Example 1211 Velocity by Flow and Channel Dimensions

This calculation can be used for a single channel or tank or for multiple channels or tanks with the same dimensions and equal flow. If the flow through each of the unit dimensions is unequal, the velocity for each channel or tank must be computed individually.

No. Channels x Channel Width (ft) x Water Depth (ft)

Problem: The plant is currently using two grit channels. Each channel is 3 ft wide and has a water depth of 1.3 ft. What is the velocity when the influent flow rate is 4.0 MGD?

Solution:

Because 0.79 is within the 0.7 to 1.4 level, the operator of this unit would not make any adjustments.

Note: The channel dimensions must always be in feet; convert inches to feet by dividing by 12 inches per foot.

■ Example 12.12. Required Settling Time

This calculation can be used to determine the time required for a particle to travel from the surface of the liquid to the bottom at a given settling velocity. To compute the settling time, the settling velocity in feet per second must be provided or determined by experiment in a laboratory.

s Settling, Velocity (fps)

Problem: The plant's grit channel is designed to remove sand that has a settling velocity of 0.080 fps. The channel is currently operating at a depth of 2.3 ft. How many seconds will it take for a sand particle to reach the channel bottom?

Solution:

■ Example 12.13. Required Channel Length

Note: This calculation can be used to determine the length of channel required to remove an object with a specified settling velocity.

• .4 ^ it ^ Channel Depth (ft) x Flow Velocity (fps) Required Channel Length = (12.12)

Problem: The plant's grit channel is designed to remove sand that has a settling velocity of 0.080 fps. The channel is currently operating at a depth of 3 ft. The calculated velocity of flow through the channel is 0.85 fps. The channel is 36 ft long. Is the channel long enough to remove the desired sand particle size?

Solution:

Required Channel Length = 3 ft x 0 85 fps = 31.9 ft s 0.080 fps

Yes, the channel is long enough to ensure that all of the sand will be removed.

Caution: Before and during cleaning activities, always ventilate the area thoroughly.

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