Process Engineering Equipment Handbook Episode 3 Part 6 ppsx

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.

The absolute separator could be guaranteed to remove 100 percent of all liquid
particles above 3 microns; and, depending on design conditions, it will remove up
to 99.98 percent of all particles less than 3 microns. This efficiency is maintained
throughout the entire flow range to design capacity.
S-18 Separators
FIG. S-21 Cyclone operation filter/separator. See text for key to components. (Source: Peerless.)
FIG. S-22 Mist extractor section. (Source: Peerless.)
The normal pressure drop through the final separation elements is limited by
design to 5 in of water column or less. The pressure drop across the primary section
will depend on operating conditions and the type of separation elements used.
See Figs. S-23 and S-24.
Vane-type separator
Line separators are designed and fabricated to conform fully to all current ASME
requirements and are usually furnished in carbon steel for most industrial
applications; however, units can be fabricated in part or entirely from stainless
steel, Monel, or other special alloy materials. See Figs. S-25 and S-26. See also Table
S-2.
Although standard units are designed for 275- and 720-psi working pressure
(412.5- and 1080-lb test pressure), vessels can be furnished in virtually unlimited
Separators S-19
FIG. S-23 Mist particle removal chart. (Source: Peerless.)
TABLE
S-2 Vane-Type Separators
Body Diameter (O.D.) 6
5
/
8
≤ 8
5
/

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
/

2
≤ 9≤ 9≤ 11≤ 11≤
G Lower head seam to
bottom of base 12≤ 12≤ 18≤ 18≤ 18≤ 18≤ 18≤
-0- Liquid capacity (gal.) 2
3
/
4
3
3
/
4
8
1
/
4
11
1
/
16
14
7
/
8
21 21
7
/
8
-0- Pressure drop (in, H
2

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).

D. 2, 1≤ 3,000# high-level shut-down connections on 14≤ O.D. and larger sizes.
E. 2≤ 3,000# vent connection (1
1
/
2
≤ on 6
5
/
8
≤ and 8
5
/
8
≤ O.D. sizes).
FIG. S-24 Absolute separator. (Source: Peerless.)
S-20
FIG. S-25 Vane-type separator performance curves. (Source: Peerless.)
FIG.
S-26 Vane-type separator (external). (Source: Peerless.)
S-21
S-22 Separators
ratings for greater pressures if required (pressures in the 20,000-lb range are not
uncommon).
Vertical gas separators
These vessels employ many physical means to separate the liquids from gases
in addition to the mist extractor. Foremost among these various separation forces
are: impingement, centrifugal force, gravitational force, and surface tension. See
Fig. S-27.
Inlet baffle. Of prime importance to the separation is the inlet impingement baffle,
which acts to eliminate heavy slugging problems set up by excess amounts of liquid

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

separation, the gas finally reaches the outlet opening of the separator clean and
dry.
Liquid control. The large liquid reservoir is adequate to store incoming slugs of
liquid during the time required for opening the liquid valve. The volume of the
vessel is large enough to allow the gas to break out of the solution and to escape
the liquid in the bottom of the separator.
Separators S-23
FIG.
S-27 Vertical gas separator. (Source: Peerless.)
S-24 Separators
Line separators
The vane-type line separator offers efficient separation of entrained liquids from a
gas or vapor stream. This separator design has been used successfully for over 25
years in chemical plants, refineries, natural gas pipelines, and all types of industrial
processing plants where efficient liquid-gas separation has been required. See Figs.
S-30 and S-31.
These separators incorporate the vane-type mist extractor as the separating
element. This unit offers a number of operating characteristics not found in other
types of separators:
᭿
100 percent removal of all entrained droplets 8–10 microns and larger
᭿
Extremely low pressure drop—less than 6 in of water column
᭿
Small housing requirement for ease of installation and economy
᭿
Flat efficiency curve with no decrease in efficiency from rated capacity down to
zero flow
FIG. S-29 Mist extractor. (Source: Peerless.)
FIG. S-28 Interbuffer—vertical gas separator. (Source: Peerless.)

