Szilas A.P. Production and transport of oil and gas, Gathering and Transportation
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200
7.
PIPELINE TRANSPORTATION
OF
OIL
The brake power, at each
q,
H,
n
points
of
the area, is to be calculated from
7.4
-
25
A
frequently accurring problem is that a rate, differring from that
of
the best
efficiency point, generally a smaller rate, must
be
transported through the pipeline
by the pump.
Fig.
7.4
-
12
shows the head curve
of
the pump
(H),
and the change of
H,=
h
+
h,
head
loss
v.
rate at the pipeline
flow.
If,
temporarily,
a
smaller rate than
q
Fig.
7.4-
11.
Iso-eficiency curves of a centrifugal pump
Fig.
7.4-
12.
q’
9a
9
Determination
of
the
characteristics
of
the decreased
delivery
the above value is to
be
transported, then two solutions are possible. One of them is,
that the gate valve, adjusted after the pump on the pipeline, is throttled
to
the extent,
that the
H‘
head, corresponding to the
q’
rate should equal the sum of the line head
loss,
H:,
and the
h’
throttle
loss.
It is clear that
a
significant proportion of the
pressure head, developed by the pump,
is
“uselessly” dissipated in this way. The
1.4.
ISOTHERMAL OIL TRANSPORT
20
1
other alternative is that the required rate
is
achieved by a decreased pumping speed.
lfthe speed is decreased from
n,
to
n,,
the operating point
4',
H"
remains close to the
affinity parabola crossing
a
point. The iso-efficiency curves, represented
in
Fig.
7.4
-
I1
show that this change is accompanied only by an insignificant decrease
in
the
pump efficiency, and that is why
it
is generally much more economic than the
control by throttling (Fishel and Howe
1979).
After Liu
(1981),
about the energy
A
0
50
100
qlq,,,,'lo
Fig.
7.4-
13.
Energy demand
of
rate control
by
throttling(1) and
by
rpm-alteration
(2).
after
Liu
(1981)
requirements of the two control methods information is given by
Fig.
7.4-
I3
that
indicates the specific energy demand
P/P,,,,
for transportation by throttling (curve
I),
and for transportation by decreased speed (curve
2)
in the function of
q/qmax.
Curve
3
shows the saving, if, instead of the Ihrottling, transportation is
accomplished, by change of speed.
It
may occur that a greater rate is to be transported than the capacity of the
existing pump. In such cases two, and less frequently more pumps are connected.
The connection may be in parallel, or in series. In
Fig.
7.4-
14
after Feizlmayr
(1975), schemes of pump stations, applying parallel, and serial connections
respectively are presented. The pumps of the station
in
part (a) are in parallel. The
station is equipped with safety devices, such as valve and tank, and also with
automatic scraper trap. In (b) station the pumps are in series, and here the change of
the scraper
is
also possible. In both cases, the prime mover of the pumps is an
asynchronous motor. The regulation
of
the rate is carried out by throttling.
In most cases the pumps in parallel are alike i.e. they can be characterized
individually by the same head curve, transporting oil from a common suction line
into a common discharge line. The resultant head curve may be plotted
so
that the
double
4x
values of the original head curve are determined at different
H,
heads. The
points, defined in this way, are connected by a solid line (see
Fig.
7.4-
15).
The new
202
7.
PIPELINE TRANSPORTATION
OF
OIL
operating point is, thus, determined by the intersection of the system curve,
H,.
and
the resultant head curve
(H,,).
Accordingly, the corresponding rate will be always
smaller than the double value
of
the rate transported by one pump, i.e.
q,l.
<
2q,,.
In
case of crude oil pipelines the parallel connection is considered to be more
advantageous, when the static portion of the flow losses is relatively high, i.e. the oil
must be transported to “hill”.
In
the opposite case (e.g.
A
I
4
f
#..
