Thermowell Vibration MonitorEntry in the TI LDC1000

Application Contest

Adjunct Professor Robert HeiderWashington University

St. Louis, MODecember 26, 2013

Application Entry, Round 1A thermowell is a bored cylinder installed as cantilever shaft in chemical process equipment and piping

to measure a process fluid temperature. A temperature sensor is installed in the bore. The well provides

a process seal and protects the sensor from the process fluid. As a process fluid passes around the

thermowell, low pressure vortices are created on the downstream side in both laminar and turbulent

flow.

An ASME standard, PTC 19.3-1974, has been used for several years to determine the proper strength of

the well due to these vortices. A serious fatigue failure at Monju (Japan) Fast Breeder reactor liquid

sodium coolant system in 1995 called the standard into question. The new standard, ASME PTC 19.3TW-

2010, explains the reasons for the failure as well as robust calculations to determine the well strength.

Due to varying process conditions, it is difficult to determine if the well is overstressed. In many

applications, the new ASME standard mandates a shorter well, which will result in increased

temperature measurement error due to heat conduction across the well.

There are many “smart” instruments used in process applications today. This proposal is to use the TI

LDC1000 inductance-to-digital converter IC to sense well vibration and provide real time diagnostic

information to plant operators. The well would have a coil embedded in the bore which would sense

well vibration. This would then provide the user with a vibration measurement, which would result in

well design to suit the application.

• What is a thermowell?• How are the well and temperature element

fabricated?• What is the problem?• How can the TI LDC1000 be used to solve

the problem?

Thermowell Background Information

Thermowells“Thermowells are essentially a circular cylinder installed like acantilever into the process piping. They allow a temperature sensor to be located within a process flow while providing a process seal and protecting the sensor from the process fluid.

The temperature sensor can be a filled capillary, a thermocouple, an RTD or a bi-metallic element for a dial thermometer.” 1

Bore sizes 0.260” or 0.385” slightly larger than the element. Well process connection can be flanged, welded, or screwed.

1 Smith, Cecil L, “Basic Process Measurements”, Wiley, 2009.

As a process fluid passes around the thermowell, low pressure vortices are

created on the downstream side in both laminar and turbulent flow.

The combination of stresses, generated by the static in-line

drag forces from fluid flow and the dynamic transverse lift forces caused by

the alternating vortex shedding, create the potential for fatigue-induced

mechanical failures of the thermowell.

Stress calculations are necessary to insure safe operation.

Thermowell Stress Calculations

ASME Thermowell Calculations

• First standard ASME PTC 19.3-1974; J.W. Murdock (1959), Fixed Strouhal1 Number.

• Improvements made to ASME PTC 19.3-19742. Variable Strouhal Number rather than a fixed, applying installation factors in the approximation of the natural frequency of the thermowell, and reviewing the frequency ratio limit of 0.8 to account for uncertainty in the natural frequency calculations.

1 Strouhal number (St) is a dimensionless number describing oscillating flow mechanisms. http://en.wikipedia.org/wiki/Strouhal_number

2 Brock, John E., "Stress Analysis of Thermowells," Naval Postgraduate School, Monterey CA (1974).

Thermowell Failure

• Thermowell fatigue failure at Monju (Japan) Fast Breeder reactor liquid sodium coolant system in 1995.

• Investigation showed thermowell was designed to ASME PTC 19.3-1974 but the failure mode was due to the in-line resonance, which is not accounted for in that standard.

New Standard• ASME PTC 19.3-1974 was used successfully in both steam and non-steam

applications, however: • Several key factors caused ASME in 1999 to completely rewrite the

standard– advances in the knowledge of thermowell behavior, a number of

catastrophic failures (example, Monju)

– The increased use of Finite Element Analysis for stress modeling.

– These factors caused many in the industry to move away from the rudimentary methods and simplified tables laid out in ASME PTC 19.3-1974 in favor of more advanced methods for predicting the thermowell natural frequency and calculating the forced frequency.

• ASME released a new standard due to the significant changes associated with the effort.

• The thermowell calculation portion of ASME PTC 19.3-1974 was 4 pages.

• New standard, known as ASME PTC 19.3TW-2010 (“TW” for thermowell), is over 40 pages due to the explanations of theory complexity of the process, costs $75 for ASME members.

