
Piping Stress Analysis –
Where do I start?
by Reid McNally, Jr. P.E.
The
following information will take you stepbystep through
the logic of the data collection effort that should occur
prior to beginning to model a piping system for a stress
analysis:
 First
of all, prior to starting to build a piping model it is
imperative to sort out what you wish to achieve in any
analysis. The following questions may assist you in determining
the reasoning for conducting a piping stress analysis:
a)
Are you interested in performing a piping
stress analysis to evaluate the stresses in a specific
piping system and to determine if these stresses are
within the range allowed by the Piping Code?
b)
Are you interested in performing a piping
stress analysis to evaluate the loads on a piece of
rotating equipment?
c)
Are you interested in performing a piping
stress analysis to evaluate the loads on a heat exchanger,
pressure vessel or tank nozzle?
d)
Are you interested in performing a piping
stress analysis to evaluate the loads on one or structural
anchors?
e)
Are you interested in performing a piping
stress analysis to evaluate the loads on one or more
pipe supports?
f)
Are you interested in performing a piping
stress analysis to evaluate the movements of portions
of the piping system due to thermal growth or contraction?
g)
Are you interested in performing a piping
stress analysis to evaluate the effects of wind loads
on the piping system and/or attached equipment?
h)
Are you interested in performing a piping
stress analysis to evaluate the effects of earthquake
loads on the piping system and/or attached equipment?
i)
Are you interested in performing a piping
stress analysis to evaluate the effects of wave loading
on the piping system and/or attached equipment?
j)
Are you interested in performing a piping
stress analysis to evaluate the effects of soil resistance
to movement for underground or buried piping system
and/or any attached equipment?
k)
Are you interested in performing a piping
stress analysis to evaluate the effects of changes
in temperature, pressure and weight on flanged couplings
and to determine if there is a tendency for the connections
to leak?
Once
these questions have been answered, then check each of the
following steps.
 Determine
which piping code will govern the design of the piping
system.
 Collect
all the plan and elevation drawings necessary to fully
document the piping routing.
 Obtain
or construct an isometric drawing of the entire piping
system. If you have several piping isometrics documenting
different parts of the piping system, make sure that the
North arrow orientation is the same on all such isometrics.
If they are different, redraw those piping isometrics
that are necessary to have all North arrow orientations
the same on all isometrics.
 Collect
all the necessary physical properties for all of the piping
components in the piping system as follows:
a)
Nominal Pipe Diameter or Actual Outside Diameter,
if the Pipe is NonStandard.
b)
Pipe Schedule or Pipe Wall Thickness.
c)
Corrosion Allowance.
d)
The Specific Gravity of the contents of the
pipe or the Weight per unit length of the contents.
e)
The Insulation Material or Insulation Density
and Thickness or the Insulation weight per linear
unit length.
f)
Piping Material or piping material density,
modulus of elasticity and coefficient of expansion.
g)
Operating Temperature (Minimum and maximum,
if applicable), Design Temperature, Upset Condition
Temperature and Base or Ambient Temperature.
h)
Operating Pressure (Internal or External),
Design Pressure and Upset Condition Pressure.
i)
Flange Rating and Flange Type or Flange Weight
and Length.
j)
Valve Type (Gate, Globe, Butterfly, etc.)
Rating or Valve Weight and Length.
k)
Elbows and/or Bends Radius or Bend Radius
Ratio, Fitting Thickness and the number of miter points,
if applicable.
l)
Reducer length, inlet and outlet diameters,
schedule or wall thickness, concentric or eccentric
and, if eccentric, the flat side orientation.
m)
Branch Connections  welding tee, weldin
contour insert, weldon fitting, fabricated tee with
the reinforcing pad thickness, extruded tee with the
crotch radius or lateral fitting data.
n)
Expansion Joint Properties – Translational
Spring Constants in force/unit length of travel –
Axial and Lateral or Shear and Rotational Spring Constants
in moment/degree of rotation – About the axis of the
expansion joint (normally considered to be totally
rigid) and about the radial axes. The length of the
bellows component is needed and in the event that
the expansion joints are not oriented along one of
the axes of the X, Y, Z axis system, the angles required
to define the skewed orientation will also be required.
Further information is required. The length between
tie rods is necessary as well as whether or not nuts
are on the tie rods to restrict extension as well
as compression in the expansion joint. The pressure
thrust area is required in the event that tie rods
do not restrain axial movements. If an expansion
joint is hinged or gimbaled, then the orientation
of the hinge or gimbal axes is required.
o)
Structural Members – Any structural member that
is welded or bolted to the piping system and is expected
to act as part of the piping system must be defined.