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

removal of 1 micron size droplets.
Low-pressure drop. Since the vane-type mist extractor is self-cleaning and contains
no small openings that can fill up and restrict the flow—such as are present in wire
mesh pads or filter screens—the pressure drop across the separator is very low. The
drop is as small as 2–3 in of water in the larger sizes.
Vari-line separators. Vari-line separators are specifically designed for those
installations where space is at a premium and piping limitations prevent the use
S-26 Separators
LINE SEPARATOR
FIG. S-31 Line separator. (Source: Peerless.)
of a straight pattern line separator, which has the gas inlet and outlet connections
on the same horizontal centerline. These units consist of the vane-type mist
extractor with internal baffling designed to permit virtually any combination of
locations for the gas inlet and gas outlet connections. The principle of operation and
performance characteristics for the vari-line separators are the same as those
described for the straight pattern line separator.
Typical inlet and outlet connections are: side in–top out, side in–bottom out, top
in–side out, top in–bottom out, bottom in–side out, and bottom in–top out.
High-pressure separators. High-pressure separators are designed for pressures in
excess of 1500 lb. There is no upper limit on design pressures. Several separators
have been fabricated with design pressures in excess of 20,000 lb. These separators
contain the vane-type mist extractor and the internal design and principle of
operation is the same as discussed for the lower pressure units.
The pressure vessel housing for these separators can be fabricated using either
A105 Grade 2 forged steel or A216 WCB cast steel. Virtually all of the separators
now used in plant designs are fabricated of forged material.
These separators have application in any service where high-efficiency separation
of entrained liquids from a gas is required.
Horizontal gas separators
Principle of operation.

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

practically zero to maximum rated flow.
Snubbers (see Pulsation Dampeners)
Stacks
Stacks can be used to conduct gases to be flared. The lit gas flame can be seen from
the top of the stack in that case. This kind of stack may be called a flare stack.
Another kind of stack is a stack that exhausts the gaseous products of
combustion, including water vapor and carbon dioxide to the atmosphere. The most
severe application stresswise for a freestanding stack might be its use in an offshore
environment due to wind loading and additional stress due to wave and water
movement on a platform. Although this book is not intended to be a dedicated design
text, it is useful for a process engineer in operations and maintenance to understand
what to look for in a stack design. The following illustrates a stack design that helps
cope with these stresses in an offshore environment. Note, however, that the design
features presented apply to onshore designs as well.
The following material* describes the methods developed to optimize the
mechanical design of a freestanding exhaust stack and its supporting structure.
These particular methods have been used for the design of three gas turbine
exhaust systems on a UK sector offshore platform currently under construction.
The driving force behind the choice of a freestanding stack was to save weight and
therefore cost. The move toward the development of marginal fields in deeper
waters will only increase the need for lighter, and therefore more cost-effective
design solutions.
Stacks S-29
* Source: Altair Filters International Limited, UK. Adapted with permission.
Although national standards that cover the basic design philosophy are available,
these have serious limitations when applied to this type of structure. The aim of
this section is to demonstrate how the limitations may be overcome by undertaking
fundamental design analysis, and also to indicate those critical areas that demand
special consideration. The detailed design analysis presented here considers the
possibility of failure due to local instability, the effect on the dynamic response of

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

, R
B
and R
C
transverse support reactions, kN
V vertical reaction at main bearing, kN
t stack section material thickness, mm
s 0.1 percent proof strength of stack material, MN/m
a semiangle subtended by imperfection
n vortex shedding frequency, Hz
S vortex shedding coefficient
x distance along stack axis, m
v transverse displacement, mm
w
s
=rpDt
2
exhaust stack weight per unit length, kg/m
E elastic modulus of stack material, GN/m
second moment of area of stack section, m
4
k elastic stiffness of supporting foundation, kN/m
g acceleration due to gravity, ms
-2
r specific weight of stack material, kg m
-3
m circular frequency for transverse vibration, rad/s
fundamental frequency in Hz
u
o

