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Optimum Design of Cryogenic Pipe Supports
Hyun-Joo, Chang
General Manager
Seung-Nam, Shin
Piping Stress Engineer
Hyundai Engineering Co., Ltd
Abstract
Cold insulation pipe supports have been widely used in a number of chemical
plants including LNG receiving terminal. This paper presents a theoretical and
practical study of optimum design of cryogenic pipe supports required to
design LNG receiving terminal. A solution for optimum design of cryogenic pipe
supports is obtained and practical results are presented.
It is shown that when we design cryogenic pipe supports, we have to consider
structural characteristics, design load, requirement from the owner and
economic aspect for each type of supports such as shoe, guide, stop and
trunnion. So, it is very important to clarify the behavior of cryogenic piping
system including pipe support during normal operation of LNG receiving
terminal. For this purpose, not only theoretical but also practical approaches
have been used to clarify the behavior of cryogenic piping system during
normal operation and initial start-up.
This design of cryogenic pipe supports has been validated by comparison with
other type of cryogenic pipe supports, and confirmed by applying to Inchon
LNG receiving terminal. It is noted that this design is efficient and applicable to
future LNG receiving terminal project.
The following issues are presented in this paper.
z Behavior of cryogenic piping system during initial start-up
z Behavior of cryogenic piping system during normal operation
z Characteristics of cryogenic piping system and pipe supports
z Requirements for cryogenic pipe supports
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z Optimization of cryogenic pipe supports
z Comparison with other type of cryogenic pipe supports
z Confirmation of cryogenic pipe supports
1. Introduction
LNG (Liquefied Natural Gas) has been widely used as a clean energy
nowadays, and there are so many large LNG receiving terminals under
construction accordingly. Among these large LNG receiving terminals, Inchon
LNG terminal in Korea is one of the largest LNG receiving terminals. We,
Hyundai Engineering Company, participated in design of Inchon LNG terminal
over 10 years. We have much experience in designing cryogenic piping, and
we would like to share this experience on this subject.
As a matter of fact, since the boiling point of LNG is such a low temperature,
what is so called cryogenic, as under -162 that extremely superior insulationproperty, durability and also stable function are required for supporting devices
such as shoe, stop, and anchor to be used at LNG receiving terminal. The
problems encountered in cryogenic piping system are as follows;
embrittlement of materials, icing around/between the cryogenic pipe support,
pipe insulation and steelwork, large displacements (due to the thermal
expansion and contraction), rapid change of phase due to large heat fluxes
(big delta T), and small latent heats of the fluids involved. Thus, extremely high
reliability is required to design cryogenic pipe support system.
From the general point of view, supports must be designed to meet all static as
well as dynamic operational conditions to which the piping may be subjected.
The support system must provide for and control, subject to the requirements
of the piping configuration, the movement due to the thermal expansion and
contraction of the piping and the connected equipment. Furthermore, the
correct and economical selection of the pipe supports for cryogenic piping
system usually presents difficulties of varying degree, some relatively minor
and others of a more critical nature. Proper selection of cryogenic pipe support
should be the objective of this paper. A good pipe support design begins with
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good piping design and layout. That means many pipe support problems may
be minimized or avoided if proper attention is given to the means of support
during the piping layout design phase. Therefore, behavior and requirements
of cryogenic piping system during normal operation and initial start-up are
presented here. This paper also provides guidelines for the design and layout
of cryogenic piping and pipe supports found in LNG receiving terminal and
related processing plant.
2. Features of Cryogenic Piping System
Heat is continuously entering the piping through the insulation and supports.
This heat will make the liquid contents boil. For this reason heat leak must be
minimized. From an economic point of view, the thermal efficiency of the piping
system must be carefully considered since the heat addition to the system will
ordinarily result in loss of product. So there must be the need for
understanding cryogenic piping system.
In order to obtain a better appreciation of the special consideration involved in
cryogenic pipe support system application, it was felt that it would be
necessary to review the behavior of materials at cryogenic temperature and
the physical and thermodynamic property of cryogenic piping and pipe support
system. These considerations are presented in this section.
