Pressure Drop Across The Tower

[Falcon03]

Hello All,

What do you expect the pressure drop across the tower reading if you have damage Trays (sieve/valve trays) ? HIGH OR LOW ?

Crude tower was opened for inspection and we found more than 10 valve trays are damaged !!!! we haven't seen any big changed on produtc quality (HGO).

 

[lastone]

Different tray manufacturers will quote different "dry tray" or liquid height pressure drops, but you should be able to make an estimate yourself. Visualize the tray in operation. Each tray has a definite amount of liquid and foam, but you know that each tray has to have enough liquid to overflow the tray weir, and make it into the next downcomer. You also remember that a liquid height is also equivalent to a pressure. Therefore the vapor must have enough pressure to overcome the normal pressure drop from the tray itself, and the height of liquid sitting on the tray.
Let's assume the dry tray pressure drop of one tray is 0.05 psi. Let's also assume your tray's weir is 3 inches, therefore the tray will be holding at least 3 inches of pure liquid. Assume HGO S.G. = 0.75. It takes 36.96" of HGO (S.G.=.75) to equal 1 psi. Therefore, 3"/36.96" = 0.08 psi. 0.05 + 0.08 psi = 0.13 psi/tray. 10 trays = 1.3 psi drop. This is an estimate of course, but hopefully it will give you something to go by.

 

[scanguy]

The change in DP is also dependant on the type and extent of tray damage. If you have missing and damaged valves but are operating at high rates your DP may not change as these valves would be running full open normally. If youu have more physical damage such as manway covers or tray panels dislodged then you may expect to see a decrease in DP as lastone has described above. However if these panels have dropped down on trays below and started to block a downcommer and restrict liquid flow the DP start to increase as flooding in that section increases. In some instances the increased loading at the blocked tray is equal to the loading loss on the damaged tray and no significant change in DP can be detected.

If for an operation standpoint there is no significant change in DP and little change in output then you probably best to leave it alone. However, if this is for knowledge prior to a turn around (planning scope and finding out you don't have to open that tower at all or ordering internals to decrease downtime) and you believe a tower may have damage or just may have history of damage as many do, you can have a TowerScan performed. This is done while the tower is operating and can provide alot of information on how the tower is operating and pin point any tray damage.
for more info:

http://www.towerscan.com/