Pipe friction loss
 

 

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INTRODUCTION

 

The Armfield C6-MKII-10 Fluid Friction Apparatus is designed to allow the detailed study of the fluid friction head losses which occur when an incompressible fluid flows through pipes, bends, valves and pipe flow metering devices.

Friction head losses in straight pipes of different sizes can be investigated over a range of Reynolds' numbers from 103 to nearly 105, thereby covering the laminar, transitional and turbulent flow regimes in smooth pipes.  A further test pipe is artificially roughened and, at the higher Reynolds' numbers, shows a clear departure from typical smooth bore pipe characteristics.

In addition to the smooth and roughened pipes, a wide range of pipeline components are fitted, including pipe fittings and control valves, allowing investigation of the losses caused by this type of connection.  A clear acrylic section of pipeline houses a Venturi meter, an orifice plate assembly and a Pitot tube, so that these can be investigated as flow measurement devices.

The C6-MKII-10 is designed to be operated in conjunction with the Armfield F1-10 Hydraulics Bench.  The unit can be used with a range of instrumentation packages including water and mercury manometers, hand-held digital pressure meters and a computer data logging pack. 

 


 

 

 


 

EQUIPMENT DESCRIPTION 

ARMFIELD C6-MKII-10 Fluid Friction Apparatus 

All numerical references relate to the diagram above 

The test pipes and fittings are mounted on a tubular frame carried on castors.  Water is fed in from the hydraulics bench via the barbed connector (1), flows through the network of pipes and fittings, and is fed back into the volumetric tank via the exit tube (23). The pipes are arranged to provide facilities for testing the following: 

bullet An in-line strainer (2)
bullet An artificially roughened pipe (7)
bullet Smooth bore pipes of 4 different diameters (8), (9), (10) and (11)
bullet A long radius 90° bend (6)
bullet A short radius 90° bend (15)
bullet A 45° "Y" (4)
bullet A 45° elbow (5)
bullet A 90° "T" (13)
bullet A 90° mitre (14)
bullet A 90° elbow (22)
bullet A sudden contraction (3)
bullet A sudden enlargement (16)
bullet A pipe section made of clear acrylic with a Pitot static tube (17)
bullet A Venturi meter made of clear acrylic (18)
bullet An orifice meter made of clear acrylic (19)
bullet A ball valve (12)
bullet A globe valve (20)
bullet A gate valve (21)

Short samples of each size test pipe (24) are provided loose so that you can measure the exact diameter and determine the nature of the internal finish.  The ratio of the diameter of the pipe to the distance of the pressure tappings from the ends of each pipe has been selected to minimise end and entry effects. 

A system of isolating valves (25) is provided whereby the pipe to be tested can be selected without disconnecting or draining the system.  The arrangement also allows tests to be conducted on parallel pipe configurations. 

Each pressure tapping is fitted with a quick connection facility.  Probe attachments with an adequate quantity of translucent polythene tubing are provided, so that any pair of pressure tappings can be rapidly connected to the pressure measurement system.

 

OPERATION 

Controlling the Flow

Water is pumped through the Fluid Friction Apparatus using a centrifugal pump mounted on the inside of the hydraulics bench.  The pump can be switched on and off using the switch indicated in the diagram shown in the equipment section of this web site.  The Control Valve should always be closed before starting the pump. 

Water flows through the connector in the channel on the bench top, through the flexible connecting hose shown in the diagram above and into the C6-MKII-10.  It will then flow through whichever of the test pipes is selected, back through the acrylic pipe section and into the volumetric tank in the hydraulics bench. 

Flow rates through the apparatus may be adjusted by operation of the Control Valve on the hydraulics bench.  Simultaneous operation of the flow control valve with the two outlet valves (gate and globe) shown in the diagram above, will permit adjustment of the static pressure in the apparatus together with the flow rate. Using the three valves in combination, it should be possible to achieve fine adjustment of the flow if required. 

