Szilas A.P. Production and transport of oil and gas, Gathering and Transportation
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polyethylene, marked as PVC
+
CPE),
and from hard polyethylene. The
I’VC‘
(polyvinylchloride) is produced by polymerizing thc vinylchlorid, a derivative of
acetylene. Ethylene is thc monomer of the polyethylene
PZ;.
Scvcrul
versions of the
above described basic plastics are manufaclured and their physical
parameters,
due
to the differences of the technologies and additives applied, may bc different.
7irhlc
6.1
-4
(after Vida
1981)
shows thc main physical paranietcra of thc above plastics
TY
Pe
H
PVC
PVC
+
CPE
HPE
I
I1
Table
6.1
-4.
Characteristics
of
Hungarian plastic
pipe
niiilciiitl\
after Vida
(1
98
I)
Density, Tensile
Hardnexh.
t1H
nioduluh
of
kg/rn’
strength,
MPa
M
Pa
ela\ticity.
M
I’a
Standard
DIN
53457
___
MS7
5546
MSz
1421
MS7
1425
I420
59.3
102.3
8
10.4
I340
44.7
103.3
636.6
900
21.2
44.7 187.8
x90
19.3
36.
I
162.5
used
in
Hungary. They are widely applied, first ofall,
in
the gas industry where they
are used
for
low-pressure gas-distribution. To illustrate the popularity of these pipes
it
is enough to mention that only
in
Europe pipelines made of HPVC totalling a
length
of
28
500
km were laid
in
1978. Application
of
the gas pipelines made
of
HPE
has considerably increased during the 1970s.
In
7 countries of Western Europe 2.7
million metric tons were used
in
1972, against
25.8
in
1979.
A
good overview
concerning the parameters and existing standards of these pipelines is given by
Muller (1977).
From the thermosetting plastics first of all the epoxy resins and the polyesthers
are suitable for pipemaking. Polyesthers are polycondensation products
of
polyvalent alcohols (glycols, glycerins, etc.) with polyvalent acids (phthalic, maloic
acid, etc.). Epoxy resins are produced by condensing diene with epichlor-hydrine in
the presence
of
caustic soda. Depending on the technology and the applied additives
several versions of these plastics are known. The
API
Specification
5LR
-
1972
describes pipes conducting oil, gas and industrial water, that are made of reinforced
thermosetting resin
(RTR)
by centrifugal casting (CC)
or
filament wound (FW).
Frequently fiber-glass is used as reinforcement, and thus the strength of the pipe
material increases while its thermal conductivity and water absorption decreases.
The changes of the strength of the thermoplastics and resins due to temperature
changes significantly differ. After Greathouse and McGlasson (1958)
Fig.
6.1
-3
shows the change
of
the tensile strength with the temperature for two types
of
pipe
material made
of
PVC and
for
two types
of
materials made
of
glass reinforced
plastics.
It
can be clearly seen that the tensile strength
of
these latter materials is not
smaller even at a temperature
of
120°C than at room temperature. There is a
h
I
LINE
PIPES
21
peculiar change however
in
the allowable pressure
in
plastic pipeline
with
time.
-
Fig.
6.1
-
4,
also after Greathouse and McGlasson, shows the increase
in
diameter of
a
PVC
pipe
in
function of the loading period at different values of tangential,
or
hoop stress:
(T
-
Pi
(r?
+
rI
)
I-
r,'-r,?
'
We can see, that contrary to steel pipes, the process of diameter increase due to the
internal overpressure is slow, and there is a critical hoop stress at which,
if
it
is
small
enough, the increase in diameter ceases
within
a given period
of
time. If, however,
0,
exceeds the critical value the pipe will
go
on swelling until
it
ruptures. Another
important factor influencing the allowable internal overpressure of the pipes is that
Fig.
6.1
-3.
Tensile strength
of
plastics
v.
temperature (Greathouse and McGlasson
1958)
I
I I
I
I
I
0
100
200
t,
300
Fig.
6.1
-4.