0
Y
-D
the dynamic
loss
is greater),
Sofety volve
I
Sofely volve
II
Volve
Control
volve
Electric motor
Pump
sofety
tOnk
Scroper lrop
Surplus
011
tOnk
Surplus
oil
Line
Or1
flce
Pressure tronsducer
Ternperoture fronsducer
Level tronsducer
scroper
signalling
ChOCk VOlVe
Fig.
7.4-
14.
Parallel and
series
connected
oil
transport stations, after Feizlmayr
(1975)
the serial joining is the more advantageous.
Fig.
7.4-
f6
shows the original and
resultant head curve
of
the series connected pumps.
At
any
qx
point the pressure
head
HIIx
of
the resultant curve
is
of double height, as compared to the original
head curve
Hlx.
The peculiar, series connected first pump of the pump stations is special
booster
pump.
This is generally placed at the deepest point of the area, occupied by tanks,
and is a centrifugal pump of big capacity, and small head pressure. Its task is to
pump the
oil
into the suction line of the transporting pump with a pressure,
exceeding that
of
the vapour, and
so
it
may prevent the cavitation damage in the
main pump. The booster pump is also able to transport the
oil
from one tank into
the other one, either for treating, or for safety reasons.
The prime movers of the pumps are generally the asynchronous electric motors
with fixed, or adjustable speed, operated by alternating current. For the speed
7.4.
ISOTHERMAL OIL TRANSPORT
203
adjustment several solutions are known. Liu (1981) describes a new, modern speed
reductor, that is operated on hydraulic principles.
In cases, when the electric energy supply may not be ensured economically. but
gas energy is available, first of all gas turbines are applied. Nowadays, Diesel engines
are used only
in
case of relatively small capacities. For the selection of the prime
movers, Wright (1981) offers practical criteria.
In
case of applying parallel
connections the motors should be selected on the basis of the power requirement of
the independent operation. Either corresponding to the disposition. or. due to the
failure
of
one of the pump units
it
may occur that only one of the units operates.
H
Hx
It
I
I
11
-
4
qI0
q
110
4
4,
411,
Fig. 7.4-
15.
Resultant head curve construction
for
parallel connected centrifugal pumps
Fig. 7.4- 16. Resultant head curve construc-
tion
for
series connected centrifugal pumps
All
the above discussed problems are valid for both oil and water, supposing that
the viscosity of the transported oil is constant. The head curves, given by the
manufacturers for water, for oil of differing viscosity may not be, however, directly
applied. Let us consider the specific energy transmitted by the ideal, frictionless fluid
to be the pump’s theoretical transportation head, and denote this value as
He.
The
actual specific energy increase developing in the real fluid flow is
H,
that is smaller
than the theoretical head by a head
loss
of
h’,
i.e.
H=H,-h’
Basically, the
loss
in the head consists
of
losses due
to
the friction and from drag. At
the
BEP
ofthe pump this latter value may be neglected, and the head
loss
equals the
product of a given energy
loss
factor, and the kinetic energy. Bobok
(1972)
indicated
that this loss factor, simultaneously to the pipe friction factor, is the function of the
Reynolds number, and the wall roughness.
At
a
given rate consequently the smaller
the actual head of the given pump is, the greater is the viscosity of the transported
oil.
Thus, the greater is the viscosity
of
the oil, the more downward the head curve,
valid for the Newtonian
oil
of greater viscosity than the water, shifts.
204
7.
PIPELINE TRANSPORTATION
OF
OIL
No
general solution is knonw, by which the head curve, measured at any pump
for water, could be applyied for oil. The most often cited method is still that of
A.
J.
StepanofT’s, that was developed on the basis of the experiments with solute casing
pump without diffuser (Stepanoff
1948).
By the pump water, and oils of
11
different
viscosities were transported, the kinematic viscosity of which ranged between the
valuesof
1
x
m2/s.Therpm ofthe pump ranged between
1240
and
1780,
and the
n,=nqo”H
0.75 value was
32-42.