• http://www.asme.org/products/codes---standards/thermowells

Resonance FatigueFailure

• Fatigue due to resonance, due to vortex shedding.

• Vortex shedding occurs at frequencies from50Hz to 1500Hz, the thermowell can experience a large number of cycles in a short amount of time.

• As the vortex shedding frequency (also referred to as the Strouhal Frequency) approaches the thermowell natural frequency, the tip displacement and stresses are greatly magnified and the thermowell can fail due to the large amount of energy it must absorb.

Minimum Velocity and Density• For slow flowing process fluids, there is not enough energy

transferred from the process fluid to the thermowell to cause fatigue failure. If the following conditions are met, there is no need to conduct frequency limit calculations as the risk of thermowell failure is negligible.

• 1. Process Fluid Velocity, V < 0.64 m/s (2.1 ft/sec)• 2. Wall thickness, (A - d) ≥ 9.55 mm (0.376 in)• 3. Unsupported Length, L ≤ 0.61 m (24 in)• 4. Root and tip diameter (A and B) ≥ 12.7 mm (0.5 in)• 5. Maximum Allowable Stress, S ≥ 69 Mpa (10 ksi)• 6. Fatigue Endurance Limit, Sf ≥ 21 Mpa (3 ksi)• Low velocities could excite the in-line resonance, cause sensor

failure due to the high vibration that exists at resonance. • If these criteria are not met, or conditions of stress corrosion,

material embitterment due to fluid interaction (which would cause a change to the fatigue endurance), the designer must fully evaluate the thermowell design.

Flow Rate

• Many process do not have a steady designed flow rate.

• Utility flows, such as superheated steam temperature measurement, will vary as the process demands.

• Design difficult, even though FEA calculations are encouraged, what basis should one use?

ASME PTC 19.3TW-2010 gives no meaningful guidance on the installation ofthermowells in an elbow. Modeling the flow in an elbow is extremely difficultdue to the turbulence and complexity. ASME PTC 19.3TW-2010 suggeststhat to be conservative, consider the entire unsupported, unshielded length tobe exposed to the flow with the forces acting perpendicular (i.e. “normal”) tothe thermowell axis.ASME PTC 19.3TW-2010 suggests that the thermowell pointed in theupstream direction is the better installation due, in part because the amountand location of the flow stream applies a smaller moment arm and force to thethermowell and the flow into the tip is more laminar. If the tip is pointeddownstream, the swirling of the fluid after passing around the thermowellcould have some cross tip components.

Installation Details

New Standard, ASME PTC 19.3TW-2010, results in a shorter and heavierwell. A temperature error results because of heat conduction from the well to the pipe surface. Heat is conducted through the extended metal tothe instrument transmitter/electronics.

Heat is conducted through the welldue to temperature difference

Vendor Calculations

• Companies will provide the calculations for a fee.

• Main difference between the older standard is the maximum length calculation.

• Thought - if this is so critical, why not supply an instrumented “smart” well that has a vibration sensor installed?

• Could the TI LDC1000 be assembled to measure the well vibration?

How is the well assembled with the temperature sensing element?

The temperature element is typically a thermocouple mounted in a spring loaded metal tube filled with magnesium oxide.

Wiring junction box, called the weather head.

The thermowell is usually piped by way of a union or pipe nipple to the weather head. This is done to thermally isolatethe wiring and electronics from the process heat.

Pipe union shown here.

The element is spring loaded, making contact with the bottom of the well.The wiring end of the element is free to move since the spring is not fully compressed.

TI LDC1000 Inductance Sensor may be used to sense vibration

Sensing coil mounted in a screwed pipe tee to measure element vibration.Wires brought out through a piping tee mounted between the well and the connector.

Vibration sensor thermowell assembly modifications.Replaced the pipe union with a screwed tee. The stock LDC1000 coilwas separated for the evaluation module, inserted in the side of the tee and wired to the evaluation module.

Vibration sensor thermowell assembly modifications.Replaced the pipe union with a screwed tee. The stock LDC1000 coilwas separated for the evaluation module, inserted in the side of the tee and wired to the evaluation module.

Well vibrates at end, element spring loaded to bottom

Process wall mount

Element vibrates at same rate as well tip.

Spring loaded element.

