In order to prevent the rotating ring from moving axially, a retaining sleeve is provided in the structure. The split ring and the ferrule rotate with the sleeve and ensure that the sleeve can withstand axial thrust. The rotating ring is made of cemented carbide (tungsten carbide or silicon carbide), and the non-rotating ring is mostly a carbon ring. The non-rotating ring is attached to the sealing surface of the rotating ring under the action of the spring force. During operation, the sealing surface maintains a stable non-contact face film.
The conventional mechanical seal uses an oil seal, and the sealing surface is made narrow due to the control of heat generation. On the contrary, the dry gas seal uses a low viscosity gas, and a wide sealing surface is used in order to obtain a stable non-contact surface gas film. This is one of the characteristics of dry gas seals.
A sealed type of dry gas seal can also suppress the leakage of the sealed gas. The structure is simple, but since the gas passing through the sealing surface leaks to the atmosphere side as it is, it is often used in those working conditions that allow gas to be discharged to the atmosphere side. . Such as air, carbon dioxide, nitrogen, etc. on the compressor using the medium.
Two series sealed type two dry gas seals arranged in series, the wake of the first stage sealing gas (on the side of the sealed gas) is connected with the atmospheric discharge port, and the seal of the stage acts as the full load of the gas pressure, and the first The secondary seal (atmospheric side seal) is almost unaffected by pressure drops. The dry gas used in the first stage primary seal is the process gas, and the dry gas used in the second stage seal is mainly nitrogen. The process gas leaked through the main seal is introduced into the torch, and a very small amount of process gas and the second stage. Nitrogen leaks at the seal are mainly discharged through the field vent line. When the first stage seal (primary seal) fails, the second stage seal acts as a backup safety seal to ensure that the process medium does not leak to the atmosphere. At the same time, when the single-stage sealing can not meet the sealing pressure requirement, the series sealing arrangement can control the secondary sealing pressure to be half of the first-stage sealing pressure, and play a partial pressure.
In the working condition, neither the process gas leaks into the atmosphere nor the condition that the isolation gas enters the machine is allowed. The intermediate labyrinth structure can be added to the two types of dry gas seal structure arranged in series, and the injected nitrogen is used as the isolation. The buffer gas, the process gas leaked through the primary seal, is completely introduced into the torch by the nitrogen gas, and the second seal leaks all of the nitrogen gas, and the first seal leak is reduced due to the flow of the buffer gas nitrogen into the leak line. Precision.
Two parallel seal types Two parallel seal types are two dry gas seal back-to-back arrangements. This structure is to inject an isolation buffer gas between the two seals to ensure that the gas leaking from the atmosphere side and the gas side to be sealed is Inert gas. This type of dry gas seal structure arranged in parallel is mainly suitable for applications where the process gas is not allowed to leak into the atmosphere, but the isolation gas is allowed to enter the machine. In order to make the buffer gas passing between the two sets of seals into a reliable sealing system, it is necessary to control the gas source pressure between the inlet seals to be 0.2-0.3 MPa higher than the process gas pressure, so as to ensure that the gas leakage direction is toward Process gas and both sides of the atmosphere. However, when the atmospheric side seal is damaged, if the buffer gas pressure is lower than the sealed gas pressure, the seal close to the seal gas side will be damaged by the reverse pressure, so the reliability is relatively poor with respect to the series seal.
Dry gas sealing working principle and system composition Dry gas sealing working principle Dry gas sealing In the non-operating state, the rotating ring and the non-rotating ring sealing surface are pressed together by spring force. During operation, the gas rotates with the rotating ring. , is drawn in from the outer diameter of the rotating ring, and enters the inside along the groove. Since the groove of the spiral groove is shallow and shallow inside, and there is no passage at the end of the groove, the gas sucked in is compressed by the sealing cymbal on the rotating ring, and dynamic pressure is generated on the sealing surface to make the two seals Disengagement between faces. As a result of the balancing of the dynamic pressure and the static pressure around the non-rotating ring, a slight gap is maintained between the sealing faces, the size of which is determined by the balance of the closing force and the separating force. The closing force is the sum of the static pressure and the spring force between the sealing faces, and the separating force is the resultant force of the static pressure between the sealing faces and the dynamic pressure generated by the grooves superimposed on the sealing faces. When the closing force and the separating force are in equilibrium, the gap between the sealing faces is controlled to be 3 to 5 μm. If the external surface interferes, the gap of the sealing surface becomes small, and the dynamic pressure at the groove rises sharply and immediately returns to equilibrium. The gap below.
Similarly, if the gap of the sealing surface becomes large, the dynamic pressure at the groove decreases, and the gap of the sealing surface can maintain the gap in the equilibrium state. The spiral groove sealing surface of such a sealing structure has the above functions of automatic separation and automatic closing, and is therefore referred to as a self-balancing seal. Compared with the traditional oil-free labyrinth seal structure, the leakage is only 1/100 to 1/1000, and due to the presence of the gas film in the gap, it can cause lubrication and cooling side effects on the seal.