2.1 Materials used in Cryogenic Piping Systems
Important consideration in the selection of materials for cryogenic piping
systems include suitable mechanical and physical properties, compatibility with
process fluids, fabricability, cost, and compliance with regulatory codes such
as ASME B31.3. It is recognized that certain materials tend to become brittle at
low temperature and maybe subject to failure which would not usually occur at
normal temperature or at elevated temperature. The transition temperature at
which certain materials become brittle is not well defined. Some ferrous
materials may pass through the transition range at normal temperature, while
others may not become brittle until it reaches low temperatures. Because of
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embrittlement of materials, carbon steel can not be used for cryogenic piping
systems. Therefore, we have to use ferrous alloys.
Table 1 - Typical Ferrous Alloys used in Cryogenic piping
AlloyMinimum
Temperature
ASME
DesignationRemark
C-Mn steel -46 A 333 Gr.1
2 1/4% Ni steel -73 A 333 Gr.7
3 1/2% Ni steel -101 A 333 Gr.3
9% Ni steel -196 A 333 Gr.8
304 Stainless
steel -254
A312
304L Stainless
steel-254 A312
316 Stainless
steel-196 A312
316L Stainless
steel-196 A312
347 Stainless
steel-254 A312
Ferrous alloys most often encountered in cryogenic piping applications are
usually classified as ferritic or austenitic types. (Please refer to Table 1.) The
terms austenitic and ferritic refer to the predominant crystallographic phases
ferrite or austenitic, which are body centered cubic (BCC) and face centered
cubic (FCC), respectively. Most of the austenitic alloy steels used in cryogenic
piping are chromium-nickel stainless steels of the AISI 300 type, such as 304,304L, 316, and 316L. Other stainless steels classified as martensitic, duplex,
and precipitation hardening also exists; however, the preceding alloys are most
commonly used in cryogenic piping for LNG receiving terminal and distribution
applications. Of the 300 Series alloys, the AISI 304 composition is the most
popular as measure by tonnage.
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As can be seen in Table 2, thermal expansion for austenitic alloy steels used in
cryogenic piping is much larger than that of carbon steel. This large thermal
expansion makes large displacements (expansion and contraction) of material.
This makes it more difficult to design cryogenic piping system than to design
hot insulated piping system.
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Table 2 - Typical mechanical and Physical properties of Ferritic Alloys used in Cryog
Alloy
ASME
spec.
Temp.
( )
Ultimate Tensile
Strength (MPa)
0.2% Offset
yield strength
(MPa)
% Elong. in
5.1cm (%)
Charpy Impact
Strength (Joules)
The
(m
C-Mn steel A 333
Grade 1
RT
-46379 207 21
95
68
2 /4% Ni steel A 333
Grade 7
RT
-73
448
517
241
27618
79
27
31/2% Ni steel A 333
Grade 3
RT
-101
689
11379
517
58618
130
30
9% Ni steel A 333
Grade 8
RT
-196
793
1172
621
931
25
27
64
34
304Stainless steel
A 312TP304
RT-254
5861724
262483
453
156102
304L
Stainless steel
A 312
TP304L
RT
-254
552
1551
255
469
45
31
81
81
316
Stainless steel
A 312
TP316
RT
-198
600
1358
262
448
45
56
-
-
316L
Stainless steel
A 312
TP316L
RT
-196586 262 45 -
347
Stainless steel
A 312
TP347
RT
-254
621
1586
469
483
50
38
81
61
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2.2 Insulation for Piping System
Most piping in liquid cryogenic service is insulated. The reasons a line would
not be insulated are that (1) its use is very infrequent and brief; (2) it is a
temporary installation;or (3) the refrigeration losses are inconsequential.
The type of insulation used for cryogenic piping includes (1) expanded foams
such as polyurethane and foamglass, (2) powder insulations such as perlite,
and (3) vacuum-insulated pipe. For an insulation system to remain effective,
the vapor barrier system must keep atmospheric moisture from entering the
insulation space and freezing against the cryogenic line. When this occurs, the
ice that is formed will degrade or destroy the insulation system.