If the structural member is a standard structural shape,
then the designation is required along with the orientation
with regards to the X, Y, Z axis system. If the structural
member is not a standard structural shape, then the
moments of inertia about each axis is required along
with the polar moment of inertia, the cross sectional
area, and the distance from the member centerline to
the outer surface. If the structural member is skewed,
then the orientation with regards to the X, Y, Z axis
system is also required.
 For
all Anchors, the following data is required. The location
of the anchor point in the piping system. A complete
definition of the equipment or structure to which the
piping system is connected. If a small piping system
is connected to a strong beam, column or anchor block,
then the anchor can be considered to be rigid. If a large
pipe, say 24”, is anchored to an 8x13 beam, relatively
flexible, then the anchor should be defined as a flexible
anchor and the flexibilities of the structural member
should be calculated and entered. If the anchor point
is a nozzle on a pressure vessel, tank or heat exchanger,
the flexibility of the nozzle may need to be entered should
the stress level in the piping system be computed to be
too high. And the stress level in the nozzle to shell
connection in the pressure vessel, tank or heat exchanger
may need to be evaluated for an over stressed condition.
In addition, equipment operating at a specific temperature
will expand or contract from the ambient conditions.
Therefore, the drawings for all connected equipment should
be obtained. The lengths from the actual anchor point
on the connected equipment are required as well as the
temperature of the connected equipment. If the connected
equipment is a piece of rotating equipment, the nozzles
are considered to be completely rigid, but the casing
will expand or contract from the ambient conditions to
the operating conditions.
 For
all restraints, the following data is required. The location
of each restraint acting on the piping system must be
defined as well as the specifics as to how each restraint
affects the piping system. The following discussion covers
restraints acting along one of the X, Y, Z axes. If skewed
restraints are in the piping system, then their orientation
with respect to the X, Y, Z axis system must be defined.
a)
Translational Restraints  First of all, the
axis along which or about which the restraint acts
must be defined. If the restraint restricts movement
along an axis (a translational restraint), then you
must be able to define if the restraint acts in one
direction along the axis or if it works in both directions
along the axis and obviously if in only one direction,
which one it is.
b)
Limit Stops  If the restraint allows a certain
amount of movement and then restrains the pipe, this
is known as a limit stop. For limit stops, the action
axis must be defined as well as how much movement
is to be allowed in the plus and minus directions
along the specified axis. Further, when the limit
is encountered, the stiffness of the resistance must
be defined. Normally, a limit stop allows movement
to a point and then stops the piping from going any
further. In some cases, when a limit stop is encountered,
the resistance to further movement is defined by a
spring constant.
c)
Imposed Movements – If a movement is to be
imposed on the piping system, the amount of the movement
and the direction of the movement must be defined.
d)
Imposed Forces – If a force is to be imposed
on the piping system, the amount of the force and
the direction of action of the force must be defined.
In addition, when the force is imposed, the force
may have a spring constant associated with it. In
other words, if a force is applied to the piping system
and the force changes as piping system movement occurs,
then the change in the force per unit of movement
(spring constant) must be defined.
e)
Dampers – In the event that a restraint acting
on a piping system allows gradual movements but resist
impulse movements, this is commonly referred to as
a damper or a snubber. In the event that dampers
or snubbers are included in a piping system, the axis
of action must be defined as well as the maximum load
that can be resisted.
f)
Frictional Resistance to Movement – When frictional
resistance can significantly influence the results
of a piping stress analysis, it should be considered.
The plane in which frictional resistance acts as well
as the dynamic and static coefficients of friction
should be defined.
g)
Existing Spring Hangers – When spring hangers
have been installed in a piping system and a new piping
stress analysis study is to be processed, the spring
constant of the spring hanger must be known as well
as the installed load, the operating load and the
minimum and maximum loads that the spring hanger will
successfully handle. It is also necessary to know
if the spring hanger is attached from above and if
lateral movement is allowed by the support rod(s)
and to what limit, if any. If the spring hanger is
actually a support, then the limit of lateral movement
should be defined and if a low friction bearing has
been placed on top of the spring support. If a low
friction bearing has been used, then the coefficient
of friction must also be defined.