Dry gas sealing system consists of buffer gas system Buffer gas system is an important part of dry gas sealing. It mainly consists of three parts: clean buffer gas supply line, gas leak line and isolation buffer gas supply line.
The cleaning buffer gas supply line is mainly for ensuring the reliability in the dry gas sealing operation, ensuring that no foreign matter or liquid is mixed into the sealing portion and enters the gap of the sealing portion, so that the sealing surface contacts and damages the sealing surface. Therefore, the pipeline for supplying the gas should be clean and dry, and the filtration efficiency of the filter on the pipeline should be 2 to 5 μm. In the case where there is a possibility of liquid mixing, a coagulation filter must be used to remove the liquid. The buffer gas that is usually used is basically the gas discharged from the compressor itself.
The gas leakage pipeline mainly detects whether the state of the dry gas seal is reliable and stable by monitoring the leakage of the leaking gas pipeline. The commonly used monitoring methods include leakage interlock alarm and leakage differential alarm. The interlocking alarm with leakage amount mainly adopts the measurement method of the flowmeter, but since the leakage of the dry gas seal is relatively small, the sensitivity of the detection is not very effective, and the leakage differential pressure alarm form is adopted, and the detection of the sealing leakage is more sensitive.
The isolation buffer gas supply line basically seals the gas while leaking, so the leakage of the sealed gas is unavoidable, and the isolation buffer gas line is mainly for preventing the lubricating oil at the bearing box from entering the dry gas sealing portion to damage the seal. The surface and the auxiliary static seal can effectively separate the sealing portion from the bearing cavity by providing a labyrinth structure between the dry gas seal and the bearing and adding an isolation buffer gas.
The control system of the dry gas seal of the control system consists of a flow control function for the supply of the sealed gas, a monitoring function for the leaking gas, and a pressure control for isolating the nitrogen. The flow rate of the sealing gas supply is mainly controlled by the regulating valve. The gas leakage monitoring is mainly implemented by the differential pressure measurement of the flow meter and the orifice plate, and the operating state of the dry gas seal is determined by the interlocking of the leakage pressure difference alarm. The pressure control for isolating nitrogen is primarily the set pressure of the self-regulating valve to ensure effective isolation at the labyrinth seal.
Problems in dry gas sealing applications: The inlet process gas pressure creates a vacuum, and the propylene compressor that draws in the lubricating oil 200,000 tons/year methanol unit and the 150,000 tons/year methanol unit has an operating condition that the inlet pressure is lower than 0 kPa during operation. Under the condition that the inlet process gas of the compressor is under vacuum, the lubricating oil in the bearing box part can be sealed over the insulating gas seal into the dry gas sealing condition, and the "O" ring of the sealing component is accelerated and aging, shortening the service period, and serious When the droplets are mixed into the insulating gas and enter the spiral groove surface of the dry gas sealing moving ring, the sealing surface is brought into contact, and the sealing surface is damaged, causing the sealing failure.
The buffer gas used in the series dry gas seal of the buffer gas not effectively vaporized propylene compressor is the second-stage gas supply source of the compressor in the process system, the pressure is 0.505 MPa, the temperature is -5.5 ° C propylene gas, through the dry In the air-tight control system, the precision filter is filtered and then enters the dry gas seal. If the temperature and the partial pressure are consistent, the liquid must be vaporized, but in fact, if the flash tank liquid level is not well controlled, there is a pipeline. It is possible to mix in the droplets. Further, impurities which are present in the content of the propylene medium component are repeatedly vaporized and liquefied, and it is easy to form a separated liquid, which is contained in the propylene gas, causing heat generation deformation of the sealing surface, causing contact failure of the sealing surface and failure.
The pre-filter is added to modify the rapid failure of the dry gas seal of the 150,000-ton/year methanol interlocking compressor of the process gas pipeline. From the perspective of the cause, it is mainly the liquid-phase phenomenon of the process gas in the circulation section and the powder present in the synthesis gas. Foreign body. The maintenance of this process mainly transforms the pipeline where the process gas enters the dry gas sealing system. The buffer gas pipeline drawn from the outlet is changed from the bottom to the side, and the liquid discharge guide is added to ensure that there is no longer any belt in the syngas. Liquid phenomenon. In addition, a pre-filter is arranged upstream of the precision filter of the buffer gas pipeline to ensure the filtration of the powdery foreign matter therein, so that the quality of the buffer gas source entering the dry gas sealing portion is significantly improved. From the operational status of a few months of investment, initial results have been achieved, but the filter replacement cycle will take time to test.
Conclusion With the wide application of dry gas sealing technology in chemical plants, the reliability and stability of shaft end seals of large high-speed running machines will be greatly improved, but the problems in dry gas sealing applications need to be improved. We are constantly exploring effective solutions.
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