Answer / carl_ellis

From Wika's FAQ:

What tests and inspections are stipulated for thermowells?

In accordance with DIN 43772 Point 4.6, all tests and
certifications should be agreed between the manufacturer and
operator.

What tests are usual or possible for thermowells?

Common non-destructive tests are the pressure test and, for
welded thermowells, the liquid penetrant test. In addition,
to test the centrality of the bore, ultrasound or X-ray
testing is possible. To test the sealing, helium leak
testing is an option. The surface finish or surface
hardness may also be tested. A material test would be
Positive Material Identification (PMI test).

What does ZFP, NDE or NDT mean?

ZFP is the German abbreviation for "Zerstörungsfreie
Prüfungen" (non-destructive examinations). The abbreviations
NDE or NDT stand for "Non-Destructive Examination" or
"Non-Destructive Testing", respectively. This is used to
refer to non-destructive inspections or tests on components
in general.

What is a hydrostatic pressure test?
The hydrostatic pressure test is a pressure and strength
test of the components of a thermowell in accordance with
the AD2000 data sheet HP30. For the test, the thermowell is
clamped into a test fixture and loaded at room temperature
with a defined test pressure and duration (e.g. three
minutes). In general, one differentiates between external
and internal pressure testing. Typical test pressures are
1.5 times the nominal pressure of the flange with external
pressure, or 500 bar with internal pressure. The test is
performed with water with a chloride content < 15 ppm. After
passing the hydrostatic pressure test, the thermowell is
marked with a "P".

What is a helium leak test?

For leak testing in accordance with DIN EN 1779 (1999) / EN
13185, helium 4.6 is used as a test gas. The test is able to
detect minimal leakage rates and is considered the most
sensitive test method for leak testing. In general, one
should distinguish between an integral and local test
method. In the integral test, leak rates (e.g. 1x10-7 mbar *
l / s) can be determined, while the local testing enables
the location of the leak to be determined using a spray
probe. After passing a helium leak test, the thermowell is
labelled with a corresponding sticker.

What is a PMI test?

The PMI (positive material identification) test proves which
alloy constituents exist in the material. There are various
common test procedures. With optical emission spectrometry
(OES) in accordance with DIN 51008-1 and -2, an arc is
generated between the thermowell surface and the test
equipment, and the spectrum of this arc enables the alloy’s
elements to be identified – both qualitatively and
quantitatively. A characteristic feature of this procedure
is the fire mark that is left on the workpiece. A test
procedure which doesn’t damage the surface is X-ray
analysis; during the X-ray the atoms of the thermowell
material are energised until they radiate themselves. The
wavelength and intensity of the emitted radiation is again a
measure of the alloy’s constituent elements and their
concentrations. Following a successful PMI test / positive
material identification test, the thermowell is marked with
"PMI".

What is a dye penetrant test?

With the penetrant test in accordance with DIN EN 571-1,
fine surface cracks and porosities in weld seams can be made
visible. After cleaning the surface to be inspected, a
contrast agent (red or fluorescent) is sprayed on. Through
the capillary effect, this agent penetrates any surface
defects there might be. After re-cleaning the surface, a
developer (white) is then sprayed on, which extracts the
contrast agent (from any hairline cracks, etc.) and through
colour contrast, enables an easy evaluation of the defects.
After passing a liquid penetration test, the thermowell is
marked with "PT".

What is a X-ray testing?

Through an X-ray test to EN 1435 or ASME Section V, Article
2, Edition 2010, for example, full penetration welds on
thermowells can be investigated with respect to
irregularities (cracks, voids, insufficient bonding). Here,
depending on the dimensions of the thermowell, up to five
X-ray images may be necessary to determine irregularities
with sizes < 0.5 mm in the full-penetration weld. An X-ray
examination can also be used to record the bore centrality
in solid body material thermowells. For this purpose, two
images of the thermowell tip at 90° to each other are required.

What is an ultrasonic test?

Through an ultrasonic test to DIN EN ISO 17640, for example,
full penetration welds on thermowells can be investigated
with respect to irregularities (cracks, voids, insufficient
bonding). To do this, the reflections of a radiated
ultrasonic signal from the interfaces of irregularities are
measured. To determine the position of the irregularities,
the ultrasound machine is set in advance with the aid of a
reference body. The ultrasonic method can also be used to
measure the wall thickness of a solid body material
thermowell, in order to determine the bore centrality.

Carl Ellis
Measure First

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