Increase
of
diameter
v.
time
of
PVC-I
pipe (Greathouse and McGlasson
1958)
22
6
GATHERING AND SEPARATION
OF
OIL
AND GAS
R-40
23.0
65.6
the pressure is constant, or changing, pulsating. Because of these specific features in
several countries standards and specifications were compiled for the qualification of
the plastic pipes.
On
the basis of the API Spec.
5LR
Table
6.1-5
shows the
minimum strength characteristics of plastic pipe materials of different grades at
temperatures of
23.0
"C
and 65.6
"C.
In
each case here strength means hoop stress.
The numbers after the
R
marking the different grades express the hoop stress
in
psi
R-45
R-50
R-60
23.0
65-6
23.0
65.6
23.0
65.6
Tdble
6.
I
~
5.
Minimum physical property requirements
for
reinforced thermosetting
resin
line
pipe
(RTRP)
after
API
Spec.
5LR
Property and test method
Cyclic long term pressure*
Cyclic short term pressure*
Long-term static* M Pa
Short-time rupture strength*
Ultimate axial tensile strength
Hydrostatic collapse strength
strength MPd
strength MPa
(burst) MPa
M
Pa
M Pa
Gradc
27-6
103
75.x
207
51.7
40.0
27.6
I03
754
207
4x.3
35.2
3
1.0
I17
89-6
24
1
62.
I
40.0
___
31-0
I17
x9.6
24
1
57.9
35-2
__
34.5
131
103
276
62.1
404
~
34.5
131
I03
276
57.9
35.2
41.4
13x
I
3x
345
77-2
35.2
Minimum hoop stress
units in case of long-lasting cyclic loading. This value is given
in
the first line of the
Table in
M
Pa units. The standard pipe sizes according to this same specification are
given
in
Tuble
6.1-6.
The favourable properties of the steel and plastic pipes are partly united in the
plastic-lined steel pipe. Due to the plastic lining of the inside wall the pipeline
becomes hydraulically smoother and thus, the throughput capacity increases. Wax
does not adhere to the pipe wall, or,
if
it
does, only to a much smaller extent. Internal
corrosion ceases.
No rust scale gets into the fluid stream and that is why the wear
and tear of the measuring devices and pipe fittings decreases. The allowable working
pressure of the pipe is determined by the parameters of the steel pipe. Polyethylene
and epoxy resin are the most frequently used lining materials. Lining can be
produced in the pipe mill, but older, already existing pipelines, especially gas pipes,
can
be
also furnished with inside plastic lining. This latter process especially requires
a special care and expertise. Against the external corrosion the outer surface of the
pipes can
be
also coated with plastics.
6.1.
LINE
PIPES
23
Table
6.1
-6.
Dimensions
and
masses
of
RTRP pipes after API Spec.
SLR
Nominal
size
do.
in.
2
(2.375)
2
I/2
(2.875)
3
(3.500)
4
(43(w))
6
(6.625)
8
(862.5)
10
(
10.750)
12
(1
2.750)
OD
d,.mm
60.3
73.0
88.9
I
14.3
168.3
219.1
273.1
323.9
ID
d,.
mm
56.8
54.2
50.7
50.2
47.6
45-
I
69.5
63.4
62.9
60.3
57.8
85.3
84.8
82.8
78.7
76.2
73.7
I
10.2
109.7
108.2
104.1
101.6
99.1
162-7
160.2
155.6
153.0
148.0
2
12.2
209.9
206.4
203.8
198.8
264.4
26 1.9
260.4
257.8
252.7
313.7
312.7
31
1.2
308.6
303.5
1
.R
3.0
4.8
5.
I
6.4
7.6
1.8
4.8
5.1
6.4
7.6
1.8
2.0
3.0
5.1
6.4
7.6
2.0
2-3
3.0
5.
I
6.4
7.6
2.8
4.1
6.4
7.6
10.2
3.4
4.6
6.4
7.6
10.2
4.3
5.6
6.4
7.6
10.2
5.
