DVGW Technology Report No. 3/10

Gas installations need to be constantly maintained to ensure their safe and reliable operation and availability. DVGW-Code of Practise G 495 describes how to maintain gas installations and presents different strategies that take into account the operating conditions and loads acting on the individual gas installations. The maintenance intervals thus defined are not, however, fully derived from the latest operational data, which would require capturing all data automatically. This, in turn, is possible only if a gas installation is fitted with the requisite metrology equipment. A DVGW-sponsored research project helped to identify the essential basic information required for implementing condition-based maintenance (CBM) of gas installations with a focus on capturing actual installation condition data as a basis for determining maintenance intervals.The availability and operating safety of gas installations is mainly predicated on maintenance. A custom-designed maintenance strategy that matches the operation and condition of the respective installation will help cut costs and ensure system availability and operating safety at the same time.

However, a more comprehensive implementation of CBM requires determining the actual condition of the relevant components and assemblies on the basis of automatically captured and analysed operational data, if possible. The DVGW research project that has just been concluded has prepared the basis for exactly this.

The DVGW subsidiary DBI Gas- und Umwelttechnik published a relevant study that collected data on the failure distribution in gas measurement and pressure reduction stations in order to identify the factors that are relevant for the determination of maintenance intervals. The project pursued the objective of defining those components of gas measurement and pressure reduction stations that are particularly relevant for maintenance, and aimed to devise new approaches to component-specific condition assessment.

The survey began by interviewing a number of company representatives so as to gain an overview of current CBM practises in Germany. It emerged that all interviewees considered the currently observed maintenance intervals appropriate and/or conservative, deeming safety features and the components essential for the functioning of the installations such as, for instance, filters/separators, pre-heaters, flow metres and gas pressure regulators especially relevant for CBM.
The failure data of gas measurement and pressure reduction stations were analysed in order to determine the most failure-prone installation components that are relevant for CBM. The data proved that most failures occurred in heat generators and odorisers, i.e. secondary structures, electrical failures and leaks being the most frequent failures.

A system of scores was created in the course of the project, which helps to determine an assessment factor on the basis of the "key variables" and "safety and function-relevant components" criteria. While this procedure is based on Code of Practise G 495, it has been amended to comprise additional criteria like the condition changes of safety and function-relevant components, and the method used to establish their condition (see diagram).

In order to be able to adequately take into account the different installation sizes, from cabinet-type installations to inlet stations, gas installations were in a first step classified on the basis of design volume flow and pressure ratio data. This ensures that operators are encouraged to equip large installations with comprehensive metrology and monitoring equipment, while preventing them from disproportionately neglecting smaller installations with rather scant metrological equipment. In determining the total points scored, the installation size is reflected by the different weighting of the three assessment groups (i.e., key variables, safety and function-relevant components).
The assessment factor AF of an installation is then determined on the basis of the system of scores. The AF reflects the condition of an installation and its expected change. As Annex 3 of DVGW Code of Practise G 495 shows, points are awarded for the different characteristics of the key variables (e. g., actual age of the installation, significance ...) or, in other words, for data reflecting the actual situation. Beyond this, points are awarded also for the condition and/or expected condition change of safety and function-relevant components on the one hand, and for the method of establishing the condition on the other.

This assessment procedure is likewise applied to function-relevant components, all of which are assessed, however (i.e. filters/separators, pre-heaters, flow metres, gas pressure regulators).
In order to determine the total points scored and the resulting assessment factor, the percentage assessment points of each group are multiplied by the respective highest number of total points scored. The score key applying to the installation classes takes into account that small installations generally are not fitted with the same metrological equipment as large installations and/or that there is a limit to the economically efficient retrofitting of metrological equipment, whereas large installations frequently enjoy a high degree of automation and RDT equipment so that the focus here is more on the "establishment and change of the condition of safety and function-relevant components".
Component-specific ratios and evaluation approaches permitting statements on the condition change of components have been created for the individual CBM-relevant components; furthermore, the study offers recommendations on the measurement parameters required to establish the condition of the installations.

When heeded, the project results permit a more careful performance of condition-based maintenance with greater relevance to the specific properties of individual installations. In other words, the project results thus facilitate the condition-based maintenance of gas measurement and pressure reduction stations, also taking into account the condition changes of safety and function-relevant components, and the method of establishing their condition. Next to creating a damage and failure statistics to identify failure-prone components, the relevant measured values should be recorded and analysed on a regular basis so as to be able to discern trends in changes in condition as early as possible. The results will be integrated into the ongoing review of Code of Practise G 495.
The system of scores is currently being validated; i.e. it is being checked for its user-friendliness, and the weighting of the "key variables" and "safety and function-relevant components" criteria in particular is being tested for its suitability for application in the field.