Szilas A.P. Production and transport of oil and gas, Gathering and Transportation
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40
6
GATHERING AND SEPARATION
OF
011.
AND GAS
Table
6.2
-2.
Dehaviour of main gas pressure regulator
types
Control
Downward valve travel will
On act ua
t
i
ny-gas failure,
On increase
of
controlled
Type code
valve will
pressure, valve will
C'untrollec
close
reverse
close
close
close
IC
upstream
open
Ila
direct
close
close
open
I
Ic
Figure
6.2
-
23
is
a sketch of a Grove-make gas-dome-type pressure regulator.
On
installation
of
the valve, dome
I
is filled through conduits not shown
in
the Figure
with gas at a pressure sufficient to exert upon diaphragm
2
a force bringing about
the pressure reduction desired. This gas is trapped
in
the dome by means
of
needle
valves. The lower surface of the diaphragm is exposed to the reduced pressure.
It
is
primarily the pressure differential
across
the diaphragm that determines the
position of inner valve
3.
The regulating action is improved by the gas space of the
1
II
0
I
t
I.
b
11
b
Ic
I1
c
Fig. 6.2 -22. Pressure-regulator
hookups
(Petroleum.
. .
Field
1956)
6.2.
VALVES, PRESSURE REGULATORS
Fig. 6.2
-
23. Grove's gas-dome pressure regulator
Fig.
6.2
-
24. Grove's
M-
I6 type pressure
regulator
41
5
Fig. 6.2 -25. Giproneftmash's
SPPK-I6
type
spring-loaded safety valve
42
6
GATHERING
AND
SEPARATION
OF
011
ANI)
GAS
dome being divided in two by capillary
4.
If
the controlled pressure rises above
a
certain maximum, a sudded pressure increase
in
the space below the capillary
will
occur and generate a considerable back-pressure. The great advantage
of
this type
of reducing valve is its simplicity and reliability of operation. Its disadvantage is that
the regulating pressure in the dome depends, of course,
on
ambient temperature.
This design is therefore best suited for controlling low-rate gas flows
in
places of
constant temperature.
The Grove Model
16
pressure regulator is likewise suited for controlling small gas
throughputs
(Fig.
6.2-
24,
e.g. to provide pressure-loaded motor valves
with
supply gas. The various sizes of this model can reduce the input pressure of round
70
bars to practically any pressure not less than
0.07
bar. Its operation resembles that
of the gas-dome pressure regulator, the main difference being that diaphragm
I
is
loaded by a spring adjusted by screw
2
rather than by the temperature-dependent
pressure of gas in a dome.
Safety valves
will
blow off fluid
or
gas whenever pressure in the vessel containing
them exceeds a given maximum.
Figure
6.2-25 shows a
SPPK
-
16
type
Giproneftmash-make spring-loaded safety valve.
If
the force acting on valve disk
2
in
the space connected
with
port
I
exceeds the downward force ofspring
3,
the valve
opens and permits the fluid to vent through outlet
4.
Opening pressure may be
adjusted by modifying the spring force by means of screw
5.
Most of the numerous other known types of pressure regulators and safety valves
embody operating principles similar to those outlined above.
6.3.
Internal maintenance
of
pipelines
On
the walls of oil and sometimes also of gas pipelines, wax deposits
will
tend to
form. Wax is composed of paraftin compounds, ceresin, asphalt and sand. The
deposit reduces the cross-section open to flow, and its removal must therefore be
provided for.
In
case of gas pipelines two types of deposits, that may cause flow resistance must
be taken into consideration.
(i)
Liquid hydrocarbons and water may condense
in
pipelines transporting gas. This liquid, which may contain solids, too,
will
reduce
on
the one hand the throughput capacity of the pipeline;
on
the other, liquid entering
gas burners together
with
the gas may cause serious trouble. The accumulation of
liquid in gas pipelines must therefore be prevented.
