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Normal Pumping

Water should flow into the wet well from a straight pipe (or channel) at least 8 pipe diameters long and coaxial with the trench to prevent deleterious cross-currents in the basin. The incoming current flows above the trench to the end wall, dives and flows upstream along the bottom of the trench. The flow that passes pump intakes joins the incoming flow near the top of the ramp.

Swirling

The narrow trench tends to keep currents evenly distributed, but swirling is sometimes greater than allowed in ANSI/HI 9.8. Although not mentioned in that publication, swirling can be controlled by adding straightening vanes (see the Figures 2 and 3 below) in the suction bell or in the horizontal pipe between the suction elbow and a dry pit pump. For wastewater applications, the vane design should allow for: (1) a sphere passageway of at least 3-in and at least equivalent to the pump's passageway, (2) at least four vanes (six are better) and (3) shedding of stringy material by making vane noses smooth, round and inclined less than 45-deg to the streamlines. As cast iron should not be welded, vanes can be bolted to cast iron flares used as suction bells. Of course, vanes can be welded in fabricated steel suction bells.

For clear water applications, the vanes can extend the full width of the flare. Four vanes work fairly well, but more are better. Similar to the wastewater vane application, vanes can be bolted to cast iron flares used as suction bells. Vanes in fabricated steel bells can be welded to the bell.

Entrance Baffle

Another means for improving performance, especially for reducing swirling and changing a "good pump environment" to an "ideal pump environment" is to reduce incoming currents with a baffle. Almost any baffle that intercepts the incoming current is beneficial, but a large, vertical rectangular baffle that forces flow under it and around its sides is superb. A horizontal baffle is easier to install, but it can be a rag catcher and force all the current to travel over or under it and not around the sides. Vertical baffles (see Figure 4) were found in model tests to be more effective than horizontal ones in eliminating swirling, and in a prototype, rags can slide down and off of them. The best width was 5/6 Dp, which for this 18-Mgal/d wet well just happens to equal D_p. The best bottom elevation was D/2 below the inlet invert, and the best location was 60 percent of the distance from the end of the pipe to the centerline of the first pump. The baffle can, however, be moved back and forth with only minor effects.

The baffle should be a thin (say, 4-in) box section, because a simple plate would probably flutter. It can be supported in many different ways. The method shown in Figure 4 is to install a box or pipe beam (that can resist both bending and torsion) above high water level (HWL). Another tactic is using a beam above HWL and another (or even the roof) above that. Another way is by means of a beam above HWL and another at the bottom of the baffle, but the lower beam would prevent shedding stringy material. The structure should be stiff enough to resist vibration due to von Karman vortices. Use stainless steel and fill box sections with concrete to resist microbial corrosion, which attacks even stainless steel in stagnant wastewater. The need for and the design of baffles should be established by hydraulic model testing.

Vortices

Strong vortices form at the trench floor under suction bells and at the trench walls about 0.28 D below the bells. As vortices tend to cause vibration and cavitation, they should either be eliminated or at least attenuated. Otherwise, impellers and casings should be made of material more resistant than cast iron. Addition of nickel to cast iron is a partial palliative, but there are other metals far more cavitation (but not vibration) resistant.  

Side wall vortices can be eliminated by fillets sloped 45-deg if sufficiently high. The ANSI/HI 9.8 recommendations for upstream suction bells are D/2 for floor clearance and 3/8 D for fillet height, thereby placing the top of the fillet D/8 below the bell rim-a safety factor of about 2.

Floor vortices can be virtually eliminated by a flow splitter with 45-deg sides and also 3/8 D high. Flow splitters with base width equal to height (side slopes of 63.4--deg) are the steepest that can keep floor vortices under reasonable control. A slope of 45-deg is preferable if it leaves room for workers' feet during installation.

In clear water applications, cones under suction bells are also effective at eliminating floor vortices. The diameter of the cone must be twice the floor clearance and the apex must be in the plane of the bell rim. Four, or preferably six, vanes can reduce swirling to much less than the maximum allowed in ANSI/HI 9.8. Some hydraulic model testing experts greatly prefer flow splitters instead of cones.

Trenches narrower than 38-in are physically too confining for workers. If there are two duty pumps in a trench, that difficulty essentially precludes the use of fillets and flow splitters for capacities much less than about 10-Mg/d. Just omit flow splitters and fillets for smaller pumping stations and use materials more resistant to cavitation than gray cast iron. Many trench-type stations that have no fillets or flow splitters but do have nickel in the iron have operated very satisfactorily for many years.

Uneven Distribution of Throat Velocities

ANSI/HI 9.8 limits the variation in throat velocities to 10 percent. Uneven and fluctuating throat velocities in the suction bell have not been a problem in trench-type wet wells with column, wet pit submersible, horizontal dry pit or vertical dry pit pumps when the vertical dry pit pump is preceded by a long-radius reducing elbow wherein the exit velocity is at least twice the inlet velocity.

