Distance from inlet bell centerline to sloping floor

Z2 = 5D minimum


Angle of floor scope

a = — 10 to +10 degrees


Angle of wall convergence

b = 0 to +10 degrees (Negative values of b, if used, require flow distribution devices developed through a physical model study)


Angle of convergence from constricted area to bay walls

f = 10 degree maximum

aCross-flow is considered significant when VC > 0.5 VX average aCross-flow is considered significant when VC > 0.5 VX average cost. Actual pumps and valves should be used to create the flow in the pump pit, and extremes of flow and submergence should be included in the test program.

Sometimes the cooling towers are on a hill, so a substantial drop in elevation provides pressure available nearer the flow area. If dry-pit pumps are used, this pressure can be utilized to reduce pump requirements. When this is done, the pump suction becomes a pipe, and the latter part of this section should be consulted.

recirculating SYSTEM—POND wet pit The intake structure in a cooling pond should be located as far as possible from the inlet pipe to the pond to generate the maximum cooling effect. If spray surface equipment is used, it should be so arranged in relation to the intake building that a minimum surface disturbance is encountered. Prevailing winds should be considered, and the building should be located on the lee (upwind) side of the

FIGURE 4 Flow rate vs. submergence over suction bell for vertical wet pit pumps. (U.S. gpm X 0.227 = m3/h, ft > 0.305 = m).
FIGURE 5 Ocean intake with velocity cap to minimize fish pickup

pond. If side and bottom areas might be easily disturbed to include silt in the flow, riprap— a wall or covering of stones thrown together randomly—should be applied to approach slopes as well as to bottom mats well beyond the inlet wing walls.

The handling of silt is usually not desirable in a pumping system. A high velocity through the pump will accelerate wear. At low velocity points in the system, silt will settle out and produce higher velocities, and more wear will occur as a result of the area blockage. If space is available, a silt-settling basin can be constructed ahead of the inlet basin. Cross baffles should be provided to slow the inlet flow to a velocity less than 1 ft/s (0.3 m/s). Most stream debris will settle out at this velocity, and the flow into the pump suction pit will be relatively clean, preventing the deposit of additional silt around the pump suction bell. If space is not available for silt beds in large-capacity installations, the main channel can be furnished with a weir across the flow path. The height of the weir should be selected to give an overflow depth above the weir of not more than one-third to one-fourth the water depth just preceding the weir. The velocity over the crest should not exceed the intake channel velocity. Weirs of this type are particularly effective when the intake channel is at right angles to the supply mainstream.

In areas where river levels vary considerably throughout the year, problems arise not only from silt and debris accumulation, but from the structures required to prevent damage to motors and electrical switch gear. If the required flow is moderate, a system known as the Ranney well (Figure 6) can be constructed. A Ranney well is a concrete silo 13 ft (4 m) in diameter, which becomes the collecting basin and pump well. It may be situated in or near a river and can be partly below and partly above water level with settings up to 100 ft (30 m). Small perforated pipes radiate horizontally from the base of the well and tap

FIGURE 6 Ranney well collects into a pool from underground strata (Ranney Method Division, Pentron Industries)
FIGURE 7 Empty spaces on nonoperating pumps cause vortexing at wall ends because of flow reversal.

into porous strata, bringing small flows of water into the main well. This water can then be pumped from the well with a deep-well wet-pit vertical pump.

Multiple Pumps Some pump requirements are easily met with 100% capacity single pumps, but more reliability usually requires two 50% or three 33% pumps, or, if the service is sufficiently critical, three 50% or four 33% pumps, and so on. There are practical limits of size for various pump types, so large flow demands will undoubtedly call for a multiple pump arrangement.

The problem arising from multiple pumps arranged in a common pit is from the probable nonuse of some of the pumps while others are operating. This can cause variables in flow patterns that may lead to eddying and vortexing (Figure 7). Installation of separating walls in the common pit may introduce additional problems because the ends of the separating walls can create eddy currents in the corners at the unused pumps. A back vent in the dividing wall will relieve this situation, provided it vents at the water surface (Figure 8b left). If walls are extended past the screens and trash racks to a forebay, this problem will not occur, but the design has then become that of a singlepump basin.