1
5.6
6.4
76
10.2
I
.5
2.5
4.2
3.6
4.3
5.6
I
.5
4.2
3.6
4.3
5.6
I
.5
1
.8
2.5
3.6
4.3
5-6
1.8
2.0
2.5
3.6
4.3
5.6
2.5
3.6
4.3
5.6
7.6
3.2
4.1
4.3
5.6
7.6
3.8
5.1
4.3
5.6
8.
I
4.4
5.
I
4.3
5.6
8.1
Spec. max.
G,,,
.
kgim
0.60
0.97
1.77
1.24
1.86
2.09
0.76
1.97
1.79
2.17
2.53
0.89
0.97
I
.49
2.09
2.6
I
3.13
1.19
I
.34
I
.94
2.64
3.57
4.32
2.53
3.87
5.2
1
618
6.04
2.98
5.66
6.70
8.04
10.43
6.26
8.68
8.34
9.98
13.03
8.94
10.43
9.98
11.92
I549
Process
of
manu-
facture
FW
FW
FW
cc
cc
cc
FW
FW
cc
cc
cc
FW
FW
FW
CC
cc
cc
FW
FW
FW
cc
cc
cc
FW
FW
FW
cc
cc
FW
FW
cc
cc
CC
FW
FW
cc
CC
cc
FW
FW
cc
cc
cc
24
6.
GATHERING AND SEPARATION
OF
OIL
AND GAS
6.1.3.
Wall
thickness
of
pipes
Though the formula applied for the calculation of the wall thickness of the
pipelines used for the transportation of oil and gas is slightly different in the different
countries (Csete 1980), the basis of these formulas
is
the same:
6.1
-
1
where
Ap
is the pressure difference prevailing across the pipe wall.
In
case of steel pipelines
e
means the figure of merit of weld seams. In case
of
seamless pipelines
e
=
1,
while for axially welded pipelines
e
=
0.7 -0.9.
For
spiral-welded pipes, in Siebel's opinion (Stradtmann 1961)
e
should be
divided by the ratio
ado,.
cr,
is the tangential stress perpendicular to the seam while
7,
is the tangential stress in a cross section of the pipe perpendicular to the axis.
In
:ase of the commonly used spiral-welded pipes the pitch of the weld seam
a
is greater
han
40"
and then
crJcr,=0.8
-0.6. The greater the
a
is, the smaller is the rate
crJcr,.
If
r
240",
then
__
=
1.
It
means that the locus of the least strength is not in the
veld seam but
in
the steel sheet itself and
Eq.
6.1
-
1
can be used by substituting e
=
1.
e
ads,
The allowable stress is
OF
no,=
-
k
6.1
-2
vhere
cF
is the yield strength and
k
is
the
sufetyfuctor,
a value greater than
I.
\ccording to the Hungarian specifications the following safety factors should be
ept
in
mind in case of oil pipes and pipelines transporting petroleum products
(I),
nd
in
case of pipelines transporting gas, and liquified gas
(2):
(1)
(2)
a) Open country, ploughland, wood
1.3
1.4
b)
At
a distance of 100-200m from inhabited areas, railroads,
arterial roads, etc.
1.5
1.7
c) Within l00m from same
1.7
2.0
d)
In
industrial and densely inhabited areas, under railroads,
arterial roads and water courses
2.0
2.5
In several countries instead of the safety factor its reciprocal value is used and
it
is
illed
design
factor.
The value of this factor according to the standards
in
12
iuntries having significant gas industry and summarized by Csete
(1980)
varies
etween
0.3
-
0.8.
Thus, the matching reciprocal value, the safety factor is
3.3
-
1.25.
In
Eq. 6.1
-
1
s,
is a supplement due to the negative tolerance ofthe wall-thickness
hich,
if
considered, is
1
1
to 22 percent of the
so
value calculated by the first term of
6
I
I
INI,
1'IPl.S
25
the equation.
s2
is a supplement added because
of
the corrosion and degradation
and its value
is
max.
