Gate Valve
Velocity Head
Pressure Head
FLUID MECHANICS
LABORATORY REPORT
To demonstrate the losses and characteristics associated with the flow through bends and fittings.
Department of mechanical engineering
HEAVY INDUSTRIES TAXILA EDUCATION CITY, UNIVERSITY
Theory:
Two types of losses occur in pipes when liquid is flowing through it. These are the major losses which occur due to friction and minor losses which occur due to bends and variation of diameter of the pipe. Losses in pipe is represented by "htotal".
htotal = hmajor + hminor
Major losses due to friction can be evaluated by Darcy-Weishbach equation i.e.
hf =f LDV22g
where f= friction factor
L= length of pipe
D= diameter of pipe
V= velocity of fluid in pipe
g= acceleration due to gravity
Whereas minor energy losses also called as minor head losses are due to bends, fittings and variation of diameter.
And general expression of minor losses is
hminor= K V22g
K is dimensionless quantity.
The apparatus we are studying have sudden enlargement, sudden contraction, bends and valves (fittings). So the factors which cause energy losses are:
The head loss in a piping system may be divided into the following two categories:
Major Losses:
They are due to viscous resistance in straight constant diameter pipes.
Minor Losses:
They appear due to localized effects which mainly arise from changes in the flow cross-sectional area (such as in nozzles, diffusers, valves, flow meters, etc.)
Or changes in the flow direction due to the use of bends and elbows.
In this experiment we are only concerned with minor losses
It is usual to express the energy head loss 'h' in terms of the velocity head in the case of bends and fittings in a pipe network i.e.
ΔH = K v2
2g
Where K = loss coefficient, V = velocity of flow.
90 degrees Elbow:
It is an engineering term which is used for an offset or change in direction of 90 degrees in piping. They always have sharp corners and are made according to a standard plus they are pre-fabricated.
Sudden Enlargement:
When there is a sudden enlargement in cross-section of the pipe at a certain place, the fluid from the smaller pipe is unable to follow the abrupt deviation of the boundary.
Sudden Contraction:
When there is a sudden contraction in the cross-section of the pipe at any place, the fluid from the larger pipe is unable to follow the abrupt change of geometry.
45 degrees Elbow
It has the same definition as the 90 degrees elbow, the only difference is that, this elbow causes a change in direction of the fluid of 45 degrees.
90 degrees Bend:
It causes the change on direction of the fluid by 90 degrees. It does not have sharp corners and is fabricated meaning it cannot be bought, it is always made according to requirement of application.
Pressure Head:
Pressure head is a term used in fluid mechanics to represent the internal energy of a fluid due to the pressure exerted on its container. It is measured in meters. It can also be said that it is the internal molecular motion of a fluid that exerts a force on its container.
Velocity Head:
The velocity of a fluid expressed in terms of the head or static pressure required to produce that velocity. Or the energy losses due to the motion of the fluid.
Purpose :
To determine the loss factors flow through a range of pipe fitting bends, a contraction, an enlargement and a gate- valve.
PROCEDURE:
Water is allowed to flow through different types of fittings.
Heads are measured before and after the flow through them.
Head losses are calculated for different fittings.
The discharge is determined through the hydraulic bench.
Velocity of flow is calculated by dividing the discharge rate by cross sectional area Hence, the loss coefficient of the fitting is determined from the above equation.
Start the pump and wait till there is water flow
Raise the swivel tube.
Adjust the bench regulating valve to provide a small aver flow through both inlet tank and overflow pipe.
Measure flow rate at each condition using stopwatch and volumetric tank.
