Szilas A.P. Production and transport of oil and gas, Gathering and Transportation
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gas heat exchanger
3,
and low-temperature separator
(5.
The low-tcmpcraturc
separator itself is shown in more detail
in
Fig.
6.4
-
45.
Wellstream enters through
choke
I.
The solid hydrates freezing out of
it
collect on the vessel bottom. Outlcts
are:
2
for gas,
3
for liquid hydrocarbons,
4
for water. Solid hydrates arc hcatcd abovc
decomposition temperature by warm wellstream flowing through coil
5.
Pressures
I
J!
I
Fig.
6.4
-
45.
Low-temperature separator. after
Mapes
(1
960)
t.
ig.
6.4
40.
<'omhliicd
~~~II~I~I-I~~~~I~~~III~II
low-
tempcrature separator.
artel
M;ipc\
(
IYhO)
and temperatures may e.g. be as follows: gas temperature upstream of choke,
30
'C;
downstream
of
choke,
0
"C.
Inflow temperature of heating wellstream,
50
'
C;
outflow temperature,
30
"C.
Such cooling of the well fluid may ensured according
to
experience by letting the wellstream expand from
140
bars to, say,
5
or
10
bars.
Low-
temperature separators are not necessarily vertical: they may be horizontal,
combined
(Fig.
6.4-46),
or spherical as well. Referring once more
to
Fig.
6.4
-
44,
the inflowing wellstream
I
is seen to be used in this setup to decompose hydrates
collected at the separator bottom. The liquid is separated from the cooled well fluid
in inlet-liquid separator
2,
for two reasons; one, to reduce the liability of the choke's
freezing up; and two, gas is easier to cool
if
separated from the liquid of greater heat
capacity, from which
it
could absorb more heat. The gas separated from the liquid
then enters into heat exchanger
3
where
it
is cooled by the cold dry gas discharged
from the separator to a temperature just above the hydrate-formation point
upstream ofthe choke. This is achieved by regulating the quantity ofdry gas passing
through the heat exchanger by means of thermostat-controlled three-way motor
valve
3.
This setup will function satisfactorily only
if
the heat delivered by the
inflowing wellstream is sufficient to melt the gas hydrates at the separator bottom,
and, on the other hand; ifthe temperature ofthe wellstream upstream of the choke is
above the hydrate-formation point at the given pressure.
Subtype (a.
1.2).
Heat required to warm the wellstream and to decompose the gas
hydrates collecting in the separator is taken from an external source.
In
this case, the
gas-to-gas heat exchanger is replaced by a line heater. Gas is heated with either
h
5
ON-t
FASI
011
FTORAGI
91
steam or hot water
in
a heat exchanger, or part of the wellstream is passed through a
coil
in
the liquid space ofa boiler; the proportion of heated to unheated wellstream is
controlled by the tempcrature upstream of the choke. The second solution is the
more up-to-date. Gas hydrates are likewise decomposed by either water or steam
taken from the boiler. This latter setup is illustrated by
Fig.
6.4-47.
The wellstream
enters through conduit
I
to
be divided by a three-way valve; part of
it
enters boiler
2,
to pass from there through choke
3
into combined separator
4.
Dry gas is dischargcd
I
Flg. 6.4-47.
Low-temperature separator. type a
I
.2.
alter Mapes
(1960)
through outlet
6.
Gas hydrates are heated to the required temperature by hot water
circulating in conduit
5.
Water is drained through outlet
7.
Hydrocarbons are led
through conduit
8
into second-stage separator
Y,
where some gas can still be
recovered from the liquid.
6.5.
On-lease
oil
storage
6.5.1.
Storage
losses
Storage losses are comprised of leakage and evaporation
loss.
Leakage can be
avoided
if
tank plates and fittings are tight and tanks are filled and emptied
with
proper care. Evaporation losses are due to the fact that the tank space above the
stored crude is saturated with hydrocarbon vapours that will escape into the
atmosphere. Evaporation losses fall into filling and breathing losses. Breathing is
due to the daily fluctuation of tank temperature. Warming of the tank makes
vapours expand,
to
escape partly into the atmosphere through the tank vents.
On
cooling, the tank will suck
in
air from the atmosphere. This process is further
complicated by rain and other precipitations. Breathing
loss
is the higher the higher
the vapour pressure of the product stored, the greater the temperature fluctuation,
and the larger the vapour space. Filling losses will occur as the rising liquid level
in
a
tank being filled expels the vapours contained
in
the vapour space. Evaporation has
the following unwelcome consequences.
