The principle of dry gas sealing when there is a hydrodynamic pressure groove on the outer side of the end face (2.5 ~ 10? M) Rotation, the dynamic pressure groove pumps high-pressure isolation gas on the outer diameter side (called upstream side) between the sealing end faces, and the gas film pressure gradually increases from the outer diameter to the groove diameter, but gradually decreases from the groove diameter to the inner diameter. With the increase of gas film pressure on the end face, the opening force acting on the sealing ring is greater than the closing force, and a thin gas film (1 ~ 3? M) to make the seal work in a non-contact state. The formed gas film completely blocks the leakage channel of the relatively low-pressure sealing medium, and realizes zero leakage or zero escape of the sealing medium.
Dry gas seal history
Dry gas seal is a new type of non-contact seal developed on the basis of gas dynamic bearing at the end of 1960s. In fact, the non-contact operation of the seal end face is mainly realized by adding a dynamic pressure groove on the dynamic ring of the mechanical seal and setting up an auxiliary system accordingly. John Crane Company of England took the lead in applying dry gas seal to gas transmission equipment of offshore platforms in the late 1970s, and achieved success. Dry gas seal first appeared to solve the problem of shaft end seal of high-speed centrifugal compressor. Because the seal runs in a non-contact way, the material of the sealing friction pair is basically not limited by PV value, which is especially suitable for shaft end sealing of high-speed and high-pressure equipment.
Application of dry gas seal in P2202 A/B pump Brief introduction of P2202 A/B pump
P2202 A/B pump [1] is the circulating pump of the reabsorber in the low-temperature methanol washing unit of the chemical fertilizer department of Anqing Branch of Sinopec. The medium is H2S-rich methanol, and the equipment model is10×12-21n. This is a horizontal single-stage pump with mechanical seal. See table 1 for the process parameters of the pump.
Table 1 P2202 pump process parameters
Improvement of seal of P2202 A/B pump
The original mechanical seal of P2202 A/B pump is imported from abroad, as shown in Figure 2. The sealing liquid is ethanol. Because the process medium (methanol) of the pump contains many impurity particles (mostly caused by pipeline corrosion), the end face of the mechanical seal friction pair is seriously worn and the life of the mechanical seal is short. The mechanical seal of methanol pump needs to be replaced frequently, which has a large maintenance workload and high maintenance cost. In this case, the mechanical seal of methanol pump was reformed in April 2008.
Fig. 2 schematic diagram of mechanical seal structure of p2202 pump.
Because the sealing liquid and medium are easy to vaporize, and the medium contains more impurities, it is easy to have a negative impact on ordinary mechanical seals. According to the technical parameters and actual working conditions of the pump, the following two improvement schemes are put forward:
(1) In order to overcome the dry friction caused by volatile medium on the end face of mechanical seal, the main seal adopts dry gas seal, and the seal type is TM 1 1A dry gas seal (Figure 3 and Figure 4), which is not affected by medium evaporation. At the same time, through the filtration control of the main seal gas, the end face of the dry gas seal is in contact with clean gas.
Fig. 3 schematic diagram of dry gas seal structure of P2202 pump Fig. 4 schematic diagram of dry gas seal control system of P2202 pump.
(2) In order to prevent impurities in the medium from affecting the normal operation of the dry gas seal, pre-buffering is used for flushing. In order to make the structure simple, the clean medium filtered at the pump outlet is directly used as the buffer, and a spiral seal is added between the pump medium and the sealed buffer to prevent impurities from entering the buffer and protect the dry gas seal from working normally.
The throttle holes on the original shell are also adjusted accordingly, and the corresponding relationship is shown in Table 2.
Table 2 Corresponding Relationship of Shell Orifice
When the dry gas seal works, the low-pressure nitrogen in the pipe network with the main sealing gas pressure of 0.7MPaG is filtered by the filter, and the filtering accuracy of the gas reaches 1μm, and then enters the sealing cavity through the self-supporting pressure regulating valve and flowmeter. In order to ensure the safe operation of the pump, when the nitrogen pressure is as low as 0.4MPaG, the methanol pump is controlled to stop by the pressure switch instrument installed behind the self-supporting regulating valve.
Pre-buffering of dry gas seal means that methanol, a medium liquid introduced from the outlet pipeline of the pump, is filled into the pre-buffering cavity of dry gas seal after being filtered by a filter, so as to protect the sealing end face of dry gas seal from being polluted by the internal medium of the pump.
Transformation effect
The dry gas seal of P2202 pump has been put into use since June 2008, 65438+ 10, and the operation condition is good, and all technical parameters are under control.
There are some improvements in the dry gas seal control system of P2202 pump:
(1) There is only one filter in the pre-buffer zone of P2202 pump because the medium of methanol washing unit contains many impurities and particles. When it is blocked, it must be stopped to clean the filter. It is suggested to connect two filters in parallel, one for opening and the other for standby. When one filter is blocked, you can switch to another filter without stopping the machine. You can also add 1 large coarse filter screen in front of the filter screen to delay the blockage of the filter screen.
(2) 1 pressure gauge should be installed on the balance pipe of P2202 pump, behind the orifice plate of buffer solution. According to the values of these two pressure gauges, the pressure of buffer solution and main sealing gas can be adjusted to the ideal state required for dry gas sealing.