Page 179 - Applied Process Design For Chemical And Petrochemical Plants Volume II

168 Applied Process Design for Chen iical and Petrochemical Plants

where this condition may not be valid, therefore, the tray
(8- 241) flow range from start-up to overload should be examined
by the designer before finalizing the physical details.
Some authors recommend clearance of Y4 in. to M in. less
or, (alternate) than the tray weir height, but always greater than !4 in. [ 1901.
The bottom of the downcomer must be sealed below
(8 - 242) the operating liquid level on the tray. Due to tolerance in
fabrication and tray level, it is customary to set the down-
comer seal referenced to the weir height on the outlet side
2. Loss through inlet weir of the tray. Recommended seals, based on no inlet weir
When an inlet weir is used, the additional resistance to adjacent to the downcomer, and referenced as mentioned
flow may be approximated by: are given in Table 8-19.
For trays with inlet weirs, seal values may be reduced if
h)d 0.3 GdU (8 - 243) necessary for high flow conditions. A good tray design is
centered about a 1.5-in. clearance distance between tray
B. Cimlar or PipaType Downcomer floor and bottom of downcomer edge.
These downcomers are suggested only where liquid flow
is relatively small for the required tower diameter, allowing
a maximum of space for bubble caps. Adequate tray spacing is important to proper tray opera-
tion during normal as well as surging, foaming, and pulsing
hdc = 0.06 (Lg/Q)2 (8 - 244)
conditions. Because the downcomer is the area of direct
connection to the tray above, the flooding of a tray carries
Iiquid Height in Downcomer
to the tray above. To dampen the response, the tray must be
The backup of clear liquid during flowing conditions adequately sealed at the downcomer and the spacing
must be determined in order to set the proper tray spac- between trays must be approximately twice the backup
ing. Tray spacing is usually set at twice the liquid height in height of liquid in the downcomer. Thus for normal design:
the downcomer. This can be adjusted to suit the particular
system conditions.
Hd = h, + h, + A + hd + ht (8 - 245) where St = tray spacing, in.
Hd = height of liquid in downcomer, in.
Downcomer Seal Once foam or froth in the downcomer backs up to the
tray above, it tends to be re-entrained in the overflowing
The importance of the downcomer seal is to prevent liquid, making it apparently lighter, and accentuating this
vapor from the tray from bubbling into the downcomer height of liquid-foam mixture in the downcomer. The
(see Figure 8-63), whether the trays are bubble cap, valve downcomer must be adequate to separate and disengage
or sieve types. If a seal weir is not included in the tray this mixture, allowing clear liquid (fairly free of bubbles)
design, then operation problems to avoid flooding, weep to flow under the downcomer seal.
ing and unstable performance, including pressure drop, A tray inlet weir tends to ensure sealing of the down-
are increased, particularly during the start-up phase. comer, preventing the bubbling caps from discharging a
The major factors governing the proper deszgnfor clearance mixture into the downcomer.
under the downcomer (see Figure M3), and the distance
between the bottom of the downcomer and the tray it is
emptying onto are [190]: (a) downcomer sealing, (b) down- Table 8-19
comer pressure drop, and (c) fouling and/or corrosive Downcomer Liquid Seal [15]
nature of the fluids. The smaller the clearance, the more sta-
ble will be the tray start-up due to the greater restriction to Tower Diameter, Seal, Outlet Weir Height minus Distance
vapor flow into and up the empty liquid downcomer. Ft Downcomer Off Tray Floor, In.
Referring to Figure 8-63, the weir height, h,, must 6 and below 0.5
always be greater than the clearance under the downcom- 7-1 2 1
er, Le., between bottom of downcomer and tray floor, hdcl. 13 and above 1.5
Always avoid too low clearance as this can cause flooding Used by permission, Davies, J. A, Pet. Refine V. 29 (1950) p. 121. Gulf
of liquid in the downcomer. There are flow conditions Publishing Go., all rights reserved.