1
mm. From country
to
country
it
differs
if
s1
and
s2
is
considered in the course
of
the calculation
of
s.
None
of
these values is prescribed in
France, FRG,
or
in the
USA.
That is why a frequently used simpler version
of
Eq.
6.1
-
1
is
s
=
d"&
.
2eo;,
oF
yield strength is the stress where the permanent
the course
of
the deformation the loading does
6.1
-3
deformation sets in
so
that
in
not increase or vary. This
phenomenon cannot be recognized in case ofeach metals and alloys. In this case the
stress belonging
to
the0.2
permanent deformation is considered
to
be the yield stress
and instead
of
oF
it
is marked with
The allowable internal overpressure of the plastic pipe can bc calculated by
applying the
IS0
formula:
where
r~,
hoop stress value can be taken from the relevant line
of
Tablc 6.1
--
5,
d,
is
the average internal diameter
(ID)
that should be calculated from
(do-
2.5).
Example
6.1-1.
Find the required wall thickness of spiral-welded pipe of
d
=
10
3/4 in. nominal size, made of
X52
grade steel,
to
be used in a gas pipeline laid
across ploughland and exposed
to
an operating pressure
of
60 bars. From Table 6.
I
-
2
the
OD
of
the given pipe is 0.273 m. According
to
Table
6.1
-
3
ofi
=
358
MPa.
By
the above considerations,
k
=
1.4, and for spiral welded pipes
e
=
1.
Hence by
Eq.
6.1 -3 the wall-thickness
is
0.273
x
60
x
lo5
358
x
10"
2XlX-----
1.4
=
3.20
x
10
=
3.20 mm
s=
______~~~
0.273
x
60
x
lo5
358
x
10"
=
3.20
x
10
=
3.20 mm
s=
______~~~
2 x
1
1.4
Table 6.1
-
2 shows the least standard wall thickness at
10
3/4 in, nominal size
to
be
3.96 mm. This thickness should be chosen.
26
6
GATHFRING AND SEPARA710N
OF
011.
AND GAS
6.2. Valves, pressure regulators
6.2.1.
Valves
Valves used
in
oil and gas pipelines may be of gate, plug
or
ball,
or
globe type.
All
of
these have various subtypes.
(a)
Gate valves
The gate valve is a type of valve whose element of obstruction, the valve gate,
moves in a direction perpendicular to both inflow and outflow when the valve is
being opened
or
closed. Its numerous types can be classified according to several
viewpoints.
In
the following we shall consider them according
to
their mode of
closure. Typical examples are shown as
Figs
6.2
-
I
-
6.2-
4.
Of
these, 6.2
-
2 and
6.2-3
are conformable to
API
Std 6D-
1971.
The main parts of a gate valve are
termed as follows: handwheel
I,
stem 2, gland
3,
stem packing
4,
bonnet
5,
bonnet
bolts 6, body
7,
wedge or disc
8,
and seat
9.
The element of obstruction
in
the valve
shown as Fig. 6.2-
1
is wedge
8.
This valve is of a simple design and consequently
1
I
Fig.
6.2-
I.
Wedge-type rising-stem
gate
valve
Fig.
6.2- 2.
Disk-type non-rising-stem gate valve.
after
API
Std
611-1971
6.2.
VALVI:S. PRESSURE REGULATORS
27
rather low-priced, but its closure is not leakproof. First
of
all,
it
is difficult to grind
the wedge to an accurate
fit,
and secondly, the obstructing element will glide on
metal on closure; deposits
of
solid particles will scratch the surfaces and thus
produce leaks. Fairly widespread earlier, this type of valve has lost much of its
popularity nowadays.
The valve in
Fig.
6.2-2
is of the double-disk non-rising-stem type. Its tightness is
much better than that of the foregoing type, because the two disks do not glide on
the seats immediately before closure or after opening; rather, they move nearly at
right angles to the same. The two elements of obstruction, machined separately, are
more easily ground to a tight
fit.
The valve
in
Fig.
6.2-3
is of the rising-stem,
through conduit type with a metallic seal.
