LOCAL HEAD LOSS IN PLASTIC PIPELINE JOINT WELDED BY ...

J. Melichar, J. Háková, J. Veselský, L. Michlík
In case of long pipelines like important water
supply pipeline or penstock into hydroelectric
power plants the way of connecting single plastic
tubes is from the mechanical and economical point
of view important. Besides classical and standard
connecting components for tube connections (e.g.
flanged connection, screwed fitting) in particular
glue connecting (eventually chemical welding
while cold) and thermal welding methods are used.
In general, butt-welding is the most common,
most simple and most reliable way of connecting
pipes for long pipeline conduits from PE and PP
materials. During this process of connecting, which
consists of warming-up and melting the ends of
elements prepared for the connection and following
their pressing down, the inner roughness – the emanation
of the material inside pipeline (see Fig. 1) is
created.
Fig. 1. The thermoplastic pipeline joint welded by the buttwelding
method; PE left, PP right.
Obr. 1. Spoj trubek z termoplastů svařovaných metodou na
tupo; PE vlevo, PP vpravo.
The size of a butt weld joint is for certain material
and wall thickness dependent on the welding
process. The exact welding techniques are recommended
for example by instructions of DVS
(Deutscher Verband für Schweisstechnik). The
inner butt weld in the tube represents the specific
kind of inner resistance and results in additional
losses of the fluid flow. The effect of local losses
caused by butt welds on total energy balance for
long pipelines is important. However, in design
practice it is often underestimated or neglected. The
adverse influence of the butt welds on head losses
for fluid flow in a pipeline and a possibility of creation
of cavitation in the jointing point under high
flow velocity is possible to eliminate by using the
300
apparatus for elimination of inner projections, alternatively
by more expensive connecting technology.
However, in design practise it is paradoxically
preferred the use of least expensive connecting
technology over a possibility to eliminate the local
losses and reduce the operating costs.
Insufficient amount of information for the determination
of a local loss coefficient ζ for the butt
weld with a projection is the main problem in design
practice. A certain simplification, for example
approximate analogy between butt weld projections
and an orifice, was considered. However, these
values of the local loss coefficient were only rough
estimates, anticipated parameters of designed systems
were often largely different from real parameters
that were determined after the construction.
Experimental works in the Czech Technical University
Prague, Faculty of Mechanical Engineering
were carried out in order to obtain quantitative values
that provide more accurate assessment of the
local losses for butt welds in jointing point of plastic
pipe from PP and PE materials. The main aim of
this research was to determine the fundamental data
on the local losses that should be usable in design
practice.
The experimental assessment of the local loss
coefficient of the butt weld in jointing point
in PP pipeline
Fig. 2 shows the pipe test that was constructed in
order to determine the local losses for the butt
welds.
The hydrodynamic pump powered by an asynchronous
electric motor provides flow of water
through pipeline. The flow rate through measuring
part of the pipe is possible to change by alternation
of the pump rotation speed through frequency converter
with full open regulation valve situated in the
end of the test loop or by closing and/or opening of
the valve at constant pump speed. The test section
of the test loop (see Fig. 3) was made of straight
polypropylene tube 90 x 8.2 βPP-H S5/SDR11
(equivalent 90 DN80/PN16), which was commercially
available from the manufacturer Georg
Fischer +GF+. These plastic tubes are supplied at
5 m sections. Allowed manufacturing tolerance of
external diameter and wall thickness is prescribed
by the DIN 8077 standard and corresponds also
with ISO 4065 standard. The average value of inner
tube diameter in joints d = 72.5 mm was ascertained
by metering.