Rules of thumb governing makeup water to loop seal

[MattNCSU03]

I am trying to determine the makeup water requirements to a large loop seal in an overflow line for condensate drain tank. To help visualize the system, I dusted off my ASCII art skills to create a P&ID:

[tt]
FWH
| ___
TNK \ /-COND
| s|_|--!-o--<-SealSupply
| \-x-/
PP-->

FWH-Feedwater Heater
TNK-Drain Tank
COND-Condenser
PP-Drain Pump
U-loop seal
!-orifice plate
o-open valve, x-closed valve
s-level switch
[/tt]

The reference plant P&ID has a normal makeup supply through an open valve and an orifice and backup through a normally closed valve that an operator would be dispatched to open in the event a seal blowout trips the level switch. Sizes for the seal water supply aren't given on the reference (but even if they we're, I'd still need to independently verify them.) The size of the loop seal for the reference is given as 14" diameter, minimum 30ft seal height. The heat balance gives condenser pressure is given as ~1.5psia and drain tank water is saturdated at 215°F.

I looked into calculating the evaporation rate but the equations I found assume that there is a breeze to carry away the water vapor. Maybe I could assume that the condenser air removal equipment removes the vapor on the condenser side of the seal and the other side is stagnant but I feel like I might be making a mountain out of a molehill. Should I just size the manual line to refill the seal in a reasonable time and the orifice to just provide 1-2 gpm? Make up is supplied from a head tank fed by the condensate system so there is no water chemistry impact to providing too much water.

 

[rmw]

Why won't vapor inside the drain tank side of the loop seal leg condensing on the pipe walls at atmospheric temperatures and draining into the loop seal keep it filled?

rmw

 

[MattNCSU03]

It might...I found an equation from nusselt that covers condensation inside tubes that I will work through on Monday. On the other hand, it might make for some good PE exam practice.