acoustic resonance in main steam line side branches resonance in main steam line side branches 1...

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Acoustic Resonance in Main Steam LineSide Branches

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Nuclear Science and Technology Symposium (NST2016)

Helsinki, FinlandNovember 2-3, 2016

Jens Conzen, Fauske & Associates, LLC (FAI)

Proprietary Property of Fauske & Associates, LLC

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Overview

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Overview

Introduction

Acoustic resonance• What is a side branch/drip leg?• What are the main mechanisms?

Why are we talking about acoustic resonance?• Two brief industry examples that caused loss in production• It happens all the time!

How do the physics work and how can one screen for it?• Vortex shedding• Column resonance• Helmholtz resonance• Screening by review, walkdown and testing

Example of a recent project with successful resolution• 1150 MWe Generating Station


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Drip leg

Acoustic Resonance

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Side Branch

What qualifies as a side branch?

A side branch is pipe attachment to the process line such as:

• A drip leg for condensate collection or system drainage• A stand pipe for a valve (sampling/instrument line or safety valve)• A dead ended branch (e.g. system in standby)


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Acoustic Resonance

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Column Resonance

Observations:1. The direction of the flow is

from the left to the right.2. A vortex shed occurs at the

leading edge of the side branch.3. The vortex frequency is

constant.4. The side branch is excited

harmonically at its quarterwave frequency (like an organpipe)

5. Pressure waves travel both upand downstream as long c > Uf

Valve closed = Dead ended side branch


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Acoustic Resonance

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Helmholtz Resonance

Same phenomenon asblowing across a bottleopening

Observations:1. Similar to column resonance2. Side branch has a neck and a

resonating cavity


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Industry Example

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Quad Cities (Column Resonance)

The implementation of an Extended Power Up-rate resulted in significant increases in steam linevibration as well as acoustic loading of the steam dryers. This led to equipment failures and fatiguecracking of the dryers. Vortex shedding coupled column resonance in the relief and safety valvestub pipes were the principal sources of large magnitude acoustic loads in the main steam system.


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Industry Example

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OKG

The implementation of an Extended Power Up-rate resulted in significant increases in steam linevibration. The vibration levels were sufficiently high to prohibit full power production. The powerlevel needed to be kept roughly 20% below full for a prolonged time period. The power uprateprogram also included a plant modernization, which resulted in the replacement of the main steamisolation valves. Vortex shedding coupled with Helmholtz resonance in the new valves were theprincipal sources of the vibrations in the main steam system.


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Engineering Physics

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Description of Physical Phenomena:

Vortex shedding is defined as:

= · ( − 0.25)

= ·

turbulent boundary layer, St = 0.33, n = 1, 2, 3, …

laminar boundary layer, St = 0.52 , n = 1, 2, 3, …

U = Flow Velocity

D = Throat Diameter

The Strouhal number (St) is a dimensionlessnumber that is related to vortex shedding.

• The higher the velocity the higher thefrequency.

• The smaller the diameter of the side branchthe higher the frequency.


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Engineering Physics

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Column resonance is defined as:

=·4

n = 1, 3, 5, … (odd numbers)L = length of the dead ended side branchc = speed of sound (ca. 485 m/s for BWR main steam system)

L

The longer the side branch the lower the frequency.


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Engineering Physics

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Helmholtz resonance is defined as:

= 2 ·

A = cross sectional area of neckL = length of the neckV = static volume of the cavityc = speed of sound (ca. 485 m/s for BWR)

A

L

V

• Larger opening provides a higher frequencysince gas can rush in and out faster.

• Larger volume provides lower frequency sincemore gas must move.

• Longer neck provides lower frequency due toincreased friction.


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Screening by Review

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Desktop Screening (Example)Screening of isometric piping drawings by applyingpreviously defined equations.

