Use of Elastomeric Duckbill Valves
3. the effluent flow continues to fall, it will enter the vertical section. The flow of salt water will increase down the riser and eventually will arrest the effluent flow. Salt water will continue to occupy the tunnel and risers in the landward direction until effluent flow is sufficient to prevent further progression of the salt water wedge. When a seabed or buried outfall is partially occupied with salt water, a steady-state density-induced circulation develops whereby salt water enters the seaward risers while effluent, or an effluent and salt water mixture, is discharged from the landward risers (Charlton, et al. , 1987). This can lead to substantially lower dilutions because the effective length of the diffuser is reduced and the discharge is no longer just treated effluent, but usually a mixture of effluent and salt water. Additional concerns with salt water intrusion are that it can introduce sediment into the diffuser, develop marine growth, and increase flocculation of effluent particles developing larger aggregates that have higher settling velocities (Larsen, 1994). Sediment Intrusion Sediment can enter the outfall numerous ways as a result of human and environmental factors. The most obvious is damage to the riser pipes or the outfall header pipe. Riser pipes are usually metal or plastic and are prone to shearing when subjected to impact loads, such as dragging anchors or fishing nets. The risers often shear below the seabed allowing the outfall to fill with bottom material. Ambient sediment transport often carries bottom material in suspension and deposits it along the diffuser pipe. Sediment dunes can cover the ports of the diffuser, which introduces sediment into the outfall. Hydraulic transients caused by starting and tripping of pumps cause pressure waves to oscillate in the outfall. The low pressure wave can produce reverse differential pressure across the ports which will introduce salt water and sediment into the outfall. An inspection video during the commissioning of an outfall in Vina del Mar, Chile captured this phenomenon. The video showed six of the twenty 250 mm duckbill valves in the closed position withstanding up to 10 m of backpressure, while the other duckbills were discharging effluent at various flow rates. Ambient conditions, such as currents and waves, often contribute to intrusion (Larsen, 1994). In shallow water typical of estuaries, waves that pass over the diffuser manifold can cause pulsing of flows through the ports or complete reversal (Grace, 1986). Waves and strong currents can erode or liquefy the seabed material in the vicinity of the outfall causing it to sink making the ports more prone to clogging. The orientation of the diffuser can make it more prone to sediment intrusion. Often, they are oriented so their axis is perpendicular to ambient currents, which improves initial dilution, but could block ambient sediment transport. It is also desirable, when possible, for the diffuser to be located on a horizontal plane, which simplifies the internal hydraulics. Th Sand Island No. 2, outfall in Hawaii had the diffuser section laid across a side-slope and oriented parallel to shore. In 1987, it was determined that the combination of the 1/9 to 1/8 slope and near-bottom water motion were responsible for sand extending over the terminal 250 m of the outfall, and blocking eighteen ports
10. press). Lee J.H.W., Karandikar J., and Horton, P.R. (1997b). Hydraulics of “duckbill” elastomer check valves. Journal of Hydraulic Engineering, ASCE (in press). Munro, D. (1981). Sea water exclusion from tunnelled outfalls discharging sewage. Water Research Center, Stevenage Laboratory, Report 7-M. Roberts P.J.W. (1996). Environmental Hydraulics , Sea Outfalls - Chapter 3, 63-110. Kluwer Academic Publishers. The Netherlands.
