Review of Design Guideline for Small Diameter Branch

8/3/2019 Review of Design Guideline for Small Diameter Branch

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Review of Design Guideline for Small Diameter Branch Connections Page 1

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2011 GMRC Gas Machinery Conference Nashville, Tennessee October2-5, 2011

By

Benjamin A. White, P.E., Southwest Research Institute

Eugene Buddy L. Broerman, Southwest Research Institute

Melissa A. Wilcox, Southwest Research Institute

Francisco Fierro, Southwest Research Institute

ABSTRACT

In all industrial applications, vibration of piping components is a concern that is oftenaddressed during the piping design phase. Effort is made to predict and avoid the mechanicalnatural frequency of large piping in order to prevent fatigue failures. However, current industry

practice is that only larger bore lines (3 inches in diameter and above) are analyzed in detail.

Sometimes only lines 6 inches and above are analyzed. Small diameter branch connections(typically 2 inch and below) are often omitted due to cost and/or a lack of available information

at the design stage. These small diameter connections are just as susceptible as large diameter

connections to fatigue failures due to high vibration; therefore, the design of the small piping

must also be considered. Current piping standards only place high level requirements on thedesign of small diameter branch connections and do not provide many recommendations related

to the design of the connections.

In 2010, GMRC in a joint project with PRCI developed a guideline which addresses the

design of small diameter branch connections. The guideline provides recommendations on the

weight placed on the branch and the length of the connection in order to minimize the possibilityof fatigue failure. The intent of the guideline is to reduce the risk of vibration and fatigue

problems associated with small bore piping without requiring a detailed study. This paper

reviews the guideline and its key components. It also discusses how it can be applied to actual

piping configurations. Lastly, several case studies are presented for the application of theguideline.


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Vibration of piping due to mechanical excitation is present in nearly any environment

where small diameter branch connections (BCs) are used. In initial piping designs, smalldiameter connections are often overlooked and not considered. However, these smaller

connections are just as susceptible to fatigue failures as the larger piping which is carefullydesigned.

Several industry standards mention high level considerations for BCs, but there is a lackof recommendations for the smaller diameter connections, especially related to the length of the

connection and allowable weight. Several industry standards such as ASME B31.3, ASME

B31.4, ASME B31.8, 49 CFR Part 192, 49 CFR Part 195 and ASTM F681-82 were reviewed. Amain focus in the standards is how to calculate the reinforcement area required for the

connection. However, reinforcement is typically not required for connections 2 and smaller as

long as the pipe has diameter less than 25% of the main pipe diameter. In most standards, a

general statement is made about including effects of vibration, outside forces, dynamic changes,etc. in the piping design. However, there is no specific discussion of how these should be

considered on a branched connection.

The Energy Institutes Guidelines for the Avoidance of Vibration Induced Fatigue and

Process Pipework provides a series of flow charts used to calculate a Likelihood of Failurefactor. This provides an alternative screening approach which can be used as a companion to the

GMRC-PRCI guideline.

The ultimate goal of the guideline is to allow for the design of small diameter branch

connections such that the branches are not susceptible to fatigue failure due to vibration. Therisk of failure is directly related to the amount of dynamic strain in the piping. For existing

piping systems, dynamic strain can be measured and used as a criterion to evaluate the risk of

failure. In practice, it is much simpler to measure vibration amplitudes on piping and compare

those to a variety of industry guidelines as a screening tool; however, this is not possible in thedesign stage.

All structural systems will have multiple mechanical natural frequencies at which they

will vibrate when impacted, much like a tuning fork. If the piping system is excited by aharmonic load such as pulsation or mechanical vibration at its mechanical natural frequency

(resonance), then vibration amplitudes will be greatly amplified, often by a factor of between 10

and 50. Because of this high amplification that occurs during resonance, the simplest solution tomost piping vibration problems is to avoid resonance.

For piping systems at the design stage, it is possible to use finite element techniques to

model the piping system. These models can be used to predict the mechanical natural

frequencies of the piping system with reasonable accuracy if appropriate modeling assumptionsare made. Predictions of vibration and dynamic stress and strain amplitudes can also be made;

however such predictions are subject to a range of uncertainty due to assumptions that must be

made regarding the excitation sources, damping, restraint stiffness values, end conditions, etc.