When the cryogenic liquid is colder than the boiling point of oxygen (-297 or
-183 ), oxygen can condensate out of the air and collect in the insulation
space. For this situation, the insulation system should be noncombustible in
the presence of oxygen. Heat leak by conduction and radiation is reduced by
the laminar radiation shielding. The heat leak by convection is reduced by the
vacuum.
When cold insulation is required, the entire system shall be fully insulated,
including all piping components, piping/tubing of insulated instruments, drains,
equipment nozzles and supports. And all metal parts which protrude through
the insulation shall be insulated.
The typical values for thermal conductivity are shown in Table 3. The expanded
foam insulation uses a covering to provide the vapour barrier protection. The
initial capital cost is usually lower than the other system, but more frequent
maintenance is required to maintain a tight vapour barrier.
Table 3 - Thermal Conductivity of Pipe Insulation Materials at Insulation Mean
Temperature of -100
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InsulationThermal conductivity
[Btu/(hft 2)]
Thermal conductivity
W/(mK)
Urethane Foam 0.012 .021
Foamglass 0.024 .042
Perlite (at atmospheric
pressure)0.018 .031
Perlite (vacuum at 1m) 7.9 x 10 4 1.37 x 10 3
Laminar radiation
shielding
(vacuum at 1m)
2.1 x 10 5 3.63 x 10 5
2.3 Flexibility Analysis for Cryogenic Piping System
Piping flexibility analysis is an important design consideration because the
large difference between ambient and cryogenic temperatures will result in
significant thermal contraction. Moreover this piping flexibility analysis should
be carried out before cryogenic pipe support design. When the amount of pipe
movement exceeds the capacity of a pipe support system, a fixed support and
more expansion loops should be designed in order to reduce the amount of
pipe movement.
The flexibility analysis of the cryogenic piping must consider the full
temperature range as well as any other conditions with severe temperature
difference which may occur during upset, thaw, or cool-down. And cryogenic
pipe support must be designed accordingly.
The analysis methods used are similar to those required for conventional
piping system. The one difference is that piping in cryogenic services contracts
rather than expands as it is the case with high temperature services. However,
since the analyst can calculate the resulting contraction, the analysis method
becomes identical to those used for conventional piping systems.
For safe design, flexibility analysis for cryogenic piping system is usually
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carried out to meet the engineering requirements of ASME B31.3, Process
Piping Code.
2.4 Requirements for Cryogenic Pipe Supports
When an un-insulated cryogenic piping is supported, a portion of the pipe
support will be at cryogenic temperature. Low temperature should be
considered when selecting the materials for the pipe support and its hardware.
For low temperature service, in addition to heat loss and gain, the problem of
atmospheric condensation must be considered, and such lines are usually
insulated with a material that has an outer covering or seal called a vapour
barrier. This barrier prevents the insulation from absorbing moisture. For this
reason it is not permissible to penetrate the insulation with load-carrying
members such as the legs of a conventional high-temperature shoe/saddle or
a pipe clamp. Since most low-temperature insulation has low compressive
strength, it is necessary to provide shields to the line the piping insulation and
to spread out the bearing area sufficiently to prevent crushing of the insulation.
Such shields should fit the outer diameter of the insulation and cover 180
degree of arc.
For cryogenic piping system, pipe support must be outside the insulation,
withstand loads from the insulation material, must be ductile at cryogenic
temperature, and has a relatively low thermal conductivity. And the vapour
barrier must be left undisturbed. Therefore, cryogenic pipe supports shall meet
the following requirements as a minimum.
a. Supports shall be lighter in weight when compared with wooden block.
b. High reliability in water and resistance to oil and corrosion Supports shall
not need and preservative treatment such as creosote impregnation.
c. High weather tightness Supports resist weathering and corrosion in long
term outdoor use.
d. Supports shall exceed in physical strength against compression, bending
and shearing.
e. Supports shall be suitable for mass production.
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f. Forming incorporated with other material shall be possible.
g. Free of grains, homogeneous and standard quality cradles shall be
obtained in large quantity at the same time.
h. Low water absorption Supports shall not incur cracks from icing during
storage or operation.
i. Heat and Flame resistance Flammability of material shall be
self-extinguished in accordance with ASTM D1692.
