Cyclone inlet section. As the mist-laden gas enters the separator, the entrained
liquids and solid particles are subjected to centrifugal force. The gas enters the
cyclone tube at two points, designated A, and sets up a swirling motion. Solid and
liquid particles are thrown outwardly and drop from the tube at point B. The
swirling gas reverses direction at the vortex C and rises through the exit portion
of the tube, designated D. See Fig. S-21.
Separators S-17
FIG. S-20 Horizontal with horizontal lower barrel and vertical configuration. (Source: Peerless.)
Mist extractor inlet section (alternate for some applications). As the gas enters the vane
unit, it is divided into many vertical ribbons (A). Each ribbon of gas is subjected to
multiple changes of direction (B) as it follows its path through the vanes. This
causes a semiturbulence and rolling of the gas against the walls of the vanes (C).
The entrained droplets are forced to contact the vane walls where they impinge and
adhere to the vane surface (D). This liquid then moves into the vane pockets (E)
and out of the gas stream. It is then drained by gravity into the liquid reservoir.
The collected liquid can then be disposed of as desired. See Fig. S-22.
Final separation element. The final separation section consists of one or more
cylindrical coalescing elements mounted vertically on support tubes. The gas and
fine mists pass from the inside to the outside of the elements. In passing through
the coalescing elements, the entrained mist particles diffuse and impinge on the
closely spaced surfaces of the element and are agglomerated into larger liquid
droplets. The larger liquid droplets emerge on the outer surface of the coalescing
element and run down the sides of the element to the liquid collection chamber. The
gas, free of liquid particle entrainment, rises and passes out of the separator
through the upper gas outlet nozzle.
Design features. Replacement of the final separation elements can be made with a
minimum of time and effort through the use of a full opening O-ring or float ring
closure. The primary separation elements (vane-type mist extractor or cyclone
section) are completely maintenance-free and self-cleaning, with no replacement or
moving parts to cause shutdown.

8
≤ 12
3
/
4
≤ 14≤ 16≤ 18≤ 20≤
In and out line sizes 2≤ 3≤ 4≤ 6≤ 8≤ 10≤ 12≤
A Face-to-face 17≤ 20≤ 24≤ 26≤ 30≤ 34≤ 38≤
B Approximate overall height 36≤ 38≤ 47≤ 49
1
/
2
≤ 52
1
/
2
≤ 60≤ 63≤
C Body length, seam-to-seam 29≤ 30≤ 36≤ 38≤ 40≤ 46≤ 48≤
D Top head seam centerline
inlet and outlet 4
1
/
2
≤ 4
1
/
2
≤ 8
1
/

O) 12–13 12–13 8–10 8 6–8 6 5
NOTES
:
1. All dimensions are ±
1
/
4
≤.
2. Base support is optional.
3. Vessels stocked with design pressure of 275 psig 150# RF and 720 psig 300# RF.
4. Vessels equipped with these accessory fittings:
A. 2,
3
/
4
≤ 6,000# gauge glass connections.
B. 2≤ 3,000# equalizer connection.
C. 2≤ 3,000# drain connection (1
1
/
2
≤ on 6
5
/
8
≤ and 8
5
/
8
≤ O.D. sizes).

in the stream. See Fig. S-28. As the slugs of liquid come into contact with the baffle,
they are deflected at an angle and are broken up by a hooked vane attached to the
edge of the baffle. This breaking up of the slugs causes them to drop out of the
stream and to the bottom of the separator. The baffle is made of extra thick material
to protect against excess erosive wear.
Rise to mist extractor. Having removed a majority of the entrained liquid or slugs,
the gas flow continues its travel to the mist extractor that is above the inlet baffle.
During this travel, a centrifugal and gravitational action takes place that separates
more of the entrained liquid. The distance the vapor (gas and liquid) must rise to
enter the mist extractor aids in the separation by supplying time necessary to
permit coalescing or the forming of small droplets into larger drops that have a
greater rate of fall than the upward velocity of the gas. By this method, maximum
separation, using impingement, centrifugal motion, and gravity, has been obtained
with a minimum pressure drop.
This settling effect, utilized in the vertical gas separator, removes all but a very
small portion of the liquid. This remaining liquid continues to rise toward the gas
outlet in the form of a fine spray. To solve this final separation problem, the mist
extractor is used.
Mist extractor. The mist extractor combines maximum scrubbing area with an
absolute minimum pressure drop. It utilizes the forces of impingement, centrifugal
motion, and surface tension to obtain its high efficiency. See Fig. S-29.
The path of the gas through the unit is constantly bending, causing the
impingement of the liquid droplets against the walls of the vane, separating some
of the entrained mist. Centrifugal force aids by throwing the heavier liquid droplets
out of the main gas stream and impinging them on the scrubbing surface. The
entrained liquid, after coming into contact with the metal surface and other liquid
droplets of the vane unit, is coalesced and adheres to the vane surface by utilizing
the forces of surface tension. Gravity and impact of the gas stream then drives the
droplets into the pockets provided at each turn of the vane where they roll down
out of the gas stream. After going through the complete process of scrubbing and