For larger diameter piping systems, vibration control is achieved by first reducing theamplitudes of the excitation forces as much as practical. Secondly, the mechanical natural

frequencies of the piping are placed above the frequencies of significant excitation by installing


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piping restraints that are sufficiently stiff and properly located. This approach avoids mechanical

resonance as much as possible. In cases where excitation forces are reasonable and mechanicalresonance is avoided, failures due to excessive dynamic strain are unlikely to occur. In cases

where mechanical resonance cannot be avoided or where excitation forces are considered

excessive, detailed structural modeling should be performed.

Small bore piping can be analyzed in the same way as larger bore piping systems.However, in practice, small bore piping typically receives less attention due to several reasons.

The first reason is that the specific details of the small bore piping are often not fully defined at

the design stage. The second reason is that the sheer quantity of lines makes it too timeintensive. The third reason is the perceived less critical nature of smaller bore piping.

Making accurate vibration and dynamic stress/strain predictions can be done for small

bore piping on a case-by-case basis when all aspects of the piping are well defined. However, if

a small bore branch piping segment is mechanically resonant, even very small levels ofexcitation will result in excessive vibration of the small bore piping.

Therefore, the design philosophy of the guideline is to reduce risk by placing the lowest

mechanical natural frequency of the branch connection above the frequencies of most significantexcitation occurring at the base of the branch line. In most cases, the lowest mechanical naturalfrequency of the branch should be at least 20% above the first, second or fourth multiple of the

compressor running speed (guidance on frequency selection is included in the guideline). The

guideline does not attempt to predict actual vibration amplitudes.

In addition to the frequency based criteria, a secondary non-resonant stress criterion isalso included. It is possible to generate dynamic stress (and strain) in the branch connection if

there is significant vibration in the mainline even if the mechanical response of the branch line is

not excited. This secondary criterion (a quasi-static stress) is based on a mainline acceleration of1.5 gs resulting in a stress of 3,000 psi 0-peak or less in the branch line (without dynamic

amplification). It is important to note that this stress criteria used is not the same as predicting

stress in the branch line due to a mechanical response of the branch (with dynamicamplification). These limits were based on the general experience of the guidelines authors and

the Industry Advisory Committee as they provided a compromise between reducing the number

or problem prone configurations while not being overly conservative. The graphs in Section 4.0

of the guideline include the limitations of both the frequency based criteria and the non-resonantstress based criteria.

As a general guideline, to reduce the risk of vibration and fatigue problems in branch

lines, it is recommended that the branch line be kept as short as possible with minimal attached

weight even if the guideline indicates higher lengths and weights are permissible. The guidelinedoes not account for any potential issues related to thermal expansion and the resulting stress that

is generated.

Pads, saddles, fabricated welding tees and other similar types of branch reinforcements

are generally not discussed in this guideline since they are typically not required or commonly

In addition, the guideline does not account for stress concentration factors that varyamong different fitting types (weld-o-let, thread-o-let, elbow-let, etc.). However, as a generalguideline, fittings with higher stress concentration factors (such as threaded connections) should

be avoided in higher risk areas such as near the compression equipment (as defined in the

guideline). A welding specification should be provided to avoid inferior welds with high stressconcentrations.


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used on smaller diameter lines (2 nominal diameter or less). However, the additional

reinforcement and stress concentration factor reduction typically provided by these types ofconnections all help to increase the reliability of typical branch lines. Figure 1-1 shows some

typical small diameter branch connections.

Figure 1-1. Typical Small Diameter Branch Connections


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The guideline provides recommendations on the allowable weight and length for various

unique configurations of small diameter BCs. The BCs covered are connections with pipediameter equal to or less than two inches where the nominal branch diameter to main pipe

diameter ratio is less than 25%.

The guideline covers the use of ferrous materials (carbon steel and stainless steel). The

development of the guideline was based on experimentally validated models and industry best

practices; however,

Six different connection configurations are covered in the guideline. Most configurations were

analyzed using Schedule 80 piping (as discussed in Section 4.0 of this paper). Some select

arrangements also included configurations with Long Welding Neck (LWN) nozzles.

following the guideline does not guarantee the avoidance of any futurevibration problems. It is recommended that the vibrations on branch connections be checked

after machine start-up in order to ensure that the vibrations are within acceptable limits.

