With the increasing requirements of modern automation control, basic parameters such as temperature, pressure, level, and flow rate in process operations must be accurate and stable. One of the most critical process control parameters is the liquid level. Especially in industrial production sites, the requirements for liquid level are even more stringent. The main applications are level switches, servo level meters, radar level meters, glass plate level meters, magnetic flap level meters, magnetostrictive level meters, ultrasonic level meters, external float level meters, float level meters, double flange level transmitters, double chamber balancing vessels; advanced neutron level meters are used in the coke towers of coking units. These instruments provide reliable parameters for process control, making process operation more straightforward and improving the qualification rate and quality of the products while further ensuring the safe production of the unit. For the steam ladle level measurement of the unit, our unit applied a double chamber balance vessel and a double flange level transmitter.
This paper presents a comprehensive analysis of the ladle level meter in the device based on practice, combined with its own work experience. It introduces the basic principles and construction of the double chamber balance vessel and double flange level transmitter for remote data purposes in the device, as well as the application and practical commissioning methods, enabling the operator to monitor the changes in level remotely and in real-time, improving the safety and stability of the device operation. At the same time, it is intended to provide a reference and reference for related work.
1. Application of double chamber balancing vessels.
1.1 Importance to ladle level measurement.
In a plant, the level of the steam spoon is an essential parameter for safe production and is mainly responsible for raising steam energy to the plant. The quality of the steam is a crucial factor in whether the equipment can run at total capacity and whether production can run stably. In the past, when the degree of automation was not high, the level measurement of the steam ladle was only at the level of in-situ observation, mainly using glass level meters for size, the level could only be observed in-situ, the change of water level needed to be observed from time to time, to grasp the real-time dynamics of the water level, manual control of water level changes, the signal of the level could not be transmitted, which brought great workload and danger to the operator.
With the improvement of automation, the measurement of water level applies a double chamber balancing vessel, using the pilot tube measurement method, so that the temperature difference between the pilot tube of the balancing vessel and the steam ladle is the same, which overcomes the influence of temperature difference on the density of saturated water and greatly reduces the error of measurement. The differential pressure is transmitted to the differential pressure transmitter through the pilot tube of the balancing vessel, which is converted into an electrical signal by the differential pressure transmitter to display the change of water level on the detection screen in real-time so that the control of water level has manual operation into automatic control. In addition, the liquid level control inlet valve adjusts the change of water level on time.
1.2 Principle and construction of the double chamber balance vessel.
The main structure of the double chamber balancing vessel
The two-chamber balancing vessel is mainly a closed vessel connected to the ladle through a circular tube, so it is called a balancing vessel; the measuring principle of the balancing vessel is designed according to the way of differential pressure, and lead pipes are leading from the bottom of the vessel and the cylinder wall, which are connected to the ladle and the differential pressure transmitter respectively.
Its main structure is shown in Figure 1. In the upper part of the vessel, there is a circular funnel at the vessel’s mouth, and the disc below the horn is the reference cup. The funnel separates the double chamber balance vessel into two parts called a double chamber balance vessel. The level measurement system of the dual-chamber balancing vessel is mainly composed of the balancing vessel, the lead pipe, the source valve, the three-valve group, the differential pressure transmitter, and the DCS.
The balance vessel consists of a condensate chamber, a reference cup, an overflow chamber, and a linkage. The condensate chamber is located in the upper part of the balance vessel. It is connected to the upper part of the spoon, mainly to receive steam generated by dripping water, where the moisture will encounter a funnel to release the heat of vaporization at the point and form saturated condensate, which will accumulate in the reference cup. In contrast, the pressure generated by the condensate in the reference cup is transferred to the negative side of the differential pressure transmitter through the pressure guide tube.
When the reference cup is complete, it will overflow into the overflow chamber. The overflow chamber is connected to the drop pipe of the ladle, and the condensate from the overflow chamber will then flow out into the drop pipe without making the room full of water. Thus, the temperature difference between the base cup and the overflow chamber is consistent, and the temperature of the spoon is also constant.
The inverted T-shaped connector is connected to the ladle at one end and the positive side of the transmitter at the other end. Its primary function is to transfer the pressure generated by the dynamic water level in the ladle to the positive side of the transmitter and compare it with the pressure on the negative side by converting the current signal to the water level in the ladle. Due to this structure of the pilot pressure, positive pressure side and negative pressure side at the same time will ensure that the temperature inside the container and the steam ladle is similar, the density of the saturated water in the container is also very close to the soaked water in the steam spoon, so that the steam ladle water level measurement greatly reduces the error, in line with the operational requirements of the production process.
1.3 Calculation of the differential pressure range of the liquid level.
The actual steam ladle level calculation mainly balances the positive and negative differential pressures within the vessel. Firstly, it is determined that the positive pressure side of the spoon is at the location of the horizontal pilot pressure of the linkage, with the flat pilot end of the link connected to the positive pressure chamber of the differential pressure transmitter. Secondly, the negative side of the ladle is determined to be the negative side of the differential pressure transmitter at the lower end of the reference cup, as shown in Figure 1.