CALCULATIONS:
BENDS AND FITTINGS (valve 4, 5 turns open)
D= 14mm
Volume
(L)
Time
(sec)
Q
m³sec
Manometer readings
(mm)
h1
h2
h3
h4
h5
h6
h7
h8
h9
4.4
31.22
1.41x10-4
243
182
198
209
152
100
75
58
22
4.5
34.13
1.32x10-4
227
179
189
199
149
103
82
62
35
4.6
40.68
1.13x10-4
201
165
173
180
142
107
90
78
54
4.55
46.65
9.73x10-5
181
153
159
163
137
112
98
90
73
4.42
60.68
7.28x10-5
158
141
143
143
135
115
106
101
90
Velocity in small bore pipe
V = 4Q/Πd2
Velocity head
V2/2g
90° Elbow
(h1-h2)
Sudden enlargement
(h3-h2)
Sudden contraction
(h4-h5)
45° Elbow
(h5-h6)
90° Bend
(h8-h9)
0.92
0.04
0.016
0.016
0.057
0.052
0.036
0.86
0.04
0.048
0.01
0.05
0.046
0.027
0.73
0.03
0.036
8x10-3
0.038
0.035
0.024
0.63
0.02
0.028
6x10-3
0.026
0.025
0.017
1:Pressure(h1-h2)vs velocity:
2:Pressure (h3-h2)vs velocity:
3:Pressure (h4-h5)vs velocity:
4:Pressure (h5-h6)vs velocity:
5:Pressure (h8-h9)vs velocity:
Part (B)
ENERGY LOSSES IN GATE VALVE
SYMBOL OF GATE VALVE:
When the wheel above the valve is rotated the lid goes down blocking the path of water regulating its flow. In this experiment we opened the valve up to two turns. Complete description diagram of gate flow valve is under.
APPARATUS:
Gate Valve:
Valves are used for controlling the flow. The gate valve, also known as a sluice valve, is a valve that opens by lifting a round or rectangular gate/wedge out of the path of the fluid. Gate valves are often used when a straight-line flow of fluid and minimum restriction is desired.
CALCULATIONS:
GATE VALVE(2 turns open)
1 bar = 105 N/m2
1 psi = 6.48 kN/m2
Volume
(L)
Time
(sec)
Q
m³sec
Differential pressure
(psi)
P1
P2
P
3.82
36.66
1.04x10-4
25
0
25
4.22
44.34
9.52x10-5
20
0
20
4.63
55.44
8.35x10-5
15
0
15
4.59
66.87
6.86x10-5
10
0
10
4.51
89.25
5.05x10-5
5
0
5
Velocity in small borpipe
Q=AV
Velocity head
V2/2g
Pressure head
H= p/ρg
0.68
0.02
2.55
0.62
0.02
2.04
0.54
0.015
1.53
0.45
0.01
1.02
0.33
5.5x10-3
0.51
6:Pressure ( P)vs velocity:
Observations
The sudden changes or the introduction of bends or valves disturbs the flow pattern along the pipe. The result is an increase in turbulence and this increases the frictional losses.
Sudden Enlargement
Sudden Contraction
Q) For which angle minor and major losses are minimum?
Ans) 70 is known as optimum angle at which the minor and major losses are minimum.If angle decreases – minor losses decrease, but you also need a longer pipe to make the transition – that means more friction losses.
Q) Discuss various factors at which minor losses depend?
Ans) Friction loss has several causes, including:
Frictional losses depend on the conditions of flow and the physical properties of the system.
Movement of fluid molecules against each other
Movement of fluid molecules against the inside surface of a pipe or the like, particularly if the inside surface is rough, textured, or otherwise not smooth
Bends, kinks, and other sharp turns in hose or piping
In pipe flows the losses due to friction are of two kinds: skin-friction and form-friction. The former is due to the roughness of the inner part of the pipe where the fluid comes in contact with the pipe material, while the latter is due to obstructions present in the line of flow--perhaps a bend, control valve, or anything that changes the course of motion of the flowing fluid.
Q) DEFINE THE CASE FOR WHICH MINOR COEFFICIENT IS EQUAL TO 1?
Ans) Pressure drops and minor loss in components correlates with the dynamic pressure and the minor loss can be expressed asPloss or hloss = ξ v2/ 2 gwhereξ = minor loss coefficient.The minor loss coefficient - ξ - ranges values from 0 and upwards. For ξ = 0 the minor loss is zero and for ξ = 1 the minor loss is equal to the dynamic pressure or head. The minor loss coefficient can also be greater than 1 for some components.
Q) Which losses are more enlargement or contraction 900 and 450 bed?
Ans) An elbow is an engineering term and they are classified as 90 deg or 45 deg, short or long radius. Bends are never sharp corners but elbows are. Pipe bending techniques have constraint as to how much material thinning can be allowed to safely contain the pressure of the fluid to be contained. As elbows are pre fabricated, cast or butt welded, they can be sharp like right angles and return elbows which are 180 degrees. Elbow is a standard fitting but bends are custom fabricated.
450 900
90° Elbow
Velocity Head
Pressure Head
Sudden Contraction
Velocity Head
Pressure Head
45° Elbow
Velocity Head
Pressure Head
Sudden Expansion
Velocity Head
Pressure Head
90° Elbow
Velocity Head
Pressure Head
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