McKinney J.D., Warnick C.C. Microhydropower Handbook Volume 1
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Length of tf
additional
LenQth of
dry area
,,
Inclined trashrack
WA)
Estimated
depth (dt) = forebay
design depth (df)
less difference
in bottom depth
8 is the angle of incline
of the rack, typically
45” or 60”
INEL21983
Figure 4.4-16.
Trashrick dimension diagram.
;
i
-
I
4.4.2.5.1 Trashrack Design Area--The design area is the clear,
--
open area in the rack through which the water passes.
The design velocity
for a trashrack is 0.5 fps.
Therefore, Equation (4.4-l), A = j,
can be
rewritten for V = 0.5 fps:
-!I-
*DT = 0.5
(4.4-7)
where
*DT =
design area of trashrack in ft2
Q
=
design flow in cfs
0.5 =
velocity through rack in fps.
EXAMPLE: As in the previous example, the design flow is 7.5 cfs.
Find the design area of the trashrack.
4,
*DT = 0.5
ADT
=15ft2 .
: '
i
4.4.2.5.2 Length of Trashrack Wetted Area--The length of the
wetted area is the length of the rack set in the incline (see
Figure 4.4-16).
To calculate the length, the depth of the water on the
rack must be estimated.
The water depth is equal to the depth of water in
the forebay less any difference in elevation between the forebay bottom and
the intake structure bottom.
In most cases, the two bottom elevations will
be the same; then the previously determined forebay depth (df) can be
I
used directly.
LW
A-
sin B
where
Lw =
length of the wetted area of the trashrack in feet
dt =
‘\,
sin e' =
(4.4-R)
depth of water in the intake structure equal to df
(from Subsection 4.4.2.4) less the difference in elevation
between the bottom of the forebay and the intake structure
trigonometric function of the angle of incline of the
trashrack, usual.ly 45 or 60 degrees.
Since the recommended angle of incline is either 45 or 60 degrees, and
since sin 13 for these angles is 0.707 and 0.866, respectively, Equa-
tion (4.4-8) can be rewritten as follows:
LW
dt
= 0, for 45-degree incline
.
.-.- . . . -__
4.4-29
(4.4-8a)
Lw = 4
0.707
LW
= 5.6 ft .
From Equation (4.4-8b) for 60 degree incline:
Lw = 4
0.866
LW
= 4.6 ft .
4.4.2.5.3 Nominal Width--The nominal width is the width of
ign area (clear area).
It does not consider the width of the meta
following equation is used to compute the nominal width:
de;
The
LW
dt
= 0, for 60-degree incline
(4.4-r&)
EXAMPLE: From the previous example, the depth of the forebay is
4 feet. Find the length of wetted trashrack for 45- and 60;degree
angles.
From Equation (4.4-8a) for a 45-degree incline:
the
1 bars
ii
.x "Is-r
N cw
(4./j-c:,
.
where
wN =
nominal width of the wetted area in ft
ADT =
design area in ft.*,
from Equation (4.4-7)
Lw =
length of the wetted area in ft, from Equation (4.4-8)
EXAMPLE: Assume from the above that Lw = 4.6 ft and that
*OT
= 15 ft'. Find WN.
15
'N = 4.6
wN
= 3.75 ft.
The nominal width must now be corrected to account for the width of
the bars.
4.4.2.5.4 Width of the Trashrack--To find the width of the
trashrack, the area of the metal bars in the rack must be added to the
design area. This is difficult to determine since the total width is not
known; therefore, the number of bars is not known. The only thing that is
known is the area of the openings between the bars (design area). It was
previously recommended that all racks be made with l/4-inch bars (except
for very small units, which can be made of l/8-inch bars). For racks made
with l/4-inch bars, the following ratios can be used to compute the
trashrack width:
e
For l/2-inch clearance between bars, the ratio is 3 inches of
'rack width for every 2 inches of opening: r = 1.50.
4.4-31
e
For 3/4-inch clearance between bars, the ratio is 2 inches of
rack width for every l-1/2 inches of opening: r = 1.33.
0
For l-inch clearance between bars, the rtltio is 5 inches of rack
width for every 4 inches of opening: r = 1.25.
For very small racks, when l/8-inch bars are used, on!y a l/2-inch opening
between bars should be allowed.
0
The ratio is 5 inches of rack width for every 4 inches of
opening:
r = 1.25.
Now the width of the trashrack can be computed using the correct ratio:
wT
=rxWN
(4.4-10)
where
w
T
=
width of the trashrack in ft
.r
=
ratio of total width to clear area width
WN =
nominal width in ft, from Equation in (4.4-9).