Configuration 1 Straight Cantilever (with or without LWN) Configuration 2 Cantilever with Elbow Configuration 3 Pressure Safety Valve (PSV) (with or without LWN) Configuration 4 Double Connection at Source Configuration 5 Straight Connection with External Restraint Configuration 6 Connection with External Restraint with Elbow

Images of these configurations are shown in Figure 2-1.

The guideline provides specifications for ten different excitation or design frequencies(related to operational speed). The determination of the frequency for the connection selection is

discussed in the guideline. Ten different operational frequencies are included: 15, 24, 30, 40, 48,

60, 80, 96, 144, and 200 Hz. Under each of these frequencies, recommendations for the branchgeometries are provided for each of the six BC configurations. Each of the recommended

geometries covers multiple nominal branch diameters (, , 1, 1 , and 2) and indicates

the maximum acceptable lengths of the branch based on the weight which is mounted on thebranch. Select configurations also include information for Long Welding Neck (LWN) nozzles.


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Cantilever

Double Connection at Source

Connection with External Restraint

PSV

1

2

3

4

5 6

Figure 2-1. Images of Various Branch Connection Configurations


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The guideline is used by reading allowable weight and/or length values from a series of

included graphs. Different graphs are included for various configuration types and designfrequencies. Each graph has multiple curves for each nominal piping diameter (1/2 through 2).

The following step by step by step process should be followed:

Step 1: Define Equipment Type (Reciprocating / Centrifugal - Compressor / Pump) Step 2: Determine if Branch is Near or Far from Excitation Source Step 3: Determine Design Frequency (typically 20% above key order) Step 4: Select Design Frequency Section of Guideline (round frequency up) Step 5: Select Configuration Type and Determine How Effective Length Defined (Le) Step 6: Determine Branch Nominal Diameter (1/2, , 1, 1.5 or 2) Step 7: Use Graphs to Determine Allowable Weight and LengthThe guideline itself includes a detailed explanation of each step described above, which is

not repeated here (see Section 3.2 of the guideline). Figure 3-1 below shows a typical samplegraph.

Figure 3-1. Sample Graph

For Configuration 3, the larger diameter lines place the PSV higher since the lengthto the PSV is based on three times pipe OD, so the 2 line has the highest and mostflexible configuration.

General Graph Notes (referring to all graphs in the guideline, not just Figure 3-1 above):

Step changes to zero in the curves on some of the graphs indicate the minimumphysical configuration length is still below the target frequency.

Some lines may cross each other on the graphs. This is due to 1) the weight of thepipe itself or 2) geometrical length assumptions based on the pipe diameter.

Note transition point from frequency

based criteria to stress based criteria.


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The design philosophy of the guideline is to reduce risk by placing the lowest mechanical

natural frequency of the branch connection above the frequencies of most significant excitation

occurring at the base of the branch line. In most cases, the lowest mechanical natural frequencyof the branch should be at least 20% above the first, second or fourth multiple of the compressor

running speed (guidance on frequency selection is included in the guideline).

The guideline is used by reading allowable lengths and weights off the numerous charts

included in Section 4.0 of the guideline. The calculation of structural mechanical responses used

to generate the graphs in Section 4.0 of the guideline were done using the commercial finiteelement software package ANSYS. The models were made using pipe (beam type) elements.

Typical ANSYS models are shown in Figure 4-1. Typical modal analysis results are shown in

Figure 4-2. The ANSYS scripting language APDL was used to automate the process of runningeach simple model iteratively to generate the data needed for the graphs.

Figure 4-1. Typical Finite Element Model

Figure 4-2. Typical Finite Element Modal Analysis Results


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There are numerous factors that impact the calculated mechanical natural frequency of a

branch connection. The most significant factors are the length, pipe diameter, and attached valveor flange weight. These factors are all included explicitly in the guideline. However, there are

several other factors that can also impact the calculations. These factors, and the assumptions

used to address them, are described as follows. It is important that the user of the guideline

understand these assumptions before selecting a branch configuration based on the guidelinerecommendations.