(ii)
If
the condensate, separated from the gas
or
water, is mixed with the lube oil of
compressors, then this mixture, sticking to the pipe wall gathers dust and iron oxide
particles, and thus, increases the roughness of the pipe wall and leads to its corrosion
(VerNooy 1980b). By applying regular pipe scraping both
of
the above effects hay
be reduced.
Deposits are usually removed by means of scrapers (go-devils)
or
cleaning pigs
inserted in the pipeline, at intervals depending
on
the rate of deposition, and moved
along by the fluid
or
gas stream.
6.3.
INTERNAL MAINTENANCE
OF
PIPELINES
43
The go-devil
(Fig.
6.3-
I)
is the type
in
longest use for cleaning the inner walls of
oil pipelines. It is used up to nominal diameters
of
12
in. Disks of leather
or
oil-
resistant synthetic rubber mounted on plates
I
ensure ‘piston action’, the moving
along
of
the scraper
in
the oil stream. Spring-loaded sawtooth rollers
2
ensure
coaxial advance. The deposits are scraped
off
the pipe wall by spring-loaded blades
Fig.
6.3
-
I.
Go-devil
Fig.
6.3
-
2.
General
Descaling
scraper
for
small-uze
pipeline>
3.
Certain scrapers are articulated, to help them negotiate sharp bends. This type of
scraper has the drawback of having a large number of parts liable to break
off
in
thc
pipe and cause all manner
of
trouble.
In
the scraper used for small-size pipes shown
as
Fig.
6.3-2,
piston action is ensured by rubber cups
I;
scraping is performed by
round wire brush
2.
In
cleaning large-size pipelines, revolving pigs are used
(Fig.
6.3
-3).
The front disk
3
of the scraper carrying piston cups
I
and round brushes
2
is
provided with tangential
jet
nozzles through which a small amount
of
fluid may
flow
behind the scraper. The jets shooting from these nozzles make the pig rotate slowly,
and help to loosen the deposit piled up by the advance
of
the scraper. Both effects
reduce the hazard
of
the pig’s seizing
up
in
the pipeline.
The scraper, equipped
with
a conic packing and spring wire brushes,
is
of
high
cleaning efficiency
(Fig.
6.3
-4).
Stucking
of
scrapers may lead to operational problems. The main reasons for this
are
(i)
the changing, and, from spot to spot, the extremely decreased pipe section, and
44
6.
GATHERING AND SEPARATION
OF
OIL
AND GAS
deformation in the pipe wall, respectively;
(ii)
the deposited layer on the pipe wall
that is too thick, and too hard; and (iii) the wear, and possible break of the pipe
scraper while passing.
In
order to avoid the disorders, caused by the first two reasons enumerated above,
the pipeline must be calipered. Before starting the operation of the pipeline, a plug,
equipped with a metal disc on its face is used for calipering. Its diameter, in
VerNooy’s opinion, should be 92.5 percent of the
ID
of
the pipeline.
For
the
21
21
-
Fig.
6.3
-
3.
General Descaling rotating scraper
for
large-size pipes
Fig.
6.3 ~
4.
Scraper with spring actuated wire brushes
measurement of the deformation, pipe wall thickness reduction, and deposition,
in
case
of
operating pipelines, the so-called caliper pig, can be used, that, while passing,
records the parameters of the measured inside cross-section (VerNooy 1980a).
Scrapers may be used also to separate
plugs of different liquids
or
plugs of liquid
and gas (‘batching’) and
it
is also used for the removal
of
the liquid, settled in
different sections
of
the gas pipeline.
If
there is no wax deposit in the pipe,
it
is simpler, cheaper and less liable to cause
trouble to insert a batching piston (pipeline pig) rather than a scraper.
A
pig of
General Descaling make, GDSS
90-4
type is shown as
Fig.
6.3-5.
Its swab cups
are made
of
neoprene. This pig will pass through any bend whose radius is at least
half as great again as the pipe radius. Branch-offs of the same diameter as the
mainline are no obstacle to its application.