Separators S-25
FIG. S-30 Line separator installation. (Source: Peerless.)
Principle of operation. The vane unit is the heart of the separator (see Fig. S-32).
As the gas enters the vane unit, it is divided into many vertical ribbons (A). Each
ribbon of gas is subjected to multiple changes of direction (B) as it follows its path
through the vanes. This causes a semiturbulence and rolling of the gas against the
walls of the vanes (C). The entrained droplets are forced to contact the vane walls
where they impinge and adhere to the vane surface (D). This liquid then moves into
the vane pockets (E) and out of the gas stream where it is drained by gravity into
the liquid reservoir. The collected liquid can then be disposed of as desired.
It is significant to note that the liquid drainage in the vane-type mist extractor
differs from the drainage in other impingement-type mist extractors, in that vane
drainage occurs with the liquid out of the gas flow and at a right angle to the
direction of flow through the separator.
The individual vane corrugations, depth and size of the liquid pockets, and the
vane spacing are critical features of the vane-type mist extractor. Many years of
testing and operating experience eventually arrive at optimum dimensions and
spacing. The slightest variation in any one of these three features will materially
decrease the capacity and performance of this type of separator.
Efficiency and capacities. The vane-type line separator (see Fig. S-33) will remove
all of the entrained liquid droplets that are 8–10 microns and larger. The efficiency
of the unit decreases on droplet sizes less than 8 microns as shown on the chart.
In order to separate these smaller droplets, the separator must be preceded by an
agglomerating or coalescing device to increase the size of the droplets so that they
can be removed by the mist extractor. Several types of agglomerating devices are
available. Some of these are capable of achieving efficiencies as high as 99
1
/
2
percent

The gas is directed into the inlet separator section (A) (see
Figs. S-34 and S-35) where most of the liquid is removed. This separated liquid
drains into the first downcomer (B). The remaining liquid is then scrubbed by the
Separators S-27
FIG. S-32 Section of vane unit on line separator. (Source: Peerless.)
FIG. S-33 Line separator performance curve. (Source: Peerless.)
S-28 Separators
FIG. S-34 Horizontal gas separator. (Source: Peerless.)
FIG.
S-35 Horizontal single barrel design. (Source: Peerless.)
mist extractor (C). This entrainment drains into the lower barrel (D) through
downcomer (E). This second downcomer is submerged in liquid, and this liquid seal
prevents the gas from following a path through the lower barrel and bypassing the
mist extractor.
Advantages of the lower barrel. The lower barrel makes it possible to get the
separated liquid away from the gas flowing in the upper barrel, thus eliminating
reentrainment. It also makes possible the installation of larger separation elements
in the upper barrel, which results in a higher capacity. The lower barrel also
provides a quieting chamber for gas to break out of solution to effect a clean
separation.
Ordinarily, liquid level in the lower barrel is controlled by torque tube level
controls and diaphragm-type valves. Connections for liquid level gauges, pressure
gauges, and drains are provided.
Capacities. Horizontal separators have high gas capacities because the mist
extractor is installed longitudinally in the vessel. This arrangement permits the
use of a large mist extractor inlet area.
Mist extractor design. This mist extractor incorporates a series of closely spaced
baffles, which combine impingement, centrifugal force, surface tension, and gravity
to effect separation. High capacity and low pressure drop are combined in this
design. The high efficiency is maintained over the entire range of flow from

the flexible foundation provided by the platform, and the determination of thermal
stresses at critical locations.
Although this design has been developed for offshore use, the techniques utilized
can be applied equally to onshore applications.
Nomenclature
V
s
design wind velocity, m/s
w design wind load per unit length of stack, N/m
L = L
4
- L
3
unsupported height of stack, m
L
o
= L
3
- L
2
spacing of upper pinned support, m
D mean diameter of exhaust stack, m
D
1
spacing of main bearings, m
k
v
vertical elastic stiffness of foundation, kN/m
R
A

