Downcomer clearance minimum

For sloped downcomers adjust W (weir length) to the correct value for the downcomer bottom edge. Downcomer clearance, //dcc, is typically a minimum of lin. (25.4 mm) and is usually 0.5 in. (12.7 mm) less than the outlet weir height to maintain a positive seal on the downcomer. Calculate the velocity under the downcomer ... 

Downcomer sealing. While pressure drop and fouling considerations set the minimum values for downcomer clearance, sealing considerations set the maximum value for downcomer clearance. If the service... 

If a clearance greater than the weir height is desired, the author recommends that it be made as low as permitted by the downcomer pressure drop and fouling criteria that the clearance not exceed weir height by more than 1 in and that the clearance be made at least 2 in less than the clear liquid backup in the downcomer at minimum vapor and liquid loads. 

My minimum downcomer clearance is 1 inch. Maybe a pressure surge will push up the downpour plate below the downcomer Maybe corrosion deposits will accumulate in this area ... 

On the other hand, if the bottom of the downcomer is too close to the tray below, then the "head loss under the downcomer" will be excessive. Typically, a minimum downcomer clearance is 1.5 to 2 inch. Too small a downcomer clearance will result in restricting the liquid flow from the downcomer. This will also cause excessive downcomer backup and flooding. Check the correct downcomer clearance on the vender tray drawings prior to the tower inspection. 

To ensure non-surging liquid flow under the downcomer, use a clearance, hdd, of at least 3 times the design for the other trays, or a minimum of 2 in. to 4 in. 

Allow 5-in. clearance (no holes) between inlet downcomer and first row of holes. The 5 in. could be reduced to 3 in. minimum if an inlet weir were used. 

The minimum clearance between the downcomer outlet and the deck of the plate is 1 inches. If the downcomer to tray clearance is greater than the liquid seal height on the tray at the minimum liquid rate, an inlet weir of suitable height should be provided to ensure that the downcomer is actually sealed in order to avoid vapor blowback through the downcomer. 

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