Categorization of Thermowells for PED

A Thermowell cannot be CE marked under the PED. Rather, it is considered to be a component part of pressure equipment or safety equipment. The material certificates, hydrostatic testing report, and supporting technical documents should be forwarded with the product to the end user for incorporation into the pressure equipment that will be CE marked according to PED.

The Pressure Equipment Directive (PED) 97/23/EC applies to pressure equipment in the form of pressure vessels, pressure accessories, safety accessories, piping, and assemblies. Equipment must fulfill certain requirements in order to fall under the jurisdiction of PED and have the CE mark affixed accordingly. It must have an identifiable pressure housing and have a rated operating pressure above 0.5 bar. It must then fall under category I through IV according to Annex II of PED.

In the case of Thermowells, which do not of themselves have an identifiable pressure housing, but rather contribute to a pressure housing, the EU Commission’s Working Group “Pressure”(WPG) has confirmed in guideline 1/40 that Thermowells are a component of an item of pressure equipment, but not a pressure accessory which should be CE marked under PED.

1/40
Question:

What does pressure bearing housing mean in the definition of pressure accessory in Article 1 paragraph 2.1.4 ?

Answer:

The term pressure bearing housing refers to an envelope in which fluid under pressure (PS > 0,5) is contained or transported (volume V > 0).

Therefore, a product whose only pressure-bearing surface is a flange or screwed fitting is not a pressure accessory (e.g. Level Switch, Flush Mounted Pressure Transmitter, Thermowell) but is a component of an item of pressure equipment under the Pressure Equipment Directive (PED) when used on such equipment.

Note :This does not apply to such devices when employed in a safety function.

See also guidelines : 1/8, 1/22, 1/25 and 7/19.

It is recognized that Thermowells may be used (as a sensor) in a safety system towards controlling temperature as a limitation of the pressure equipment. WPG Guideline 1/20 clarifies that measurement systems or equipment alone are not safety accessories under PED:

1/20
Question:

When is a measuring or control system considered as a safety accessory under the PED?

Answer:

A measuring system alone cannot be considered as a safety accessory, as a safety accessory as defined in PED necessarily requires :
- a measuring or detection function and
- an activation function for correction, or shutdown, or shutdown and lockout.

See also Guidelines: 1/25 and 2/16

WPG Guideline 1/25 further confirms that sensors alone can not be considered as safety accessories and should not be CE marked (due to PED):

1/25
Question:

Are the sensors which are used as part of a safety system to protect pressure equipment covered by the PED ?

Answer:

A sensor alone does not meet the definition of a pressure accessory, as per Article 1.2.1.4 (see guideline 1/8), nor the definition of a safety accessory, as per Article 1.2.1.3. Consequently, no CE marking (due to the PED) is to be put on the individual sensor.

The conformity assessment procedure and essential safety requirements of the directive relate to the complete safety system. The requirements to the sensor may be different depending upon the safety concept employed (for example redundancy or fail safe, see Annex I point 2.11.1).

Note: For the purpose of this guideline, sensor means "element of a measuring instrument or measuring chain that is directly affected by the measurand" as defined in the International Vocabulary of Basic and General Terms in Metrology, prepared by BIPM, OIML, ISO, IEC.