The same velocity rules apply to multiple arrangements as to single-pump basins. Odd arrangements should be avoided even when they look invitingly symmetric—fan-shaped, round, radial, peripheral—all have directional problems that are not easily overcome. A basic pit design, consisting of a number of equally-sized pumps in a common pit basin with flow entering parallel and straight in at 1 ft/s (0.3 m/s) or less, would not need to be model tested to assure reliability (Figure 8a left). If separating walls are required for structural support, and they are properly shaped and vented, no model test is recommended. The pumps should be located at the extreme rear of the pit so the whole approach assumes the characteristics of a suction pipe. Individual pump manufacturers may vary the location of the pump relative to the pit bottom, velocity of inlet spacing, and so on. It has been found that some of these variations require additional splitters or baffles below the pump, up the back wall behind the pump, or centered in the flow ahead of the pump. If so, a model test should be run and the additional pit cost weighed against other alternatives (changing the pit shape, pump location, pump size, and pump speed).

A dry-pit pump installation will have the pumps located either in a dry well at or below wet well water level (Figures 1e and 1f) or directly above the wet well and using a suction lift (Figures 1g and 1h), which calls for priming equipment. The additional cost of priming equipment (vacuum pumps, and so on) may be partially offset by the additional space and valve requirement of the first option. In either case, the suction piping in the wet well

FIGURE 8 Using basics of good pit design precludes the need for model testing (Hydraulic Institute Standards, 13 th Edition, 1975—out of print).

should be treated in the same fashion as the pump suction bells in wet-pit installations as far as spacing, direction, and velocity of flow are concerned.

when the pump is installed at an elevation that may be below the water level in the suction pit, a valve must be installed at the suction of the pump. The temptation to reduce the inlet size to use a smaller valve should be avoided. Pipe size at a pump inlet may decrease into the pump down to the pump suction size, but it would not be reduced below that size and then have to increase again as it enters the pump. The pipe in the wet well should preferably have a bell end and project downward. The minimum water level above the top edge of the pipe or the lip of the bell should be at least 5 ft (1.5 m) for a recommended entrance velocity of 5 ft/s (1.5 m/s). The bell mouth should protect downward to assure uniform inlet flow and to attain maximum submergence.

A wet-pit intake style that closely approximates a suction pipe arrangement and uses what is essentially a dry-pit pump has been expanded by the U.S. Department of the Interior, Bureau of Reclamation to an elbow-type suction tube design. This incorporates a formed concrete suction inlet with a swing of 135 degrees in the vertical plane and a gradual decrease in area to the suction eye of the pump (Figure 9). The resulting design saves considerable excavation in the wet well area, reduces losses in the pump,

FIGURE 9 Improved 135° design of Bureau of Reclamation elbow suction tube inlet (U.S. Department of the Interior, Bureau of Reclamation)

and allows a smaller, higher-speed pump to be used. With higher velocities, debris dropout is reduced so silt buildup does not occur as readily and a smaller trashrack area remains effective longer. These inlets are independently self-sufficient and may be grouped into multiples as long as the forebay is designed to adhere to the basic design rules for wet-pit approach channels. Additional guidelines on the proportions of formed suction intakes are included in the Hydraulic Institute Pump Intake Design Standard.

Considerations for either single- or multiple-pump pit design relate primarily to even flow and low velocity into the pump. At the leading edge of the pump impeller (suction) vanes, this velocity will be increased and the direction of flow violently changed. To make the transition from pit flow to pump flow is the work of the pump designer. Some pump designs include ribs in the suction bell; others do not. It is obvious that the transition from parallel flow at 1 ft/s (0.3 m/s) to right-angle rotating flow at 15 to 20 ft/s (4.6 to 6.1 m/s) requires a high degree of skill in matching the suction bell to the impeller. If the pump is not designed to handle a wet-pit installation as described in previous paragraphs, a turning vane pit may be required.

Suction Pit Turning Vanes Figure 1c illustrates how turning vanes are used to guide the flow of water into the inlet of a vertical volute dry-pit pump. Figure 1k shows the same for the suction bell of a vertical diffuser wet-pit pump. If these pumps were connected to an inlet sump without turning vanes, as illustrated in Figures 1a and 1j, a wider and longer channel would be required to feed the suction bell from all directions. The pumps with turning vanes an narrower inlet channels may be desirable as multiple pumps can be spaced closer together providing screen width requirements do not dictate spacing.