4. (Grace, 1997). Another seven ports were blocked with other items such as tin cans. The total cost to evacuate sand and to completely restore the outfall was US$1,168,000. DETRIMENTAL EFFECTS ON OUTFALL PERFORMANCE Two detrimental effects on outfall performance result when salt water and sediment has intruded and occupies a portion of the outfall. First, the hydraulic capacity of the outfall is reduced, and secondly, the dilution efficiency of the diffuser is reduced. Reduced Hydraulic Capacity The hydraulic capacity of the outfall is reduced because the headloss of the system is increased as a result of sediment occupying a portion of the outfall. One of the components of system headloss in an outfall is the friction loss in the outfall pipe, and can be calculated by the Darcy Formula, Equation 2. h = f (L/D) (V 2 /2g) (2) where: h = Headloss Due to Friction, m f = Moody Friction Factor L = Length of Outfall, m D = Diameter of Outfall Pipe, m V = Outfall Pipe Velocity, m/s g = Gravitational Acceleration, m/s 2 For a diffuser that has 1/4 of the pipe area occupied by sediment, for example, the headloss would increase by a factor of two considering the effective outfall diameter is reduced by 13%, and the velocity head is increased by 78%. A diffuser that is half full of deposits would have eight times the friction loss. These scenarios do not consider an increase in the Moody Friction Factor. Usually, effluent outfalls, whether gravity-driven or pumped, only have a limited available driving head to discharge peak flow. The reduced hydraulic capacity of an outfall can require additional pumping operations or overflows to bypass outfalls or retention basins. Pumping is undesirable because of the energy and maintenance costs. Discharge from overflow/bypass outfalls is undesirable since they are usually close to the shore, do not provide as much dilution, and can heavily pollute the beaches and coastline. Reduced Dilution Efficiency Multiport diffuser substantially increase the dilution of effluent compared to single-point outfalls by distributing the wastefield over a wider spacial area. They achieve high initial dilution and can effectively meet water quality standards in the near and far field. To illustrate, consider the “plume equation” for a buoyant plume in stagnant ambient. The centerline dilution (Roberts,1996) can be expressed:
5. S m = H 5/3 (3) Q 2/3 where, S m = Centerline Dilution, H = Water Depth, Q = Port Flow The effectiveness of a multiport diffuser is evident from Equation 3. Increasing the number of ports reduces the individual port flow, which reduces the Q 2/3 term, thereby generating higher initial dilution. Conversely, when some of the ports of the diffuser no longer discharge as a result of salt water or sediment intrusion, the initial dilution is compromised. For example, a diffuser that has half of the ports blocked will result in “Q” increasing by two, resulting in a 37% reduction in initial dilution, Equation 3. The reduced dilution efficiency can result in noncompliance of water quality standards in the mixing zone, and higher bacterial concentrations on nearby beaches and coastlines. THE DUCKBILL VALVE The duckbill valve, such as the Tideflex valve, is manufactured completely of vulcanized rubber reinforced with nylon or polyester fabric, similar to a truck tire. Available sizes range from 25 mm to 2400 mm. The upstream end of the valve is circular and contours to a flattened portion known as the “duckbill.” Figure 1 shows cut-aways of a duckbill valve illustrating the operating principle. The elasticity of the rubber keeps the bill in the closed position in the absence of backpressure. Reverse differential pressure assists in maintaining a positive closure. Positive differential pressure progressively opens the valve as effluent flow increases. By modifying the elastomer and fabric matrix, specific characteristics such as jet velocity and headloss can be controlled. The duckbill valve has no moving parts that can corrode or be fouled which allows it to perform exceptionally well in the adverse conditions of marine waters. The duckbills can also be fabricated with integral wire-reinforced rubber risers and Figure 1 – Operating Principle of Duckbill Valves Figure 2 - Open Area of a Fixed Orifice and Duckbill Valve
