Inlet weirs

Sedimentation Tanks These tanks are an integral part of any activated-sludge system. It is essential to separate the suspended solids from the treated liquid if a high-quality effluent is to be produced. Circular sedimentation tanks with various types of hydraulic sludge collectors have become the standard secondary sedimentation system. Square tanks have been used with common-wall construc tion for compact design with multiple tanks. Most secondary sedimentation tanks use center-feed inlets and peripheral-weir outlets. Recently, efforts have been made to employ peripheral inlets with submerged-orifice flow controllers and either center-weir outlets or peripheral-weir outlets adjacent to the peripheral-inlet channel. 

Inlet weir on tray blocking flow from tray above. Limited capaeity. High pressure drop as rates were increased. Design and inspection error. 

For most trays, liquid flows across an active area of the tray and then into a downcomer to the next tray below, etc. Inlet and/or outlet weirs control the liquid distribution across the tray. Vapor flows up the tower and passes through the tray active area, bubbling up through (and thus contacting) the liquid flowing across the tray. The vapor distribution... 

These contribute to the uniform distribution of liquid as it enters the tray from the downcomer. There are about as many tray designs without weirs as with them. The downcomer without inlet weir tends to maintain uniform liquid distribution itself. The tray design with recessed seal pan ensures against vapor backflow into the downcomer, but this is seldom necessary. It is not recommended for fluids that are dirty or tend to foul surfaces. The inlet weir is objectionable for the same reason. 

The first row of caps next to the weir or inlet downcomer must be set back far enough to prevent bubbling into the downcomer. The inlet weir prevents this, although it can be properly handled by leaving about 3 in. between inlet downcomer and the nearest fece of the first row of caps. 

The height of an inlet weir, if used, should be 1 to l)f in. above the top of the slots of the bubble caps when installed on the tray. 

If inlet weirs are used they should have at least two slots %-in. by 1-in. flush with the tray floor to aid in flushing out any trapped sediment or other material. There should also be weep or drain holes below the downcomer to drain the weir seal area. The size should be set by the type of service, but a minimum of M-in. is recommended. 

Design uses tapered segmental downcomer and inlet weir. 

Area between downcomer and inlet weir for up-flow of inlet liquid. All trays included access manway. 

Holes for drainage must be adequate to drain the column in a reasonable time, yet not too large to interfere with tray action. Draining of the column through the trays is necessary before any internal maintenance can be started or before fluid services can be changed, when mixing is not desirable. The majority of holes are placed adjacent to the outlet or downcomer weir of the tray. However, some holes are placed in the downcomer inlet area or any suspected low point in the mechanical layout of the column. 

Figure 8-101 presents a generalized representation of the form useful for specific tray capacity analysis. Instead of plotting actual vapor load versus liquid load, a similar form of plot will result if actual vapor load per cap (here the cap row relative to inlet or outlet of tray is significant) versus the liquid load per inch or foot of outlet weir length. 

Slope the trays downward from liquid inlet to outlet, with the total drop from inlet to outlet weir not exceeding one-half the calculated gradient. 

Cascading the tray by using weirs as dams to divide the tray in steps, each step or section of the tray having no significant gradient from its inlet to outlet. This is usually only considered for trays 10 ft in dia. and larger, as it adds considerably to the cost of each tray. 

The head loss in liquid flowing down the downcomer, under its underflow edge (and up over an inlet weir, if used) and onto the tray is important in determining the back up of liquid in the downcomer. There are many suggested relations representing this head loss. 

When an inlet weir is used, the additional resistance to flow may be approximated by ... 

The bottom of the downcomer must be sealed below the operating liquid level on the tray. Due to tolerance in fabrication and tray level, it is customary to set the downcomer seal referenced to the weir height on the outlet side of the tray. Recommended seals, based on no inlet weir adjacent to the downcomer, and referenced as mentioned are given in Table 8-19. 

For trays with inlet weirs, seal values may be reduced if necessary for high flow conditions. A good tray design is centered about a 1.5-in. clearance distance between tray floor and bottom of downcomer edge. 

A tray inlet weir tends to ensure sealing of the downcomer, preventing the bubbling caps from discharging a mixture into the downcomer. 

Enlarge the clearance between the downcomer face and the inlet weir (or equivalent), (see Figure 8-92 or 63) to 1.5 to 2 times the dimensions used for the other trays. 

Inlet weir (downcomer side) length (if used), 4.0 ft... 

Inlet weir height above tray floor, 3 in. 

Note that if an inlet tray weir is used, the (hw + how) group is replaced by the corresponding (hw + how ) calculated for the inlet weir using the same algebraic relations. 

Downcomer pressure loss. Qearance between bottom of downcomer and plate = 1-in. max. Underflow area = (9.5 in.) (1 in.)/144 = 0.065 ft. Because this is less than the downflow area (of 0.334 ft ), it must be used for pressure drop determination. No inlet weir used on this design. 

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. 

Tray Thickness (Net Required for Bubble Cdps) Type of Flow Split, Cross Inlet Weirs (Y No) ... 

Aerated tray, liquid pressure drop or equivalent clear liquid on tray, in. tray liquid Height of clear liquid on inlet side of tray, in. Height of clear liquid at overflow weir, in. 

Velocity of liquid flotving between segmental downcomer and inlet weir, ft/sec Vapor velocity through equivalent net tray area, based on tower area minus twice downcomer area, ft/sec also... 

Figure 11.24. Segment (chord) downcomer designs, (a) Vertical apron (b) Inclined apron (c) Inlet weir...

---