Comparing depth of pit bottom below pumps with and without turning vanes, the following is to be noted. The excavation beneath a vertical volute dry-pit pump with turning vanes can be slightly less than the excavation beneath the same pump with no vanes but with a suction bell. The velocity approach into the closed portion of the channel beneath the pump can be as high as 3 ft/s (0.9 m/s) if turning vanes are used at the design flow; but it should be limited to 1.5 ft/s (0.46 m/s) with no turning vanes. Although the suction bell design requires a wider channel than a vaned inlet (approximately two bell diameters), this is not wide enough and the channel must be made deeper to meet the lower velocity requirement for this type of inlet. When turning vanes are used with a vertical diffuser wet-pit pump, the pit must be excavated deeper than would be required if no vanes were used. This additional depth is required to form an elbow in the narrower channel and provide equal flow distribution to the impeller. Design velocity at the inlet vanes is 3 ft/s (0.9 m/s).

The setting of the lip of the suction bell and the pump impeller below design low-water level for volute dry-pit pumps must be the greater of the dimensions required to

• Prevent vortexing (ANSI/HI dimensions)

• Provide adequate NPSH at the centerline of the impeller

• Provide a level of water sufficient for the unit (impeller) to be self-priming

As an alternative to the use of turning vanes under a vertical volute dry-pit pump, the long-radius suction elbow inlet illustrated in Figure 1b offers some advantage in reducing the width and depth of excavation under the pump. The inlet velocity to the long-radius elbow, which is usually formed in concrete, is preferably no greater than 3 ft/s (0.9 m/s) at the design flow rate.

A decision to use turning vanes should not be based on a guarantee that there will be an increase in pump efficiency. The design of the vanes—their number and spacing—is still an art more than a science, and it is difficult to prove pump performance in the field. Turning vanes can be effective in eliminating underwater vortices, a problem sometimes associated with suction bells without turning vanes. It has been observed during model testing that the suction bell, as illustrated in Figure 1a, must be placed closer to the back wall than normally recommended for open channel inlets (similar to that in Figure 1j), to prevent underwater vortexing. The flow of water into a closed channel from an open pit containing water of considerably greater depth creates an unequal flow pattern in which the maximum velocity is along the floor. Also, there is little, if any, flow to the back side of the suction bell down from the top of the channel. Unless the bell lip is close to the back wall, flow along the floor and from the front only will overshoot the inlet and roll over, back, and up into the bell, forming an underwater vortex. Although turning vanes can prevent this, they may not prevent uneven flow distribution up into the pump impeller unless they are properly designed. They may even cause hydraulic and mechanical unbalance, which could result in noise, vibration, and accelerated wear of the pump bearings. For this reason, model testing of the turning vanes is recommended.

Screens and Trashracks Although it may not be feasible economically to eliminate all refuse from a pumping system, it probably will be necessary to limit the size and amount of debris or sediment carried into the system. Depending on the probable source of debris, such as a river subject to flooding with considerable flotsam in the runoff and with a very loose bottom, or a lake at constant level without disturbing inlet flows near the structure and with a solid bottom, the protection needed may include only a bar trashrack plus rotating, flushed, fine-mesh screens. If sediment deposit is likely, a settling basin may be required.

The designer must note that, if this equipment is useful, it will pick up debris and gradually increase the velocity through the openings as the net area decreases with blockage. When this occurs at the trashrack, the water level differential will build up, causing a waterfall with increased velocity and turbulence on the pump side of the rack. In addition,

FIGURE 10 Trashrack with raking mechanism: (1) Large enclosed trash hopper contains debris discharged by rake. Hinged door in end of hopper opens wide for debris removal. (2) Heavy-duty, single-drum hoist, push-button-controlled with two separate cables—one for carriage, one for rake teeth. (3) Walking beam actuated by hydraulic cylinder controls position of rake teeth. (4) Fixed sheave. Cable operating over stationary sheave raises and lowers rake carriage. (5) Discharge guide flanges assure positive positioning of rake over trash chute prior to dumping. (6) Dead plate, or apron, integral with superstructure guides rake to discharge point—prevents trash from falling off rake prematurely. Design permits operation over 3.5-ft. (1.1 m) high hand rail. (7) Self-centering rack-guided carriages allows rake to ride over obstructions in water during lowering cycle. Debris of all types picked up on lifting cycle rather than forced to bottom of channel. (8) Rake mechanism assures positive removal of debris with maximum carrying capacity. Hydraulic relief valve provides automatic overload relief. Teeth automatically open if overload occurs, permitting load to drop off rake. No cable failures due to overload. (9) Wide, flanged rollers ride on at least two rack bars (Envirex, Inc., a Rexnord Company)