The flow path through the pipe friction network is controlled using the system of isolating valves shown in the diagram above.  By opening and closing these valves as appropriate, it is possible to select flow through any combination of pipes.

 

Volumetric Flow Measurement 

The service module incorporates a moulded volumetric measuring tank which is stepped to accommodate low or high flow rates.  A stilling baffle is incorporated to reduce turbulence. 

A remote sight gauge, consisting of a sight tube and scale, is connected to a tapping in the base of the tank and gives an instantaneous indication of water level.  The scale is divided into two zones corresponding to the volume above and below the step in the tank.  A dump valve in the base of the volumetric tank is operated by a remote actuator.  In operation, the volumetric tank is emptied by lifting the dump valve, allowing the entrained water to return to the sump.  When test conditions have stabilised, the dump valve is lowered, retaining the water in the tank.                          

 Timings are taken as the water level rises in the tank.  Low flow rates are monitored on the lower portion of the scale corresponding to the small volume beneath the step.  Larger flow rates are monitored on the upper scale corresponding to the main tank.

When very small volumetric flow rates are to be measured, the measuring cylinder supplied with the F1-10 should be used rather than the volumetric tank.  When using the measuring cylinder, diversion of the flow to and from the cylinder should be synchronised as closely as possible with the starting and stopping of a watch; do not attempt to use a definite time or a definite volume. 

Pressure Measurement 

The head loss due to pipe friction is measured by taking pressure readings at different tapping points on the pipe network.  In order to measure the pressure loss along a pipe, the pressure measurement device is connected between a pair of tappings, using the tubing and connectors. 

Each pressure point on the apparatus is fitted with a self-sealing connection.  To connect a test probe to a pressure point, simply push the tip of the test probe into the pressure point until it latches.  To disconnect a test probe from a pressure point, press the metal clip of the side of the pressure point to release the test probe.  Both test probe and pressure point will seal to prevent loss of water.

Operation with Manometers

Flexible tubes are connected to the inlets at the bottom of the manometer and the quick release connectors are fitted to the other ends of the tubes.  Connect the manometer tubes to the pipe network at two tappings with a high pressure drop (e.g. either side of a partially closed valve) and start the pump.  Water will be forced through the manometer, expelling the air in the pipes.

When all air bubbles have been expelled, disconnect the manometer from the pipe network.  The quick release fittings will seal keeping the tubes full of water.

The pressurised water manometer incorporates a Schrader valve which is connected to the top manifold.  This permits the levels in the limbs to be adjusted for measurement of small differential pressures at various static pressures.

The hand pump will be required to effect reduction of levels at high static pressures. Alternatively the foot pump may be used.

Operation with Hand Held Pressure Meter

Fit the quick release fittings supplied with the C6-MKII-10 to the ends of the tubes on the hand held pressure meter.

It is important to expel any air which may be trapped in the pipes of the pressure meter before taking readings.  Connect the meter tubes to a convenient pair of tappings and switch on the pump.  Carefully undo one of the nuts holding the tubing to the pressure meter until liquid is expelled from the joint.  Bleed the tube to expel any air.  Tighten the nut and repeat for the other tube.

When taking readings with the hand held pressure meter, it is important that the meter is zeroed before taking a set of results.  Switch on the pump, close the outlet valves and then close the control valve to leave the system at a high static pressure.  When the reading on the meter has stabilised, press the zero button to reset the meter. 

SPECIFICATIONS 

Test Pipe Diameters: 

                        1.         19.1mm x 17.2mm

                        2.         12.7mm x 10.9mm

                        3.         9.5mm x 7.7mm

                        4.         6.4mm x 4.5mm

                        5.         19.1mmx 15.2mm (artificially roughened)

 

Distance between tappings:           1.00m

 

EXPERIMENT A - Fluid Friction in a Smooth Bore Pipe

Objective

To determine the relationship between head loss due to fluid friction and velocity for flow of water through smooth bore pipes and to confirm the head loss friction factor f. 