• Requires good documentation• Use computational tools such as MathCad• If vibrations are already present use a “stencil” or

ruler to “walk down” the drawings to check forcritical lengths an openings


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Screening by Testing or Measurement

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Pre-Power Uprate TestingSubscale testing (typically 1/8th scale) with air and Mach number scaling to check for elevatedacoustic vibration potential in side branches and safety relief valves.


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Screening by Testing or Measurement

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Component TestingSubscale testing of components (typically 1/5th scale) such as valves is possible to replicateundesirable conditions , to experiment with improving design changes, and to validate them.


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Recent Project

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1150 MWe Generating Station (GS)

GS was experiencing noticeable and audible vibrations on the main steam system piping. Noisy andstrong vibration were reported in particular near the main steam valves. The valves also exhibitedmore than normal wear and required regular maintenance.

A vibration assessment was performed that consisted of an analytical approach (desktop screening)as well as vibration measurements (testing) during power operation.


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Recent Project

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Desktop Review Results

Based on the desktop review it was found that acoustic column resonance could occur at the mainsteam valves (M1, M2), their up- and downstream drain pipes (U1, U2, D1, D2), as well as the riserpipes (R1, R2). GS has two loops on the steam side. 1 stands for line 11 and 2 stands for line 12. Thefollowing schematic illustrates the measurement locations.


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Recent Project

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Field Testing

• Performed a “walk by” of the complete steam line pipingbefore the vibration measurements took place.

• Noticed an audible tone near the main steam valves.

• The microphone indicated 387Hz, which was a possiblefrequency (within scatter) according to the desktopscreening for the 3 locations and also in agreement withpreliminary measurements made by GS.


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Recent Project

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Field Testing Observations

• The time domain signal appears clear and harmonic. May be an indicator for acoustic resonance.• The strongest magnitude of vibration was measured in location D1. The peak value exceeds the

maximum allowable value. Since D1 is in close proximity to the main steam valve, it must beassumed that the valve is experiencing a strong oscillatory force. That would justify theincreased maintenance.

• Amplitudes of vibration were generally less at reduced power operation.• The frequency of vibration did not change during the reduced power (reduced flow velocity)

operation which may be indication acoustic resonance.• GS provided documentation that during plant condition “single line operation” the flow velocity

is about 30% higher. The vibration is not audible at this condition. Indication for passing troughresonance.


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Recent Project

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Field Testing Observations

• D1 is a drip leg downstream of the main steam valve. It is a vertical DN150 pipe with a DN50horizontal bypass to a dump tank for condensate removal.

• It was possible to activate the bypass during power operation. This would add disturbance toquarter wave inside the drip leg and the vibration measurement should be affected. This testwould demonstrate if D1 is the source of the vibration.

Levels at D1

D1

Bypass


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Recent Project

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“The Test”

• Vibration levels dramatically dropped after bypass opening.• The opportunity to test for the root cause on the operating plant rarely exists. The insights from

this test are invaluable.

Bypass “open”


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Recent Project

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Conclusions

The test data in combination with the analytical assessment indicate that D1 is undergoing a secondmode acoustic column resonance. All drain stubs (D1, D2, U1 and U2) are of identical designs, i.e.DN150 and 1.2m long. It is expected that all drain pipes are producing an acoustic standing wavedue to similarities in geometry and flow velocity. However, D1 is experiencing the strongestresonance. The flow is accelerated in the venturi style valve so that the velocity may be slightlyhigher at D1 (compared to U1) and provides a better excitation. The flow velocity at D2 and U2 isslightly lower due to higher line losses.


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Recent Project

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Resolution

Operations: Single line operation during part load production to increase flow velocity.

Hardware modifications: 3 options below

• (1) was installed to increase throat diameter and lower shedding frequency• Excitation source was successfully decoupled and vibrations lowered to acceptable levels


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Thank you for listening. Time for questions!

[email protected]

acoustic resonance in main steam line side branches resonance in main steam line side branches 1...

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