FIGURE 10 Trashrack with raking mechanism: (1) Large enclosed trash hopper contains debris discharged by rake. Hinged door in end of hopper opens wide for debris removal. (2) Heavy-duty, single-drum hoist, push-button-controlled with two separate cables—one for carriage, one for rake teeth. (3) Walking beam actuated by hydraulic cylinder controls position of rake teeth. (4) Fixed sheave. Cable operating over stationary sheave raises and lowers rake carriage. (5) Discharge guide flanges assure positive positioning of rake over trash chute prior to dumping. (6) Dead plate, or apron, integral with superstructure guides rake to discharge point—prevents trash from falling off rake prematurely. Design permits operation over 3.5-ft. (1.1 m) high hand rail. (7) Self-centering rack-guided carriages allows rake to ride over obstructions in water during lowering cycle. Debris of all types picked up on lifting cycle rather than forced to bottom of channel. (8) Rake mechanism assures positive removal of debris with maximum carrying capacity. Hydraulic relief valve provides automatic overload relief. Teeth automatically open if overload occurs, permitting load to drop off rake. No cable failures due to overload. (9) Wide, flanged rollers ride on at least two rack bars (Envirex, Inc., a Rexnord Company)

the increase in velocity may pull more debris through the bars than can be tolerated. It is best to rake these racks (Figure 10) frequently enough to keep the differential head across the rack below 6 in (0.15 m). The spacing of the bars should be such that objects that cannot be pumped would be excluded from passing through. This, in general, will call for the bar spacing to be in proportion to the size of the pump. A pump manufacturer can determine the maximum size sphere a pump will handle, and the bar spacing should be limited to 50% of that value. The size of the bar, the lateral distance between supports, and the pier spacing will influence the rate of debris accumulation and the allowable design differential head.

Rotating screens (Figure 11) will remove trash of a much smaller size because the accumulation is continuously removed and the open area is kept uniform. Finer screening than that required by the pump may be necessary in installations where the liquid pumped must pass through small openings in equipment serviced, such as condenser tubes or spray nozzles (Figure 12). Screens are usually installed in conjunction with trashracks so large, heavy pieces will not have to be handled by the screens. Because velocity through

FIGURE 11 Traveling water screen (Envirex Inc., a Rexnord Company)

the screen is limited to 2 ft/s (0.6 m/s) unless environmental considerations require lower velocities, the pit cross section may be determined by screen requirements. If flow is such that a maximum-width screen available would be too long (deep) for practical or economic reasons, two screens may be employed with a center pier. In this case, the distance to the pump should be increased 50% over single-screen distance. Piers should be rounded (radiused) on the upstream side and ogived (tapered to a small radius) on the downstream side. Any corners at the sidewalls should be faired at small angles to the opening and wall to prevent pockets where eddies can form.

The trash collected by the screens must be disposed of. Traveling screens carry trash up into a hood above the operating floor, at which point a series of spray nozzles wash the trash into a trough leading into a disposal area (Figure 13). The nozzles are supplied by pumps sized for 200 to 300 gpm (45 to 68 m3/h) with pressures of 60 to 100 lb/in2 (413 to 690 kPa). These pumps are normally deep-well turbine multistage units. They are located in the clear well, if possible, close to a wall. If this is not possible, they can be suspended in the circulating water pit, to one side and ahead of the main pumps. Care must be taken that they do not disturb the flow to the larger pumps. Their submergence requirements are usually less than those of the main pumps, and this allows use of the pump setting that gives the least interference with either pump flow.

It is possible to have these pumps also dewater the pit. This will require additional piping and valves and a more careful location of the pump because it will, of necessity, be close

to the bottom of the pit. A small chamber off the main pit, located far enough from the main pumps to avoid eddying, will be required.

The direction of the flow from the forebay through the screens and into the pump area should be continuous. Avoid right-angle screens, through which flow must change direction at least once and possibly twice. If screens must be at an angle to the flow into the pumps, increase the screen-to-pump distance by 100%. Environmental considerations may increase the possibility of problems in this area.

Environmental Considerations Suction pit requirements will vary according to whether hydraulic or structural standpoints are being considered. Both of these may also be in conflict with environmental considerations.