Double flange level transmitter for steam ladle level measurement
The positive and negative pressure sides are generally first connected to a special three-valve manifold for the instrument for easy commissioning and maintenance later. The zero and full levels of the ladle are then determined as well as the range. Thus, the zero level of the spoon is the lowest level of the continuum, and the whole level is the highest level of the continuum.
The lowest value of the differential pressure is the whole liquid level, and the maximum value of the differential pressure is the zero liquid level of the ladle. The maximum value of the differential pressure is the range of the liquid level. Combined with actual working experience, the density here uses the value of saturated water at the time when the steam ladle was put into use after the temperature and pressure had stabilized.
The differential pressure range of the liquid level is the pressure generated by the height of the water column between the zero level and the full level. The differential pressure range of the liquid level can be easily calculated according to the height difference between the corresponding positions of the zero and full water levels on the balance vessel.
1.4 Commissioning methods and procedures.
The balance vessel should be commissioned to determine the range and zero point before being put into service.
(1) Firstly, we open the drain valve connected to the ladle, open the steam and water phase valves connected to the balancing vessel and the ladle, and flush all the pipelines connected to the vessel to prevent them from being blocked, causing measurement errors.
(2) We have to determine the actual position of the zero and total liquid levels in the vessel wall of the balance vessel and measure the vertical height difference between the two positions. Then, the density of saturated water at high temperature and pressure after stabilizing the process conditions is used to calculate the differential pressure range of the liquid level.
(3) Open the two drain valves on the differential pressure transmitter, followed by opening the balancing valve on the three-valve manifold, filling the balancing vessel with water, and then slowly opening the two positive and negative pressure chamber valves, to remove the dirt and air from the pressure guide tube.
(4) Testing the signal cable connecting the differential pressure transmitter to the control room and confirming that it is correct before sending power.
(5) Under the premise of ensuring that the positive and negative pressure chambers are full of condensate, close the balance valve, open the positive and negative pressure valves, and at the same time open the steam and liquid phase valves connected to the balance vessel and the steam ladle.
At this point, the positive pressure chamber and the vertical pressure guide tube connected to the balancing vessel are filled with water, and the negative pressure chamber to the upper edge of the reference cup of the balancing vessel is filled with water, i.e. the level is a full range and the transmitter output value corresponds to 20mA.
(6) We can use the programmer to connect the transmitter to view the differential pressure value at this time, through the programmer on the transmitter and then enter the differential pressure value of the zero level, that is, the zero point to do 100% range negative migration, that is, the zero point is a negative value, corresponding to the output of the transmitter 4mA current signal.
The primary consideration for the commissioning of the balance vessel is that the positive and negative pressure chambers must be filled with water, the range calculation of the transmitter, and the negative migration of the zero point. In the actual production, pay attention to the changes in the liquid level. If the error is too large, the zero point of the differential pressure should be corrected in time.
2. Application of double flange level transmitters.
The double flange level transmitter can be used for differential pressure, flow, and liquid levels. It consists of a differential pressure transmitter, a capillary tube, and a flange assembly with a sealing diaphragm filled with silicone oil in the capillary tube. The sealed diaphragm is in direct contact with the process medium, and the pressure acting on the diaphragm is transmitted to the positive and negative pressure diaphragm of the differential pressure transmitter via the silicone oil in the capillary tube to which it is connected, which is processed by the transmitter and converted into a standard current signal of 4 to 20 ADC, corresponding to the change in level.
What is the Calculation of the level differential pressure zero point and range?
The range and zero point of the double flange level transmitter are relatively simple, and because of the difference in its structure, installation and commissioning are simplified. Firstly, in the design drawings, the vertical height between the horizontal centerline of the upper and lower flange of the ladle-mounted level meter is found. Then, the pressure generated by this height is calculated using the density of silicone oil.
This pressure is then used as the zero point value for 100% negative migration, resulting in a negative value for the zero-level differential pressure. The range value of the liquid level is to use the density of saturated water corresponding to the stability of the process when the steam ladle is put into use and the density of silicone oil to do the subtraction method to calculate the pressure generated at the same height in front, i.e. the difference in pressure between saturated water and silicone oil at this time as the range. In this way, the level zero point and a full double flange level transmitter range are determined.
In practice, the measurement of ladle level, double chamber balancing vessels, and double flange level transmitters are widely used in production. The double chamber balancing vessel is relatively complex in terms of mechanism. It requires an understanding of its internal structure before it can be applied to the site for commissioning and installation. For the double flange level transmitter, BCST has many years of experience using double flange level transmitters in the steam ladle level measurement, and our double flange differential level transmitter can be used in high-temperature, high-pressure areas. The practice has proved that the measurement of the steam ladle level reaches the process index requirements while reducing the maintenance of the instrument, improving efficiency, and ensuring the device’s stable operation.