Self-Cleaning

The wet well is cleaned by dewatering it rapidly (pump-down) with the last pump. Choose a time when the inflow is about half the capacity of the last pump. As the water level falls below the top of the ramp, a hydraulic jump is formed. As the water level continues to fall and the hydraulic jump approaches the foot of the ramp, the currents wash floating material to the last pump where it is entrained in the fluid and pumped out. As the jump progresses along the floor, it suspends all remaining debris and the currents wash the debris into the pump.

Consider, for example, a wet well featuring three equal variable-speed pumps (two duty pumps) with 25-in suction bells having an entrance velocity of 4-ft/s-an 18-Mgal/d facility. A single duty pump would have a capacity of about 10-Mgal/d (15.5-ft³/s). Assume cleaning occurs when the inflow is 7.8-ft³/s. At the bottom of the ramp, the velocity is about 17-ft/s (from Figure 5), and the depth of flow is only 0.3-ft. The flow splitter ends at Node 15 and the average water depth drops accordingly. The Froude number never falls much below 6 and the sequent depth (height of jump) remains above 1.3-ft. As the last bell should be no higher than D/2 below the sequent depth to prevent loss of prime, the bell should be at an elevation no higher than 1.3 - 25/ (2x12) = 0.26-ft above the upstream floor. It should also be D/4 or 0.52-ft above the floor below, so the floor must be lowered by 0.52 - 0.26 = 0.26-ft.

From www.coe.montana.edu/ce/joelc/wetwell.   Available free.

Bed characteristics and definitions for the trench at pump-down in the figure above.

Cleaning would be adequate if, at the foot of the ramp, the depth of water were 0.1-ft or more with a velocity of 12-ft/s or more and if the Froude number at the end of the wet well were at least 3. The cleaning performance indicated by Figure 5 (depth 0.27-ft, velocity 18-ft/s at foot of ramp and Froude number 6.2 at end of trench) is superb.

Cleaning Time

The time can be calculated by estimating the volume to be discharged and the net flow rate into and out of the basin. The volume in the sewer pipe is that between the draw-down curve and the original depth. If 18-Mgal/d (27.9-ft³/s) fills a 3.0-ft pipe, 7.8-ft³/s fills it to a depth of 1.25-ft. (See Figure B-5 in Pumping Station Design [3].) Critical depth is 0.88-ft from UnifCrit2.2 [5], so draw-down at the end of the pipe is 1.25 - 0.88 = 0.37-ft. If the draw-down curve is roughly 600-ft long at the end of pump-down, the volume, V1, in a parabolic wedge is

      V₁ , Approximate volume of draw-down......... 600 x 3 x 0.37 x (1/3) ≈ 220-ft³

      V₂, Volume above trench = 1.85(4.17+8.0)(1/2)21.04.......................240

      V₃, Top of trench to 0.3-ft above floor = 4.97 x 4.17 x 17..................350

      V₄, Empty two column pumps = (18/12)² π/4 x 2 x 30......................110

Pumping at 15.5-ft³/s with an inflow of 7.8-ft³/s gives a net discharge pumping rate of 7.7-ft³/s. The volume in the wet well is 220 + 240 + about half of 350 = 635-ft³. The time required to pump this amount is 635/7.7 = 82 s. The pump probably loses about 15 percent of its capacity at low submergences, so the rest of the water (110 + 350/2 = 285-ft³) is discharged at a net flow rate of 15.5 x 0.85 - 7.8 = 5.4-ft³/s. This part takes about 53 s. The total time is about 2.3 minutes.           

If two pumps are used to discharge the 635-ft³, the net pumping rate is 27.9 - 7.8 = 27.1-ft³/s, and the 82 s shrinks to 23 s for a net cleanout time of about 1.3 minutes.

Grease

Grease accumulates on walls between high and low water levels and must be occasionally removed. It clings tightly to concrete and may have to be scraped off. It can be more readily washed off walls coated or lined with plastic with a water jet of about 25- to 30-gal/min at a nozzle Pitot pressure of about 90-lb/in.². Although expensive, coating is  well worth the cost.

Easy access to all wall areas with the water jet helps facilitate removing grease. The jet must be close to the wall to be effective, so attach a nozzle to the end of a long tube to make a water "lance" that can reach to within a yard or so of the surface to be washed. Access hatches make washing possible, but they are a nuisance. A better solution is an inside walkway the full length of the wet well (as shown in Figure 4), but the entire space must then be well-ventilated and the vented air treated-all of which results in additional cost. However, ventilation and treatment protects concrete from corrosion and does allow easy access (not readily achieved otherwise) to the suction bells for clearing out trash or for other needs, so the walkway can be justified in many pumping stations.