2.5 Consideration of Cryogenic Pipe Supports
High density cradle type of cryogenic pipe supports shall incorporate a molded
heavy density layer bonded with a stainless steel weather shield and
assembled with a steel cradle. The high density layers shall be stepped and,
together with the metal jacketing, sufficiently extended to facilitate
incorporation within the adjacent insulation system. All Joints between
supports and insulation shall be tightly fitted together and staggered with as
few voids as possible in order to avoid icing due to heat leakage.
Cryogenic pipe supports shall meet the design requirements in respect of
compressive strength under sustained load, thermal conductivity, coefficient of
friction, service temperature and flammability.
3. Optimization of Cryogenic Pipe Support
As reviewed in the previous section, an extremely high degree of reliability is
required in recent days in the field of pipe supporting system design such as
LNG receiving terminal.
Conventionally, wooden heat insulators have been used for piping system
supports in these plants. However, these materials involve difficulties of
availability and unstable quality. Furthermore, this material is very heavy and
expensive. And often delivery is very long. Therefore, this kind of wooden
block can not meet the requirements mentioned above. So we have to find and
develop a better one. Urethane block made of high density polyurethane foam
which has low thermal conductivity is a better cryogenic pipe support among
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various kinds of materials. They have been already used, and well received in
various plants. And we are going to present optimized cryogenic pipe supports
made of high density polyurethane foam.
3.1 Polyurethane Cradle Supports
Shoe type of support mainly consists of polyurethane cradle and a steel
load-bearing plate. It is used for sliding supports, guide supports, hanger
supports, stanchion, trunnion and etc to avoid the condensate and formation of
ice, around each support, which would restrict free movement of the piping.
Additionally, under certain thermal conditions, direct contact between the pipe
and the structure could produce local brittleness of the structure itself.
Figure 1 shows typical cryogenic pipe support detail drawing, where B is cold
insulation thickness.
Fig. 1 Cryogenic Pipe Support Detail Drawing
Cradles shall be high density polyurethane foam which shall possess a unique
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cellular structure. And each cold insulated pipe supports shall have a vapour
barrier. Easy assembling and finishing polyurethane cradle to the pipe line is
also required. Design strength shall be based on ultimate compressive
strength with a minimum safety factor of 5, or that which results in a 1%
deflection, whichever is less, and shall have the following properties;
a. Polyurethane foam shall satisfy the flame spread requirements of UL94.
The minimum percentage of weight retention of the foam when tested in
accordance with ASTM D3014 shall be 75%.
b. Average density of PUF cradle shall be verified by dividing the weight of
the cradle by its volume. Average density shall be within 5% of the
specified density, for both 224kg/m3 and for 320kg/m3 PUF cradles.
Average density for 160kg/m3 shall be within -0% and +10%.
c. Minimum value for the ultimate compressive strength for samples taken
from the core i.e., within the middle 60% of the thickness for all densities
shall be within 10% of the specified values.
d. The thermal conductivity of the polyurethane foam at -160 , in accordance
with ASTM C177, shall be within +/-5% of the values specified in Table 4.
Samples shall be taken from the core within the middle 60% of thickness,
where it is practical.
Table 4. Mechanical Characteristics of High density Polyurethane
Pipe SizeCore
Density
Stress at 1%
deflection
Minimum
Compressive
Strength
Thermal
Conductivity
(W/mk)
1/2 to 8 160 kg/m3 3.2 kg/cm2 18.5 kg/cm2 0.022
10 to 72 320 kg/m3 12.8 kg/cm2 70.4 kg/cm2 0.032
- Finish
Cradle ; Protective coating
Bearing Plate and Shoe ; Painted after pickling or hot dip galvanized
Masking ; The bore of the cradle is completely covered with masking tape
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Bonding ; The bearing plate is bonded to the cradle at the shop and the
cradle is bonded to the pipe by field fabricator.
- Service Temperature Limit ; -196 to 80
- Size Range ; 1/2 through 72 pipe size
Pipe support type varies in accordance with insulation thickness. Figure 2
shows type selection for pipe according to insulation thickness which has been
adopted for Inchon LNG receiving terminal in Korea.
