The gasifier is the core equipment for the gasification process of water, coal, and slurry. They are synthetic gas in the gasifier’s excitation cooling chamber to complete the cooling, ash removal, and humidification. The reliability of the gasifier device-level measurement instrumentation directly affects the safe operation of the gasifier.
We have modified the pressure-taking device, thus extending the service life of the telemeter. In addition, it reduces the labor intensity of the staff and meets the needs of the device cycle operation. These will achieve savings in maintenance costs. This paper analyses the factors that may affect the measurement, from the selection, installation, routine maintenance, and other aspects of the remote level meter.
1. Fouling and slag blockage
1.1 Measurement failure
When syngas and molten slag enter the water bath of the cooling chamber, the water in the room contains a large amount of slag media. This slag accumulates for a long time at the lower flange pick-up tube, causing deposits on the surface of the measuring diaphragm, resulting in inaccurate measurements.
1.2. Reducing the effects of slag clogging
To reduce the impact of slag blockage, the original design was to incline the lower flange pressure line of the gasifier level by 45°. However, it requires installing a flushing water device transmitted remotely via the differential pressure transmitter. The flushing water is introduced from the upper pipeline where the measuring diaphragm box is connected to the process flange and uses de-oxygenated water from the gasification unit. However, the pump supply pressure is around 8.5 MPa.
1.3. Flushing effect
We generally do not depressurize the flushing water to improve the flushing effect. It is because the pressure and flow rate of the flushing water influence the flushing effect. When flushing, we generally do not use continuous flushing. We can measure according to the level gauge work. We can also flush regularly (usually once a week). Before we wash, we have to release the gasifier level meter interlock to prevent the level display from fluctuating too much during the flushing process, causing the gasifier to jump.
2. Measuring membrane box erosion
Due to the high pressure of the flushing water, whether we use continuous flushing or regular flushing. The high-pressure water jet from the flushing water connector will cut the surface of the membrane box, resulting in linear dents in the lighter cases and cutting the box open in the heavier cases, causing damage to the chest; at the same time. It will also cause the level meter to fail to display.
3. Flush water connection leakage
During the operation of the gasifier, we use the flushing water connections provided by the transmitter manufacturer. Due to vibration, corrosion, thermal expansion, and contraction, it will easily cause the joint to loosen and produce leakage. We need to tighten the treatment daily. The joint is located between the measuring diaphragm device and the process flange, which is not easy to fasten in the small space. We can lengthen the size of the joint according to the actual situation on-site so that it can be dry attached.
The strength of the joint is reduced due to the corrosion of the system over a long period. Sometimes, it will appear as a fracture phenomenon, and because of the poor measurement medium, the gasifier connected to the process root valve – generally can not. The process root valve of the gasifier connection is generally not completely closed. If the root valve is closed, a large amount of high-temperature black water will leak, forcing a stoppage. Therefore, every time we carry out a routine inspection of the gasifier, we have to check the flushing device and deal with any problems in time.
4. Diaphragm box corrosion
The membrane box material is generally 316 stainless steel. Stainless steel has better corrosion resistance. The membrane box is mainly formed on the surface of the stainless steel, an invisible oxide film, which is then passivated. The formation of the passivation film results from exposure of stainless steel to the atmosphere and oxygen reaction or contact with other oxygen-containing environments.
If the passivation film is broken, the stainless steel will continue to erode. In many cases, the passivation film is only partially destroyed on the metal surface, forming small holes or pits, creating small, irregularly distributed pits of corrosion that can cause leakage over time. At the same time, the black water in the cooling chamber contains, for example, Cl- ions. At a temperature of around 270°C, pitting corrosion occurs, which can easily cause corrosion leakage from the film box.
5. Hydrogen embrittlement of membrane cartridges
The gasifier contains hydrogen, which at high temperatures and pressures can easily cause hydrogen embrittlement. Hydrogen is not a corrosive medium, but it is very porous. The tiny hydrogen molecules will enter the metal’s crystal lattice, causing it to blister. It is also known as “hydrogen embrittlement” (the phenomenon of hydrogen embrittlement varies from metal to metal due to its intrinsic structure).
The transmitter sensor diaphragm is approximately 0.1mm thick and will rupture if hydrogen penetration is severe. Diaphragms made from materials such as Hastelloy or 316L are very susceptible to seepage and rupture.
6. High temperature and vacuum environment
During the baking phase of the gasifier, the opening of the extractor will cause a certain negative pressure to be reached in the gasifier. At this time, the transmitter membrane box is in a vacuum state. In the transmitter, to transfer the pressure of the measured medium, its filling medium requires a small expansion coefficient, non-corrosive, incompressible, and high-temperature resistant properties.
The transmitter’s isolation diaphragm is corrosion resistant and has a slight stiffness. In the transmitter in a high-temperature environment, the volume of fluorine oil will expand, then the expansion of the additional pressure generated by the distortion of the remote isolation diaphragm. In addition to the vacuum environment, the remote isolation diaphragm protrudes outwards, deforming the diaphragm and making the measurement inaccurate, affecting the transmitter’s everyday use.
We can remove and clean the antenna of the waveguide radar when the gasifier is being serviced. Once the fouling has been removed, we can continue to use it. If the waveguide cable is damaged, we can replace it.
There are currently no domestic manufacturers using radar level meters under this operating condition. The cycle time for continuous operation of a single domestic gasifier is generally around two months. If the waveguide cable of the guided wave radar is fouled, then the gasifier operation will not significantly affect the measurement during the cycle.