Condensate problems

[Lijantropo]

Good morning,

The background is this:
A process tank uses steam to maintain the temperature of the fluid. The condensate leaving the tank passes trough a steam tramp and then, it flows toward the deaerator.

The steam is around 140 psia, so the condensate will have enough pressure to flow until the deaerator (23 psia).

Yesterday I heard something from an experienced engineer.
He said the steam control valve can be closed (when the temperature in the tank reach the set point) and in this case the pressure in the line (after valve) is almost zero, so the condensate will remain in the line. When you open the valve again, the fluid hits the condensate in the line and this could cause many problems.

Ok, my question is: when you are closing the valve you restrain the flow, so the velocity increases and the pressure drops. But if the line es large enough, what happens with the fluid forward?

I think it will be able to reach the deaerator while the pressure in the line equals the pressure in the deaerator. I mean, the pressure in the line can not be zero, because in this case the fluid in the dearetor will flow through the line, but, as I said before, he is an experienced engineer and that makes me doubt.

regards

lij

 

[steamdog]

The control valve controls only the temperature in the tank, so that it could produce a lower pressure in the coil than after the steam trap. This is VERY common, happens in heat exhangers, too, anywhere where there is a pressurized return system and a modulating steam control valve. It is called stall.

So, yes, you may have condensate in the line and maybe water hammer or uneven heating. The only way to prevent this is to force the outlet of the trap to atmospheric pressure and use a pump to get the condensate back to the de-a.

 

[Lijantropo]

Thank you Steamdog.

I looked for information about stall and I found this links:

http://www.spiraxsarco.com/resources/steam-engineering-tutorials/condensate-removal.asp

http://www.armstronginternational.c...ity-understanding-and-solving-equipment-stall

Regards,

Lij

 

[Homayun]

Presuming that the control valve is one the outlet of the coil and that the Steam Trap is closed to the Control Valve, the line we are talking about here should be short. So in that case I don't expecte that much problem..And the line after the stea trap should be fully liquid.

 

[TBP]

Homayun - If the control valve in on the condensate outlet of the coil, then it also performs the duty of "trap". (That's really all a steam trap is, when you get right down to it - an automatic valve.) I've installed HXs like this, and it works just fine. No control is needed on the steam inlet. The steam is simply "ON", as long as the isolation valve is open.

If you try to have the steam pressure force the condensate back to the DA, I would expect some pretty impressive water hammer. As steamdog suggested, dump the condensate to a vented tank, and pump it back to the DA. Spirax-Sarco has excellent literature on this type of installation.