2. FLUID FRICTION IN PIPES, VALVES AND FITTINGS
A variety of studies will be conducted with this equipment, including the determination
of:
a) the friction factor - Reynolds number relationship for pipe flow,
b) valve coefficients (Cv values) and characteristics (f(x) vs x) for gate valves,
and globe valves,
c) friction losses for various fittings.
Since the piping systems used for a, b, and c contain substantial lengths of pipe, it will be
necessary to correct the measured pressure drops for the lengths of straight piping
between the fittings. These corrections should be calculated assuming the pipe is smooth.
The main apparatus is a flow loop with ten (10) separate lines, each incorporating
different types of fittings. The pressure differences across the piping and from the
venturi meter are measured with variable reluctance differential pressure transducers.
The transducer output is a voltage signal that is collected by Voltage Meters and
Demodulators, which are connected to a SCB-68 National Instruments Shielded I/O
Connector Block for DAQ Devices with 68-pin connectors. The Connector Block is then
used to send voltage signals to a NI PCI-MIO-16E-4 I/O Terminal installed in an IBM
Intel computer. The resulting signals are then picked up by LabVIEW 8.2 and displayed
in “Fluid Friction.vi” in a series of graphs and indicators. A cold junction-compensated
(CJC) thermocouple is used to measure the temperature of the fluid. The CJC occupies
channel ai0 and the thermocouple is attached to channel ai1 and ai9 of the connector
block.
Each transducer is calibrated individually. The Demodulators are set to zero by allowing
the individual transducers to be open to atmospheric pressure (with both bleed screws
open). Once stabilized, the spans of the Demodulators are set at 10V with the bleed
screws closed and air applied at the appropriate pressure. The exact procedure will be
discussed by the TA.
To measure the flow rate for comparison with the Venturi meter, a bucket, stopwatch,
and scale are required. A 3/32” Allen wrench is also necessary for adjusting the pressure
transducer bleed screws. The inside diameter of the tubing is 0.527 inches.
The equipment is fairly complex and you will have to spend some time tracing the flow
lines and thinking about what you are going to do. Don’t turn anything on until the
demonstrator has given his/her approval.
From your experimental measurements on the Venturi meter, calculate the coefficient of
discharge, C, in the relation:
w  CA 2
2 (  p )1
1  4
where W is the mass rate of flow,
A2 is the cross-sectional area of the throat of the Venturi,
1 is the density of the fluid just upstream of the throat,
(-) is the pressure difference across the Venturi,
 is the ratio of throat diameter to inside pipe diameter (14.3 mm and 25.3 mm
respectively)
The valve coefficient and valve stem function are defined as:
Q  Cv f ( x)
where
Q
Cv
f(x)
S
Pv
x
Pv
S
= flowrate (US gallon/minute)
= valve coefficient (usgpm/psi0.5)
= dimensionless stem function (0, closed; 1, fully open)
= fluid density/water density
= pressure drop over valve (psi)
= stem position (fraction open)
A possible f(x) vs x relationship is of the form f(x) = xm.
In examining the valve performances the data should be used to find the best-fit values
of Cv and m.
References
2.1 Anon, “Flow of Fluid Through Valves, Fittings and Pipe”, Technical Paper No. M409, Crane Ltd., 1950.
2.2 Perry’s Chemical Engineers’ Handbook, 6th Edition, 1984.