h)
New Spring Hangers to be Designed – When spring
hangers are to be sized and selected, the number of
spring hangers to be located at that support point
must be defined. Usually one spring hanger is to
be placed at each support point. Occasionally because
of restricted headroom, a trapeze assembly with two
spring hangers providing support will be used. In
addition to the number of spring hangers, the desired
manufacturer should be defined as well as the desired
maximum load variation.
i)
Rotational Restraints  If the restraint restricts
movement about an axis (a rotational restraint), then
axis about which rotation is restrained must be defined.
j)
Imposed Rotations – If a rotation is to be
imposed on the piping system, the amount of the rotation
and the direction of the rotation must be defined.
k)
Imposed Moments – If a moment is to be imposed
on the piping system, the amount of the moment and the
direction of action of the moment must be defined.
In addition, when the moment is imposed, the moment
may have a spring constant associated with it. In other
words, if a moment is applied to the piping system and
the moment changes as piping system rotation occurs,
then the change in the moment per unit of rotation (spring
constant) must be defined.
 Special
Effects such as cold spring must be defined. First, the
location of the cold spring in the piping system must
be specified. This must include the direction or directions
of the cold spring. Cold spring along the X axis, the
Y axis and the Z axis can be placed in a piping system.
Then it must be specified whether the cold spring is a
Cut Short or a Cut Long. In addition, the amount of the
cut short or cut long must also be specified.
 Special
Loading Conditions
a)
Wind Loading – When wind loads are to be considered
in an analysis, the piping components on which the
wind loads are to be applied must be identified.
TRIFLEX calculates wind exposure and does not apply
wind loads on a piping component when the axis of
the component and the wind are coincident. To define
wind loads, the direction of the wind loads with respect
to the X, Y, Z axes must be defined. Then the magnitude
of the wind loads must be quantified as a wind speed
or a pressure per unit of surface area and a shape
factor or a load per unit of length of the piping
component.
b)
Wave Loading – When wave loads are to be considered
in an analysis, the piping components on which the
wave loads are to be applied must be identified.
TRIFLEX calculates wave exposure and does not apply
wave loads on a piping component when the axis of
the component and the wave are coincident. To define
wave loads, the direction of the wave loads with respect
to the X, Y, Z axes must be defined. Then the magnitude
of the wave loads must be quantified as a wave speed
or a pressure per unit of surface area and a shape
factor or a load per unit of length of the piping
component.
c)
Uniform Loads such as Snow and Ice  When
uniform loads are to be considered in an analysis,
the piping components on which the uniform loads are
to be applied must be identified. TRIFLEX applies
uniform loads on a piping component as defined by
the analyst. To define uniform loads, the direction
of the uniform loads with respect to the X, Y, Z axes
must be defined. Then the magnitude of the uniform
loads must be quantified as a load per unit of length
of the piping component.
d)
Seismic Loads  When seismic loads are to
be considered in an analysis, the magnitude of the
loading must be quantified and a decision as to the
analysis method to be employed must be made. When
seismic loads are to be evaluated in a static analysis,
they are to be defined as a percentage of gravity
along the X, Y, and Z axes. The percentages should
be identified and the various combinations of loading
conditions should also be identified.
e)
Soil Interaction – When soil loading is to be
considered, the piping components on which the soil
interaction is to be modeled must be identified. The
analyst can elect to calculate and enter spring constants
to simulate the soil stiffness. Stepped stiffnesses
may be entered if required because of the movement and
the soil properties. Alternatively, the analyst may
employ the guidelines published in the B31.1 Power Piping
Code. When using these guidelines, the following data
will be required: Soil Density, the Type of Backfill,
the Depth of the Trench, the Width of the Trench, the
Load Coefficient, the Horizontal Stiffness Factor and
the Axial Friction Coefficient.
 Once
all the physical data has been collected, the Global (overall)
Axis System (X, Y, Z) must be oriented on the isometric
drawing for easy reference. (The standard righthand
rule axis system is used with Y being the vertical axis.
All weight calculations are based upon gravity exerting
a negative Y force on the piping system.)
 Now
you are ready to begin assigning data point numbers to
all pertinent piping components in the piping system.
All such data point numbers should be placed on the isometric
drawing. A data point must be assigned to any location
in the system for which output data is desired. The data
point describes the specific location in the system and
the preceding segment of the piping system. Assign a
data point number at each blind end or nozzle (which begins
a Branch). Even if an Anchor is totally free to move
and rotate, it will still be specified as an Anchor point
at the beginning of a branch.



