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Mechanical Seal Stuffing box pressure and Vapour Margin

Mechanical Seal Stuffing box pressure and Vapour Margin

Mechanical Seal Stuffing box pressure and Vapour Margin

(OP)
It is well known that the vapor suppression margin needs to be maintained between the seal stuffing box pressure and the vapor pressure of the liquid being pumped to avoid flashing. Given the water is at 260 C in the pump. The stuffing box pressure is 5290 Kpa(g) and the Vapor pressure of water at 260C is 4843 Kpa(a).

Now assuming if the pump was depressuired for some reason and the stuffing box pressure fell below the V.P (ie 4843-14.7) we would have flashing. However I have two questions associated with this senerio.

1. What actually happens to the temperature while the stuffing box pressure is dropped. My take is the temperature also drops (i.e as per Clausius-Clapeyron equation. However what I do not understand is why would not water condense in this senerio with dropping temperature. Why does it rather flash/boil?

2. My other question is if I look at the enthaply - entropy diagram for water/steam. For the above conditions it seems water exits as liquid (i.e for the above pressure and temperatures). However in order to flash water, atleast as per the H-S diagram it would require a massive pressure drop. Which seems counter intutive to the phenomena of boiling (i.e when surrounding pressure in the above case stuffing box pressure drops below the vp pressure of the liquid), that margin is quite small compared to the drop that the H-S diagram requires in terms of pressure for water to flash.

Not sure where I am getting disconnected in understanding the pressure and temperature relation above in terms of flashing.

Any help would be appreciated.

Thank You in Advance
UB  

RE: Mechanical Seal Stuffing box pressure and Vapour Margin

(OP)
My correction to the second question in my above post.

2. In the H-S diagram water will exist as liquid (because of the API seal flush 23) which maintain a temp of around 75- 80 C on the seal faces) as part of seal cooling. However, at that temp (ie. of 80 C after seal cooling) the pressure would have to drop quite high to cause flashing in the stuffing box. Which is counterintutite of the concept of stuffing box pressure dropping below vapor pressure. Because in this case the margin is small while following the H-S diagram the drop has to be quite high.

Thanks
 

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