On
the basis of his
and
1.88
x
1.0
0.8
’’
0.6
0.4
1
.o
0.8
0.6
0.4
‘7
0.2
0
lo2
lo3
lo4
cl
1
o5
-
*P3
Fig. 7.4
-
17. Multiplication factors
of
oil
transportation. after Stepanoff (1948)
experiments he determined the diagram, shown
on
Fig.
7.4
-
17
that defines the
multiplication factorsf,, andf, in the function of the formula
NRe=q/(dpv)
that is
proportional to the Reynolds number.
d,
is the impeller diameter of the pump.
Moreover, he established that between the values valid for the water,
q,
and
H,,
and those, valid for the oil of
v
viscosity,
q,
and
H,
the following relation may be
introduced:
7.4
-
26
By applying the above relations, the parameters of the operating point of the head
curve valid
for
water may be converted for fluids of other viscosities. Since according
to Falk
(1967),
with a good approximation, the zero-capacity head,
H,,
is
independent from the viscosity, i.e.
it
is constant, the head curve valid for the oil may
be plotted between the points
q,,
H, and
q,,,
Ha, with a good approximation.
Anderson
(1971)
describes a nomogram, from which, in case of given
q,
H and
v,
the correction factorsf,,f, and
f,
can be directly read.
Example
7.4-5.
Let, at the
BEP
be q,=1.2m3/min, HW=l00m,
q=O.8,
and
d,=300mm. Let us determine the transportation parameters for an oil of
v=0.76 x
m2/s viscosity and
p=850
kg/m3 density:
=877.
002
Re-
0.3
~0.76
x
N-
7.4.
ISOTHERMAL OIL TRANSPORT
205
From
Fig.
7.4
-
17
On this basis
fH=095
and
f,=0.58
and
H,
=
0.95
x
100
=
95 m
qo
=
058
x
0.8
=
0.46
According to Eq. 7.4
-
26
q,,
=
q.,
(2)'
.'
=
0.02
(&)
'
'5
=
0.01
85,
m
3/s
and the power requirement determined by Eq. 7.4
-
25
is
0.01
85
x
95
x
850
x
9.8
1
0.46
p=
~~ =3.19~ 104=31.9kW.
In
case of pseudoplastic fluids, corresponding to the power law, the Reynolds
number is to be calculated by Eq.
1.3
-
15:
If
the crude is thixotropic pseudoplastic
it
must be also considered, that after the
storage
in
tank, i.e. after relaxation,
or
after flowing for a longer duration, that is,
after a significant shear load period, does the oil get into the pump. The flow curves
also change in the function
of
the shear time (see
Fig.
1.3
-
2).
From these curves the
one, corresponding to the estimated shear time value, respectively the
p" and
n
constants
of
the exponential equation simulating the above curve, must be
substituted into the above relations.
In case
of
the characteristic curves of the pump, transporting oil, the possibility of
the change in the viscosity of the oil must be also considered. The two main reasons
for this are, that, on the one hand, the viscosity of the given quality oil may change
due to temperature variations, and, on the other hand, the quality of the transported
oil may change. The change in temperature may be temporary,
or
regularly
expected. Temporary change may be caused by the temperature alteration of the oil,
stored in tank at the first pump station, e.g. because
of
the modification in the
treating technology. Regular alteration is caused by the seasonal change of the
pipeline surrounding
soil's
temperature. During the summer warmer, while during
the winter period colder oil gets in to the pumps. Shifts in the operating point may be
also caused by the change of the
q
-
H,
system curve due to the alteration in flowing
temperature.
The selection of the pump unit, and the control of its operation deserve special
attention,
if
the pipeline transports oil slugs of different viscosities (Menon 1976). In
Fig.
7.4
-
18
three pairs of curves are shown. For all the three curves,
h
means the
heavy, and
1
the light products (Diesel
oil
gasoline, resp.).
The curves
I
are valid for the same pump operating at
n
speed.
H
head is replaced
by
Ap
pressure difference. Curves
I!
are the system curves of the pipe where, instead
206
7.
