Service

Thermowells

How high is the permissible pressure loading for thermowells?

In the Appendix to DIN 43772 are loading diagrams, from which, depending on temperature and medium, can be taken, the maximum allowable pressure load for the different geometries. If the conduit geometry does not correspond to DIN 43772, individual calculations can be performed in accordance with ASME PTC 19.3 or Dittrich / Klotter, which as static results include the max. pressure loading.

What is the maximum permissible temperature for thermowells?

The maximum temperature depends on the materials used and the standards which must be met.  So, for example, a standard stainless steel can be used in air up to about +900 °C, the maximum operating temperature is approximately +600 °C and an approval can be made up to +450 °C.

What are suitable materials for thermowells for negative temperatures?

The first choice for high-temperature applications should always be stainless steel, such as 1.4404 or 316L. (Approval per AD2000 W10 down to -270 °C) Carbon steels should be considered carefully in detail, through the effect of the drop-off effect.

What is the difference between one-piece and fabricated thermowells?

Fabricated thermowells are manufactured from tubes which are sealed by a solid welded tip (for example) at the process. Single piece thermowells are manufactured from a complete element of bar stock (round or hexagonal).

When are one-piece or fabricated thermowells typically used?

Fabricated thermowells are generally recommended for low to medium process loads. One-piece  thermowells are suited to the highest process loads, depending on their design. Thus internationally or in the petrochemical industry, one-piece thermocouples are now used almost exclusively.

What are the factors influencing the response times of thermowells?

Put simply, one can say that the more stable a thermowell is constructed, the slower it reacts to temperature changes. In order to optimise the response time, there are thin wall thicknesses and low air space between sensor and the bore's interior walls. Further optimisations in design are pocket-drilled bottoms and effective insertion lengths of more than 100 mm.

What is meant by double-certified materials, such as "SS 316/316L"?

Dual certified materials fulfil the requirements of the individual materials.  The material SS316 has, per ASTM A182, a maximum carbon content of  0.08 %;  the material SS316L (L=low carbon) has a maximum carbon content of 0.03 %. Steel alloy, with, for example, C=0.02 %, fulfil both requirements and can be marked with SS316/316L.

What do the markings on  'sealing faces to ASME B16.5' mean?

RF - Raised Face:
Sealing faces with a standard roughness "Stock Finish" 125-250 AARH to B16.5
RFSF - Raised Face Smooth Finish:
< 125 AARH (not defined in B16.5)
RTJ - Ring Joint Groove/RJF Ring Joint Face  < 63 AARH to B16.5

Obsolete descriptions were in accordance with ANSI:
- Stock Finish 250-500 AARH
- Smooth Finish 125 -250 AARH
- Mirror Finish
- Cold water finish
without definition of the roughness.

Why do older themocouple designs often have a spherical tip?

In the past, HSS drills were used with a tip angle of 118 ° for the production of thermowells.  In order to achieve a possible uniform wall thickness, the tip was ball-shaped or spherical in shape.  The current state of production technology enables the use of special deep hole drills, which allow a nearly flat bottom to the bore.  For this reason, modern thermowells (eg DIN 43772) with a flat tip shape can be made.

What is the minimum insertion length of a thermowell?

The insertion length of a thermowell will be specified through the thermometer  used. In general one can assume a length of  60-100 mm for mechanical thermometers from a minimum total length. Electrical thermometers need an insertion length of at least 35 - 50 mm. Each individual case should be checked, though.

What is the maximum insertion length for a thermowell?

For multi-piece thermowells, the maximum length is limited by the manufactured lengths of the tubes, which is about 5-6 meters.  One piece thermowells are made of solid material and limited by the production length of the drill hole, which, for each product is between 1,000 mm and 2,000 mm. . Longer one-piece thermowells have to be made by welding individual elements together.

What is the appropriate sensor length for a thermometer within a thermowell?

For mechanical thermometers, the sensor must have no contact with the bottom of the bore, rather it must be arranged with an air gap of 2-5mm.  For electrical thermometers, the sensor is spring-loaded, since the sensor tip must be touching the bottom of the bore, where the sensor yields approximately 2-5mm.

What should be the insertion length for thermowells in pipes?

Generally, it must be ensured that the sensor of the thermometer has the medium flowing past it. This is generally achieved by having the thermowell tip in the middle third of the pipeline.

Why do modern thermowells mainly have female threads for thermometer connections, and not male threads as in older specifications?

The risk of damage with  female threads is less than with male threads.Since the replacement of thermowells is always fraught with difficulties. Since it allows the thermometer to be removed without difficulty while the plant is running, this configuration is recommended. In the past, most thermometers were used with union nuts that fitted the male threads on the thermowell.

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.

Why do some users specify a polished thermowell surface, and others define a high roughness or knurling of the area in the flow?

This depends on the usage of the thermowell.  A polished surface has a higher corrosion resistance than a rough surface.  The rough or knurled surface has an advantage with respect to the vibrational excitation by the Karman vortex street, meaning such thermowells can withstand higher flow rates than smooth thermowells.

Do thermowells need to be CE marked?

Thermowells must not be CE marked, in principle. An exception as a result of its special design is the model TW61 thermowell with DN>25, suitable for orbital welding. This must be CE marked in accordance with the Pressure Equipment Directive (97/23/EC).

Are there any GOST certificates for thermowells?

No. GOST certificates only exist for measuring instruments and a thermowell is only considered a component part of a thermometer.

What information is needed in order to perform a thermowell calculation in accordance with ASME PTC 19.3 TW-2010?

For this one needs the following information:
- Temperature
- Pressure
- Flow rate
- Density of the medium
- Insertion length
- Ø of the bore
- Root diameter
- Tip diameter
- Tip thickness
- Interior diameter of the adapter
- Height of the adapter

Further information can be found in our Technical Information IN 00.15 "Strength calculation for thermowells" in the download area of www.wika.de.

Can the calculations per ASME PTC 19.3 TW-2010 be used for one-piece and fabricated thermowells?

No. The calculation per ASME PTC 19.3 TW-2010 is only used for one-piece thermowells in tapered, straight or stepped designs from solid materials, such as Model TW10, TW15, TW20, etc.

Which models from the current DIN 43772 correspond to the old DIN 16179 and DIN  43763?

DIN 16179
BD = Form 5
BE = Form 6
BS = Form 4
CD = Form 8
CE = Form 9
CS = no longer specified
DIN 43763
Form A = Form 1
Form B1-B2-B3-C1-C2 = Form 2G (partial)
D1-D2-D3-D4 = Form 4 and neck tube
Form E1-E2-E3 = Form 3 (partial)
Form F1-F2-F3 = Form 3F (partial)
Form G1-G2-G3 = Form 3G (partial)

previously not standardised: Form 2F, 4F, 7