(i)
Hydrocarbon vapours escaping into the
92
6.
GATHERING AND SEPARATION
OF
OIL
AND GAS
atmosphere are lost for good,
(ii)
the light-fraction content, and hence, the value per
ton, of the stored product decreases,
(iii)
the environment is polluted by the escaping
vapours; moreover;
(iv)
an explosion hazard is created, and
(v)
the air entering the
tank accelerates the corrosion of the inside tank wall.
Several ways to reduce
or
altogether stopevaporation losses are known.
In
order
to assess the economy of the solution proposed,
it
is necessary to determine the
losses, e.g. by closing all apertures of the tank into the atmosphere, save one, and by
measuring the quantity and composition of the gas escaping through the remaining
aperture over a longer span of time.
For
estimating evaporation losses of light
hydrocarbons
in
big tanks, a procedure requiring no separate measurement has
been devised (O’Brien
1951).
The method is based on
Figs
6.5
-
I
-
6.5
-3.
Figure
6.5
-
I
shows Reid vapour pressures
v.
temperature for various hydrocarbons.
As
6,s.
ON-LEASE
OIL
STORAGE
F,
C
N,'
mz
in1
90
76
60
62
,-55
:,"
,?a
35
31
2R
24
21
17
14
7
0
5
10
l5
vt
,10~~'
Fig.
6.5
-
2.
Breathing
loss.
after O'Brien
(
1951
)
regards evaporation
loss,
it is the surface temperature of the oils stored
in
the tank
that is relevant.
A
statistic
of
data collected all over the
USA
has revealed surface
temperatures to exceed mean annual temperatures by
5.5
"C
on an average.
Figure
6.5
-2
is a plot of breathing losses over one year v. tank volume, for various vapour
pressures. In compiling this diagram tanks have been assumed to be painted a
silvery colour and to
be
half full; the vapour space has been assumed to expand by 20
percent.
Figure
6.5-3
is a plot of filling
loss
v.
tank volume
for
various vapour
pressures. It has been assumed that the vapour escaping during filling has the same
gasoline content as the vapour space. The tank is painted a silvery colour and is half-
full on a long-term average.
Example
6.5
-
1.
Find by O'Brien's method the breathing
loss
over one year and
the filling loss during the filling
of
lo6
m3
of
liquid in a tank of
8700
m3 capacity,
if
the liquid in the tank is gasoline
of
90
kN/m2 Reid vapour pressure and its mean
surface temperature is
25°C.
By
Fig.
6.5-1,
vapour-space pressure is
pv
=63
kN/m2. Accordingly, breathing
loss
over one year is
485
m3/year
(Fig.
6.5
94
h
GATHERING AND SEPARATION
OF OIL
AND GAS
P"
kN/m'
101
90
76
69
62
55
:85
41
38
35
31
28
2b
21
17
14
7
Fig.
6.5
~
3.
Filling
loss.
after
O'Brien
(195
I)
-2).
By
Fig.
6.5-3.
the
loss
factor
410/41
is
04)0162,
that is,
1620
m3
for a liquid
throughput of
lo6
m3.
Losses in an upright cylindrical tank may be reduced: (a) By keeping temperature
fluctuations within limits.
(i)
The tank is painted a silvery or aluminium colour.
Evaporation losses of a crude of
0.837
kg/mJ gravity for tanks of different colours
are
listed
in
Table
6.L
1.
(ii)
The tank is partly or entirely covered
with
a heat-
insulating layer.
If
covering is partial,
it
is applied to the tank roof.
(iii)
The tank
is
cooled by the evaporation of a sheet of water on the tank roof, or by sprinkling with
water.
A
so-called water roof may reduce evaporation losses by
25
to
40
percent,
sprinkling by more than
50
percent.
(iv)
The tank is built underground. An earth
cover of
30
to
40
cm above the tank may prevent all breathing losses. (b) By closing
the tank to the atmosphere, that is, by choosing a tank whose permissible internal
pressure is higher than the vapour pressures that may arise. Evaporation losses
will
cease
if
pressure at the liquid surface is higher than the vapour pressure of the liquid.
(c) By special tank-roof designs. (d) By means of vapour-recovery installations.
h
ON-l
I
AS1
OII
SlORAOl
Table
I.
