Exp. (8) : Minor Losses Introduction: Energy losses in pipe flows are the result of friction...
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Exp. (8) : Minor Losses Introduction: Energy losses in pipe flows are the result of friction between the fluid and the pipe walls and internal friction
Exp. (8) : Minor Losses Introduction: Energy losses in pipe
flows are the result of friction between the fluid and the pipe
walls and internal friction between fluid particles. Minor
(secondary) head losses occur at any location in a pipe system
where streamlines are not straight, such as at pipe junctions,
bends, valves, contractions, expansions, and reservoir inlets and
outlets. In this experiment, minor head losses through a pipe
section that has several bends, transitions, and fittings will be
measured.
Slide 3
Exp. (8) : Minor Losses Purpose: To determine the loss factors
for flow through a range of pipe fittings including bends, a
contraction, an enlargement and a gate-valve.
Slide 4
Exp. (8) : Minor Losses Apparatus: Energy Losses in Bends and
Fittings Apparatus. It consists of: - Sudden Enlargement - Sudden
Contraction - Long Bend - Short Bend - Elbow Bend figure 1:minor
losses apparatus - Mitre Bend
Slide 5
Exp. (8) : Minor Losses Figure 2 : Schematic drawing of the
energy-loss apparatus
Slide 6
Exp. (8) : Minor Losses figure 3 : Minor Losses Apparatus with
hydraulic bench
Slide 7
Exp. (8) : Minor Losses Apparatus: (Cont.) Flow rate through
the circuit is controlled by a flow control valve. Pressure
tappings in the circuit are connected to a twelve bank manometer,
which incorporates an air inlet/outlet valve in the top manifold.
An air bleed screw facilitates connection to a hand pump. This
enables the levels in the manometer bank to be adjusted to a
convenient level to suit the system static pressure. A clamp which
closes off the tappings to the mitre bend is introduced when
experiments on the valve fitting are required. A differential
pressure gauge gives a direct reading of losses through the gate
valve.
Slide 8
Exp. (8) : Minor Losses Theory: The energy balance between two
points in a pipe can be described by the Bernoulli equation, given
by where pi is static pressure (in Pa) at point i, g is specific
weight of the fluid (in N/m3), zi is the elevation (in meters) of
point i, Vi is the fluid velocity (in m/s) at point i, g is the
gravitational constant (in m/s2), and hL is head loss (in
meters).
Slide 9
Exp. (8) : Minor Losses The term pi/ is referred to as the
static head; zi is the elevation head; and Vi 2 /2g is the dynamic
(or velocity) head. The summation of the static head and the
elevation head, pi/ + zi, is referred to as the piezometric head.
The piezometric head is what is measured with the piezometer
(manometer) board on the apparatus for this experiment.
Slide 10
Exp. (8) : Minor Losses Head loss, hL, includes the sum of pipe
friction losses, hf, and all minor losses, where hi is the minor
head loss (in meters) for the ith component and n is the number of
components (fittings, bends, etc.). Pipe friction losses are
expressed as the Darcy-Weisbach equation given by where f is a
friction factor, L is the pipe length, and D is the pipe diameter.
Pipe friction losses are assumed to be negligible in this
experiment.
Slide 11
Exp. (8) : Minor Losses The energy loss which occurs in a pipe
fitting (so-called secondary loss) is commonly expressed in terms
of a head loss (h, meters) in the form:
Slide 12
Exp. (8) : Minor Losses Where K = the loss coefficient and v =
mean velocity of flow into the fitting, For the expansion and
contraction, the V used is the velocity of the fluid in the
smaller-diameter pipe. Because of the complexity of flow in many
fittings, K is usually determined by experiment. For the pipe
fitting experiment, the head loss is calculated from two manometer
readings, taken before and after each fitting, and K is then
determined as
Slide 13
Exp. (8) : Minor Losses Due to the change in pipe
cross-sectional area through the enlargement and contraction, the
system experiences an additional change in static pressure. This
change can be calculated as
Slide 14
Exp. (8) : Minor Losses To eliminate the effect of this area
change on the measured head losses, this value should be added to
the head loss readings for the enlargement and the contraction.
Note that (h1 - h2) will be negative for the enlargement and will
be negative for the contraction.
