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Ex Special Interest Group Briefing Note: Ex Inspection Sample Nomination

Author : HARVEY T. DEARDEN, HTS ENGINEERING GROUP

24 May 2021

The Institute of Measurement and Control Explosive Atmospheres Special Interest Group (Ex-SIG) aims to promote good practice and support continuing professional development in the Ex discipline through a range of activities and publications. The group produces briefing notes to help inform members on key topics. This article is one such briefing note on Ex inspection sample nomination.

(Ex-SIG Briefing Notes are first released to members of the SIG before being made publicly available)

Sample Ex inspections per IEC60079-17 are constituted not to detect random failures but rather systematic issues; issues that are liable to be common to an asset type and deployment circumstance. (Note there is no requirement for all assets to be subject to a periodic detailed inspection.)

Sample inspections should not be expected to reveal faults of a random nature, such as loose connections, but should be used to monitor the effects of environmental conditions, vibration, inherent design weakness, etc.

IEC 60079-17: 2017 4.3.3

In this respect the important thing about an inspection sample is that it should be representative; this means that the assets selected should reflect both the range of equipment types employed, and their duties (if these may vary significantly across an installation).  If there is a confidence that a given subset of a wider population suffers more severe conditions of use, then a sample drawn from that subset may be taken as conservatively representing the total population. 

In terms of systematic issues, for a given equipment type, a single asset that had operated in the harshest duty and was of the greatest age could be taken as conservatively representative of all those items of the same type that were on less severe duties or were younger.  This may appear counter-intuitive, with a residual concern that one might be ‘lucky’ with the selected asset, but luck implies a random aspect which is specifically excluded. Additional units might be selected as a confidence boosting measure, but there would be no additional value in terms of identification of systematic effects.

Simple nomination of X% of assets is often employed but this may involve an unwarranted degree of disturbance and may not produce a properly representative sample.  Detailed inspection is necessarily invasive and will require disturbance to the installation; there is the very real possibility that the inspection itself may compromise the protection, particularly in respect of cover fitting and sealing, which may be made more difficult through the ageing of the asset or subsequent painting for example.

If there is some uncertainty in identifying appropriate duty subsets, or the degree to which these may or may not overlap, then it may be appropriate to expand the sample size.

Post initial detailed inspection at commissioning, the focus of subsequent detailed sample inspections pertaining to safety (as distinct from loss of service) should be those faults that:

a) Represent a potential ignition hazard AND

b) Would not be revealed through functional failure (prompting intervention) AND

c) Would not be revealed by close/visual inspection.

Note that in this respect there is no requirement for a detailed inspection of a single circuit field item if the protection is Ex ‘i’, since degradation of the field asset would not generate an ignition source. If the field item is unsuitable in some way, this should be revealed by a close/visual inspection.

Note that there is an obligation here to investigate or isolate zoned equipment that has failed in service with appropriate urgency (possibly linked to zone of use and the nature of the equipment), since the fault may have produced a potential ignition source.

The sample should be drawn from the population on the most severe duty on which the equipment type is deployed, or from a selection of such duties if different duties are identified as being severe for different reasons. One population subset might suffer high vibration and another high temperature cycling. If there was a subset that suffered both, the selection should be made from that subset. The selected sample should be drawn from the oldest of those assets within this subset population.

So, the assets selected for inspection should, as far as is practicable, be the oldest on each identified distinct severest duty subset.

Note that key to effective sampling is the identification of relevant duty subsets. Although a high vibration environment might be perceived as presenting higher severity than one with high temperature, they may present entirely different degradation mechanisms.  If an environment presented high temperature AND high vibration AND included the oldest asset from the three environmental sets (high vibration, high temperature, high vibration and temperature) then that oldest asset could be selected for detailed inspection as conservatively representative of all three.

If all the assets of a given type operate in essentially the same environment, the oldest within that population would be the preferred sample to represent that population.

In selecting sample units for detailed inspection, consideration should also be given to practical concerns. If the preferred asset happens to be relatively inaccessible then it may not be a practical candidate for selection; as long as the selected asset(s) are well towards the oldest age and do not exhibit any significant degradation through ageing, and such ageing mechanisms are considered progressive, then there can be appropriate confidence that the oldest asset itself will not have suffered significant systematic degradation. 

The disturbance to the installed population may be minimised by making a point of undertaking a detailed inspection of those assets that must be disturbed for reasons of routine maintenance or repair, or those that are being retired.

Detailed inspection is a significant burden on plant operation and maintenance and it is well to use a suitably discriminating approach in the nomination of the sample to avoid unwarranted disturbance and demand on inspection resources.
This document is distributed by the Ex-SIG as an information service to the SIG membership. No guarantee is made by the institute or the author(s) concerning the accuracy, reliability or completeness of the information provided. This document should not be construed as providing advice. Readers should satisfy themselves of the applicability of the information provided. Readers make use of the information provided at their own risk.


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