Fig. 2 Type Selection for Pipe and Insulation Thickness
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3.2 Bearing Plate
The material for the bearing plate which prevents crushing of the insulation
shall be carbon steel (ASTM A36 or equivalent) fully killed open-hearth, electric
furnace, or basic-oxygen steels. Steel band strapping seals are to be pusher
type seal.
3.3 Adhesive, Protect ive Coating and Seal
The adhesive shall be applied to a thickness of 0.015inch (0.38mm) when
Fosters 81-84 is used. Sufficient adhesive shall be used to fill any gaps or
voids in the surfaces to be bonded. The bond adhesive shall be allowed to cure
overnight at room temperature. If the adhesive material recommended by the
PUF manufacturer is other than the specified one, the substituted adhesive
material and applied thickness must be properly tested prior to being used. All
surfaces of the polyurethane which requires adhesive bonding, protective
coating of seal shall provide an appropriate anchor profile. Any waxy, smooth
surfaces such as mold release film must be removed prior to the application of
adhesive or protective coating.
a. Adhesive
The polyurethane cradles shall be bonded to the bearing plate/bearing plate
assemblies by the polyurethane foam (PUF) manufacturer. Multilayer cradles
are also bonded together by the polyurethane foam (PUF) manufacturer. The
adhesive for the above bonding is normally Fosters 81-84, manufactured by
the Foster Products Division of the H.B. Fuller Co.
b. Protective Coating
Monolar mastic 60-91 (gray) adhesive/coating available from the Foster
products Division of H.B. Fuller Co. and H.B. Fuller licensees to be applied to a
dry thickness of 0.034 inch (0.86mm). The manufacturer shall supply
approximately 10% of the quantity of protective coating used in the shop
fabrication of cold insulated pipe shoe for field repair of minor breaks in the
protective seal.
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c. Seal
The interface joints between the cradle and bearing plate shall be completely
sealed with Butyl rubber sealant, to prevent water ingress. Sealant is normally
Childers CP-76, Childers Products Company, Fosters 95044 (Fuller Company,
Foster Products Division) or equivalent.
The interface surfaces between upper and lower cradles shall be completely
sealed with Childers CP-76, Foster 95-44 or an equal sealant.
d. Masking Tape
The inside radius surfaces of the cradle shall be completely covered with
masking tape.
3.4 Beam Width and Allowable Moving
The anticipated movement at each support point dictates the basic type of
support required. Each type of support selected must be capable of
accommodating movements obtained by piping flexibility analysis. Both
longitudinal and horizontal movement must be evaluated.
Because of large displacements (expansion and contraction) of material used
for cryogenic piping system, displacement control becomes very important.
These displacements due to thermal contraction can be predicted by piping
flexibility analysis. For this reason supporting one line from another is
forbidden for cryogenic piping. The Figure 3 shows recommended beam width
and its allowable moving, which has been adopted for Inchon LNG receiving
terminal in Korea. Therefore, detail design should be applied in consideration
of pipe temperature under contraction and distance from anchor point. And
special length support is available upon request for need or more allowable
moving.
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Fig. 3 Beam Width and Allowable Movement
In addition to displacement control, the cryogenic pipe supports has to slide
smoothly in order to avoid icing around/between the cryogenic pipe support
and pipe insulation. Thus PTFE sliding plate shall be used to minimize
horizontal forces caused by frictional resistance for cryogenic piping system.
3.5 Field installation Check Point
Based on the experience, we have the field installation check point as follows;
a. As soon as the package is opened, check the support assembly if there is
any damage. And if the damage is small such as coming off of coating, the
damage should be repaired at the field.
b. Clean the surface of pipe to remove all the foreign objectives adhered
such as rust, vapour, oil, dust and etc.
c. As the supports are installed at the center of existing beam or at off-set
position depending on the requirement, the installation position shall be
determined and clearly marked.
