Technical information

Wake frequency calculation
WIKA data sheet IN 00.15

Applications■■ The wake frequency calculation for thermowells serves

as mathematical proof of the strength with respect tostatic and dynamic stress in relation to the operating
temperature and pressure

Special features■■ Wake frequency calculation per ASME PTC 19.3 TW-2016

for standard bar stock thermowells as an engineering
service■■ Recommendations for structural changes to thethermowell where the permissible stress limits areexceeded can be derived

FEA representation of a thermowell in flow with stressrepresentation at the tip and at the root

Description
The wake frequency calculation per
ASME PTC 19.3 TW-2016 is used for solid drilled bar
stock thermowells in tapered, straight or stepped designs,
such as models TW10, TW15, TW20, TW25 and TW30 orthermowells solid-machined from forgings.
The process data required to perform a calculation as per
ASME PTC 19.3 TW-2016 is as follows:
SI unit
ImperialOthers---

For recommendations concerning structural changes where
the permissible stress limits have been exceeded, thefollowing additional information will also be required:---■■ Inside diameter of nozzle

Flow rate

m/s

ft/s

---

Temperature°C°F---

Density of medium
Pressure

Dynamic viscosity 1)

kg/m³
barmm²/s1) Optional for ASME PTC 19.3 TW-2016

WIKA data sheet IN 00.15 ∙ 01/2019

lb/ft³psi

ft/1000s

WIKA guarantees that the calculation is executed on thebasis of the ASME PTC 19.3 TW-2016. The end user is
responsible for the consistency between the real processdata and the data on which the calculation is based.
Generally, no warranty can be given by WIKA for thecalculation of the results per ASME PTC 19.3 TW-2016. The
results are of informative character.

cP■■ Height of nozzle (shielded length)■■ Inner diameter and wall thickness of pipeline/vessel

Page 1 of 7

Basic information about ASME PTC 19.3 TW-2016
ASME PTC 19.3 TW-2016 is divided into dynamic and static
calculation results.
For low density gases, the frequency limit is typicallyrmax = 0.8. For other gaseous medias, the steady operation in
the range between r = 0.4 ... 0.6 around the in-line resonanceis not permitted. For liquid media, in many applications, thenewly-introduced limit frequency of rmax = 0.4 for the in-line
resonance typically applies.
In-line resonance:r = 0.5 (drag oscillation)

Main resonance:r = 1 (lift oscillation)

rmax

00.1

0.2

0.3

0.4

rmax

0.5

0.6

0.7

0.8

0.91

r = fs/fn

Section 6-8.2:
0.8

Low-density gases NSC > 2.5 nd Re < 105
Section 6-8.3 and 6-8.4:
Cyclic stress condition

0.4

0.6

0.8

All other cases:
0.4

The evaluation of the dynamic results is made using thedamping factor NSC (the Scruton Number NSC has a direct
relationship to the permissible frequency ratio rmax from thewake frequency fs to the natural frequency fn). Simplified, forgaseous media, a characteristic value is NSC > 2.5; fluids
typically have an NSC < 2.5.
The Scruton Number NSC in the calculation is dependant on
the intrinsic damping factor, the density of the thermowellmaterial, the process medium and the tip and bore diameterof the thermowell.

The static results of ASME PTC 19.3 TW-2016 are generated
from the maximum permissible process pressure (dependent
upon the process temperature and the geometry of thethermowell) and the bending stress in the area of thethermowell root. The bending stress is caused by the incident
flow on the thermowell, and is dependent on the shielded
length of the flange nozzle.

Whether the frequency ratio, r < 0.8, can also be used as
an evaluation limit with other process media, is determinedthrough a consideration of the permissible stresses in thethermowell material with respect to the actual stresses atresonance. In addition, an evaluation of the strength of thethermowell material with respect to the flexural fatigue stressin the area of the thermowell clamping is carried out.

WIKA data sheet IN 00.15 ∙ 01/2019

Page 2 of 7

Remedies using structural changes, when the permissible frequency ratio, rmax ,is exceeded
By exceeding the maximum permissible limit frequency, rmax,
for the “In-line”- or main resonance, the following structuralchanges may be a solution:a) Shortening the insertion length
This is the most effective method (and the recommended
method from ASME PTC 19.3 TW-2016) for theimprovement of the frequency ratio r.
b) Increasing the root diameter
By increasing the root diameter, the natural frequency fn isincreased, optimizing the frequency ratio r.
c) Increasing the tip diameter
By increasing the tip diameter, the vortex shedding
frequency fs is reduced, optimizing the frequency ratio r.
d) Support collars
Support collars or other means of support are outside
the scope of the standard. The use of support collars
is not generally recommended, as rigid support can beobtained only with an interference fit between the support
collar and the installed piping, ASME PTC 19.3 TW-2016
points 6-7-(e). On customer request, support collars canbe used, and are designed to provide an interference
fit with the process connection. The thermowell will be
designed in accordance with the design and calculation
criteria of ASME PTC 19.3 TW-2016, however, this falls
outside the scope of ASME PTC 19.3 TW-2016. The
operator is responsible for the rigid support of the collar inthe nozzle, which may mean that a reworking of the collar
is needed. A guarantee for support collar solutions isgenerally not given by WIKA!

