Outlet Weirs

The water outlet s siphoid baffle must be submerged to a depth roughly equal to half the useful depth of the separator. This is a minimum as priority is now given to oil removal and not desanding. It can be built of asbestos board, of Epikote-coated steel or of plastic. 

Hydraulic loads of 30 to 50 per meter of outlet weir are defined for throughput 

Q throughput flow race in cubic meters per second L developed length of the weir in meters H height of the water head in meters. 

Tire water falling downstream from the weir has no adverse effect on the physicochemical purification operation. It may, however, cause foaming and aerosols, or even odors. 

The means of preventing this kind of trouble (and the less often the sludge is pumped out, the more frequent it is) is to install a scraper screw across the whole width of a single concentration pit or to have a number of square truncated pyramid shaped concentration pits all across the width of the basin with an equal number of suction devices. 

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FIG. 14-24 Performance of two crossflow plates operating at 0.13 bar pressure and total reflux. Test mixture etbylbenzene/styrene. Spacing between plates is 0.50 m, and outlet weir height is 38 mm. Ut = superficial vapor velocity, pc = vapor density. [Billet, Comad, and Giuhh, I. Chem. E. Symp. Ser. No. 32, 5, 111 (1969).]... 

As noted, the weir crest is calculated on an equivalent clear-liquid basis. A more realistic approach is to recognize that in general a froth or spray flows over the outlet weir (settling can occur upstream of the weir if a large calming zone with no dispersers is used). Bennett et al. [AIChE J., 29, 434 (1983)] allowed for froth overflow in a comprehensive study of pressure drop across sieve plates their correlation for residual pressure drop /jf in Eq. (14-87) is represented by Eqs. (14-115) through (14-120) ... 

For sieve or valve plates, h = h , outlet weir height. For bubble-cap plates, h = height of static seal. Tbe original references present vaH-dations against laboratoiy and small-commercial-column data. Modifications of tbe efficiency equation for absorption-stripping are also included. 

Sieve plate—4.0-mm (%2 in) holes, hole-active area = 0.048, 40-mm (1.57-in) outlet weir... 

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... 

Vapor rises up through risers or up-takes into bubble cap, out through slots as bubbles into surrounding liquid on tray. Bubbling action effects contact. Liquid flows over caps, outlet weir and downcomer to tray below. Figures 8-63-67, 79, and 81. 

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. 

Notched outlet weirs (usually 60° V-notch) are only used for low liquid flow rates, and the head over this type of weir with notches running full [13]. 

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. 

Tower Diameter, Ft Seal, Outlet Weir Hei t minus Distance Downcomer OH Tkay Floor, In. 

The static slot seal is the fixed distance between the top of the outlet weir and top of the bubble cap slots. 

Tray Spacing, 24 in. Type outlet weir End Note Tray spacing was set when the 6-ft, 0-in. diameter was determined. 

Weeping A condition occurring when the vapor rate is not large enough to hold all the liquid on the tray, so that part of the liquid flows over the outlet weir while the rest falls through the holes. 

The term, hi, represents the hydrostatic head on the tray, while (hw + h w) is the liquid seal at the tray outlet weir, expressed as clear liquid. The factor, p, can be obtained from the upper curve in Figure 8-126 [193]. 

A liquid flow rate exceeding 50-60 m /hr-m of outlet weir (537.9-645.5 ft /hr-ft of outlet weir length)... 

Intensifies as outlet weir height increases and as the liquid flow rate increases. 

Allow 3-in. clearance (no holes) between outlet weir and adjacent row of holes. 

The downcomer area and plate spacing must be such that the level of the liquid and froth in the downcomer is well below the top of the outlet weir on the plate above. If the level rises above the outlet weir the column will flood. 

A simplified schematic of a particle in a centrifuge is illustrated in Fig. 12-3. It is assumed that any particle that impacts on the wall of the centrifuge (at r2) before reaching the outlet will be trapped, and all others won t. (It might seem that any particle that impacts the outlet weir barrier would be trapped. However, the fluid circulates around this outlet corner, setting up eddies that could sweep these particles out of the centrifuge.) It is thus necessary to determine how far the particle will travel in the radial direction while in the centrifuge. To do this, we start with a radial force (momentum) balance on the particle ... 

Water that drops down a foot or two from the outlet weir can cause the emission of odourous gases from the sewage. This is usually a problem only after primary sedimentation. 

M perfectly mixed, isothermal CSTR has an outlet weir. The flow rate over the weir proportional to the height of hquid over the weir, h, to the 1.5 power. The weir height is. The cross-sectional area of the tank is A. Assume constant density. 

Deoiled water passes from the plate pack into an interceptor bay, over an outlet weir, and into the outlet channel as shown in a very early design (Fig. 1). This particular design, which consists of a concrete-lined pit buill in the ground, was utilized for refinery wasle-water service. 

Oil is removed by a fixed-level skim pipe which is located above the highest level of the water outlet weir. This level prevents water from leaving through the oil outlet skim pipe. 

The clean water flows horizontally through the unn and falls into the clean-water outlet chamber overflowing the adjustable water-outlet weir plates. The height of this water weir as measured from tank inside bottom decides ihc working level of the coalescer unit A sheen baffle is located just before (lie water-outlet weir. The sheen baffle captures small oil droplets that might pass undet the oil-retention baffle. The buildup of oil film at the sheen baffle is so slow that no skimming device is required This oil buildup gets hack into the separation chamber when the unit is shut down for planned maintenance purposes... 

The clear liquid backup is divided by the froth-density to give the froth height if this exceeds the tray spacing plus the outlet weir height, the tray is deemed to be flooded. 

Possibly 90 percent of the trays seen in the plant are of these types. Perforated tray decks all have one feature in common they depend on the flow of vapor through the tray deck perforations, to prevent liquid from leaking through the tray deck. As we will see later, if liquid bypasses the outlet weir, and leaks through the tray deck onto the tray below, tray separation efficiency will suffer. 

Liquid flows across a tray deck toward the outlet weir. The liquid overflows the weir, and drains through the downcomer, to the tray below. 

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