PIPELINE TRANSPORTATION
OF
011
of
head
loss
H,
pressure loss
Ap,
is represented. Curves
111
indicate the power
of
the
prime movers, in the function
of
the transported flow rate. Curves
I
and
11
have four
intersections, i.e. four operating points.
At
the
A
point heavy fluid flows
in
the
pipeline, while light fluid in the pump. The pumped liquid rate is
qA,
while the
pressure increase, developed by the pump is
Ap,.
The pump gradually fills up the
pipeline with light product, and during this interval, the operating point slowly (e.g.
during some hours) shifts from position
A
to
D.
At
the
D
operating point the pump
transports light liquid ofq, rate, and the same quality ofoil is moving
in
the pipeline.
111.
I
I
I
I
-
qA
qBqD
qC
q
Fig.
7.4-
18.
Characteristic operating points
of
batch-transport, after Menon
(1976)
The transportation parameters further change when heavy oil gets into the pump.
In
this case the operating point is suddenly shifted to
C
position. The discharge
pressure raises to
Ap,,
the rate to
q,
values and positive pressure wave may be
generated. The operating point,
C,
is then gradually shifted since the pipeline is
being filled up with heavy liquid.
At
the
E
operating point, the pump transports
4s
rate of the heavy liquid, and along the total pipeline this type of fluid flows. Further
change may occur
if
light product gets into the pump. Then, the discharge pressure,
supposing that the suction pressure remains the same, suddenly decreases by a value
of
(LIP,-
Ap,),
and the transportation is continued with the parameters
of
the
A
operating point. Due
to
the rapid pressure change negative pressure wave may
develop.
For transportation reasons the least favourable is the case when the flow
resistance
of
the pipeline is the greatest, and the pressure increase developed by the
pump is the smallest. This phenomenon occurs, when, in the total length of the
pipeline heavy liquid
of
unfavourable flow behaviour is moving, while
in
the pump
light liquid is still to
be
found. The pump is to
be
selected on the basis
of
4
and
Ap
parameters
of
this operating point. If, by the pump selected in this way,
a
greater rate
of
light oil than the
q,
is
to
be
transported, then
it
should be taken into
consideration at the selection
of
the prime mover. The maximum transportable flow
rate by the selected pump
is
qc.
The motor power corresponding to this value is
P,.
To fulfil the task that the selection, arrangement, and the operational capacity
of
the pumps should be optimum, is
of
extreme economic importance. Feizlmayr
7.5.
ISOTHERMAL OIL TRANSPORT SYSTEM
207
(1975),
while describing the Transalpine Pipe Line (TAL), says that the annular
electric energy demand
of
the system, when
fully
developed, is about
100
million
DM.
By the optimum operation
of
the booster pump stations an annual
3.4
percent
savings in electric energy were achieved, as compared to the previous non-optimized
operation.
7.5.
Isothermal
oil
transport system
The main parts
of
the transport system are the pipelines, the oil transport starting
station, the booster stations, and the terminal. Their location, and, to a certain
extent, their selection, is discussed in the previous Chapters. The main accessories
and their tasks of the up-to-date
oil
transport systems with tankless booster stations
will
be discussed further.
Beside the pumps by-pass line with
check
oaloe
is installed.
It
has to prevent that
after the stopping
of
the transport a pressure greater than the allowed valve
developes. In the upper section
of
Fig.
7.5-1
(Feizlmayr 1975) the profile of a
:b
h1
----
---+
FIN
I
I
A+
‘Ikrn
*
Fig.
7.5
-
1.
Overpressure protection by check valve in approximately horizontal terrain, after Feizlmayr
(1975)
pipeline, built on a near horizontal terrain (line
I),
the head curve
of
the allowed
internal overpressure (line
If);
and the piezometric curve
of
the steady state flow
(lines
III)
are plotted.
At
points
A
and
B
there are pump stations. Here, only station
B
is schematically denoted by a pump, and the line is furnished with check valve.