TI LDC1000 evaluation module,held in place with a lab clamp.

Stock coil

Wiring and coil held in with electrical tape for evaluation.

Vibration calculation program output for test well.Wake frequency 94 Hz, natural 216 Hz.

Sensor data from evaluation program from striking thermowell tip.

Made use of MATLAB script file subroutines supplied to analyze vibration% TI LDC1000 test script file. This script streams inductance % proximity data from the LDC1000 to MATLAB, strips the bias % and dislays the frequency spectrum of the vibrati on. % Rev.1 Robert L. Heider - Washington University; 1 9-December-2013 clear close all pause(2); % give some time to clear display % open the serial USB port Timeout=5; sport = LDC1000_open( 'COM4' , Timeout ) %set sample rate > vibration per sampling theorum Fs = 20000; L = 40000; % What version are we using? LDC1000_version(sport) % Set sample rate in LDC1000 Fser=LDC1000_setsamplerate(sport,Fs); % Start collectiong dta [Rp, T]=LDC1000_streamdata(sport, 1, L, L); %2^10 % Close port ret = LDC1000_close( sport ) % Analyze data % remove bias for better frequency definition Rp = Rp - mean(Rp); % Calc fft Ts = 1/Fs; NFFT = 2^nextpow2(L); Y = fft(Rp,NFFT)/L; f = Fs/2*linspace(0,1,NFFT/2+1); % plot the results figure semilogx(f,2*abs(Y(1:NFFT/2+1))); title( 'Single-Sided Amplitude Spectrum of Rp(t)' ); xlabel( 'Frequency (Hz)' ); ylabel( '|Rp(f)|' ); grid;

0 0.5 1 1.5 2 2.5 3 3.5 4

x 104

9000

9500

0 0.5 1 1.5 2 2.5 3 3.5 4

x 104

3230

3235

3240

9100 9150 9200 9250 9300 9350 9400 9450 95003230

3235

3240

Display from evaluation MATLAB call, bursts from striking well.

10-1

100

101

102

103

104

0

5

10

15

20

25Single-Sided Amplitude Spectrum of Rp(t)

Frequency (Hz)

|Rp(

f)|

FFT spectrum shows well wake, 94 Hz, and natural 216 Hzfrequencies.

Comments

• 2 sensors should be mounted at 90 degrees from each other, to sense both wake and natural frequencies. This could be incorporated in electronics to alert operations of unsafe operation.

• This test only shows concept. Additional work is required to assemble a system for process use.

• The converter has applications to analyze other equipment vibrations.

Conclusion

• The Texas Instruments LDC1000 inductance to digital converter can be used to sense thermowell vibration.

Respectively submitted:

Robert L. HeiderAdjunct ProfessorDepartment of Energy, Environmental & Chemical Engineering Washington University in St. LouisOne Brookings Drive, Campus Box 1198St. Louis, MO 63130-4899Phone: (314) 935-6014Fax: (314) 935-7211email: heider@wustl.edu

26 December 2013

Bill of Material

Texas Instruments LDC1000 Evaluation Module

Pyromation T48G-014(3/4)-SL-6HN31 thermocouple element and weather head

Pyromation S4C1208 screwed thermowell

JMS Southeast, Inc. Thermowell Design Report, ASME 19.3 – TW 2010

Miscellaneous pipe fittings

https://www.asme.org/products/codes-standards/ptc-193-tw-2010-thermowells

http://www.jms-se.com/Apps/Tools/ThermowellDesign/Home.aspx

http://www.controleng.com/single-article/back-to-basics-thermowell-protection/3c5bf2adeae5931923d8c3d1d457d280.html

http://www.ti.com/ww/en/analog/dataconverters/inductance-to-digital-converter/index.html?DCMP=sva-psp-ssp-ldc1000-en&HQS=sva-psp-ssp-ldc1000-vanity-en

Website Links, References

D.W. Richmond, "Selecting thermowells for accuracy and endurance", InTech (February 1980).

Ian H. Gibson, “Optimal selection of thermowells”, ISA Transactions 34 (1995) 209-216.

Rosemount White Paper - Thermowell Calculations00840-0200-2654, Rev AB, January 2012

Smith, Cecil L, “Basic Process Measurements”, Wiley, 2009.

http://en.wikipedia.org/wiki/Strouhal_number