Tailpipe Construction for Air Scour Application

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The video above is part four of a four-part series regarding filter nozzle/strainer construction.  Here, I will discuss tailpipe construction for air scour applications for monolithic or steel plate underdrains.

To watch Parts 1 – 3 of Filter Nozzle Construction click below:

Air Blanket

As scour air is introduced into the plenum of false floor underdrain systems, it collects and builds an air blanket evenly across the filter footprint and downward from the top of the plenum, displacing the water.  In the video, one may see an air blanket in a test column in our Orthos Research Center.

Scour air enters the nozzle through metering holes or slots manufactured in the nozzle tailpipe.  The size and location of these holes determine the depth of the air blanket at a specific air flowrate per nozzle.  Once air scour is terminated, the residual blanket above the top metering hole is evacuated through an air bleed hole located at the top of the nozzle tailpipe.

Orthos has developed a proprietary, calibrated software model that calculates blanket depths at varying air flowrates for a specific nozzle tailpipe construction.

Air-Water Divide Line

Recently, Orthos was contracted to supply replacements of all nozzles at a competitor’s problematic installation.  We input the nozzle tailpipe dimensions into our model, which immediately raised a concern about the location of the air-water divide line relative to the existing nozzle’s air metering holes.  At the two scour air flowrates desired by the utility, the predicted blanket depths were near the large metering hole at the bottom of the tailpipe.  Engineering personnel performed a test in our Orthos Research Center and confirmed the model output.

Any Elevation Difference Will Create Serious Problem

Air Water Divide Line Issues

Graphic A

Elevation Difference Slots

Graphic B

When the air-water divide line is near or within a metering hole or slot, maldistribution of scour air may occur across the footprint of a filter—this is not proper design.  Though contractors try their best to have a false floor to be completely level, in reality it’s not.  Most industry specifications allow ±2mm. Graphic A shows that a floor elevation variance can undesirably create significantly different air flowrates from nozzle-to-nozzle.  Nozzles without air metering holes and instead constructed with air metering slots only at the tailpipe bottom will also likely create maldistribution (see Graphic B)

Effective design locates the air-water divide line between air metering holes or an air metering hole and nozzle tailpipe bottom—slots are not used.  For this example project, Orthos delivered nozzles with a dual air-metering hole tailpipe that provides buffers for variance of delivered air flow rate.

For concurrent air-water backwash applications, the air blanket is compressed within the plenum when backwash pumps are turned on, which adds some design complexity.  Orthos Engineering staff will select the proper combination of metering hole quantity, location, and size to ensure excellent distribution during concurrent backwash as well as the air-only initial phase.  Designs may be easily confirmed in our Orthos Research Center.

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Remember Orthos’ HUGE life cycle cost benefits– At the end of the filter’s life cycle (~25 years), in contrast to a complete plastic block or folded sheet metal underdrain overhaul, nozzles are simply replaced potentially saving $Millions.

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