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Module 1.26

Testing Strategy 1

Frequency regime and bandwidth

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2Guided Ultrasonics Ltd. - Module 1.26 - Rev 3 - 02 Apr 2007

Before Data Collection Check presence and condition ofequipment and tools;

Select location and prepare pipe

surface if necessary;

Check probe orientation, attach ring

with correct orientation and check

that transducers contact the pipe;

Connect leads, turn on instrument,

start software, check ring and

instrument are detected;

Check capacitance and coupling; Complete collection screen inputs,

set range and choose protocol;

Collect.

After Data Collection Check status and raw data. Rectify anyproblems;

Measure visible features and mark on

results;

Classify other features using:

amplitude, shape, symmetry, frequency

behaviour, and orientation;

Auto fit DACs, adjust if necessary;

Check suspicious echoes and/or reverb

(move ring position if required);

Classify unidentified signals and confirmwith other techniques;

Write comments for reporting;

Attach photos and print/export report.

Wavemaker G3 usage checklist

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Toneburst excitation

Instead of a large pulse to excite the waves, we use tonebursts:

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Toneburst excitation

This allows us to chose the frequency and bandwidth of the signal:

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Time traces

The raw time traces are processed to show only the envelopeover distance

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Excitation signal parameters

Frequency used just like in standard UT, except that our transducers

work over a frequency range and not just at a single

frequency

Bandwidth

like choosing a narrow or wide band probe in standard UT

We use frequency regime instead of frequency: a number which normalises the wavelength to pipe diameter

a high number corresponds to high frequency, and a low

number to low frequencies

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7/257Guided Ultrasonics Ltd. - Module 1.26 - Rev 3 - 02 Apr 2007

Frequency regime

High frequency regime (>6) gives higher sensitivity

shorter propagation distances

Low frequency regime (

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Automatic collection

By default, the software always samples a range offrequencies

Results are presented at interpolated frequency

Frequency regime and the bandwidth of the displayed datacan be changed dynamically after data collection

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Frequency regime

Sweep function:

Use the Frequency regime slider and Animate

buttons to quickly sweep through a range of interpolated

frequency regimes

Animate buttonFrequency slider

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Frequency regime

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Frequency regime

Limits:

Transducer spacing sets an optimum frequency

regime (can be seen from status screen)

Limits are automatically set by the software

Level 1 operators are recommended to work within

the positive frequency regime (fr > 0) only.

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Frequency regime

Recommended frequency regimes:

Recommendations can be found in G3 manual (Table

3.2)

Practice using this and tables 3.3 and 3.5 to determinethe optimum spacing for the following cases:

16 pipe with welded supports

2 pipe with welded supports

Discuss the implications

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Frequency regime

Compromise between range and sensitivity:

Higher frequency regimes give higher sensitivity but

shorter propagation range Lower frequency regimes give lower sensitivity but

longer propagation range

Desired balance depends on the required sensitivity for

a specific application

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Bandwidth

Narrow frequency bandwidth meansMore power

Long time signal (many cycles)

Broad frequency bandwidth means

Better resolution

Short time signal (few cycles)

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Bandwidth

The number of cycles (in time) determines the bandwidth

broad band narrow band

Time

Amp

litu

de

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Dead zone

The transducers cannot receive any signals while theyare transmitting.

This creates the dead zone close to the ring that

cannot be inspected Exact length of dead zone depends on the excitation

signal bandwidth and frequency during collection

When planning ring location always consider the

worst case (the lowest frequency of the test)

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When the frequency regime is lower, the transmissiontime and the dead zone are larger

Dead zone

Low frequency regime High frequency regime

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Broad Band Narrow Band

Dead zone

When the bandwidth is narrower, the transmission timeand the dead zone are larger

Broad band Narrow band

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Dead zone

Display in software automatically activated

Green area is dead zone

Grey area is near field (more later)

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Collection protocols

There are times when it is beneficial to optimise the system forthe pipe that is being screened

Special collection protocols can be selected from the Collect

Screen

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Collection protocol

Short range screening Designed for use on short sections

Frequency is set as high as possible for current ringspacing

Bandwidth is increased to maximise resolution andminimise dead zone

High noise filter Designed for use on pipes that vibrate wildly

Increased averaging is selected Additional hardware filtering is engaged

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Summary

Ability to change the frequency regime and bandwidth isimportant:

We want to achieve a balance between High Sensitivity

Good Axial Resolution

Short Dead ZoneVersus

Range

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Frequency regime for thick pipes

Depending on frequency regime of excitation andthickness of pipe the second torsional mode T(0,2) can

appear

Generally the pipe wall thickness would have to beover 50mm for this to become important

Level 1 operators can only work in the frequency

regime where only T(0,1) is present

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-4.0 -2.0 0.0 2.0 4.0 6.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Distance (m)

Amp(mV)

+

F1

-

F1

Frequeny regime for thick pipes

Example of test when T(0,2) IS NOT present

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25G ided Ultrasonics Ltd Module 1 26 Rev 3 02 Apr 2007

-4.0 -2.0 0.0 2.0 4.0 6.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Distance (m)

Amp(mV)

+F

1

-F

1

Frequeny regime for thick pipes

Example of test when T(0,2) IS present.

Documents

1.26 Strategy