Influence of colour
to
which
tank
I\
piiirited
on
brealhing
loss.
for
oil
of
X37
hpni' grabily
C'olour
of
paint
Evaporation
loss
percent per year
I
.24
1.14
I
03
0.X3
6.52.
Oil
storage
tanks
Oil
tanks on a lease are installed partly at well testing stations. and partly
at
the
central oil gathering station
of
the lease. They are usually made of steel sheet
or
concrete. Most
of
them are above-ground, but some are buried underground. Pits
dug
in
the soil,
as
the simplest means
of
liquid storage, are used temporarily,
if
at all,
next to exploration and after well blowouts. Oil-storage pits are
to
be dug
in
level
ground,
if
possible. The pit walls and bottom are lined with stamped clay (cf.
Fig.
6.5
-
4).
In
order to minimize seepage losses, a layer of water
10
-
IS
cm high is poured
Fig.
6.5
-
4.
Storage pit
in
under the oil. Pits serving as mud pits or
for
the storage offracturing fluid are
first
injected with a silicon compound and then
with
a polymer expanding in water.
In
such pits, seepage
loss
is less than
0.1
percent ofthe liquid volume stored
(Oilund
Gas
Journal
1969).
A
similar treatment
will
reduce
oil
seepage,
too.
The most widespread type
of
on-lease oil-storage tank is upright cylindrical,
made
of
sheet steel. The sheet used is usually smooth, seldom corrugated. Smooth
sheet is joined by bolting. riveting
or
welding. Bolted and riveted tanks can be taken
apart without loss
of
sheet, and re-erected elsewhere. Welded tanks can be installed
faster; they require less sheet for a given volume. However, their dismantling and re-
erection entails a considerable loss
of
sheet. In welded tanks, rings are either butt- or
lap-welded. (cf.
Fig.
6.5
-5;
butt welds (a), telescopic lap-welds (b) and alternating-
ring lap-welds (c)). Bottoms are flat; roofs are usually conical. The necessary wall
thickness of upright cylindrical tank shells can be calculated
in
view
of
the
maximum liquid pressure to
be
expected as
'P
'hply
Gal
uark
s=
-
=
~
96
6
GATHLRING AND SLPARATION
Ob
OIL
AND
OAS
(0)
(b)
(C)
Fig.
6.5-5.
Hoop
welding
patterns
of
tanks
(Petroleum.
.
.
Oil
1955)
where
r
is radius,
h
is the maximum height
of
liquid column in the tank,
p,
is the
maximum liquid gravity
to
be expected,
o,,
is maximum permissible stress, and
k
is a
weld-seam factor.
Exarnple6.5-2.
Let h=3m, d=5m, p,=1000kg/m3, a,,=140MN/m2. What
wall thickness is to be chosen?
-
By Eq. 6.5-
1.
2.5
x
3
x
1OOO
x
9.81
S=
=
5.3
x
m
=O53
mm
.
140
x
lo6
Providing the tank with the necessary rigidity and properly joining the sheets
demand a wall thickness
of
3
-
4
mm
at
least, depending on tank size and steel grade.
Up
to
about 600m3 capacity, tanks are made
of
sheet
of
constant thickness,
determined by the above considerations, whereas larger tanks are telescoped (higher
rings are made
of
thinner sheet). The quantity
of
metal needed
to
build a tank of
given volume will be different for various height-to-diameter ratios. Metal volume
is
and since
we have
d:
7t
4
v=
-h
6.5
ON-LEASE
011.
STORAGE
97
/.
The least metal volume for
a
given tank volume may be determined by calculus. The
derivative
of
V,,
with respect to
h
is
dV,,
=
q
a72
+s3)
dh
2
h2
which implies that metal consumption will be a minimum
if
m,
Mg
2
20
200
180
160
6.5
-
2
Introducing this result into the above volume formula we get the most favourable
tank height:
6.5
-
3
where
k'
is the overlap factor.
-
It can be shown in similar way for telescoped tanks
(Shishchenko and Apriesov
1952)
that
6.5
-4
where
ah,
=gal
k".
The value
of
k"
ranges from
0.72
to
0.77.
It
accounts for strength
reduction due
to
welding and riveting.