8. effluent, thereby arresting the steady-state circulation. The effluent, having a lower density, spread along the soffit of the tunnel above the saline wedge and discharged from all of the risers. The effluent continually mixed with the salt water until the outfall was completely purged. In the early years of outfall operation, flows from the treatment plant may be low and insufficient to purge salt water from a conventional outfall, and it will operate at a reduced efficiency. Duckbill- fitted outfalls, given the purging characteristics, will allow all of the ports to discharge effluent, thereby maintaining the dilution efficiency of the diffuser. Flexible Components Buried outfalls usually have plastic or metal riser pipes that protrude above the seabed. Many outfalls have sediment inside the diffuser pipe solely from the riser pipes being damaged or completely sheared from the outfall. The damage is usually a result of boat anchors, fishing nets, trawls, etc. which the plastic and metal risers cannot easily deflect without damage. Duckbill valves can be fabricated integrally with all-rubber, wire-reinforced risers and/or elbows. The rubber construction provides flexibility and the wire-reinforcement provides durability. These components are able to deflect and return when subjected to impact loads. Having all-rubber components above the seabed will reduce or eliminate damage to the diffuser as a result of boat traffic. CASE STUDY - RICHMOND OUTFALL The Richmond outfall in San Francisco, California was constructed in 1976. The outfall is 1.83 m diameter, 2800 m long outfall with a 340 m long diffuser originally having 140 ports ranging from 63 mm - 102 mm in diameter (Grace, 1997). The outfall is located near a navigational channel and discharges in 9 m depth into the San Francisco Bay. The original port configuration had vertical plastic risers with 90 o elbows that extended 0.58 m above the crown of the outfall pipe. Extensive damage to the outfall pipe and diffuser was found during a 1992 inspection. Several holes were found in the top of the pipe, one manway was ripped out, and nearly all of the risers had been sheared off. This led to the entrance of mud into the outfall occupying three-quarters of the pipe volume in the last 270 m of the outfall. The cost to remove the mud and repair the damaged pipe and risers was US$600,000, and was completed in 1994. To try and prevent further damage from boat traffic, navigational buoys were installed at each end of the diffuser. To prevent bay mud from through the ports, the plastic risers Figure 5 - Duckbill Valves Installed on the Richmond, CA Outfall Diffuser Ports
1. MARINE WASTE WATER DISCHARGES 2000 – 28.11.00/01.12.00, GENOVA, ITALY Use of Elastomeric “Duckbill” Valves for Long-Term Hydraulic and Dilution Efficiency of Marine Diffusers Michael J. Duer, P.E. (1) ABSTRACT Many worldwide marine outfalls suffer from reduced hydraulic capacity and dilution efficiency caused by salt water, sediment, and marine organisms occupying a portion of the outfall pipe. The open ports in the diffuser cannot prevent backflow, and therefore allow intrusion during periods of low flow and hydraulic instabilities. The cost of evacuating the sediment from the outfall and restoring it to service can be significant, often costing thousands up to millions of dollars. The cost is compounded if the intrusion mechanisms recur. Variable orifice duckbill valves prevent intrusion of salt water and sediment resulting in considerable savings in operation and maintenance cost over the life of the outfall. The variable orifice inherently produces significant hydraulic advantages by minimizing headloss at peak flow, and generating enhanced initial dilution at lower flows. Compared to conventional diffusers, the duckbills produce a hydraulically optimized diffuser that is safeguarded against common failure mechanisms. The practical, hydraulic, and dilution enhancements of multiport diffusers with duckbill valves are presented. A case study on the Richmond outfall in San Francisco, California illustrates the cost associated with removal of sediment and the complete restoration of the outfall. KEYWORDS Outfall, Diffuser, Dilution, Duckbill Valve, Intrusion, Headloss, Jet Velocity INTRODUCTION The primary objective of a marine outfall is to provide adequate dilution of effluent to minimize the impact on the receiving waterbody, and to protect nearby beaches and coastlines from high bacterial concentrations. Equally important is the ability to discharge peak flows with a limited amount of driving head (Grace, 1978). The common practice in outfall design is to locate the multiport diffuser as far away from shore, and in water as deep as practical given the limitations of available driving head, construction difficulties, and cost. The most important item in a diffuser regarding hydraulic capacity and dilution efficiency (1) Red Valve Company, Inc., 700 North Bell Ave., Carnegie, PA 15106 USA, (412) 279-0044, (412) 279-5410 fax, [email protected]
