wetted area for column trays
wetted area for column trays
(OP)
Sir,
How do you calculate distillation column wetted perimeter for reflief valve sizing?
Thank You
How do you calculate distillation column wetted perimeter for reflief valve sizing?
Thank You





RE: wetted area for column trays
If so, I generally assume the normal liquid level at the bottom of the column added to the with the total liquid holdup in all the trays. Multiply that combined level by the circumference of the column.
Wetted Area = 2*pi*r*(N*l + L) + [surface area of bottom head of vessel]
pi = 3.14159...
r = radius of column
N = # of trays
l = liquid holdup per tray (I've used ~3" in the past)
L = Highest opertional liquid level in bottom of column.
Anyone out there handle this differently?
RE: wetted area for column trays
something like Wetted area=2*pi*r*(N*l+L)-(average area of bubble caps per tray)+surface area of bottoms liquid.
This is more accurate, but it's better to overcompensate when it comes to safety devices. In which case, your formula is more than reasonable.
"Scientists dream about doing great things. Engineers do them." -James Michener
RE: wetted area for column trays
The formula in my original post is based off the surface area for a cylinder and should only be the wetted *sides* of the column. The trays themselves are not exposed to the fire, but the walls of the vessel are exposed. We're not dealing with cross-sectional areas so simply subtracting the bubble cap areas isn't mathematically valid.
RE: wetted area for column trays
RE: wetted area for column trays
I think whammett is referring to the volume of the bubble caps.
RE: wetted area for column trays
Yes you add the wetted area of the sump to the wetted area of the trays. But this area is the outside area of the fluid along the vessel wall. Therefore bubble caps volumes/areas/whatever have no place in the equation.
"Column sump" level is in my equation above as variable "L", the tray level are "N*l". Note they're being added together and multiplied by the circumference of the column.
RE: wetted area for column trays
As stated earlier, it is usually assumed that the liquid on the trays ends up in the column sump (see API STD 521, table 5) because the fire scenario is evaluated assuming that the column is blocked in and all mass and heat flow to and from the column has stopped. One could add the liquid height on the trays (all of them) to the level of the column sump but one could also calculate the volume of the liquid on the trays and relate that to column sump liquid height.
Note that in your equation you do not use liquid hold-up (volume in my interpretation) of the trays but liquid height (weir height?) on the trays.
RE: wetted area for column trays
"Scientists dream about doing great things. Engineers do them." -James Michener
RE: wetted area for column trays
Either way, I would never advise someone to take the bubble caps into account. This "accuracy" adds nothing to the safety of a device that already has some rather arbitrary safety factors embedded in its design equations.
RE: wetted area for column trays
"Scientists dream about doing great things. Engineers do them." -James Michener