Slide 15
Exp. (8) : Minor Losses For the gate valve experiment, pressure
difference before and after gate is measured directly using a
pressure gauge. This can then be converted to an equivalent head
loss using the equation 1 bar = 10.2 m water
Slide 16
Exp. (8) : Minor Losses Procedure: It is not possible to make
measurements on all fittings simultaneously and, therefore, it is
necessary to run two separate tests. o Part A: 1)Set up the losses
apparatus on the hydraulic bench so that its base is horizontal by
adjusting the feet on the base plate if necessary. (this is
necessary for accurate height measurements from the manometers).
Connect the test rig inlet to the bench flow supply and run the
outlet extension tube to the volumetric tank and secure it in
place.
Slide 17
Exp. (8) : Minor Losses Procedure : (Cont. part A) 2) Fully
open the gate valve and the outlet flow control valve at the right
hand end of the apparatus. 3) Close the bench flow control valve
then start the service pump. 4) Gradually open the bench flow
control valve and allow the pipework to fill with water until all
air has been expelled from the pipework.
Slide 18
Exp. (8) : Minor Losses Procedure : (Cont. part A) 5) In order
to bleed air from pressure tapping points and the manometers close
both the bench valve and the test rig flow control valve and open
the air bleed screw and remove the cap from the adjacent air valve.
Connect a length of small bore tubing from the air valve to the
volumetric tank. Now, open the bench valve and allow flow through
the manometers to purge all air from them; then, tighten the air
bleed screw and partly open both the bench valve and the test rig
flow control valve. Next, open the air bleed screw slightly to
allow air to enter the top of the manometers, re-tighten the screw
when the manometer levels reach a convenient height.
Slide 19
Exp. (8) : Minor Losses Procedure : (Cont. part A) 6 ) Check
that all manometer levels are on scale at the maximum volume flow
rate required (approximately 17 liters/ minute). These levels can
be adjusted further by using the air bleed screw and the hand pump
supplies. The air bleed screw controls the air flow through the air
valve, so when using the hand pump, the bleed screw must be open.
To retain the hand pump pressure in the system, the screw must be
closed after pumping.
Slide 20
Exp. (8) : Minor Losses Procedure : (Cont. part A) 7) If the
levels in the manometer are too high then the hand pump can be used
to pressurise the top manifold. All levels will decrease
simultaneously but retain the appropriate differentials. If the
levels are too low then the hand pump should be disconnected and
the air bleed screw opened briefly to reduce the pressure in the
top manifold. Alternatively the outlet flow control valve can be
closed to raise the static pressure in the system which will raise
all levels simultaneously. If the level in any manometer tube is
allowed to drop too low then air will enter the bottom manifold. If
the level in any manometer tube is too high then water will enter
the top manifold and flow into adjacent tubes.
Slide 21
Exp. (8) : Minor Losses Procedure : (Cont. part A ) 8) Adjust
the flow from the bench control valve and, at a given flow rate,
take height readings from all of the manometers after the levels
have steadied. In order to determine the volume flow rate, you
should carry out a timed volume collection using the volumetric
tank. This is achieved by closing the ball valve and measuring
(with a stopwatch) time taken to accumulate a known volume of fluid
in the tank, which is read from the sight glass. You should collect
fluid for at least one minute to minimize timing errors. ( note:
valve should be kept fully open.)
Slide 22
Exp. (8) : Minor Losses Procedure : (Cont. part A ) 9) Repeat
this procedure to give a total of at least five sets of
measurements over a flow range from approximately 8 - 17 liters per
minute.
Slide 23
Exp. (8) : Minor Losses Procedure: o Part B: 10) Clamp off the
connecting tubes to the mitre bend pressure tappings (to prevent
air being drawn into the system). 11) Start with the gate valve
closed and open fully both the bench valve and the lest rig flow
control valve. 12) open the gate valve by approximately 50% of one
turn (after taking up any backlash).
Slide 24
Exp. (8) : Minor Losses Procedure : (Cont. part B ) 13) For
each of at least 5 flow rates, measure pressure drop across the
valve from the pressure gauge; adjust the flow rate by use of the
test rig flow control valve. Once measurements have started, do not
adjust the gale valve. Determine the volume flow rate by timed
collection. 14) Repeat this procedure for the gate valve opened by
approximately 70% of one turn and then approximately 80% of one
turn.
Slide 25
Exp. (8) : Minor Losses Data & Results: The following
dimensions from the equipment are used in the appropriate
calculations. Internal diameter of pipework d = 0.0183 m Internal
diameter of pipework at enlargement outlet and contraction inlet d
= 0.0240m
Slide 26
Exp. (8) : Minor Losses Data & Results: Tables.doc Minor
Losses (excel)