Method

To obtain a series of readings of head loss at different flow rates ,through one or more of the smooth bore test pipes.

Theory

Professor Osborne Reynolds demonstrated that two types of flow may exist in a pipe. 

1)         Laminar flow at low velocities where h u

2)         Turbulent flow at higher velocities where h un

You may need to refer to your notes and the friction loss section on this web site for more information.

Where h is the head loss due to friction and u is the fluid velocity, sometimes shown as v in some text books.  These two types of flow are separated by a transition phase where no definite relationship between h and u exists. 

See diagrams below. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 Furthermore, for a circular pipe flowing full, the head loss due to friction may be calculated from the formula:

                                         or                          Eqn 1            

 

(Note that some text books may show v2 instead of u2, but they are both the same thing.) 

bullet L is the length of the pipe between tappings,
bullet d is the internal diameter of the pipe,
bullet u , is the mean velocity of water through the pipe in m/s,
bullet g is the acceleration due to gravity in m/s2 and
bullet f is the pipe friction coefficient.

Note that the American equivalent of the British term f is λ where λ = 4f. 

Reynolds' number, Re, can be found using the following equation: 

                                                                  Eqn 2                         

 

where μ is the dynamic viscosity (1.15 x 10-3 Ns/m2 at 15°C) and ρ is the density (999 kg/m3 at 15°C).

Having established the value of Reynolds' number for flow in the pipe, the value of f may be determined using a Moody diagram, a simplified version of which is shown below.  You may have a more detailed version in your notes.  There is also a detailed version in “Chris’s web site”.  You will find the link in my useful links section.

 

 

 

 

 

 

 Equation 1 can the be used to determine the theoretical head loss if you know the value of f for the pipe.  Alternatively you can rearrange the same equation to determine f for the pipe being used in your experiment.  

Equipment Set Up 

Additional equipment required: Stop watch, Internal Digital Calliper. 

Arrange the valves on the C6-MKII-10 to allow flow through only the test pipe under observation.

Procedure

Prime the pipe network with water.  Open and close the appropriate valves to obtain flow of water through the required test pipe.

Take readings at a number of different flow rates, altering the flow using the control valve on the apparatus, (ten readings is sufficient to produce a good head-flow curve). 

Measure flow rates using the volumetric tank.  For small flow rates use the measuring cylinder.  Measure head loss between the tappings using the portable pressure meter or pressurised water manometer as appropriate.

Obtain readings on all four smooth test pipes if you have the time. 

Measure the internal diameter of each test pipe sample using a Digital calliper.  

Processing Results

All readings should be tabulated as follows:

 

Volume
V

[litres]

Time
T
[Secs]

Flow
rate
Q
[m3/s]

Pipe
Diam.
d
[m]

Velocity
u
[m/s]

Reynolds
No
Re

Measured
Head Loss

h

[m H2O]

 

Friction factor
f

 

 

Vx10-3
T

 

4Q
d2

ρud
μ

 

(hC-hD)

 

 

 

 

 

 

 

 

 

 

Plot a graph of h versus u for each size of pipe.  Identify the laminar, transition and turbulent zones on the graphs.

Confirm that the graph is a straight line for the zone of laminar flow h u . 

Plot a graph of log h versus log u for each size of pipe.  Confirm that the graph is a straight line for the zone of turbulent flow h  un.  Determine the slope of the straight line to find n. 

Estimate the value of Reynolds number at the start and finish of the transition phase.  These two values of Re are called the upper and lower critical velocities. 

Confirm that the head loss can be predicted using the pipe friction equation provided the velocity of the fluid, the value for f and the pipe dimensions are known. 

It is assumed that the dynamic viscosity μ is 1.15 X 10-3 Ns/m2 at 15°C and the density ρ is 999 kg/m3 at 15°C.  However, you should measure the actual temperature of the water and adjust these values accordingly.

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Last Edited :  10 March 2015 10:42:11