Question

The flame (flashback) arrestors, used, in accordance with the norm EN 730-1, for gas welding equipment subject to the PED, are to be considered as safety accessories or pressure accessories as per PED?

Discussion

In order to perform a correct classification, it needs to identify the function of these devices, the final goal that is achieved with their entering in service.

As first step, it is to be clarified that these devices, beyond the function of flame arrestor and preventing any gas flow reversal, can have also the function to stop the gas flow and prevent contamination by means of suitable dust filter located at the gas inlet. Their primary function is the protection of the plants devoted to the technical gas distribution (for welding, oxy-fuel cutting and related processes), preventing the gas flow reversal (as check valves) and the flashback.

In general, they are equipped, in combination with the flame arrestor, with a temperature sensitive cut-off device that, according to the EN 730-1 definition, cuts-off the gas flow when the temperature reaches a predetermined value. This last device cuts-off the gas flow before the temperature at the flame arrestor reaches a point where the flame transmission across the flame arrestor takes places.

As soon as the cut-off device becomes active, the valve continues to be part of the pressure boundary of the upstream system sharing with it the allowable pressure and temperature limits (which in general will be higher than the values existing when the device became active). The valve, therefore, has not the function to protect the upstream pressure system from exceeding the allowable limits (pressure and temperature), but it also needs to be protected by a suitable safety device. The valve shall be able to withstand the pressure of the system where is installed until the upstream safety device becomes active.

From the above, it follows that, according to PED, these valves are to be considered as pressure accessories and not as safety accessories, since they are not designed and placed on the market as ultimate means of protecting the pressure system against exceeding the PED allowable limits (pressure and temperature), but only as protecting means of the equipment for gas welding

The hazard analysis, properly performed, shall document this function, by proving that the failure of this device would not have detrimental effects on protecting the system where it is installed against exceeding the PED allowable limits, what is ensured by the adoption of a specific safety accessory suitable for this purpose.

Normative background

The safety accessories are defined by the article 1, clause 2.1.3, and discussed in clause 1.4 of article 3 (technical requirements), in annex I, clauses 2.10 (protection against exceeding the allowable limits of pressure equipment) and 2.11 (safety accessories) and in annex II.

The annex II states that “The safety accessories defined in Article 1, Section 2.1.3, and referred to in Article 3, Section 1.4, are classified in category IV. However, by way of exception, safety accessories manufactured for specific equipment may be classified in the same category as the equipment they protect.”

The guide-line approved by the technical committee dealing with the subject, directly or less, are: 1/20, 1/42, 1/43, 2/16, 2/29, 2/32, 2/33, 3/17, 5/6, 8/2, 8/13, 9/7.

The guide-line 1/20 clarifies the conditions for which a safety device is to be classified as safety accessory as per the articles above:

“In order for a control system to be classified as a safety accessory, it shall be designed and placed on the market as an ultimate means of protecting pressure equipment from exceeding allowable limits, and therefore it shall meet the corresponding essential requirements of Annex I, section 2.11.”

The guide-line 2/16 states the following:

“In general pressure regulators are pressure accessories.

Only in the case where they fulfil the definition of safety accessory and consequently have a specified safety function, they are to be considered safety accessories and they shall meet requirements of Annex I, section 2.11.

When a pressure regulator is installed in an assembly where the design pressure of the system downstream of the device is lower than the pressure which can occur upstream of the device, and the system downstream is not protected by a safety accessory, the manufacturer of the assembly must ensure that this pressure regulator fulfils the requirements of a safety accessory.

Note: It is foreseeable that some pressure regulators without specific safety function could be inadvertently used as safety accessories. The manufacturer of the pressure regulator must include an appropriate warning in their instructions for use.”

The guide-line 2/32 clarifies that “according to the definition in Article 1 paragraph 2.1.3, a safety accessory is designed to protect pressure equipment against exceeding the allowable limits.”

The guide-line 2/33 states what follows:

“Question: When a safety accessory consists of a safety chain which itself includes “items of pressure equipment” (for example a valve or a cylinder), in which category shall this “equipment” be classified ?

Answer: When items of pressure equipment are integrated in a safety chain, they are considered as parts of the safety chain and therefore fall under the hazard analysis of the safety chain, which include the pressure containment aspect of this item. When the hazard analysis of the safety chain shows that the failure of an individual item of pressure equipment within the chain would have no detrimental effect on the safety function to be ensured (i.e. fail-safe), the requirements of a category lower than category IV for the said "item of pressure equipment" can satisfy the requirement resulting from the hazard analysis of the safety chain.

Its integration in the safety chain is achieved by using the category IV or the category of the equipment for which the chain is specifically designed.

Note 1: This does not preclude the use of standard CE-marked items of pressure equipment as parts of a safety chain.

Note 2: A safety accessory, even when it is a safety chain, cannot be classified as an assembly.”