Sealing issue
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The inlet pressure of the hydrogen feed pump is 0.5 MPa, and the outlet pressure is 19.3 MPa, while the pressure of the external flushing oil is only 0.9 MPa. Shouldn’t the pressure of the external flushing oil be higher than that of the medium? ； Also, the inlet pressure of the turbine medium is 14.5 MPa, while the pressure of the seal flushing oil is only 3.3 MPa. I’ve been struggling with these issues for a long time; could some expert help me figure them out?
This post was last edited by zhangchao2012 on 2017-10-1 22:40. It belongs to the pressure-type series sealing PLAN53B; the pressure of the flushing oil is higher than that of the medium. If there is a leak in the seal at the medium side, the sealing fluid will enter the pump chamber.
Shouldn’t the pressure of the external flushing oil be higher than the pressure of the medium? The pressure of the flushing oil is higher than that in the sealing chamber; the pressure in the sealing chamber is not equal to the inlet pressure or the outlet pressure!
The sealing flush pressure is higher than the inlet pressure. Because there is pressure reduction via a balance disc at the outlet side (the balance disc usually has a labyrinth seal in the axial direction), and moreover a balance tube connects the inlet side to the area between the balance disc and the outlet mechanical seal, the flushing pressure of the mechanical seal at the outlet side is, just like that at the inlet side, slightly higher than the pump inlet pressure.
1 To be precise, the pressure of the external flushing oil must be higher than the pressure in the pump’s sealing chamber. For multi-stage pumps with support at both ends, the pressure in the sealing chamber is generally 0.2 Mpa higher than the inlet pressure; therefore, the pressure of the external flushing oil needs to be 0.2 Mpa higher than this chamber pressure, which means it is roughly 0.9 Mpa； 2. As for the turbine, once you understand its principle, it becomes clear; the turbine can be considered as a pump in reverse operation – high-pressure liquid enters at the high-pressure outlet of the pump and exits at the low-pressure inlet. It is estimated that the chamber pressure of this turbine is around 3.1 Mpa
Both the feed pump and the balance pump use the BB5 design; a balance tube is used to equalize the pressure at the pump’s inlet and outlet. The pressure in the sealing chamber can be considered to be equal to the inlet pressure, so the pressure in the sealing chamber of the feed pump is 0.5 MPa. The pressure of the external flushing oil is generally sufficient if it is 2 bar higher than this value. The principle is the same for turbines. The pressure at the turbine outlet is determined by the process flow of the unit, and it may be 28 bar.
Can it be understood in this way: if it is a double-support pump, then the mechanical seal at the pump inlet must seal against the pressure at the primary inlet, which is 0.5 Mpa; the outlet section of the pump is connected to the inlet section via a balance tube, and the mechanical seal at the outlet section also needs to seal against the pressure at the primary inlet, namely 0.5 Mpa. For external flushing, a pressure 2 bar higher than 0.5 is sufficient. A flushing pressure of 0.9 is sufficient.
The oil sealing pressure needs to be only 0.05-0.1 Mpa higher than the pressure in the sealing chamber.