Home - Effluent Diffuser Systems - Hydraulic Optimization

Optimized Jet Velocity and Dilution, Increased flow capacity, Even Flow Distribution among ports

    Tideflex Diffuser Valves (TFD) not only provide backflow protection, but they are also inherently a variable orifice – they progressively open/close with increase/decrease in differential pressure and flow. The hydraulic characteristics of fixed orifices is very inefficient with a linear jet velocity profile and headloss that is a function of the square (double the flow quadruples the headloss). The graphs below compare the open area, jet velocity and headloss characteristics of TFD’s to fixed-diameter ports.

Compared to fixed orifices, there are significant hydraulic advantages from the variable orifice characteristic of the TFD’s:

    1. Higher jet velocity at low flows improves initial dilution

    As flows decrease, the open area of the TFD decreases and the TFD’s generate higher jet velocity at lower flows compared to fixed-diameter ports. The optimized jet velocity yields higher initial dilution because the momentum flux is maximized. For marine discharges, the improved dilution can sometimes result in the plume trapping below the water surface.

    This is also beneficial for brine discharges where the effluent is denser than the receiving waterbody. In these cases, momentum is solely responsible for initial dilution. The jets are discharged at some angle above horizontally, then fall to the seabed since they are negatively buoyant. Higher jet velocities will prolong the trajectory and create more turbulence so mixing is maximized

    2. Elliptically-shaped jet Improves initial (near-field) dilution

    TFD’s discharge an elliptically-shaped jet compared to a circular shape of fixed diameter ports. The elliptical jet improves initial dilution since; 1) ambient fluid reaches, and dilutes, the centerline of the jet faster, 2) the jet has more surface area and turbulence between the jet and ambient water results in faster mixing and 3) the jet undergoes “axis-switching” whereby a vertical jet will transition to horizontal and back and forth as the jet moves away from the port.

    3. Lower headloss at peak flow increases flow capacity

    As flows increase, the open area of the TFD increases and the TFD’s have less headloss at peak flow compared to fixed-diameter ports. This maximizes the hydraulic capacity of the outfall. Thus higher flows can be discharged by gravity and will reduce the need for effluent pumps. With many outfalls, there is often a recommended or required minimum jet velocity that must be maintained. This often leads to an extremely high headloss at peak flow with fixed diameter ports. With TFD’s the jet velocity at lower flows can be achieved without having excessive headloss at peak flow.

    4. Less variability in jet velocity and headloss thru range of flows

    The TFD’s variable orifice generates higher jet velocity at low flows and less headloss at peak flows as discussed above. This results in less variability of jet velocity and headloss thru the flow range. Having more stable hydraulics reduces upstream level fluctuations and keeps a more uniform backpressure on the pumps of pumped outfalls.

    5. More even flow distribution among ports

    Multiport outfalls can have two ports or hundreds of ports spaced along the outfall diffuser pipe. The nature of manifold hydraulics results in more flow discharging out the ports closest to shore and less out of the ports near the end of the outfall. TFD’s result in a more even flow distribution among ports because the TFD’s, being a flexible variable orifice, are self-regulating and the headloss characteristics is linear (double the flow, double the headloss). Fixed-diameter ports ‘ open area does not change, they are not a variable orifice, are not self-regulating, and the headloss characteristics is a function of the square (doubling the flow quadruples the headloss). Many outfalls have a wide range in flow rates so even flow distribution is problematic with fixed diameter ports.

    6. Significantly improved salt water purging characteristics

    Before marine outfalls are commissioned, they are full of salt water. For the outfall to operate properly, the salt water must be purged. This is done by putting effluent to the outfall and “pushing” the salt water wedge to the end of the outfall and out all of the ports. This is difficult when the treatment plant effluent is less dense than the salt water, typical of municipal and most industrial effluents. With fixed diameter ports, the flow rate required to push the salt water wedge to the end of the outfall can be significant. The reason is a steady-state circulation pattern develops whereby salt water is being discharged out the landward ports and salt water from the sea is being pulled into the outfall thru the seaward ports. With TFD’s on the ports, this circulation pattern cannot develop and the outfall purges salt water at an extremely low effluent flow.

US Patent No. 6,367,505
Canada Patent No. 2,366,605