radial displacement at flange/shell intersection, mm
f
o
rotation at flange/shell intersection
t
F
flange thickness, mm
The design of gas turbine exhaust systems on offshore platforms generally falls
within well-proven parameters. The gas-carrying duct is suspended inside an
external structural steel framework and connected via a system of mounts and
guides to allow for thermal growth.
Extending the ductwork or stack above the steelwork such that the stack itself
carries structural loads would appear to be a simple extension of proven designs.
When the design of such a system was undertaken in practice, this was shown to
be a long way from the truth.
Offshore oil and gas production platforms are well known for providing a
particularly hostile environment for mechanical equipment operation. The North
f
m
=
2p
IDt=
p
8
3
S-30 Stacks
Sea is probably one of the harshest examples of an offshore environment. Wind
strengths are uncommonly high, with wind speeds of 45 m/s not unusual. In
addition to the high wind strength, the rapid changes and gusting make conditions
extremely unpredictable. Humidity levels are also high leading to problems with

The 45 tons of stack support structure is located at a high level on the platform.
For this reason it is necessary to provide a further 45 tons of steel in the topside
structure.
A freestanding exhaust system has a significant proportion of the upper ductwork
unsupported by steelwork, as shown in Fig. S-36. The main support would be at
the base of the freestanding section, with a system of guides for ductwork below
this support as necessary.
The design study considered the option of both 10 m and 20 m of freestanding
stack. Figure S-37 shows the relationship between the free length of stack and
platform steelwork saving for a single stack of 2 m diameter. The design and
fabrication costs can then be compared directly with the savings in steelwork to
show the net savings. Table S-3 shows the results. The cost savings can be seen to
be substantial. With three identical stacks being utilized on the platform in
question, savings of £440,000 can be achieved with a platform weight reduction of
156 tons. This equates to a saving of 35 percent when compared with the total
installed cost of the fully supported exhaust system.
Stacks S-31
S-32 Stacks
FIG. S-36 Exhaust stack showing alternative support arrangements. (Source: Altair Filters
International Limited.)
FIG. S-37 Relationship of free stack length to weight saving. (Source: Altair Filters International
Limited.)
TABLE S-3 Net Cost Savings for Various Lengths of Freestanding Stack
Free Stack Net
Length Stack Manufacturing Platform Steelwork Saving Percentage
(m) Cost (£) Saving (£) (£) Saving
0 110,000 0 0 0
5 125,000 63,000 48,000 11
10 130,000 126,000 106,000 25
15 135,000 161,000 136,000 32

interaction between pairs, rows, or groups of chimneys. In this case the three
systems are positioned in a row at close pitch. The use of aerodynamic devices such
as helical strakes to alter the response to gust loading of a single stack are well
proven. Deeper investigation into the effectiveness of such arrangements on
multiple arrays has again emphasized the serious limitations of available codes.
Designers should, however, look closely at the possible impact of aerodynamic
effects from nearby structures before finalizing their design. In this section
consideration of structural aspects only are considered. The aerodynamic interaction
of the three stacks on the platform in question played a major part in the decision
to limit the freestanding height to 10 m above the steelwork.
Because of the unique aspects of this design and the considerable uncertainties
in available design codes, a decision was made to develop directly applicable
analysis methods from fundamental principles.
See Table S-3.
Steady-State Wind Loading
The wind loading on an isolated exhaust stack will depend on its geographic
location, height, and the nature of the surrounding terrain. The design wind speed
and aerodynamic force on the freestanding stack design have been determined in
accordance with the recommendations of BSI (a British standard) CP3, Chapter V,
Part 2 (1972). Typical values for an offshore environment are a design wind speed
of 53 m/s, which for a 2-m-diameter exhaust stack corresponds to a drag force of
w = 3.6 kN/m. For a stack mounted on pinned supports at three discrete levels
as shown in Fig. S-38 the support reactions are not statically determinate.
It is necessary to integrate the general expression for the bending moment and
Stacks S-33
introduce the boundary conditions in the resulting displacement function. This
yields four simultaneous equations that may be solved to give the individual
reactions in the form:
and
where

3
2
2
1
2
21
31
1
2
31
2
21
2
1
248
4
12
3
=
-
Ê
Ë
ˆ
¯
+-+
()
+
()
{}
-+ -

3
=
-+ -
()
-+ +
()

()
{}
K
LLLL LLLL
LL LL
2
3
2
1
2
31 2
2
1
2
21
31
2
21
2
4
=
+
()

{}

V
D
wL L
L
RL L RL L
A
B
=-
Ê
Ë
ˆ
¯

()

