Define frequency of inspection

The frequency of inspection is how often inspection of a lot is carried out. It should be varied depending on the following factors, the first three of which provide an option for

enhancing RBI:

C equipment criticality (see B.3.5.1);

C lot size and ASL (see B.3.5.2);

C confidence in historical records (see B.3.5.3), and C other constraints (see B.3.5.4).

In practice, the frequency of inspection should be within the limits defined in IEC 60079-17 (see section 2.9.2), where either the equipment records are unknown or poor quality, or where no records exist (such as in a new installation). In the absence of any specific information, an annual frequency of inspection should constitute a reasonable starting point providing inspections are not restricted by infrequent shutdown intervals (see B.3.5.4).

B.3.5.1 Equipment criticality

Equipment criticality is a key factor in determining the level of inspection for a lot. When applied correctly, it should result in high risk equipment located in high risk areas being inspected at a higher level of inspection than other equipment located in other areas.

For example, in an RBI strategy, lots in Zones 1 (i.e. high risk areas) should be inspected more frequently than those in less critical areas (i.e. Zone 2 and non hazardous areas).

This will result in a more targeted, effective and balanced approach, which should meet the requirements of risk-based Ex inspection.

Equipment criticality is influenced by ignition risk, severity of the environment and equipment age, as described in B.3.5.1.1-B.3.5.1.3.

B.3.5.1.1 Ignition risk

The ignition risk due to the simultaneous presence of a potential source of ignition (caused by a fault in Ex electrical equipment) and a flammable atmosphere (i.e. ignition risk) is a key factor in determining Ex electrical equipment criticality and the necessary level of inspection. The methodology presented in this section provides an estimation of ignition risk when using Ex electrical equipment that may be subject to some degradation of its type of protection. The methodology enables discrimination between equipment whose protection is no longer assured when operating in a Zone 0 hazardous area compared to a lower risk situation when similar faulty equipment is operating in a Zone 2 hazardous area.

The ignition risk is the product of the probability of a source of ignition being present and the probability of a flammable atmosphere being present. There follows guidance on this determination.

Probability of source of ignition being present

The probability of equipment used in a zone becoming faulty which leads to it constituting a source of ignition in that zone is defined as:

C low ignition risk;

C medium ignition risk, and C high ignition risk.

Probability of flammable atmosphere being present

The probability of a flammable atmosphere being present is defined by reference to the pertinent hazardous area classification; the methodology of EI Area classification code for installations handling flammable fluids refers to the following zones:

C Zone 2 (low risk): That part of a hazardous area in which a flammable atmosphere is not likely to occur in normal operation and, if it occurs, will exist only for a short period.

C Zone 1 (medium risk): That part of a hazardous area in which a flammable

atmosphere is likely to occur in normal operation.

C Zone 0 (high risk): That part of a hazardous area in which a flammable atmosphere is continuously present or present for long periods.

Strictly, Ex electrical equipment is not required in non-hazardous areas as there is no probability of a hazardous area being present in routine operations; however, in the case of offshore installations which are compact, it might be prudent to consider external non-hazardous areas as Zone 2 areas as they may contain Ex electrical equipment that is energised in a major hazards emergency.

Ignition risk

The ignition risk is the product of the probability of a source of ignition being present and the probability of a flammable atmosphere being present and forms the basis for an RBI strategy. This is defined by three levels and determined by Table B.4:

Table B.4: Ignition risk

Probability of flammable atmosphere Probability of

source of ignition Zone 2

(Low) Zone 1

(Medium) Zone 0 (High)

Low Low Low Low (Note 3)

Medium Low Medium Low (Note 3)

High Medium High Low (Note 3)

Key

Low risk Medium risk High risk Notes:

1. Table replicated as Table B.6.

2. The table assumes correct selection of Ex type of protection in the first instance.

3. For Zone 0, use of intrinsically safe systems (Ex 'ia') means that there is an inherent low ignition risk.

Table B.4 should be used as a starting point in determining the frequency of inspection in an RBI sampling strategy on the basis:

high risk > medium risk > low risk

The guidance that follows in B.3.5.1.2-B.3.5.1.3 should be used to adjust the frequency of inspection, e.g. Ex electrical equipment exposed to a severe environment category might be moved from low ignition risk to medium ignition risk.

Note that the sampling methodology does not take into account the consequences of fires/explosions to evaluate the ignition risk; it is assessed only by their probability.