The optimal tank shapes given by the above formulae may be slightly modified by
standard steel sheet sizes. On the other hand, as regards gauging the contents
of
a
tank, it
is
better if the tank is taller, because a given error in level determination will
cause the less error in volume, the slenderer the tank. In sizing tanks it should be
kept in mind that tank weight per unit volume is the less the larger the tank
(Fig.
6.5
v,,
m3
Fig.
6.5-6.
Typical data
of
Soviet tanks, after Shishchenko and Apriesov
(1952)
98
6
GATHERING AND SEPARATION
OF
OIL
AND GAS
-
6).
It
is therefore more economical,
in
so
far as this is a prime consideration, to use
a few big tanks than numerous small ones. Prescriptions concerning tanks are laid
down in API Stds 12B, 12C,
12D
and 12F. Aluminium tanks are treated
in
API Std
12G.
Let us point out that tanks suitable for dismantling are not usually made
in
sizes larger than 1600 m3. Larger tanks are of welded construction. Still, there is no
objection to the making of small tanks by welding.
In
most upright cylindrical steel tanks, vapour pressure above the liquid must not
exceed
0.4-0.5
kN/mZ. For radially ribbed decks. this value may attain
0.2-0.5
0
3
Fig.
6.5
-
7.
Upright
steel
tank
bar, but such designs need substantially more steel.
-
If
a tank is to withstand a
higher pressure
-
a few bars, say
-
then a horizontal cylindrical shape is adopted.
The cylindrical shell is closed on both ends by an ellipsoidal or spherical dome. Such
tanks should be made in a shop and not on-site. For oils of particularly high vapour
pressure, the spherical and spheroidal tanks usual in the storage of light distillation
products should be employed. The fittings
of
upright cylindrical tanks are shown in
Fig.
6.5-
7
as manhole on deck
I,
clean-out port on shell
2,
inlet studs
3,
breathing
valve
4,
gauge hatch
5,
water drain
6,
outside ladder or stairs
7,
inside ladder
8,
drain
stud, with swivelling suction pipe
9,
level gauge
IO,
hookup to foam-type fire
extinguisher
I
I.
Breathing valves, installed on the roof apertures of tanks, open at
0.15-030
kN/m2 pressure differential, both positive and negative. In the mechanical
breathing valve shown as
Fig.
6.5-8,
valve
I
opens to overpressure
in
the tank,
valve
2
opens to vacuum. Valve capacity depends on the maximum inflow/outflow
rate of liquid into/from the tank (cf. Table
6.5
-2, after Shishchenko and Apriesov
1952). In some instances, the gauge hatch is designed as a mechanical-type breathing
valve
(Fig.
6.5
-
9)
;
I
is the datum level of liquid level gauging. Overpressure is
released by the entire hatch cover pivoting about
2;
vacuum is compensated through
disk valve
3.
In order to eliminate the hazard of the mechanical valves’ sticking or
seizing, hydraulic valves are used in some instances
(Fig.
6.5
-
10).
Antifreeze liquid
6.5.
ON-LEASE
OIL
STORAGE
99
poured
in
cup
I
will be raised in annulus
2
by overpressure and in annulus
3
by
vacuum. This type
of
valve is designed
so
that the liquid will not be spilled out of
annulus
2
or
3
until the maximum operating pressure
of
the mechanical valve has
been exceeded. Breathing valves are often provided with flame arresters, in order to
help localize tank fires. Breathing valves will reduce evaporation losses by
15
to
20
percent, depending on the nature of the crude stored.
Figure
6.5
-
I I
shows schematically two types of liquid-level gauge. The device
in
part (a) permits an approximate determination of liquid level. Line
2,
hanging float
Fig.
6.5
-
8.
Mechanical breathing valve after
Shishchenko and Apriesov (1952)
Fig. 6.5
-9.
Gauge hatch closed by mechanical
breathing valve
Fig. 6.5-
10.
Hydraulic safety valve
I,
emerges through liquid-filled airlock
3
to
pass in front ofcalibrated tape
4.
Index
5
will give an approximate reading
of
liquid level in the tank. Its purpose is to provide
first-glance information to the operator rather than to permit accurate liquid-level
measurement. In design (b), the bobbing of float
I
is limited by guide wires
2,
which
makes for a higher accuracy in measurement. The perforated tape attached to the
Table 6.5
-
2. Diameters
of
mechanical
breathing valves (after Shishchenko
and Apriesov
1952)
Pumping rate Valve diameter
0-
50
50-
100
100-200
200-400
50
100
1
50
200