In most all cases, the assumptions described below were made to add conservatism to the

guideline. Therefore, if the graphs in the guideline are used to work backwards and predict a

mechanical natural frequency of a given branch connection, the graph should typically under-predict the actual mechanical natural frequency of the branch connection. It is difficult to

quantify an exact level of conservatism since it will vary from case to case. However, care was

taken not to make the guideline overly conservative and too limiting on the design. It is typicallypossible to predict piping mechanical natural frequencies within a +/- 10% margin. An

additional 10% margin is typically included to provide separation between the excitation

frequency and the structural response frequency (resulting in the 20% criteria noted in Section1.0 of this document). The testing done for this guideline normally fell within that 10% margin

of error. The following assumptions were made to add conservatism to the guideline:

All connection types were analyzed using schedule 80 pipe geometries (which is theminimum recommended schedule). Thicker schedules are typically slightly more

conservative. For Long Welding Neck (LWN) nozzles, which have significantly

higher wall thicknesses, separate graphs are included for select configurations. Withvery long LWN branches, the weight of the fitting itself becomes critical (which is

included in the graphs).

The model included only the branch line (not the mainline). The model of the branchline was terminated at the attachment point to the mainline. An end condition stiffness(six directional spring) was modeled at the termination point to account for flexibility

in the branch fitting and in the wall of the mainline. Larger diameter, thin walled

mainlines can have significant shell flexibility. The assumed stiffness values (for allconfigurations and connection types) were based on the most flexible combinations

based on the ranges of interest. The assumed stiffness values were 1.0e6 lbf/in in all

three translational directions and 1.5e6 in-lbf/radian in all three rotational directions.

Stiffness values of all connection types (weld-o-let, thread-o-let, elbow-let, etc.) areassumed to be equal. From the laboratory tests and FEA work, stiffness values fordifferent connection types, sizes, and ratings were calculated. Comparing the

calculated translational stiffness values it was found that the average stiffness value

was 1.9e6 lbf/in, the median value was 1.9e6 lbf/in as well, and the minimum andmaximum were approximately 1e6 lbf/in and approximately 3e6, respectively (this is

for the fitting stiffness only and some values were just outside this range). This is

considered to be a minimal amount of scatter of the stiffness values because a change

of approximately 5-10 times was generally found to be required to significantly shift aresponse. When the pipe wall flexibility is also included, the translational stiffness

values are similar, but the rotational stiffness values are reduced. This is discussed

further in Section 4.0 of this paper.


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Flanged joints were assumed to be rigid. The connection stiffness values were chosensuch that the results will be conservative. While the joint stiffness in a flange doesaffect the branch natural frequency, the effect is not significant and is accounted for in

the conservative connection stiffness.

Because of the modeling process and assumptions made, the analysis becomes lessaccurate as frequencies increase. At higher frequencies (above 100 Hz) the pipingsystems are stiffer and the mechanical responses become more dependent on factorssuch as flange and gasket flexibilities, specific fitting and flange geometries, etc.

In addition to the frequency based criteria, a secondary non-resonant stress criterionwas also included. It is possible to generate dynamic stress (and strain) in the branch

connection if there is significant vibration in the mainline even if the mechanicalresponse of the branch line is not excited. This secondary criterion (a quasi-static

stress) is based on a mainline acceleration of 1.5 gs resulting in a stress of 3,000 psi 0-

peak or less in the branch line (without dynamic amplification). It is important to note

that this stress criteria used is not

The most conservative geometries were selected for the analysis in most cases. Thismeans that the most flexible configurations (length ratios) were modeled to generatethe graphs. Therefore, the actual configurations should be equal to or stiffer than what

was modeled. This is discussed in further detail in the guideline Appendix A.

the same as predicting stress in the branch line due to

a mechanical response of the branch (with dynamic amplification). These limits werebased on the general experience of the authors and the Industry Advisory Committee

as they provided a compromise between reducing the number or problem proneconfigurations while not being overly conservative. The graphs in Section 4.0 of the

guideline include the limitations of both the frequency based criteria and the non-

resonant stress based criteria.

All Long Welding Neck (LWN) configurations assumed 600# flange ratings todetermine the outside diameter and the wall thickness of the LWN fitting.

For configurations with external restraints (pipe clamp or support), the restraint wasassumed to have a minimum stiffness of 10,000 lbf/in in all three translational

directions. Actual restraint stiffness values can vary significantly.