()
{}
1
4
1
43
4
31 32

R
KK KK
KK
wR

frequency for an isolated cylinder is given by the empirical formula
For a 2-m-diameter cylinder supported in a turbulent airstream having a mean
velocity V
s
= 53 m/s, the coefficient S will have a value of 0.25, and the frequency
of vortex shedding from the sides of the exhaust stack will be 6.2 Hz.
For practical installations it is known that the flow pattern will be three
dimensional with additional vortices being generated by the flow over the top of the
stack. The shedding frequency for such vortices may well be different from the value
calculated above. To avoid any difficulties associated with aerodynamic excitation
it is prudent to ensure that the fundamental frequency for transverse vibration of
the exhaust stack is well above the primary vortex shedding frequency.
Unfortunately the formula given in BS 4076 calculates the natural frequency for
transverse vibration of a cantilever mounted on a rigid foundation. While this is
suitable for most land-based chimney designs it will yield an unduly optimistic
estimate for an exhaust stack supported on pinned joints. The error will be
increased further when consideration is also given to the flexibility of the supporting
structure. It is clear that a more detailed analysis of the vibration behavior is
required.
Examination of the bending moment diagram for inertia loading of an exhaust
stack supported at three levels suggests that the natural frequency for the
fundamental mode of transverse vibration will be almost entirely determined by
the geometry of the upper sections. Accordingly for the purpose of frequency
calculations only the two upper supports and corresponding stack sections have

n
SV
D
s
=

1
2
36
2
4
p
r
.
IDt=
p
8
3
BB
L
L
LL
LL
A
L
L
B
L
LL
k
o
oo
o
o
o
o

24
46
432234
sin
B
EIk
L
AB
L
LL
K
o
o
oo
o
==
-
+-
12 2
21
3

v
w
EI
xKLxALxBL mt
o
s
ooooooo
=+++

and where
S-36 Stacks
FIG. S-40 Idealization of exhaust stack for vibration analysis. (Source: Altair Filters International
Limited.)
where
and
For a fixed foundation k = 0 and these expressions can be simplified with the
fundamental frequency for transverse vibration being given by
The variation in f
o
as the free length of a 2-m-diameter exhaust stack is increased
from 5 to 20 m is shown in Fig. S-41. There is no doubt that the natural frequency
f
ED g
L
L
L
KK
A
A
L
L
KK
K
AA
o
o
o
oo
=◊

9
1
2
4
7
512
15
1
3
2
4
5
2
2
9
22
pr
.

Y A B A AB B
L
L
KK
K
A
k
BAABB
o
oooooo
=+ + + ++

3
22
9
222
X
L
L
KKBK
L
L
LL
LL
o
o
oo
o
=+
Ê
Ë
ˆ
¯
-+
()
+
Ê
Ë
ˆ
¯
+
+

15.1 Hz.
It is interesting to note that the corresponding natural frequency for a 10-m-high
cantilever mounted on a rigid foundation is 19.6 Hz. The difference between these
values emphasizes the importance of ensuring that the recommendations of the
relevant standard have been interpreted correctly.
For the pin-jointed configuration with a free length of 10 m the reduction in
natural frequency with support flexibility is shown in Fig. S-42. It is convenient to
express the flexibility of the support in terms of the tip deflection obtained with a
rigid foundation.
For the proposed design it is important to note that even for a relatively stiff
foundation, with a flexibility
1
/
k
= 145 MN/m, which is typical of the support
structure, the natural frequency falls to 10.9 Hz. It follows that the stiffness of a
conventional support structure will be sufficiently low to have an adverse effect on
the ideal dynamic response of the stack assembly.
The significant commercial benefits available from the correct choice of the design
for offshore gas turbine exhaust systems and their supporting structures is evident.
By using a freestanding exhaust the cost savings are shown to be substantial.
No comprehensive standard is available to assist the designer for this application.
Should a design analysis be undertaken without appreciating the limitations of
existing standards, the consequences could be disastrous.
These limitations have been highlighted, and a number of analysis procedures
presented to illustrate how the deficiencies may be overcome. Use of the proposed
procedures will allow a detailed and comprehensive design analysis to be completed
for a freestanding stack. The method is simple to apply and allows parameter and
optimization studies to be carried out within commercially viable time scales.
Freestanding stacks of even greater length can be achieved by prudent selection

Alfalfa (Dehydrated)
Alumina Ore
Ammonium Nitrate
Barium Carbonate
Barium Sulfate
Bark Ash
Barley
Bauxite
Bentonite
Bisphenol “A”
Blood (Dried)
Bonemeal
Borax
Boric Acid
Brewers Grits
Burnt Lime
Calcium Carbonate
Calcium Chloride
Calcium Silicate
T-1
* Source: A.O. Smith Engineered Storage Products Company, USA. Adapted with permission.
The 1

in (25

mm) flange
FIG.
T-1 Generic storage tank (TecTank SealWeld tank). (Source: Peabody TecTank.)
T-2