6. elbows to replace “hard-piping” above the seabed. These components are durable and are able to deflect and return when subjected to impact loads minimizing or eliminating damage to the outfall. Hydraulics of Duckbill Valves Extensive hydraulic tests were performed on various sizes and constructions of valves by Utah State University Water Research Laboratory (Abromaitis and Raftis, 1995). The largest duckbills tested were 900 mm and 1200 mm in diameter. One inherent characteristic of the duckbill valve is the variable orifice, which enhances diffuser hydraulics compared to conventional fixed diameter ports. Open area, jet velocity, and headloss graphs are shown in Figures 2 through 4 for a 100 mm fixed diameter orifice and a duckbill valve. Figure 2 shows the duckbill progressively opening with increasing flow until it reaches an asymptotic open area beyond peak flow. The open area of the fixed orifice is constant independent of flow. The duckbill has a non-linear jet velocity verses flow profile. It generates higher velocity at low flows as a result of the smaller effective open area, and lower velocity at peak flow as a result of a larger effective open area, Figure 3. The jet velocity of a fixed diameter port is a linear function of flow. The total headloss of the duckbill and fixed orifice is a function of the jet velocity squared, Figure 4. At low flows, the higher jet velocity of the duckbill creates higher headloss, but creates less headloss at peak flow due to the reduction in jet velocity. Typically, the higher headloss at low flow is not detrimental and the enhanced jet velocity improves initial dilution. It is at peak flow where headloss is a major concern, and the reduced headloss with duckbill valves is advantageous. Advantages of Reduced Peak Flow Headloss Duckbill valves prevent intrusion at low flows. This gives the designer the option to concentrate on sizing the duckbills to minimize the headloss at peak flow. Instead of a nominal 100 mm fixed orifice, for example, a nominal 150 mm duckbill could be utilized to reduce the peak flow headloss Figure 3 - Jet Velocity Comparison of a Fixed Orifice and Duckbill Valve Figure 4 - Headloss Comparison of a Fixed Orifice and Duckbill
2. are the ports. The quantity, diameter, spacing, and orientation are iterated during design to produce the most effective configuration. Maintaining the originally designed capacity and dilution efficiency requires that the outfall flow full and not be restricted by salt water and sediment that has intruded into the outfall. Flow rates for wastewater treatment plants can range in magnitude of 10:1 due to wet weather, population growth, tourism, etc, making the outfall susceptible to intrusion during periods of low flow. Through its design life, an outfall will be exposed to severe environmental conditions such as large waves and high currents that make it increasingly difficult to keep the outfall free of salt water and sediment. Conventional multiport diffusers typically incorporate a series of equally spaced fixed orifice ports at the seaward end of the outfall. Fixed orifices consist of holes cast or drilled in seabed outfalls or riser pipes that protrude above the seabed from buried and tunneled outfalls. Selecting the quantity and size of ports is a complicated task in that the port size must be maximized to pass peak flows with the available head, but be minimized to generate sufficient jet velocity at low flows to abate intrusion of salt water and sediment. This contradiction is further complicated considering the headloss of a fixed orifice is a function of the flow rate squared. Common methods to abate intrusion of conventional diffusers during periods of low flow are to isolate the ports with caps or to install reducers and orifice plates to generate higher port velocities. However, the peak flow can no longer be discharged with the same amount of driving head, and the initial dilution of the diffuser is reduced if the effective length of the diffuser is decreased. These methods require expensive diving operations to continually modify or remove the restrictors. FAILURE MECHANISMS Salt Water Intrusion Salt water usually occupies the outfall prior to being commissioned and can intrude into the outfall during operation if the plant flow rate is low. Since salt water is typically two percent heavier than effluent, it will intrude into the diffuser ports when the port densimetric Froude number, Equation 1, falls below unity (Charlton, et.al. , 1987). F = Vj (1) √( g’d) where: Vj = jet exit velocity; g’=(dN/N)g, dN = N a - N, N a = ambient water density, N = effluent density, and d = the diameter of the port. For seabed outfalls, salt water will creep into the outfall through the ports and a saline wedge will develop in the bottom of the pipe. The wedge will continue to move in the landward direction until the effluent flow is sufficient to prevent further progression. For buried and tunneled outfalls, salt water will creep into the horizontal section of the riser and, if