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For the purpose of developing the guideline, three (3) unique branch connection

configurations (and a few variations of those three configurations) were fabricated and used forlaboratory testing. These small diameter branch connection configurations were instrumented

with accelerometers and strain gauges. Each test configuration was mounted to a shaker table toprovide a base excitation. The base excitation of the shaker table could be adjusted for bothamplitude and frequency. During the testing, information was collected on the mechanical

natural frequencies of each configuration and on the vibration and dynamic strain amplitudes as a

result of the base excitation. Static stiffness testing was also performed. Figures 5-1, 5-2, 5-3

and 5-4 show the test configurations.

Figure 5-1. Test Configuration


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The following basic test configurations were fabricated:

A 1 nominal diameter valve with 2 mainline connections (see Figure 5-2). A 1 nominal diameter long piping run with multiple elbows and a single excitation point

with rigid end restraint (see Figure 5-3).

A 1.5 nominal diameter elbow-let with cantilevered valve and piping (see Figure 5-4).In addition to the test configurations shown in Figure 5-1 that were built exclusively for

testing for the guideline, eight (8) supplementary existing small diameter branch connection

configurations were also impact tested for mechanical natural frequency data. These existing

configurations were located at Southwest Research InstitutesMetering Research Facility (MRF)in San Antonio, TX. Representative configurations are shown in Figure 1-1 of this paper.

The data gathered during this testing process was used to validate the finite elementmodels used to generate the guideline graphs. As noted previously, because of the conservative

nature of the many assumptions made (see Section 4.0), if the graphs in the guideline are used towork backwards and predict a mechanical natural frequency of a given branch connection, thegraph should typically under-predict the actual mechanical natural frequency of the branch

connection.



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From the laboratory testing done on the three test configurations (built for the purpose of

developing the guideline), it was determined that when the excitation amplitudes of the base(mainline piping) were at a level of 0.2 inches per second (ips) 0-pk or greater, and the vibration

occurred at the same frequency as the first mechanical natural frequency of the branch line, the

resulting dynamic strain in the branch line was almost always excessive (enough to result in a

high risk of failure in most typical piping systems). Vibration levels on mainline piping are oftenin the range of 0.2 to 0.5 ips and often higher.

Based on this, it was concluded that it was not practical to develop the guideline using

dynamic strain levels if the branch connection had a mechanical natural frequency near the

frequency of excitation. Therefore, frequency avoidance (or at least avoidance of the frequenciesof significant excitation) was determined to be the key to avoiding failures of small diameter

BCs. The guideline is based on the idea that the natural frequency of each branch should be

shifted such that it will not be excited by the primary excitation source. Measured dynamicstrain should be below 100 strain pk-pk for long life.

Table 5-1 summarizes the measured vibration and strain amplitudes for the various tests

(base excitation levels were typically 0.1 ips but were varied to result in reasonable branchvibration amplitudes). Table 5-2 summarizes the measured damping characteristics.

Table 5-1. Summary of Vibration and Strain Measurements

Configuration 1st peak (Lowest Mechanical Response)

Frequency(Hz)

ips 0-pk

strainpk-pk strain/ips 0.2 ips

EOL Horiz. 67 0.19 395 2035 407

EOL Axial 69 0.16 71 432 86

EOL Vertical 65 0.17 49 283 57

EOL Horiz.+ mass 14 0.68 1415 2072 414

EOL Axial + mass 15 0.60 53 88 18

TOL Horiz. 14 0.62 240 387 77

TOL Axial 6in 8 1.09 210 193 39

TOL Axial 12in 8 1.09 173 159 32

TOL Axial 20in 8 1.09 178 163 33

TOL welded 9 1.43 35 24 5

WOL Horiz. 93 0.53 980 1843 369

WOL Vertical 82 0.63 778 1244 249

WOL + mass 84 1.09 1196 1097 219


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Table 5-2. Summary of Damping Values

In addition to the laboratory testing and finite element modeling using beam elements,more detailed finite element modeling was performed using three-dimensional, solid-type

elements again using ANSYS software. This modeling was done primarily to validate the

assumptions made for the stiffness values used at the location where the branch line connects to

the mainline. For the beam element modeling, the mainline itself was not included, so atermination stiffness (in three directions of translation and three directions of rotation) was

needed. This stiffness represents the flexibility of the fitting itself (weld-o-let, thread-o-let,

elbow-let, etc.) and the flexibility of the shell wall of the mainline piping. Figure 5-5 showssome typical solid-type finite element models that were used in the guideline development.