EXAMPLE: From the previous example, WN = 3.75. Assume that the
rack is constructed of l/4-inch-wide :lars with 3/4.-inch op~;ling~.
Find the width of the trashrack.
wT
=rxWN
w
T
= 1.33 x 3.75
wT
= 5 ft.
4.4.2.5.5 Bundle Size--To size the rack bundles, you must use
the wetted dimensions of the trashrack. (Lw = length of wetted area, and
wT
= width of trashrack, as shown in Figure 4.4-16).
I
Width of Bundle--CJivide the trashrack width (W,) into
convenient widths, each approximately 12 inches wide.
This sets
the width of the bundle and the number of bundles.
(Remember, it
is advisable to have an extra bundle.)
0
Length of Bundle--The length is the sum of the wetted length
plus the extra length required to bring the rack to the top of
the intake structure.
Figure 4.4-11 shows the rack extended to
the walkway so that debris can be easily raked onto the wai kuray.
To determine bundle length, go back to the forebay section
(Subsection 4.4.2.4) and determine how high the top of the berm !s above
the design flow level.
Add 6 inches to 1 foot to that distance so that the
intake structure is above the berm.
This distance is the additional height
required for the bundle to reach the top of the intake structure (see
* .
Figure 4.4-16). The distance will have to be divided by the sin 6 of the
angle, as was the case for Lw [Equation (4.4-8)]:
?I
LA = sin 0
where
L* = additional length in feet
HA = additional height in feet
(4.4-11)
sin 0 =
trigonometric function of the angle of incline of the
trashrack, usually 45 or 60 degrees.
As with Equation (4.4--a), the equation can be rewritten for 45- and
60-degree inclines:
I
/-
HA
b. .
= 0' for 45 degrees
HA
LA = c.866' for 60 degrees.
(4.4-lla)
(4.4-lib)
EXAMPLE:
Determine the length and width of trashrack bundles from the
previous examples, where WT = 5 feet, and Lw = 4.6 feet for a
60-degree angle of incline.
The forebay berm is 2.5 feet above the
design flow level.
Width of Bundle:
5 feet total; therefore make six bundles, each
one foot wide (one extra bundle).
Length of Bundle:
The design intake structure is 6 inches above
the forebay berm; therefore, HA = 2.5 ft + 0.5 ft (6 in.) =
3 ft.
From Equation (4.4-lib), for a 60-degree incline:
LA
3
=0.866
LA.
= 3.5 ft .
Thus,
LT
= LW + L*
(4.442)
where
B
,’
L
/’
LT =
total length of bundle in feet
Lw =
wetted length of bundle in feet, .from
Equation.(4.4-8)
LA =. additional length of bundle in feet, from
Equation (4.4-11)
LT
= 4.6 + 3.5 ft = 8.1 ft.
Add a few extra inches so that the rack will be above the
walkway,
say to a total of 8.5 feet.
Make the rack bundle 1 foot
wide and 8 feet 6 inches long.
4.4.2.5.6 Trashrack Supports--Figure 4.4-17 shows some typical
supports for the rack bundles.
Each bundle can easily slide into its
frame.
The frame should be spaced with inside dimensions at least 1 inch
.
wider than the bundle.
This will help to ensure that the bundle does not
bind in the frame.
The frame is constructed with 2- to 3-inch angle iron as shown in the
figure. All connections are welded except where they are bolted to the
intake structure.
4.4.2.5.7 Width of Trashrack Support Structure--The sum of the
bundle widths plus the additional width required for support frames equals
the width of the trashrack support structure.
EXAMPLE: From previous examples, W, = 5 feet; five bundles require
six frames. Allow 2-l/2 inches additional width per bundle.
4.4-35
----- _-. ,._
-... -- __-.. . .
.-
. ._
Angles welded
to embedded
channel
Walkway-2 x 12 planks
bolted to frame
I
2-
or 3-in. angles
welded back-to-back
bundle
bolted to slab
Wing
wall
INEL 2 2329
Figure 4.*4-17.
Penstock
intake structure, showing typical trashrack
supports, cleanout pipe, and
wing walls.
Therefore, 6 x 2.5 =
15.. Add an additional 15 inches to the width of
the bundles.
Thus, total width
= 6 feet 3 inches.
4.4.2.5.8 L.ength of the Trashrack Support Structure--The lenq::;,
of the trashrack support structure Ls, in Figure 4.4-16, cati be found 2,
multiplying the total length of the trashrdck by the cos.ine of 8.
2
=
length of 'n-take strl;ctu-e requi:red for t.-arlsra-!c i'r' 'i-:-
c
z
i.-,’
total icngth of trs:tiracj\ ,in feet