A
pipeline batching ball serving also
for
the removal
of
liquids from gas lines is the Piball, likewise
of
GD make
(Fig.
6.3
-6).
6.3.
INTI<RNAl MAIN1 IINANC‘I:
Ot.
I’II’I.I.INI:S
45
The ball of synthetic rubber can be inflated with a liquid through valvc
I
so
that its
diameter exceeds by
2
percent the
ID
of
the pipe. Expcrinicnts havc shown that the
Piball removes all but
0.02-0.04
percent
of
thc liquid
in
thc pipc. Its advantagcs
include longer life owing to its spherical shape and inflatability.
It
isconipatiblc
with
Fig.
6.3
-
5.
General Descaliny
GDSS
90
-
4
pipeline
plg
I
Fig.
6.3
-6.
General Descaling
‘Pihall’
lease automation and will readily pass through pipe fittings. Scrapers fitted with
polyurethane blades, inflatable polyurethane pipeline balls and polyurethane-foam
balls are also used; practical experience has shown these to be rather long-lived
(Zongker
1969).
A
fairly complete removal of liquid may be achieved by means
of
the so-called
sypho-pig, shown schematically in
Fig.
6.3-
7.
The device, fitted with four swab
cups
I,
is pushed forward by dry gas in the pipeline. Part
of
the dry gas, entering
through
orifice
2,
passes through the tortuous paths indicated by arrows into
chambers
3
and
4,
where it dries the pipe wall by evaporating the liquid film
46
6
GATHtRING AND SEPARATION
OF
011.
AND GAS
adhering
to
it, and carries the vapour into chamber
5.
Liquid
in
chamber
5
is
expelled by the gas through tube
6
and jetted through injectors
7
in
front
of
the pig.
The jets, causing turbulency
in
the liquid pushed forward by the pig, prevent solids
from settling out
of
it
and from seizing the pig
(PLI
Staff
1970).
In
some cases, balls
or
cylinders made of paraffin and asphalt kneaded together
or
of
a substance slowly
soluble in oil are also used
for
paraffin-removal.
1
5
Fig.
6.3
~
7.
Sypho-pig, after
‘PLI
Star
(1970)
Fig.
6.3
-
8.
Scraper trap
in
front of separator station
Scrapers are introduced into the flowlines
of
wells at the wellhead, and retrieved
at scraper traps installed next
to
the separator station
(Fig.
6.3-8).
During the
scraping operation, the well fluid is directed through gathering line
I
into the first
separator, with valve
2
closed and valves
3
and
4
open. After the arrival
of
the
scraper, valve
2
is opened; valves
3
and
4
are closed. The cut-off pipe section
containing the scraper is bled
off
through relief valve
5;
the scraper can be removed
after opening cover
6.
-
Figure
6.3
-
Y
shows launching and receiving scraper traps
installed at the beginning and at the end, respectively,
of
an oil pipeline. During
normal operation, valves
I
and
2
of
the launching trap [part (a)
of
the Figure] are
closed, while valve
3
is open. After depressurizing through valve
4
the section cut
off
by valves
I
and
2,
cover
5
is removed and the scraper
is
inserted into oversize barrel
6.
After replacing cover
5,
valves
I
and
2
are opened and valve
3
is closed. Instead
of
h
3
INTt
R\AI
MAINTF\AYC
I
OF
PIPF
I
IYI5
47
gate valves, handwheel-operated plug valves are often employed
in
these
applications. The operation of the receiving scraper trap [part
(b)
of the Figure] is
much similar to that of the launching trap.
-
Long pipelines are often provided
with
intermediate scraper stations
(Fig.
6.3-
IO),
where the worn scraper arriving
on side
/
is retrieved and
a
new one is inserted on side
2
(cf.
F'iq.
7.4-
14).
'-=
2-
1-1~.
6.3-
10.
Intermediate scraper trap on
a
pipcline
1
Fig.
6.3
-
I
I.
General Descaling
pipe
ball feeder
Pipeline balls may be fed into the line by remote control.