A design to accommodate fish limitations was mentioned briefly in a previous paragraph. Fish react to a horizontal velocity but are not aware of a pull in a vertical direction. Thus, to keep them from entering the inlet, a horizontal flow must be established at a velocity low enough to permit fish to escape.

Intakes that take their flow directly from a river may have a high velocity that would trap fish. Even if the velocities are lowered to reasonable screen levels—2 ft/s (0.6 m/s)— fish may still be drawn into the screen area and carried up to trash disposal.

When the source of a water supply system is a body of water containing fish, steps must be taken to prevent undue disturbance and destruction of the fish. A site survey should determine

• The intake location furthest from natural feeding areas and from attractive, or "trap," areas

• The number of species involved

• The size range of each species and whether they are anadromous or settled

Sites for intakes should not be selected near feeding areas for large schools of fish (kelp beds, coral reefs, and similar attractive spots). Sheltered spots most suitable for intake flows may also be most attractive to fish.

Next, total flow, probable intake size, and the velocities at inlet, through screens, and at trashracks should be determined. Variations in flow throughout the year and temperature ranges in winter to summer should be available.

The best source of information about local fish is marine biologists who have studied the local areas. They may not only have information on fish habits, feeding patterns, population, and so on, but may also have test information about the fish swimming ability. If they do not already have this information, they can probably run a survey to develop the data.

The most difficult problem to overcome is related to small fish. Screen openings must be held to a minimum, and under velocity conditions, small fish have much less swimming-sustaining ability than larger fish, both in speed and in duration time. In a given steam flow (such as is generated by pumps with inlet water going through screens), a fish must have the ability to sustain a given speed against this flow for a certain length of time. When it weakens, it will fall into the current flow and will be impaled against the screen and destroyed. If the fish senses the velocity early enough, and has an alternate route, it can use darting speed to escape. Or it can follow another attraction (cross velocity flow into a separate chamber or a light attraction to the chamber) and be removed on an elevator or pumped out to a safety channel (Figure 14). Migrating fish need a continuation channel to restore their interrupted journey.

In designing an intake, it is necessary to keep the velocity below 0.5 ft/s (0.15 m/s) through the screen to avoid drawing fish into the screen. For a tube inlet away from shore, a horizontal velocity cap (Figure 5) should be placed over the inlet. This will prevent fish from being subject to a vertical velocity and will allow them to maintain a horizontal velocity that will direct them away from the inlet. Alternatively, a cross flow can be created that will propel or attract the fish to one side of the inlet area. From there, they can be directed into a bypass pool and lifted back to their own living area, or they can be sent around the plant to a downstream location. If they are anadromous, they can be sent to an upstream rendezvous. Piers and screens should be kept flush across their inlet face to prevent attractive pockets where fish can hide and be drawn into the screens when they weaken.

FIGURE 14 (a) Fish pump prevents entrapment on traveling screen (Detroit-Edison). (b) Fish escape allows fish to bypass screen area.

Inlet screen areas should be in small sections rather than one long face so a fish will not be trapped in the center and find it too far to swim to safety after it realizes its predicament. The maximum deterrent flow is about 10 ft/s (3 m/s), but this may be too much disturbance for the flow to the pumps. Smaller areas allow short-term limits for enticing fish away from the inlet, and survival will be much higher.

Fish congregating at an inlet or in a forebay pool can be crowded or herded to an outlet point by the use of vertical nets or horizontal screens. In a direct channel, horizontal moving screens can route fish past a sloping (relative to stream flow, say 35 degrees) moving screen. This results in directing fish to a narrow outlet at one side, leading to an outlet channel away from the main inlet.

The Environmental Protection Agency has had a major impact on plant design since the passage of the Federal Water Control Act of 1972 (as amended) in the US. Enforcement of Section 316a regarding thermal effluent has softened somewhat as later studies indicate that the effects of heat distribution on marine life are variable. Section 316b, however, covers every aspect of best-known technology and is applicable to any facility using a water intake structure.

Young fish must be kept from impinging on inlet screens. This can be accomplished by

• Lowering the velocity through the inlet screens

• Diverting or attracting the fish to other areas

• Providing restraints at inlets

• Using fish buckets and elevators to remove the fish before they can enter the plant area

• In short, by helping the fish avoid contact with the intake to the greatest extent possible

Obviously, trash and fish must be handled in separate areas, and screen wash pressures must be lowered to prevent harm to the fish.

