Technical information

Wear-resistant coatings of thermowells
WIKA data sheet IN 00.44

Abrasive wear for thermowells
Abrasive wear, or abrasion, is the term used withthermowells to describe the loss of material caused by themechanical action of a solid on the thermowell caused bya flowing medium. Typical applications are, for example,
an FCC unit (Fluid Catalytic Cracking unit) in a refinery or
coal-dust pipes in power plants.
In order to prevent abrasion, attention must be paid to asuitable material selection when designing the thermowell.
Less sensitive to abrasive wear than the most commonlyused stainless steel are metallic cobalt-chromium based hardalloys.
The most commonly used hard alloy for thermowells isStellite® 6. This can be used as solid-body material, in
welded or sprayed form. Alongside Stellite® 6, Stellite® 12 isalso used.
Stellite® is a registered trademark of Kennametal Stellite.

Damage to abrasively loaded thermowells (examples)

WIKA data sheet IN 00.44 ∙ 12/2018

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Coating with Stellite® through melting
Coating with Stellite® through melting is the highest quality
method, since the Stellite® powder is firmly welded to thecarrier material of the thermowell body. Therefore, thismethod is highly recommended for heavy-duty applications
in refineries of the oil and gas industry.

< 3rd layerStellite®
Laser cladding< 2nd layer< 1st layer

We distinguish between 2 processes:
Basic material■■ Laser cladding

With this process, the Stellite® powder is introduced
into a laser beam and welded to the thermowell surface.
The precisely meterable energy input enables a very
low-distortion coating. By building-up in several layers
welded to each other, large layer thicknesses are possible.

Laser beam
Stellite® powder

Inert gas

Thermowell■■ Plasma Transfer Arc (PTA)

With the PTA process (Plasma Transfer Arc), an arc iscreated between a tungsten electrode and the thermowellbody. The Stellite® powder is introduced to the arc andmelted onto the thermowell material.

Tungsten electrode
Powder nozzle

ArcStellite® powder
Inert gas

Thermowell

WIKA data sheet IN 00.44 ∙ 12/2018

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Coating with Stellite® in spray process
Coating with Stellite® through spraying is a process through which the Stellite® powder is permanently adhered to the surfaceof the thermowell body. This procedure is recommended for normal applications, such as in wastewater treatment plants.
Here we also distinguish between 2 processes:■■ High Velocity Oxygen Fuel spraying (HVOF)

The HVOF process (High-Velocity-Oxygen-Fuel) involves
continuous combustion under high pressure, using awide variety of fuels. The Stellite® powder is fed into theescaping gas jet and accelerated by it. On impacting on
the thermowell surface, the layer is applied through theadhesion of the powder particles.

Spray nozzle
Flame

Stellite® powder
Thermowell■■ Atmospheric Plasma Spraying (APS)The APS process (Atmospheric Plasma Spraying)features an arc ignited between an electrode and thecathode, through which the plasma gas is passed. TheStellite® powder is introduced into the plasma flame
emerging from the nozzle, and it is melted by the hightemperatures, impacts the thermowell and adheres to it.

ElectrodeNozzle

Stellite® powder
Flame

Thermowell

Coating process in overview
Coating process

Resistance

Layer thickness

CostsLaser cladding++++> 3.2 mm possible€€€

Atmospheric Plasma Spraying (APS)++< 1.6 mm€

Plasma Transfer Arc (PTA)High Velocity Oxygen Fuel spraying (HVOF)WIKA data sheet IN 00.44 ∙ 12/2018++++

1.6 mm (standard)< 0.8 mm€€€

Application(typical application)
Oil and gas industry
Oil and gas industry
Wastewater

Pulp industry

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Recommended length of the Stellite® coating
In principle, it is possible to coat the thermowell with Stellite®
over its entire insertion length, whereby the area of theprocess connection (flange or thread) is basically excluded
from the coating.
Since the length of the thermowell shielded by the flange
nozzle is not directly exposed to the abrasive process loads,limiting the coated length should be considered for economic
reasons. In general, an overlap of 25 ... 75 mm is considered
as sufficient.

Shielded area byflange nozzle without
direct flow
Recommended
overlap:
approx. 25 ... 75 mm
Abrasively loaded
area with direct flowfrom the process

Wake frequency calculation
ASME PTC 19.3 TW-2016, in Section “1 - 2 Scope”, excludes
coated thermowells from the scope of the standard.
Original text from ASME PTC 19.3 TW-2016:“Thermowells … including flame spray or weld overlays,at any place along the length of the shank or at the tip areoutside the scope of this Standard.”
For this reason, any wake frequency calculationcommissioned can only be informative in nature.

12/2018 EN© 12/2018 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 00.44 ∙ 12/2018

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