Several linked column .. method for design pressure setting. (design basis) 1

[jamesbanda]

I'd like some perspective on setting of design p's for tall columns which are linked..as i approach a re-engineering exercise on a proces design..

col 1: RV P 100 Psig top
Col 1 bottom Design P = Pressure from (Tray liquid loading at flood) + Pmax liquid level in sump + P margin.

Col 2,
Ptop = vapour loss to P1+ Relief P atP1
Option 1: P base = P sump col2 + P liquid col 2+ P liquid col 1(assumping all liquid is transferred) + margin
or
Option 2: P base = P sump col 2+ pliquid col 2 + P back pressure through col 1 to relief + margin.

i think option 2 is viable becuase it is reasonable to assume in a fire pumps will be stopped due to burning of fire safe valves..

i've been told we dont allow for Pbase = PRV at top + col full of liquid becuase we hydrotest in verticle and all our relief cases are only fire.. no other crediable case..

But i'd like to see what is convention for other firms with tall columns..

 

[sshep]

Hey James,

It wasn't obvious how your two columns are linked without a picture.

Our standards sound very close to option #2. The liquid height to use at the bottom is stated in our standards as either the LSHH, or the height of the liquid with the tower in a slump condition. The simplest estimate in my opinion is the LSHH plus slump height- since the LSHH will cause a slump if it trips the reboiler heat.

Even though all the towers at the site I just came from are designed for hydrotest in the vertical (and are frequently tested that way even in columns >100m tall), the calculation for maximum bottoms pump pressure is based on the LSHH or slump height.

best wishes,
sshep