Entrainment of marine organisms too small to be restrained by normal screens causes further problems in areas where such organisms normally develop. If the intake structure must be located in such an area, extensive information must be gathered at the site. Analyzing as much data as are available, in conjunction with plant flow and location requirements, may give the designer some idea of which equipment to select.

Small slot-width wedgewire screens (Figure 15) are now being furnished in larger sizes, and, with their very low inlet velocity and backflushing capability, offer one good approach to reducing entrainment. Sand, chemical, and cloth filters may suit some situations, but backflushing and cleaning problems make them less attractive costwise. Equipment may consist of any combinations of

FIGURE 15 V-shaped slotted screen provides velocity control to avoid attracting fish and good air or water backflushing for debris cleanup (Johnson Division, UOP).

1. Stationary screens a. Bar racks in various attitudes (Figure 10), with or without automatic rakes b. Air bubble, lighting, or electric heaters c. Underground, as in Ranney wells (Figure 6)

2. Moving screens a. Vertical or horizontal (Figure 11)

3. Transportation devices a. Elevators b. Pumps (Figure 14)

c. Baskets on vertical screens d. Attractive escape areas (Figure 15)

e. Velocity changes

4. Remote intakes a. Ocean outfall with velocity cap (Figure 5)

b. Ranney well (Figure 6)

If the source is a river, the angle of the intake structure relative to the direction of flow is important in modifying the impact of these design requirements. A case in point is shown in Figure 16. The screen house at a low angle to the river flow (instead of the usual 90 degree inlet) allows the river current to provide a swim-by attitude for fish while a low-velocity screen approach is maintained. If the pump house is in line with the screen house,

FIGURE 16 Angling the intake structure to the river flow allows fish to swim by Belle River power plant (Detroit-Edison)

minimal disturbance will be felt by the pumps. A tendency for eddies to form (and create vortices) can be minimized by placing wing walls upstream and downstream to control velocity.

Testing Model Pit Design When the basic rules for good pump suction pit design are adhered to, no model test will be required to ensure proper operation of the pumps and pumping system. The substance of these rules is to keep a straight-in approach at a constant low velocity from the water source to the pump chamber. The ANSI/Hydraulic Institute Standard dimensions and charts satisfy these criteria for the average pump in general application.

Site layout problems may make the ideal solution impossible. Structural and environmental requirements may outweigh hydraulic requirements in some instances. When the ideal pit may not be possible or economically feasible, a model test should be considered. It should be noted that the ideal dimensions are a composite covering not only a range of specific speeds but also a complex melding of pump design philosophies. Some variation from ideal dimensions should be expected from individual pump manufacturers.

Pump manufacturers are not in a position to guarantee the pump pit design. Differences of opinion between the structural and hydraulic pit design engineers and the pump design engineers may best be resolved by performing a model pit test (Figure 17). Refer also to Section 10.2.

Vortexing The real problems resulting from improper pit design occur largely on the water surface in the form of vortices, or cones, produced by localized eddies on the surface of the water. If this disturbance continues, the flow of water will carry the underwater part of the vortex down toward the pump suction bell and ultimately into the pump (Figure 18). This introduces air into the impeller and will affect the mechanical radial balance of the impeller by interrupting the normal solid-liquid flow pattern. This type of disturbance will produce hydraulic pulsations in the pump flow and mechanical overloading of bearings and impeller guides.

Underwater vortexing sometimes occurs in round pits or in pits where the pump suction bell is at some distance from the rear wall. Flow past the suction bell strikes the rear

FIGURE 17 Model pit test setup with fixed screens and pumps and valves for variable flow, to scale (Flowserve Corporation)
FIGURE 18 Surface vortex drawing air into pump suction bell

wall and rolls back toward the bell, forming an eddy current that disturbs the normal flow into the pump. In a round pit, a cross baffle below the pump bell may reduce this effect. Where the pump is some distance from the back wall, a wall can be installed near the pump, or a horizontal baffle at suction bell level behind the pump will also reduce the disturbance. In all cases, the distance between the suction bell and the bottom of the pit should not be more than one-half the suction bell diameter, and one-third the diameter is preferable.

