DESIGN OF DE-SILTING TANK

Design Discharge = 4 m3/sec

Additional Discharge (1) = 20% for flushing of silt from de-silting tank

Additional Discharge (2) = 20% overload

THEREFORE

Design Discharge for De-silting Tank = 4 x 1.20 x 1.20

= 5.76 m3/sec

CONSIDER

Smallest removable particle size = 0.2 mm

MAXIMUM FLOW THROUGH VELOCITY

v = a x d ½

where, d = Particle size to be removed, in mm

a = Factor varying with particle size(44 in this case)

v ==

44 (0.2) ½

19.67 cm/Sec

= Say 0.19 m / sec

SETTLING VELOCITY

For 33 gm/l and 0.2 mm particle size,

Settling Velocity w = 3.48 cm / sec

Due to turbulence, the velocity reduces by w’

Where w’ = 0.132 v / h

H = Depth of water

= 4.0 m (Assumed)

v = Settling Velocity

Settling Velocity in presence of Turbulence = W – w’

= 0.035 – 0.132 x 0.0348 / 4.0

= 0.0325 m / sec

WIDTH OF DE-SILTING TANK

Q = Av = b x h x v

b = Q / hv

= 5.76 / (4.0 x 0.19)

b = 7.5m

LENGTH OF TANK

l = vt t is retention period

h = w t1 t1 is settling time

In no case t1 > t for 100% settlement

Considering t1 = t, we have L = h x v / w

Length of Tank = 4.0 * 0.19 / 0.0325

= 23.39 m

= 25 m (Say)

THEREFORE, FINAL DIMENSION OF DE-SILTING TANK

Length = 25 m

Width = 7.6 m

Height = 4.0 m