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d. Remove the masking tape which is adhered on the cradle bore.
e. Apply the adhesive uniformly on the cradle bore, then, press the support
assembly against the pipe and fasten firmly together by using the steel
band until the adhesive harden.
f. The time to release the steel band is depended on the open air
temperature. When the temperature is over 18 , the steel band may be
released after 12 hour duration.
g. Touch up the portion with protective coating agent where the protective
coating is come off.
3.6 Thermal Bowing owing to Two Phase Flow
Consideration of the cryogenic fluid properties has an effect on the piping
arrangement. Because the cryogenic fluid is colder than ambient air, the
continuous heat leak from ambient air to the piping system is a design
consideration. Because of rapid change of phase due to large heat fluxes
caused by this kind of heat leakage, there is the temperature difference
between top and bottom of the pipe cross section and two phase flow. The
effect of two phase flow is much more complicated than that of single phase
flow. This is attributed to the fluctuations of flow rate, density and pressure
gradients, as well as oscillations due to compressibility of the partial gas fluid.
This continuous heat leakage also causes thermal bowing, which should be
avoided.
When a cryogenic liquid line is initially put in service, the warm piping will
cause liquid flash-off, which could restrict the flow during the two-phase flow
transient period. When it is possible to pre-cool the lines, the piping can be
sized for liquid phase flow, which will result in small piping. If rapid cool-down is
required, the piping must be sized for two-phase flow. This rapid cool-down
also causes thermal bowing. Undesirable heat transfer and heat loss is
therefore reduced.
Considering unexpected thermal bowing and fluctuations of flow rate, pipe
support span for cryogenic piping shall be much shorter than that of
hot-insulated piping. When practical, a support should be located immediately
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adjacent to any change in direction of the piping.
4. Conclus ions
This paper has shown in such a way as to ensure proper support under all
operating and environmental conditions and to provide for expansion /
contraction, PTFE sliding plate, thermal bowing, and insulation protection for
cryogenic piping system.
In conclusion, it appears that the following points represent a reasonable point
of cryogenic pipe support design from the theoretical and practical study and
by applying to Inchon LNG receiving terminal
1. Cryogenic pipe supports shall be designed to minimize thermal
conduction which could adversely affect the fluid in the pipe and/or the
surrounding structure.
2. Cryogenic supports shall be designed taking into account warm-up and
cool-down conditions. So piping flexibility analysis is necessary before
cryogenic pipe support design. Adequate systems shall be used in order
not to induce additional stresses on insulation material.
3. At support location, insulation material shall be high density foam
(160kg/m3 or higher), and a maximum deflection of 1% on insulation
cradle shall be respected.
4. Because of large displacements (expansion and contraction) of material
used for cryogenic piping system, supports selected must be capable of
accommodating movements.
5. PTFE sliding plate shall be used to minimize horizontal forces caused by
frictional resistance for cryogenic piping system.
6. Considering unexpected thermal bowing and fluctuations of flow rate,
pipe support span for cryogenic piping shall be much shorter than that of
hot-insulated piping.
All of the foregoing topics are very important and must be studied to design
cryogenic piping system from the support point of view and to provide a
general understanding and the basis for cryogenic pipe support design guide.
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Further work on this topic includes cryogenic pipe support subject to surge
force and steady state vibration like pulsation.
REFERENCE
1. Paul R. Smith and Thomas J. Van Laan ; Piping and Pipe Support
Systems, Design and Engineering, McGraw-Hill Book Company
2. Piping Design and Engineering, ITT Grinnell Industrial Piping, Inc.
3. Ernest Holmes ; Handbook of Industrial Pipework Engineering,
McGraw-Hill Book Company
4. MSS SP-58, Materials and Design of Pipe Supports
5. MSS SP-69, Selection and Application of Pipe Supports
6. MSS SP-89, Fabrication and Installation of pipe Supports
7. BS 3974, Specification for Pipe Supports, Part 1, 2 and 3
8. ASME B31.3, Process Piping
9. M. W. Kellogg, Pipe Support Components and Fabricated Assemblies
10. N.H.K Spring Co., Ltd, Inspection Report for Cryogenic Pipe Support,
M.W. Kellogg Type
11. Mohinder L. Nayyar ; Piping Handbook, McGraw-Hill Book Company