Figure 5-5. Solid Type Finite Element Models

Table 5-3 shows the calculated translational and rotational stiffness values from themodeling for various fitting types. Table 5-3 shows stiffness values for the fittings only (no

mainline pipe wall flexibility). Table 5-4 shows stiffness values for both the fitting and the

mainline pipe wall flexibility.


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Table 5-3. Stiffness Values for Various Fitting Types (Fitting Flexibility Only)

Table 5-4. Stiffness Values for Mainline Pipes (Fitting + Pipe Wall Flexibility)


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Design of a simple cantilevered branch connection with single valve.

Example #1

1 nominal diameter piping with 600# block valve. Mounted on a 1,000 rpm reciprocating compressor discharge pulsation bottle.

Step 1: Define Equipment Type

: Reciprocating Compressor.

Since the branch is mounted directly to the discharge bottle, it would be considerednear (within 20 feet of bottle line connection for a reciprocating compressor from

Section 3.2 of the guideline).

Step 2: Determine if Branch is Near or Far from Excitation Source:

Step 3: Determine Design FrequencyThis is based on 20% above the 4

: 80 Hz from Table 3-1 of the guideline.th

order = (rpm/60) * 4 * 1.2 = 80 Hz.

Step 4: Select Design Frequency Section of Guideline:80 Hz can be used directly (no rounding up required since 80 Hz appears in guideline).

Configuration Type #1 best describes this layout. Defined length is from mainline

outside diameter to center of valve. See Figure 3-3 of guideline.

Step 5: Select Configuration Type and Determine How Effective Length is Defined (Le):

Step 6: Determine Branch Nominal Diameter

: 1.

Step 7: Use Graphs to Determine Allowable Weight and Length

Locate the appropriate graph for Configuration #1 at 80 Hz on page 44 in Section 4.0 ofthe guideline. Locate graph line for 1 nominal piping (green dashed line). Figure 6-1 of

this paper is a reprint of the plot that can be found in the guideline.

:

Option A - For a known valve weight (with flange & bolt weights included) of 55

pounds, the length should be 4 or less (from mainline OD to valve center).

Option B - For a minimum required length of 5, the maximum allowable valve weight is

40 pounds.

Option C - If neither Option A or B works, consider a different branch line size ormoving the branch line farther away from the compressor unit. External bracing could

also be considered. Bracing this type of branch back to the vessel is typically the best

bracing solution.

L

W


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Figure 6-1. Graph for Example #1



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Design of a cantilevered branch connection with PSV with LWN.

Example #2

2 nominal diameter piping with 600# PSV (LWN inlet / Sch. 80 outlet). Mounted on piping (50 from a 900 rpm reciprocating compressor bottle).


: Reciprocating Compressor.

This branch would be considered far from the compressor per Section 3.2 of the

guideline since it is more than 20 feet from manifold bottle line connection.


Step 3: Determine Design Frequency

This is based on 20% above the 2

: 36 Hz per Section 3.2 of the guideline.nd

order = (rpm/60) * 2 * 1.2 = 36 Hz.

Step 4: Select Design Frequency Section of Guideline:

Use 40 Hz (rounding the 36 Hz up to nearest guideline design frequency above 36 Hz).

Configuration Type #3 best describes this layout. Defined length is from centerline of

PSV outlet elbow to top of stack. See Figure 3-5 of guideline.



: 2.


Locate the appropriate graph for Configuration #3-LWN at 40 Hz on page 34 in Section

4.0 of the guideline. Locate graph line for 2 nominal piping (solid blue line). Figure 6-2of this paper is a reprint of the plot that can be found in the guideline.

:


pounds, the length should be 6 or less (from PSV outlet elbow centerline to top ofstack).

Option B - For a minimum required length of 15, the maximum allowable valve weight

is 60 pounds.

Option C - If neither Option A or B works, consider external bracing. Bracing this type

of branch back to the vessel is typically the best bracing solution.