e) ScrutonWell® design
The ScrutonWell® design can be used on solid-machined
thermowells with flange connection, in Vanstone design
or for weld-in or screwed process connection. This design
reduces the amplitude of oscillation by more than 90 % 1)
and allows an easy and fast installation of the thermowell
without expensive and time-consuming rework on site, atthe same time. The WIKA ScrutonWell® design has beentested and approved by the independent laboratories
TÜV SÜD NEL (Glasgow) and the Institute for Mechanics
and Fluid Dynamics (Technical University of Freiberg).
For detailed information please see datasheet SP 05.16.
Calculation of ScrutonWell® design based on
ASME PTC 19.3 TW-2016■■ Maximum permissible pressure load with original stem
dimensions■■ Maximum permissible bending load with modified stem
dimensions■■ The dynamic part of the wake frequency calculation is
not required because of the damping of the oscillation
by more than 90 %Thermowell model TW10 in ScrutonWell® design

For fabricated (welded tube) thermowells
ASME PTC 19.3 TW-2016 is not applicable. Pleasecontact a WIKA representative to provide calculations
based on Dittrich/Klotter.

1) Journal of Offshore and Mechanics and Artic Engineering Nov 2011, Vol 133/041102-1
by ASME

WIKA data sheet IN 00.15 ∙ 01/2019

Page 3 of 7

Performing a standard wake frequency calculation in accordance to
ASME PTC 19.3 TW-2016
This simplified flow diagram shows
the procedure of performinga standard wake frequency
calculation in accordance to
ASME PTC 19.3 TW-2016 step-bystep. The diagram addresses a failed
frequency ratio only. Consult the WIKA
operating instruction for a complete listof possible error codes.
Because of variation in thermowelldesigns in combination with various
process parameters not all wakefrequency calculations can follow thisstandard procedure.
If the illustrated procedure will
not bring you to a good result,please contact your WIKA sales
representative for support, because a
individual engineered solution may berequired.

Wake frequency calculationin accordance toASME PTC 19.3 TW-2016

TW is outside of
the scope of thePTC 19.3 TW-2016

Yes

Please contact
your WIKA sales
representive for support

Stelliteoverlays,
rough surface, support
collar, spiral ridges designs,stepped bore or different
shank design
specified?
No

Wake frequency
calculation toASME PTC 19.3 TW-2016

Support collar
orScrutonWell® design

Yes

Result passed?
NoSuggested
unsupported length(L) > nozzle projection +wall thickness +2" (50 mm)

Yes

No

WARNING!
In case of “loop-situation”,
please contact your WIKA
sales representive forsupport.

Change unsupported
length to Lopt andrecalculate

Result passed?

Yes

No

Increase tip and/or root
diameter and recalculate

Result passed?

No

Yes

END of wake frequency
calculation
TW passed

WIKA data sheet IN 00.15 ∙ 01/2019

Page 4 of 7

Design details
Determine 1st support collar location
The thermowell support collar location will be calculated as:
Nozzle projection – 1" (25.4 mm)
For example:14" (355.6 mm) nozzle projection. The 1st support collar
would be located at 13" (330.2 mm) from the flange face.
The nozzle projection length is defined as the length frompipe/tube outside diameter to height of fitting (flange facing or
sock-o-let, etc.).

Nozzle projection

Nozzle projection
lengthDetermine the number of support collars and locations
If the 1st support collar location is less than 5" (127 mm) only
one support collar is required.
If the 1st support collar location is 5" (127 mm) or more, a
2nd support collar is required and located at 1st support
collar location divided by two. For nozzle length greaterthan 30" (762 mm), please consult your WIKA sales
representative.
Example No. 1 – Two support collars
Nozzle length is 14" (356 mm). Support collar 1 located at
14" (356 mm) – 1" (25.4 mm) = 13" (330 mm). Since thisnumber is greater than 5" (127 mm), two support collars will
be required. Therefore 13" (330 mm) / 2 = 6.5" (165 mm).
Support collar 2 located at 6.5" (165 mm).
Example No. 2 – One support collar
Nozzle length is 4.5" (114 mm). Support collar 1 locatedat 4.5" (114 mm) – 1" (25.4 mm) = 3.5" (89mm). Since thisnumber is smaller than 5" (127 mm), one support collar will
be required.

