type ⑥Acoustic type ⑦Radiation type ⑧Optical type; Electrode-type level gauge. Features: Used for the over-limit indication and control of water level in small boilers. Stainless steel electrodes are used to extend the service life. It is employed for controlling the upper and lower limits of wastewater in urban sewage treatment tanks.

Yes, which two methods are generally used for controlling the water level in boilers?

The boiler uses a relatively high differential pressure! But generally, a electrical level gauge is also added! It seems safer!

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Generally, two measurement methods are used. Controlling the liquid level in the air tank is crucial, quite challenging, and represents a key aspect of boiler control. High temperature and pressure are characteristic of air tanks; when there are significant fluctuations in steam flow and water inlet flow, it is possible for false signals to appear regarding the liquid level. Therefore, two different measurement methods are usually employed.

Generally, two electrical contact level gauges and a balance vessel are used to measure liquid levels; one forms a circuit based on the electrical conductivity of the liquid, while the other measures the pressure difference between positive and negative pressures

False liquid levels can be overcome using impulse control!

It would be great if there were diagrams of two-chamber equilibrium containers and single-chamber equilibrium containers

Yes, three sets of gauges based on different principles are required to ensure that water level in the steam drum can be monitored in case the differential pressure type balance vessel fails to function properly. However, there are still some safety risks; if these gauges don’t work properly, things can go wrong. It is still up to those experienced thermal engineers to oversee this situation

For the detection of the drum liquid level, multiple level gauges are generally used for comprehensive measurement; our plant employs 7 such detection methods. Four differential pressure transmitters are supplied via 3 sets of double-chamber balance vessels (one balance vessel serving two transmitters), which transmit the level signals to the DCS; these signals are used in a three-out-of-two configuration for three-element control. 2 sets of electric contact level gauges send the contact signals to the digital display, which is used as a reference in the control room. There is also 1 set of camera surveillance for the dual-color level gauge to provide a more intuitive visual display. As a side note: the drum level dual-chamber balance vessel is different from ordinary balance vessels, as it features temperature and pressure compensation. Thus, making the measurement more accurate.

In drum operation, maintaining the water level balance in the drum is the most important task. The drum water level is affected not only by changes in the feedwater volume but also by the steam generation rate. Therefore, using a single-parameter control method for water level adjustment is not ideal; it is necessary to employ a method that takes into account the difference between steam and water flow rates in order to control the water inflow and thus maintain a more stable water level in the drum. This method of controlling the drum water level is commonly referred to as three-transistor control. In fact, three-transistor control is a form of cascade control, with the drum water level being the main parameter of this control system. In this control loop, the output signal from the water level regulator serves as the setpoint for the cascade regulator, while the difference between the steam flow rate and the feedwater flow rate, after being added together, acts as the measurement value (the secondary parameter) for the cascade regulator. As long as there is a deviation between the measured value and the set value, the cascade regulator outputs a signal to control the variation in the inlet flow rate.