Guidelines for Grit Sampling and Characterization by Water Environment Federation;

Author:Water Environment Federation;
Language: eng
Format: epub
ISBN: 6186988
Publisher: Water Environment Federation
Published: 2017-08-15T00:00:00+00:00

6.7.2 Equipment and Setup

To obtain the required fluid velocity (i.e., 1.2 m/s [4 ft/s]), as discussed above, one of two methods are proposed: (1) an external pump or (2) an obstruction to flow. The pump presents the simplest way to ensure that the velocity requirement is met and allows the highest degree of control. The flow obstruction is potentially easier to install in a remote location, but it would also be prone to uncontrolled events such as insufficient flow velocity during low flows or excessive headloss, potentially leading to flooding, or the obstruction being washed away by the flow during high flows.

Using the pump, discharge velocity can be directly compared to the required fluid velocity. For example, if a 2-in. discharge pipe is used, the pump would need to deliver at least a flow of 151 L/min (40 gpm) to maintain the required discharge velocity. If the discharge velocity is too low, a flow reducer may be used to increase the velocity. For example, if the pump discharge flow is only 95 L/min (25 gpm), the flow would need to be restricted to a 3.8-cm (1.5-in.) nozzle. The nozzle would increase the dynamic head the pump must produce. It would, therefore, be necessary to confirm that the additional head does not reduce the discharge flow to the point where the target velocity of 1.2 m/s (4 ft/s) is no longer met (in other words, below 83 L/min [22 gpm]) using standard hydraulic calculations and the pump curve. The discharge from the mixing pump should be directed perpendicular to the flow direction, and should be set up such that the full width of flow is affected by the discharge and a grit particle at any location would be suspended as it passes the mixer pump discharge. Ideally, the mixing pump discharge should be as close as possible to the sampling pump intake without directly feeding into it. The goal is to locate the sample pump suction in an area of maximum turbulence and flow velocity to ensure that most of the grit at the sampling location is in suspension.

Most conduits are small enough to be mixed by a single large bubble-diffuser compressed air mixer that is using a single large bubble diffuser connected to a dedicated compressor. For large conduits, more diffusers may be used. The diffuser(s) would be temporarily installed in the conduit, upstream of the sample pump intake and close enough to ensure maximum homogeneity, but far enough to avoid air entrainment. In this application, the appropriate bubble release frequency would need to be determined. Most likely, it would be at the maximum of every 3 seconds or faster. Because this is a new application for these mixers, some discussion with the equipment supplier would be required and some trial and error can be expected.

One option would be to use a portable electric compressor that plugs into an external electric power supply. The challenge would be to make sure that the power available on-site matches the power requirement of a compressor that can supply enough air for the application.

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