Figure
6.3
-
I/
shows the
GD
hydraulic ball feeder. After removal of cover
I,
several balls can be inserted into
pipe section
2.
At
the desired instant, a hydraulic control unit
3
(not shown
in
detail
in the Figure) actuates release device
4
which lets a ball
roll
into the pipeline through
open valve
5.
It
is possible to monitor the passage of the ball through suitably
chosen pipe sections. The sections in question are perforated and a fitting is installed
in the perforation which includes a small feeler reaching radially into the flow
section. The passing ball will touch the feeler; a device attached
to
this latter will
mechanically or electrically signal the passage
of
the ball (cf. Section
7.2.3).
48
6.
GATHERING AND SEPARATION
OF
OIL
AND GAS
6.4.
Separation
of
oil
and gas
6.4.1.
Equilibrium calculations*
The oil and gas composing the well fluid are usually separated
in
separators.
A
separator is essentially a vessel whose interior is kept at the prescribed separation
pressure and temperature. Separator temperature is either determined by the
temperature of the inflowing well stream and the ambient temperature,
or
adjusted
by means of heating
or
cooling equipment. The prescribed separator pressure is
usually maintained by a pressure regulator installed in the gas line
of
the separator.
Oil
and gas emerge from the separator through separate outlets. The composition
and relative abundance of the separation products can be predicted either by
laboratory tests,
or
-
in a fair approximation
-
by theoretical considerations
and/or using diagrams based on practical experience. In calculations based on
auxiliary diagrams
it
is assumed that the well stream and the discharged oil and gas
do not vary in composition with time; that separation is of the flash type; and that
the system is
in
thermodynamic equilibrium at the given pressure and temperature.
The calculations are based on the following three equations:
n=
n,+
n,
6.4-
1
6.4
-
2
zin
=
nLxi
+
n,yi
Yi
K.=
-
Xi
6.4
-
3
K,
is the equilibrium ratio of the ith component in the liquid-gas system. By Eq.
6.4
-
1,
then, the total number of moles in the system equals the number of moles
in
the
liquid and gas phases taken separately. By Eq.
6.4-2,
the total number of moles of
any component in the system equals the number
of
moles of that component taken
separately in the liquid and gas phases. The above considerations imply
m m
xi=
1
yi=
1
z,=l
i-
1
i=
1
i=
I
Dividing both sides
of
Eq.
6.4
-
2
by
n,
we get
Let
nJn
=zL
and
n,/n
=
z,
;
then
zi
=
ZL
xi
+
Zy
yi
.
6.4
-
4
6.4
-
5
By Eq.
6.4
-
4,
the sum of the mole fractions of the components
is
unity, separately in
the liquid and gas as well as in the entire liquid-gas system. By Eq.
6.4-
5,
the mole
*
Largely after Amyx
(1960)
6.4.
SEPARATION
OF
OIL
AND
GAS
49
fraction
of
a given component in the system equals the sum
of
its mole fractions
in
the gas and liquid phase, taken separately. Introducing
yi
and then
xi
from Eq.
6.4
-3,
we get
and
These are the fundamen
6.4
-
6
6.4
-
7
al
equations of equilibrium in separation.
In
practice, separation is usually performed in several stages. Stages are series-
connected, the last stage being the stock tank. Three-stage separation,
for
instance,
Fig.
6.4-
I.
Schematic diagram
of
three-stage separation
takes place in two separators and one stock tank
(Fig.
6.4-1).
The numerical
suffixes attached to the symbols introduced above refer to the respective separator
stages. The liquid discharged from the first separator enters the second separator.
The mole number of this stream is
nL1 =ZLlnl.
The mole number of the liquid entering the third separator (which is the stock tank
in
the case considered) is
nL2=zL2nLl=zLlZL2nl.
The mole number of the liquid obtained from the third separator is
nL3=zL3nL2 =zLlZL2zL3nl
.
The mole number
of
the gas discharged from the first separator is
nv1
=zvlnl.
4