The use of the suction bell diameter as a basis for spacing should be carefully evaluated. It will be seen that there is nothing magical in this relationship, especially when several pump manufacturers all use different bell diameters. The real criterion is the allowable velocity at the suction bell. It has been found that very-low-head pumps are much more sensitive to bell velocity over 6 ft/s (1.8 m/s). For example, the velocity head loss at the bell inlet with a high velocity may be such a large percentage of total pump head that efficiency could drop as much as 10%. A good design rule for safe operation can be related to pump head. For pumps having up to a 15-ft (4.6-m) head, the suction bell velocity should be held to 2.5 ft/s (0.76 m/s); up to 50-ft (15-m) head, 4 ft/s (1.2 m/s); and above 50-ft (15-m) head, 5.5 ft/s (1.7 m/s). These values should be used for any substantial amount of pumping, but for occasional short-term pumping, they can be exceeded without destroying the pump.

Vortices may be broken up and effectually nullified by arrangements of baffles and vanes, or they may be prevented from occurring initially by a proper pit design. The only way to determine what type of baffling should be used and its effectiveness is by model testing. Methods of eliminating vortexing are discussed in Section 10.2.

Vortices are usually generated when the flow direction of the liquid to be pumped changes or when there is high velocity past an obstruction, such as a gate inlet corner, screen pier, or dividing wall. In combination, these two causes invariably generate vortexes. For this reason, the pump suction pit should be immediately preceded by a straight channel in which the velocity does not exceed 1.25 ft/s (0.4 m/s). Satisfactorily operating pump pits with higher velocities are rare and should not be put into operation without the assurance of a model study.

An additional condition likely to generate vortexes is a multiple pump pit with individual cells in which only a portion of the pumps will operate simultaneously. The dead space behind the non-operating pump will have flowing water tending to reverse direction and form eddies. Eddies and vortices can be avoided by eliminating or venting the walls at the rear of the pit. It also helps to position the pumps at the extreme rear of the pit. Alternatively, expensive modifications to the pit, such as splitter walls and baffles, may be required (Figure 7).

Round pits tend to generate vortices, especially when the pump is centered in the pit. These vortices usually occur around the pump column because of eccentric inlet flow. Special cases of the round pit are tolerable either when a Ranney well (Figure 6) is used or when there are booster pumps in the pipelines (Figure 19). In the Ranney well, the ratio of pump size (and flow) to pit size (and capacity) is such that very low velocity exists, as in a lake inlet. Water comes into the Ranney well all around the periphery. These conditions of direct flow and low velocity prevent vortexing. Booster pumps installed in a circular can

FIGURE 19 Booster pump suspended in a steel well (or can) that requires a minimum space for suction pit (Flowserve Corporation)

(suction tank) must be centered in the can, and all inlet velocities to the can and flow in the can and into the pump must be uniform and high enough to provide fluid control. This velocity will vary from 4 to 6.5 ft/s (0.76 to 1.98 m/s).

Vortices are not generated by a pump or pump impeller and so do not fall into clockwise or counterclockwise rotation because of the pump rotation. Also, in a pump pit, vortices do not have a directional rotation induced by the rotation of the earth and therefore are not of opposite rotations above and below the equator, as is the case with "bathtub vortexing," which occurs in tanks being drained without pumps.

Submergence Centrifugal pumps in intake pumps must be submerged deeply enough to provide

• A pressure sufficient to prevent cavitation in pump first-stage impellers, referred to as NPSH (it is assumed that the proper pump has been selected to perform satisfactorily with available NPSH)

• Prevention of vortexing and associated pit flow problems detrimental to pump operation

A pump may have adequate submergence from a pressure standpoint and still be lacking in sufficient depth of cover above the suction inlet to prevent surface air from being drawn in. Any wet-pit pump must have its suction inlet submerged at all times, and for continuous pumping every pump will have a fixed minimum submergence requirement. Because this relates to velocity, there are two basic parameters: the suction inlet diameter and the depth of water above the inlet lip. As pump size (and flow) increase, the inlet velocity may stay constant as the bell diameter increases, but at the same time, the impeller distance above the suction inlet becomes larger, so a fixed submergence value would lead to increased surface velocity, peripheral drawdown, and an increase in air intake.

Basically, submergence must of necessity increase with pump size. For the final determination, some balance must be struck between submergence and pit width to satisfy an average flow velocity of 1 to 1.25 ft/s (0.3 to 0.4 m/s) and maintain reasonable economic balance between excavation costs, concrete costs, and screen costs without neglecting ecological requirements and still fulfilling the primary need for circulating water in adequate quantities.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

Get My Free Ebook

Post a comment