Air cooled steam condenser , pressure equalizing line. 2

[D.E.R]

Hello ,

Your help will be appreciated. I'm trying to find out what is the purpose of the pressure equalizing line (called also "balancing line"). this line is bypaasing the cndenser so the the condenser inlet is connected with the condensate tank - schematic drawing attached.

What will happen without this line ? is it active all the time or not suppose to active at all ?

thanks in advanced ,

Best regards.

 

[georgeverghese]

The pressure equalising line is used to maintain the vapor space pressure in the condensate storage tank at above the saturation vapor pressure of the steam condensate at the operating temp of the condensate pool in the tank.

This helps the operator to subcool the condensate as much as the overheads condensor would allow, and yet maintain a decent tank pressure. Without this balance line, the tank pressure will collapse to the saturation vapor pressure of the condensate product from the condensor, and that would be subatmospheric.
This excess pressure head on the tank also helps the operation of the condensate transfer pump by increasing NPSHa. This also helps to enable subcooling the condensate at the condensor, as that is the only way to ensure all the steam has condensed (since this is a single phase chemical component).

So if the condensate has a very low sat vap pressure of p1 corresponding to condensate product temp of t1, and if tank pressure is p2, then p2 >>p1.

A more sophisticated version of this balance line is used in overheads condensors operating at total condensors in distillation columns - flow though this balance line is controlled by a CV to maintain a constant condensate drum pressure or constant dp between column and tank. In your case, this bypass flow is seen to be managed by a manual throttle valve in a fixed position, which may mean you have more flow thorough this line than you really need sometimes. At other times, the bypass flow may not be enough.

 

[D.E.R]

Many thanks for your clear reply.

Actually in my specific design there is no control option at all ( controlled valve or manual valve ) . p&id attached .

in what case the flow will be from the condensate tank to the inlet duct ? is it possible that the saturated vapor pressure of the condensate is higher than the pressure in the inlet duct ?

Best Regards.

 

[georgeverghese]

Which inlet duct are you referring to : the hot steam bypass inlet or the cold condensate feed inlet?

Strange you dont have even a manual throttle valve on this hot gas bypass - pressure balance line; I suspect you must have a large elevation difference between the max liquid level in the tank and the top of the U bend of the liquid seal on the condensor common exit line, probably 10-11 metres at least.

 

[D.E.R]

Hello ,

I'm referring to steam inlet duct. The pressure equalizing line is connecting between the vapor section in the condensate tank and the main steam inlet duct to the ACC.

In the P&ID the flow direction is appeared as two directions : hot steam from the main inlet duct to the condensate tank (vapor section) and the opposite.

the second scenario means that the pressure in vapor section of the condensate tank is higer than the inlet steam pressure to the ACC header , is that possible ?

 

[georgeverghese]

Cannot see how this could be a two way flow line - hot steam would always flow into the storage tank. The first diagram you posted shows a unidirectional flow line here. Resolution on the second drawing you posted seems to be poor - cannot see many details, including flow arrows.

 

[D.E.R]

Hello ,

I agree...it was very helpful discussion.

thanks!

 

[don1980]

As steam collapses into liquid (in the air cooled condenser) you want the condensate to freely drain from the condenser to the collection drum. Without this equalization line, you'd likely see sluggish control and operations of this system, as the pressure in the collection drum rises and falls, which then causes surges of condensate drainage followed by no drainage. This sluggish cyclical operation is prevented by either: (1) sizing the condensate drain line to be self-venting, or (2) install a pressure equalization line. Obviously, the process designer chose alternate #2. Note that there's no continuous flow from the steam header through this equalization line. instead, it's simple a pressure balancing (equalization) line.