WIKA data sheet IN 00.15 ∙ 01/2019

Support collar 2

Support collar 1

Page 5 of 7Typical installation through a nozzle
Detailinterference fit between
support collar andinside diameter ofnozzle≥ 45 mm (1.75")
12.7 mm (½")

see “Detail”

4-point support collar“A”

tapered insertion
length Ured“A”

Process

Section “A”-“A”

Determine the support collar outside diameter from the pipe size and schedule
NPS

UOM

Outside diameter of support collar

1"

inch

1.107

1 ½"2"

mm

inch

mm

inch

mm

SCH.10

SCH.40

SCH.STD

SCH.80

SCH.XS

SCH.160

SCH.XXS

28.1

26.9

26.9

24.6

24.6

21.0

15.5

1.692

43.0

2.167

55.0

1.059

1.620

41.1

2.077

52.8

1.059

1.620

41.1

2.077

52.8

0.967

1.510

38.4

1.949

49.5

0.967

1.510

38.4

1.949

49.5

0.825

1.348

34.2

1.697

43.1

0.609

1.110

28.2

1.513

38.4

Determine the suggested maximum root diameter based on nozzle size and schedule
NPS

UOM

Suggested root diameter at support collar

1"

inch

0.938

1 ½"2"mminchmminch

mm

SCH.10
23.8

1.500

38.1

1.875

47.6

WIKA data sheet IN 00.15 ∙ 01/2019

SCH.40
0.875

22.2

1.375

34.9

1.750

44.5

SCH.STD
0.875

22.2

1.375

34.9

1.750

44.5

SCH.800.813

20.61.250

31.8

1.625

41.3

SCH.XS
0.813

20.6

1.250

31.8

1.625

41.3

SCH.160
0.688

17.5

1.125

28.6

1.500

38.1

SCH.XXS
0.500

12.7

1.000

25.4

1.250

31.8

Page 6 of 7

Design specifications to ASME PTC 19.3 TW-2016
Description

Tapered and straight designs

Stepped design

63.5 mm (2.5")

609.6 mm (24")

127 mm (5")

609.6 mm (24")

9.2 mm (0.36")

46.5 mm (1.83")--Minimum

Insertion length L

3.175 mm (0.125")

Bore diameter dTip diameter B

Taper ratio B/A

Step ratio B/A for B = 12.7 mmStep ratio B/A for B = 22.2 mm
Bore ratio d/B

-

0.16

per ASME PTC
19.3 TW-2016

in WIKA datasheetsLU

Tip diameter

BVdB

A

Root diameter

1-

0.5

0.8

0.583

0.875--

2--0-

Maximum

6.7 mm (0.265")--

Insertion length
Bore diameter

6.1 mm (0.24")

0.71

3 mm (0.12")

Min. wall thickness (B-D)/d20.9 mm (0.825")--

Length ratio Ls/L

Minimum-

2

Cross-sectional ratio L/B

Marking

0.58Maximum

0.63 mm (0.12")-If the thermowell dimensions, based on customer
requirements or for specific applications, lie outside therequirements of ASME PTC 19.3 TW-2016, the calculation
results can only be used for informative purposes.
On these grounds, a guarantee cannot be given by WIKA.

Q

Provision of calculation data
The examples in the following table show how the process
and geometry data should be provided as an excel
spreadsheet for further electronic processing by WIKA.
Sample table including calculation data for 6 measuring points
Tag no.

T

Pv

rho

TW-0301

220

1.5

23.6

2.4

TW-0305

2351019.66.1

TW-0303

TW-0307

TW-0309

TW-0311in °C
220220235400

in bar
1.5
1030

31.5

in m/s
25.7
13

8.9

31.9

LegendTag No. Measuring point No.
TTemperature
PPressurev
Flow raterho
Density of process medium

in kg/m³

Dyn. viscosity Model
in cP

Dimensions in mmLØd

ØA

ØB

Tt

NID

NL

Material
(EN)

2.0

0.017

250

8.5

2519

6.4

38.3

220

1.4435

8.9

28.3

10.1
L
ØdØA
ØB
Tt

0.013

TW100.015

TW100.014

0.013

0.017

TW10TW10TW10TW10Insertion length
Bore size
Root diameter
Tip diameter
Tip thickness

250250355355355NID
NLModel

8.5

8.5

8.5

8.5

8.5

25

25

25

2525

19

19

19

19

19

6.4

6.4

6.4

6.4

6.4

38.3

38.3

38.3

38.3

38.3

220

220

220

220

220

1.44351.44351.45711.45711.4571

Nozzle inside diameter
Nozzle length
WIKA thermowell model

01/2019 EN© 03/2004 WIKA Alexander Wiegand SE & Co. KG, all rights reserved.
The specifications given in this document represent the state of engineering at the time of publishing.
We reserve the right to make modifications to the specifications and materials.

WIKA data sheet IN 00.15 ∙ 01/2019

Page 7 of 7

WIKA Alexander Wiegand SE & Co. KG
Alexander-Wiegand-Straße 30
63911 Klingenberg/GermanyTel.
+49 9372 132-0
Fax+49 9372 132-406
info@wika.de
www.wika.de