A more practical approach to Table B.4 using common equipment types is set out in Table 3.1.

B.3.5.1.2 Severity of the environment

The severity of the environment in which Ex electrical equipment is used should be determined using Table B.5. Where water is an environmental factor, it could be in the form of ‘green’ water, as encountered on floating production, storage and offloading

units (FPSOs), or deluge system water. Mechanical damage could refer to drill floor areas of offshore installations.

For the purpose of varying the frequency of inspection, the severity of the environment should be classified as:

C Severe – if at least one environmental factor is considered severe.

C Moderate – if no environmental factor is considered severe, but at least one is considered moderate.

C Benign – in all other cases.

Table B.5: Severity of the environment

Environmental category

green water (e.g. FPSOs) Inside Inside, in a controlled environment Dust/sand Dust layer visible in

≤1 day

Dust layer visible in

>1 day but <1 week Dust layer visible in

>1 week UV radiation Outside with high

sunlight Outside with moderate

sunlight Inside Ambient temperature (°C) >40 or <-20 N/A -20<T<40 Temperature cycling Outside or inside

enclosure having high

Vibration High (e.g. offshore

installation drill floor) Moderate Low (e.g. inside an enclosure) Key N/A = not applicable

B.3.5.1.3 Equipment age

For the purpose of varying the frequency of inspection, equipment should be divided according to the following three age classes:

C ≤5 years old;

C >5 years old but ≤20 years old, and C >20 years old.

Equipment age may affect its protection; where possible, such criteria should be referenced to design life.

For simplicity and to avoid defining small lots, a pragmatic approach would be to select a single conservative age for all equipment on a particular installation, equal to the age of the installation.

These criteria are similar to those used in defining lots (see B.3.1.4).

B.3.5.2 Lot size and ASL

A consequence of requiring broadly consistent statistical confidence in inspection results for a particular ASL is that samples randomly selected from large lots typically comprise only some 15% of a lot; whereas, for small lots, random samples may typically comprise some 25% of a lot. Some risk analysts may consider that 15% is too small an amount to sample, given that inspections may be carried out annually, or maybe less frequently (e.g. petroleum refineries having four-yearly shutdowns); if so, the frequency of inspection could be reduced and/or other level of inspection criteria could be adjusted,

e.g. the ASL could be reduced. Reducing the ASL increases Pa(10%), which indicates the confidence in the sampling plan being applied to the lot (see B.3.3.4).

B.3.5.3 Confidence in historical records

The frequency of inspection may be varied using knowledge of the fault modes and frequencies of faults of equipment; in practice, the frequency of inspection should be set based on known performance.

This confidence in historical records database should be classified as:

C High – there is a database where all equipment and its history is recorded and there is a confidence level of at least 70% in its accuracy.

C Moderate – there is either a database where all equipment and its history is recorded and there is a confidence level of at least 40% in its accuracy, or a person highly competent with such equipment has validated/challenged the historical data.

C Low – in all other cases.

Once several inspections have been carried out, confidence in historical records should improve as the exhaustiveness of the database improves; further, confidence in its accuracy should increase and more history should be recorded. When this is so, the level of inspection should be reviewed.

B.3.5.4 Factors governing frequency of inspections

In the absence of any specific information, an annual frequency of inspection should constitute a reasonable starting point providing inspections are not restricted by infrequent shutdown intervals. Applying the change rules (see B.3.9.3) to such a sampling plan would take five acceptable inspections to go from normal to reduced inspection (i.e. five years), two unacceptable inspections to go from normal to increased inspection (i.e. two years) and five acceptable inspections to revert from increased to normal inspection (i.e. five years).

For process plants, an annual frequency of inspection for Ex detailed inspections (which requires the equipment to be isolated) might not be possible owing to infrequent shutdown intervals (e.g. four-yearly). In such cases, selection of a sampling plan requires further consideration. An alternative approach would be to consider that the annual sampling plan is applied four times at each shutdown (the number of times being equal to the number of years between inspections); each sample would be considered as a separate inspection and tested against the unadjusted rejection criterion. This approach has the advantage of more quickly invoking the change rules.

In the same way that circumstances may necessitate equipment being allocated to specific lots (see B.3.1.6), moveable equipment (hand-held, portable and transportable) or SCEs should be given particular frequencies of inspection. For moveable equipment, the frequency of inspection should consider factors such as the pattern of use and working environment.