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

chloride attack by the saliferous atmosphere.
From a mechanical viewpoint major problems are caused to a tall slender
structure by the flexibility of the platform. Consideration of its dynamic response
compared with the wind-induced excitation is therefore of paramount importance.
Thermal effects due to the hot exhaust gases are a further factor for consideration.
The design must consequently take account of thermal stresses at critical locations.
Choices of construction materials must be carefully considered. In this instance
stainless steel grade 316 L was chosen as the most suitable to meet all the project
requirements.
Choice of Design Philosophy
A typical design for an offshore gas turbine exhaust system is shown in Fig. S-36.
The exhaust ducting is surrounded on all sides by a substantial steel framework.
Upright structural members would typically be 254 mm ¥ 254 mm ¥ 73 kg/m
universal column and horizontal members 457 mm ¥ 191 mm ¥ 67 kg/m universal
beam. This structure would then be diagonally braced with 168 mm ¥ 8 kg/m
circular hollow section. A 2-m-diameter exhaust duct would require approximately
9 tonnes of such steelwork for every 5 m of stack height.
The steelwork would support the dead weight of the exhaust system and
withstand all dynamic loads due to wind. The mounting system connecting the duct
to the steelwork enables the loads to be transferred while allowing for thermal
expansion of the system during operation. Normally the fixed support would be near
the base of the stack and be capable of supporting the dead weight. Longitudinal
thermal growth would then be vertically upward, with a system of guides allowing
vertical movement while providing horizontal restraint.
The total exhaust stack length for the design in question is 24.4 m. The weight
of platform steelwork required to fully support such a system would consequently
be 45 tons. Recent North Sea developments have tended to be on more marginal
fields, and therefore consideration of capital costs versus revenue has become
crucial. With a typical installed platform cost of £3500 per ton of steelwork weight
saving now takes a high profile.

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

20 145,000 182,000 147,000 35
NOTES
1. The percentage saving relates to the total installed cost of a fully supported system. This is £425,000 made
up from a stack cost of £110,000 plus 90 tons of steel at £3,500 per ton.
2. The stack manufacturing cost includes fabrication and design by an acoustic equipment manufacturer.
3. All prices quoted are UK pound sterling, and based on 1990 rates.
Design Parameters
The following base design parameters are defined as an example:
Lower stack connection 59.1 m above sea level
Upper stack termination 83.5 m above sea level
Freestanding stack length 10 m
Turbine type Coberra 6462
Nominal power output 23 MW
Exhaust gas mass flow 90 kg/s
Power turbine gas exit temperature 475°C
Design wind speed 53 m/s
Material of construction Stainless steel AISI 316L
From the turbine exhaust flow and system pressure loss limitations the duct
internal diameter was sized at 2 m. Under maximum flow and temperature
conditions the mean gas velocity in this duct is 60 m/s. Previous work on free-
standing exhaust systems had highlighted the lack of a comprehensive standard
that covered the structural aspects of such a system. Further investigations for this
project confirmed this situation. The most applicable standard is BS4076 (1978).
This specification offers guidelines for the design of freestanding chimneys, but has
definite limitations. The most serious of these with respect to this design is that it
makes no allowance for nonrigid foundations. The empirical formulae used are also
based entirely on using carbon steel as the construction material. It is not clear
what modifications would be needed to make these formulae applicable to stainless
steels or other metallic alloys.
A further consideration highlighted by this standard is the warning on the

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

Figure S-39 shows the bending moment diagram for a stack with a free length of
10 m.
The maximum bending moment of 182 kN occurs at the main support where the
transverse reaction R
C
= 86 kN. Introducing a typical value k
v
= 2 MNm
-1
for the
vertical stiffness of the supporting steelwork at the main bearing, it is interesting
to find that the corresponding vertical force (V = 1 kN) is low when compared
with the self-weight of the exhaust stack when the design wind load is applied.
Consequently this element can be ignored during subsequent analysis.
K
LL L
kD
EI
LLLLL
LL
kD
EI
LL LL
v
v
4
43 4
1
2
3

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

()

()
{}
K
LL
kD
EI
LL
LL
kD
EI
LL LL
v
v
3
32
1
2
22
2
31
1
2
31
2
21
2
4
12

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

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

+
()
-+
()
+
()
-
()

()
{}
K
LL
LLLL LL
1
32
3
3131
2
21
2
=
-
()
-
()
-
()

()

{}

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

KK
KK
wRwLRR
A
B
CA
B
=
-
-
=
-
-
=
23 14
31
42
31
4
S-34 Stacks
FIG. S-38 Stack idealization for steady wind load. (Source: Altair Filters International Limited.)
The loading conditions have been determined in accordance with the
recommendations of BSI CP3, Chapter V, Part 2 (1972), which is appropriate to the
proposed location in the North Sea. For locations in the vicinity of North America
it might be more appropriate to use the equivalent American National Standard
(ANSI A58.1 1982).
Effects of Gust Loading
In addition to steady drag forces the exhaust stack is required to sustain the effects
of gust loading. Appendix B of BS (a British standard) 4076 (1978) highlights
procedures for avoiding aerodynamic excitation. In particular, the vortex shedding