It
has stem indicator
/
as a typical
structural feature. One of the advantages of this design is that the inner
wall
of the
valve is exposed to no erosion even
if
the fluid contains solids. The elements
of
obstruction cannot wedge
or
stick. Overpressure
in
the valve is automatically
released by safety valve
2.
Its drawback is that, owing to the metal-to-metal seal,
Fig.
6.2
-
3.
Full-opening rising-stem
gate valve, after
API
Std
6D-1971
Fig.
6.2
-
4.
Grove’s
G-2
full-opening
floating-seat
gate
valve
28
6.
GATHERING AND SEPARATION
OF OIL
AND GAS
tightness may not always be sufficient
for
the purposes envisaged
in
the
oil
and gas
industry. Furthermore, the
tit
of
the disks may be damaged by scratches due to solid
particles. The biggest valves
of
this type now in use are made
for
lo00
mm nominal
size and
66
bars operating pressure rating (Laabs
1969).
Figure
6.2-
4
shows a through conduit-type, non-rising-stem floating-seat valve.
The so-called floating seats
3,
capable of axial movement, and provided with O-ring
seals
2,
are pressed against the valve gate by spring
1.
The pressure differential
developing on closure generates a force which improves the seal. (Concerning this
mechanism cf. the passage on ball valves in Section 6.2.I+b).) Small solid par-
ticles will not cause appreciable damage
to
the sealing surfaces, but coarse metal
fragments may. For high-pressure large-diameter applications, variants of this type
ofgate valve with the valve body welded out
of
steel plate are gaining in popularity.
The valves described above have either rising or non-rising stems. Rising stems
require more space vertically. In a rising stem, the thread is on the upper part. likely
to
be exposed to the outer environment. In
a
non-rising stem, the thread is on the
lower part, in contact with the well fluid.
It
is a matter ofcorrectly assessing the local
circumstances
to
predict whether this or that solution will more likely prevent
damage to the threads. In rising-stem valves the handwheel may or may not rise
together with the stem. In modern equipment, the second solution is current (cf. c.g.
Fig.
6.2-3).
Turning the stem may
be
effected directly, by means of a handwheel
mounted on the stem (as in all types figured above). but also indirectly by
transferring torque
to
the stem by a spur
or
bevel gear
(Figs
6.2-5
and
6.2-6).
Fig.
6.2
-
5.
Spur-gear gate-valve stem drive
v
Fig.
6.2
-
6.
Bevel-gear gate-valve stem drive
h
2
VALVES.
PRESSURE
REGULATORS
29
The valvc may, of course, be power-operated,
in
which case the drive may be
pneumatic. hydraulic
or
electric.
Figure
6.2-
7
shows a rising-stem valve with
elcctric motor and a bevel gear drive. Gate valves used in oil and gas pipelines
usually
have flange couplings, but
in
some cases plain-end valve bodies are used
with
flanges attached by welding, or threaded-end ones with couplings.
At
low
operating pressures. the outer face of the flange, to lie up against the opposite flange,
Fig.
6.2
~
7.
Bevel-gear gale-valve stern drive. operated
by
electric
motor
is either smooth or roughened. Ring gaskets used are non-metallic, e.g. klingerite.
At
higher pressures,
it
is
usual to machine a coaxial groove into the flange, and to insert
into
it
a metallic gasket, e.g. a soft-iron ring.
-
Operating-pressure ratings of gate
valves range from
55
to
138
bars according to API Std
6D-1971.
The standard
enumerates those ASTM grades of steel of which these valves may be made. -Gate
valves are to be kept either
fully
open or fully closed during normal operation. When
partly open, solids
in
the fluid
will
damage the seal surfaces in contact with the fluid
stream.
(b)
Plug
and
ball
valves
In these valves, the rotation by at most
90"
of an element of obstruction permits
the full opening or full closing
of
a conduit. The obstructing element is either in the
shape of a truncated cone (plug valve)
or
of a sphere (ball valve).
Figure
6.2
-8
shows