Jan 2019
Gas Distribution and Gas Transmission
Derivation of a Risk Based Approach to High Pressure Filter & Pig Trap Closure Inspection Frequencies
Jan 2019
Mar 2021
SGN, Cadent, National Grid Gas Transmission, Northern Gas Networks and Wales and West Utilities
SGN Keith Ellison Project Lead Jon Todd Northern Gas Networks (NGN) Luke Hollis Cadent Daniel Wyatt Wales & West Utilities (WWU) James Gilliver National Grid
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Network Innovation Allowance
GT - Reliability and maintenance
Gas Distribution Networks
Since the inception and introduction of the pressure systems safety regulations in 1999 the UK gas networks have inspected their filter population and pig trap population on fixed time periods of 6 and 12 yearly intervals for visual and MPI respectively in accordance with the current Written Scheme of Examination (WSoE).  During this period the Networks are not aware of a single instance of a loss of containment caused through corrosion or fatigue crack growth on either asset type.

The majority of the filter population are manufactured from cast steel.  The pig traps closures which are considered to be a critical component of the pig trap vessels are, historically, also manufactured from cast steel.  It is worth noting though that in recent years there has been a move to the ‘band-lock’ closure which is manufactured from forged material.

When inspections are undertaken defects are frequently identified in the cast steel materials. Defects found then must be assessed to determine if they are within each operator’s allowable defect size limits.  If they are not within ‘safe’ operating limits a number of options are available to the Networks, typically:
  • Increased monitoring 
  • Grinding repair
  • Weld repair
  • Replacement
  • Hydrotest revalidation

These options can be very expensive to implement in terms of cost, time and HSE implications.
Due to the nature of the pressure vessels, particularly those manufactured from cast steel, there is increasing suspicion that many of the defects found during inspection are original manufacturing defects. Unfortunately, without the initial inspection records, it is impossible to substantiate this suspicion, but the project will look to prove that the defects are stable i.e. not propagating or growing at an extremely slow rate. In addition, it is also believed that the allowable defect size limits and the allowable grinding limits are unnecessarily conservative.  The implications of this are:
  • Unnecessary inspections are being undertaken 
  • Unnecessary assessments are being undertaken
  • Unnecessary remediation and replacements are being undertaken
as a result of the above, significant additional costs are being incurred along with an additional unnecessary increase in HSE risk from additional driving and on-site activities.
The objective of the scope of work are;
  • Development of a risk based inspection regime for cast steel filter bodies and cast steel pig trap closures. The main risk contributor being considered will be fatigue crack growth.
  • Development of less conservative allowable defect sizes.
  • Development of less conservative grind repair limits
Development of a new tool, effectively a ‘live’ examination specification which will allow Users to update their WSoE’s to reflect the revised Examination Dates which would consider past and future anticipated operational duty.
The Networks currently have approximately 1687 High Pressure (HP) Filters & 200 Pig Traps & have traditionally inspected these filter and pig trap populations on fixed time periods of 6 and 12 yearly intervals for visual and Magnetic Particle Inspection (MPI) respectively to fulfil compliance with the Pressure Systems Safety Regulations (PSSR) & are managed in accordance with the current Written Scheme of Examination these examinations are costly & when Defects are identified these then have to be assessed in accordance with the Defect Management Procedure DAM1 to determine if they are within the Networks allowable defect size limits, which result in either Increased monitoring, Grinding repair, Weld repair, Hydrotest revalidation or Replacement.