7. similar to that shown in Figure 4. Further reduction in peak-flow headloss is accomplished with larger nominal size duckbills. Reducing the port headloss allows the following design options to be considered to further optimize the outfall design: 1. When particle deposition in the outfall is a concern, a smaller outfall diameter can be utilized which will produce higher pipe velocities thereby abating deposition and improving the scouring of sediment. 2. The peak capacity of the outfall will be increased if the port headloss is reduced. This will reduce the number of diversions to overflow pipelines, and in some cases can allow the overflow outfall to be smaller in scale. 3. A considerable cost savings could be realized by being able to utilize smaller or fewer pumps. The savings in energy costs and maintenance over the life of the pumps can be significant. 4. The length of the outfall can be increased such that the additional pipe friction loss does not exceed the amount of headloss saved by the duckbills. The diffuser will be in deeper water, which will improve initial dilution. Enhanced Initial Dilution Independent hydraulic testing of duckbills was conducted to determine the effect duckbills have on initial dilution (Lee, et.al. , 1997a). The duckbill valves enhance initial dilution compared to fixed orifices by generating higher jet velocity and an elliptical plume geometry. The increase in jet velocity generates greater turbulence increasing the momentum flux, which contributes to higher initial dilutions. The elliptical jet improves dilution by providing a more rapid dispersion of the plume since ambient fluid reaches the plume centerline faster than it would with a circular jet. The elliptical jet also undergoes “axis-switching” whereby the major axis transitions from vertical to horizontal and back until the jet decays to a circular shape. Uniform Flow Distribution The University of Hong Kong testing, along with Utah State test data, yielded two dimensionless relationships for effective open area and headloss of duckbill valves. These correlations were incorporated into a manifold hydraulics program and a simulation was run for an existing fifteen port outfall. The results showed that the duckbill valves provided a more uniform distribution of flows along the diffuser compared to the fixed orifice diffuser (Lee, et.al. , 1997b). Improved Salt Water Purging Completely purging salt water from conventional outfalls can be a difficult task, especially for buried and tunneled outfalls, often requiring sustained flows near the peak capacity of the outfall (Munro, 1981 and Howard, 1994). However, additional tests at the University of Hong Kong determined that duckbill-fitted outfalls purge salt water at very low flows, much smaller than the flow required to purge a conventional outfall with fixed orifice ports (Lee, et al. , 1997a). The duckbill valves prevented salt water from entering the seaward risers while the landward risers were discharging
9. were replaced with duckbill valves that were fabricated as one-piece units with flanged, all-rubber wire-reinforced 90 o elbows, Figure 5. The duckbills were fastened with nylon bolts and nuts. Inspections have been conducted every year since the repair and retrofit in 1994. During that period, only one duckbill valve had been separated from the diffuser, and was subsequently replaced. During each inspection, all 140 of the duckbills were discharging normally and the peak capacity and dilution efficiency has been maintained. CONCLUSION Owners of marine outfalls are often forced into large maintenance expenditures as a result of salt water, sediment, and marine organisms intruding into the outfall pipe. Elastomeric duckbill valves maintain the long-term operation of outfalls by preventing intrusion. The inherent variable orifice characteristic of the duckbills optimizes the hydraulics by enhancing initial dilution, reducing peak- flow headloss, and providing a more uniform flow distribution. The valves are implemented in new diffuser designs and can be retrofitted to existing outfalls that are operating at a reduced efficiency. REFERENCES Abromaitis A. T. and Raftis S.G. (1995). Development and evaluation of a combination check valve/flow sensitive variable orifice nozzle for use on effluent diffuser lines. Water Environment Federation 68 th Annual Conference, Miami Beach, Florida. Charlton J.A., Davies P.A. and Bethune G.M.H. (1987). Seawater intrusion and purging in multi- port sea outfalls. Proc. ICE Part 2, 83 , 263-274 . Grace R.A. (1978). Marine outfall systems: planning, design, and construction . Prentice-Hall, Inc., Englewood Cliffs, New Jersey, U.S.A. Grace R.A. (1986). Sea outfalls - a review of failure, damage, and impairment mechanisms. Proc. Instn of Civ. Engrs , Part 1, 80 , 553-557. Grace R.A. (1997). Returning impaired marine outfall diffusers to full service. J. Environmental Eng. , 123 (3), 297-303. Howard, R.A. (1994). Saline intrusion in deep riser outfall systems. Proc. International Specialized Conference on Marine Disposal Systems , IAWQ 195-206. Larsen T. (1994). Diffuser design for marine outfalls in areas with strong currents, high waves, and Sediment transport. Proc. International Specialized Conference on Marine Disposal Systems , IAWQ 271-277. Lee J.H.W., Karandikar J., and Horton, P.R. (1997a). Hydraulics of duckbill valve jet diffusers. Proc. 13 th Australian Coastal and Ocean Engineering Conference , Sept. 1997, New Zealand (in
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