Twin from ProjektGB0301.pdf
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15.0.0.1
The TwinPipe system
Overview
Introduction
This section describes:
- the principles
- the special design rules
- the properties
of the TwinPipe system.
If the design rules and other properties coincide with those of the single pipe system, referen-
ce will be made to this.
Pipe dimensioning, pressure loss calculation etc. are not included in this section.
Contents
System description
15.1
Installation methods
15.2
Straight pipe runs
15.3
Directional changes
15.4
Branches
15.5
Other components
15.6
Excavation of trench and expansion zones
15.7
Heat loss
15.8
Surveillance
15.9
ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. +45 76 23 30 00
Design · 2003.01
15.1.0.1
TwinPipes
System description
The TwinPipe
system
Like the single pipe system ALSTOM TwinPipe system is a bonded system, in which
service pipe, insulation layer, and casing pipe are cast together in a sandwich construction to
a firm unit.
In this system flow and return pipe are embedded in the same outer casing.
The bonded pipe principle means that the expansions or contractions, occurring in the steel
pipes as a result of temperature variations are transmitted through the insulation to the outer
casing.
Consequently, the actual movements take place between the outer casing and the sur-
rounding sand and due to the friction the movements are considerably reduced, compared to
non-buried systems.
The frictional forces are converted into axial stresses in the steel pipes and the ability to
absorb these is the basis of the function of the bonded TwinPipe system.
Control of expan-
sion movements
The exposure of the two steel pipes to
varying temperature influence results in an
irregular longitudinal expansion of the two
pipes.
In order to ensure the pipe system against
too large movements between the steel
pipes, fixing bars are welded onto the ends
of all straight pipe runs.
The fixing bars are only placed at the ends of
the pipe runs where the expansion move-
ments are largest.
On the rest of the pipe run the two steel
pipes are secured in relation to each other by
means of power transmission through the
insulation foam.
The TwinPipe system has built in fixing bars
in all preinsulated fittings components except
preinsulated venting valves.
However, on preinsulated branches there are
only fixing bars on the branch pipes (See
15.5.0.1).
Straight pipes and curved pipes are not
delivered with fixing bars.
ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. +45 76 23 30 00
Design · 2003.01
15.2.0.1
TwinPipes
Installation methods
Overview of
installation
methods
ALSTOM bonded TwinPipe system is installed after 2 different installation methods:
- Heat prestressing (I)
- High axial stress installation (IV)
The installation methods influence the first movement of the pipe system and the maximum
axial stress in the TwinPipe system.
Detailed description of the principles of the installation methods are found in section 1.3 of this
manual.
Conditions of the
calculation
Due to the influence between the steel pipes the basis for calculating first movements and fric-
tional lengths is another than the one for single pipes.
Basis of calculation for table values:
- Flow temperature (Tf)
130°C
- Return temperature (Tr)
70°C
- Soil temperature
10°C
- Average temperature difference
90°C
- Heat prestressing temperature
55°C
Values, corresponding to a tempeature variation of ± 45°C are used as key figures.
The bonded TwinPipe principle means that the PUR-foam bonds the casing and the two iden-
tical steel pipes together in a secure sandwich construction.
When calculating frictional length and expansion movement an average temperature for flow
and return (T
f
+ T
r
) / 2 is applied.
This simplification is possible, because the two steel pipes have the same dimension.
To calculate the stress in the individual steel pipe use the actual medium temperature in the
steel pipe.
Prestressing (I)
Due to prestressing to mean temperature the
first movement after backfilling will be a con-
traction, corresonding to
l
45
.
The subsequent movements within the tem-
perature difference range will be 2 x
∆
l
45
.
Pipe runs longer than L
45
are locked by fric-
tion and consequently without expansion
movement.
∆
L
45
+112,5 N/mm²/10˚C
The stress in steel pipes is 2.5 x
∆
T (N/mm
2
).
Prestressing to 55°C makes the TwinPipe
stress-free when flow as well as return pipe
have this temperature.
-112,5 N/mm²/100˚C
l
45
If both pipes are cooled to 10°C, then both
the average and the actual temperature are
10°C.
l
45
So the stress is:
2.5 x (55 - 10) = 112.5 N/mm
2
.
Design · 2003.01
ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. +45 76 23 30 00
15.2.0.2
TwinPipes
Installation methods
High axial
stresses (IV)
If high axial stresses without relief are
allowed, then heating within the temperature
difference range will result in a first move-
ment, corresponding to 4 x
l
45
.
As is the case with other installation methods
the subsequent expansion movements will
be 2 x
∆
L
45
L
45
l
45
.
Pipe runs longer than 2 x L
45
are locked by
friction and consequently without expansion
movement.
∆
+112,5 N/mm
2
/10˚C
Calculation of stresses, see the installation
method heat prestressing.
4 x
∆
l
45
- 225 N/mm
2
/100˚C
Full load, flow = 130°C and return = 70°C
means that the average temperature is
(130 + 70) / 2 = 100°C.
∆
l
45
However, here there is no heat prestressing,
so the average stress is:
2.5 x (10-100) = -225 N/mm
2
.
References
Design:
General, system definitions
1.2
installation methods
1.3
Frictional lengths and first movements
15.3.0.1
ALSTOM Power FlowSystems A/S · Fredericia DK · Tel. +45 76 23 30 00
Design · 2003.01
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