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
=

Stacks S-35
FIG. S-39 External reactions and bending moment diagram for exhaust stack subjected to design
wind load. (Source: Altair Filters International Limited.)
been modeled as a uniform beam mounted on pinned supports as shown in Fig. S-
40a. This simplification has an immediate advantage in that the support reactions
are statically determinate with values given by:
Calculation of the fundamental frequency for transverse vibration of the beam
system has been undertaken using Rayleigh’s method, in which the maximum
kinetic energy of the vibrating system is equated to the maximum bending strain
energy. It is necessary to select realistic displacement functions that satisfy the
boundary conditions of the problem. The supporting structure is flexible. This will
affect the transverse displacement, and hence the natural frequency of the vibrating
stack.
Suitable displacement functions for a foundation with stiffness k are illustrated
in Fig. S-40. For vibration with a circular frequency m, these may be written
In section AB
where
and in section BC
where
Equating the maximum values of the kinetic and bending strain energies, putting
w
s
=rpDt and , the fundamental frequency for transverse vibration is
obtained as:
f
ED g
L
X
Y
=

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

==
Ê
Ë
ˆ
¯
+
-
=
Ê
Ë
ˆ
¯
-
+-
Ê
Ë
ˆ
¯
43
2
34

v
w
EI
xLxLxALxBL mt
o
s
=-+++
()

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
=+++

()
24
2
4334
sin
R
w
KL R
w
KL
LL
LL
K
L
L
A
oBo
o
oo
==
+
-
=-
Ê
Ë
ˆ
¯
22
1
2

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
=◊

+
Ê
Ë
ˆ
¯
-+
()
+++
Ê
Ë
ˆ
¯
-+ +
-
+
()
1
2
36
1
1
6
615 10
104
45
26
15
1
3
1

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
=+ + + ++

+
Ê
Ë
ˆ
¯
-+ +
-
+
-
+++
{}
104
45
26
15
12
5
1
3
1
9
1
2
4
7
512
15
2
5
1

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

-
Ê
Ë
ˆ
¯
È
Î
Í
˘
˚
˙
1
1
6
615 10
5
12
1
5
2
5
2
Stacks S-37
FIG. S-41 Variation in transverse vibration frequency with free length of exhaust stack. (Source:
Altair Filters International Limited.)
FIG. S-42 Effect of foundation flexibility in reducing natural frequency of transverse vibration.
(Source: Altair Filters International Limited.)
of the freestanding stack with a height of 20 m is too low for this configuration to
be an acceptable design solution. The natural frequency of a stack with a free length
of 10 m, mounted by pinned supports on a rigid foundation, is calculated to be

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

of configuration, material type, thickness, and support arrangement.
Steam Generator and Steam Supply
A steam generator consists of a boiler (see Some Commonly Used Specifications,
Codes, Standards, and Texts, at the back of the book, for boiler specifications) its
fuel system, and all controls and accessories. Steam supply is now a sophisticated
science in itself, especially with supercritical steam now increasingly used to boost
efficiency on steam-turbine cycles and other operations. Service factors on steam
valves, lines, and other accessories are consequently more severe.
S-38 Steam Generator and Steam Supply
T
Tanks
Tanks are used for storage in process plants and refineries. Tanks for
petrochemicals, oil, and other potentially explosive products are highly specialized.
A more common kind of tank is used for bulk dry storage of products in agriculture
and a variety of other process industries. A variety of designs exist. The principle
behind one of these types (trade name TecTank SealWeld) follows.
General Storage Tanks*
TecTank SealWeld tanks (see Fig. T-1) from Engineered Storage Products Company
are designed for simple precision assembly. The critical flange-to-shell fabrication
process incorporates positioning clamps and automated welding. Dual weld seams
are used to increase the joint strength and to avoid warpage and distortion. During
jobsite panel fit-up, flange connections are secured with structural bolts, not rivets.
(See Fig. T-2.)
The interior welding of the assembled silo can proceed without the problems that
plague traditional field-welded silos. Location costs are reduced, and the welding
can be accomplished under controlled conditions. The welded interior of the product
zone is free of ledges and gaskets.
Carefully controlled tank coating processes ensure the product within the tanks
is not contaminated. Some examples of products stored are:
ABS Pellets

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