NOTE #1: From the referenced graph (Configuration #3-LWN at 40 Hz on page 34 ofthe guideline), it shows the allowable weights/lengths for 2 nominal piping as less thanthose for smaller line sizes. The reason for this is that Configuration #3 assumes that the

piping upstream of the PSV (L1) is equal to three times pipe OD. Therefore, the larger

pipe sizes have the PSV placed higher above the mainline (and, therefore, more flexible).

NOTE #2: For PSV configurations with different inlet and outlet pipe diameters, check

each diameter separately and use the minimum of the two results.

L1

W

L


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Design of a branch connection with valve, elbow & restraint.

Example #3

1 nominal diameter piping with 600# block valve. Mounted on piping (30 from a centrifugal compressor inlet).


: Centrifugal Compressor.

This branch would be considered far from the compressor per Section 3.2 of the

guideline since it is more than 25 feet from manifold bottle line connection.


Step 3: Determine Design Frequency

This is based on the far classification for a centrifugal compressor.

: 15 Hz from Table 3-1 of the guideline.

Step 4: Select Design Frequency Section of Guideline:15 Hz can be used directly (no rounding up required since 15 Hz appears in guideline).

Configuration Type #6 best describes this layout. Defined length is summation of all

individual lengths from mainline outside diameter to piping restraint. See Figure 3-8 ofguideline.



: 1.


Locate the appropriate graph for Configuration #6 at 15 Hz on page 23 in Section 4.0 ofthe guideline. Locate graph line for 1 nominal piping (green dashed line). Figure 6-3 of

this paper is a reprint of the plot that can be found in the guideline.

:


pounds, the total length should be 20 or less (from mainline outside diameter to piping

restraint).

Option B - For a minimum required length of 30, the maximum allowable valve weight

is 30 pounds.

Option C - If neither Option A or B works, consider a larger branch line size or movingthe restraint closer to the mainline. Bracing this type of branch with an external restraint

too close to the branch connection could result in excessive stress.

Le = L1 + L2 + L3

L1 L2

L3W


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The installation of an external restraint (support, bracing, etc.) will add stiffness to the

branch line raising its minimum mechanical natural frequency and reducing the risk of excessivevibration. Because there are an almost unlimited number of potential branch line configurations,

it is not possible to recommend specific restraint configurations that will cover all feasiblelayouts. However, the following general guidelines are provided.

The stiffer the restraint is, the more effective it will be. A minimum restraint stiffnessof 10,000 lbf/in in all three translational directions was assumed for the graphs in theguideline.

A good restraint should be triangulated in multiple planes to provide stiffness inmultiple directions. A restraint mounted to a tall, single slender support will typically

be very flexible and ineffective in controlling vibration in directions other than theaxial direction of the tall support.

Simple weight supports and springs typically provide very little vibration control.

It is typically preferred to brace the branch line back to the mainline piping. Bracingthe branch line to a very stiff external support can increase bending stresses due to

relative displacement from vibration and thermal expansion. A relatively stiff external

support should not be installed too close to the branch connection.

Strap type clamps are typically more effective than U-Bolt type restraints. If U-Boltsare used, they should be used in pairs to prevent rotation.

Gusset plates can add stiffness but also add high stress intensification factors andshould be used with caution. If used, any gussets should attach to a reinforcing pad

and not directly to the mainline.

To prevent fretting, all clamps should be lined with a resilient liner material(Fabreeka or similar).


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99.. RREEFFEERREENNCCEESS

1. Design Guideline for Small Diameter Branch Connections

2.

49-CFR Part 192,

, Release 1.0, GMRC-PRCI,

March 2011.

Transportation of Natural and Other Gas By Pipeline: MinimumFederal Standards

3. 49-CFR 195,, United States, Department of Transportation, PHMSA, 2009.

Transportation of Hazardous Liquids by Pipeline

4. ASME B31.3,, United States,

Department of Transportation, PHMSA, 2009.

Process Piping

5. ASME B31.4,, American Society of Mechanical Engineers, 2008.

Pipeline Transportation Systems for Liquid Hydrocarbons and OtherLiquids

6. ASME B31.8,, American Society of Mechanical Engineers, 2009.

Gas Transmission and Distribution Piping Systems

7.

ASTM F681-82,

, American Society of

Mechanical Engineers, 2007.

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Transcript of Review of Design Guideline for Small Diameter Branch