If
the
flow
stops, and the check valve, in the line, by-passing the pump
B,
closes, then
the pressures in the
A
-
B
lines are equalized, and a pressure head, approximately
equal to the average value,
h,/2,
develops in it. This value is apparently smaller than
the allowed values, determined by the
I1
line. If no check valve were applied, the oil
could flow back from the pipe section after
B
pump, and
so,
in unfavourable cases,
the pressure at the suction side would exceed the allowed maximum value.
In
Fig.
7.5-2
(after Feizlmayr
1975),
pump stations
A
and
B,
set up on steep terrain are
208
7.
PIPELINE TRANSPORTATION
OF
OIL
shown. The task of the check valve, installed beside station
B,
is to prevent an over-
pressure in the section, following station
A,
after stopping the transportation.
If
the
check valve fails, the oil, through safety valve
I,
flows into safety tank
2.
Figure
7.5-3
shows the characteristic curves
of
a given shorter section of
TAL (Trans Alpine Pipeline), after Feizlmayr
(1975).
The marking of the lines is the
same, as in case of the former Figure. Due to the great changes in the terrain, the
piezometric curve slope, without pressure regulation, after
C
mountain peaks,
A
I,
km
*‘**“
Fig.
7.5
-
2.
Overpressure protection by check
valve
in
hilly terrain, after Feizlrnayr
(1975)
Fig.
7.5
-
3.
Creation
of
the desired piezornetric
line
by
pressure regulator, after Feizlrnayr
(1975)
t
1
Fig.
7.5
-
4.
Creation
of
the desired piezornetric line by pressure regulator at the pipeline end section, after
Feizlmayr
(1975)
would
be
greater than the value
of
pressure curve
Ill,
required to ensure the flow
rate, and for this reason, “non grata” free surface flow would develop (see Section
7.4.1).
That is why, at the
A
point,
pressure regulator values
are installed, that in the
given pipe section provides the desired pressure curve slope.
Figure
7.5-4
shows the
7.5.
ISOTHERMAL OIL TRANSPORT SYSTEM
209
last section of this very same pipeline. In order to avoid the open flow, at the
A
end-
point pressure regulator valve is installed, which ensures the corresponding pressure
gradient in the line section after pump
1.
Without this the flow would be open after
several mountain peaks.
Generally, the pressure regulator valves do not offer protection against over-
pressure, due to quick pressure waves.
Figure
7.5-5
shows a longer section of the
TAL. At the low points,
A
and
B
pressure limiting safety values
denoted by
1,
are
h
m
1600
1400
1200
1000
800
600
400
Fig.
7.5-5.
Pressure regulation
of
a
TAL
pipe line section,
after Feizlmayr
(1975)
applied, that prevent pressures, higher than by the
If
line determined values, in the
pipeline. The liquid rate responsible for the excess pressure is led into
safety tanks,
2.
Several types of the safety valves are known. The simplest devices of this group
of
instruments are spring loaded (e.g. see
Fig.
6.2
-25).
Their disadvantage is that
for
their opening considerable excess-pressure is required, and their operation is also
affected by the downstream pressure. The devices, equipped with pilot valves,
accurately open, their operations is not influenced by downstream pressure, their
functioning is, however, slow. The valves controlled by instruments (e.g.
in
Fig.
6.7
-
9),
may be accurately set, and the prescribed pressure may be changed by remote
control. Their operation however, requires external energy sources. The expansible
tube
type
regulator, developed by Grove, seems to prove to be a rapid and reliable
safety device requiring no external energy source (Cohn and Nalley 1980). One
of
its
versions is planned to be applied to the oil pipelines
of
the
USSR
(Pipe Line
.
. .
The scheme
of
the system is shown on
Fig.
7.5-6.
The pressure
I,
through the
throttling valve
2,
acts upon bladder
4,
positioned in accumulator
3,
that separates
the oil
from
the gaseous nitrogen above
it
in tank
5.
The pressure of this gas has an
impact upon the bellows valve
6,
and the elastic flexo tube
7,
installed in it,
1979).
14