- 12 May 2005 -

Benefits of on-line relief valve testing

As the frequency with which it is necessary to shut down pressurised equipment is reduced, so the intervals between relief valve examinations need to be similarly extended, or systems developed to allow the examinations to be carried out on-line. Tony Nicholls, managing director of Furmanite International, specialists in maximising asset uptake, looks at a recent development that allows the latter to be achieved — with significant benefits to operators.

As plant operators seek to meet the on-going challenge of ensuring max-imum return on investment, the desire to increase asset uptime and extend intervals between shutdowns is a fundamental goal. While periodic thorough examination of items of pressurised equipment and associated protective devices is a requirement under the Pressure Systems Safety Regulations 2000, developments in examination tools and techniques, such as Risk Based Inspection (RBI), have meant that individual equipment items can be examined less frequently while still complying with the regulations, and in some cases non-invasive on-line examination techniques have now replaced the traditional visual internal inspection.

Clearly such developments are of significant benefit in extending production runs. But they can only be fully capitalised upon if the same can be achieved for the examination of the protective devices — generally a relief valve or bursting disc plus pipework from the pressure vessel to the relief valve and on to atmosphere or the relief collection header (known collectively as the relief stream).  Evidently, techniques are needed either to allow these protective devices or relief streams to be examined less frequently too, without compromising safety, or to examine them on-line without disrupting production.

Moreover, not only is extending runs and avoiding unscheduled downtime a high priority for any operator, but so too is keeping planned outages — an unavoidable necessity — as short as possible, with immense pressure to complete maximum tasks in the minimum time possible and the shutdown often tightly scheduled to the last hour.  With this in mind, the ability to avoid the risk of delays by removing some of the tasks from the shutdown altogether is to be welcomed — pointing, once again, to the benefits of on-line examination and re-certification procedures.

A breakthrough

On-line maintenance and repair techniques from pressure leak sealing to pipeline intervention are widely accepted and applied these days, as means to avoid unscheduled downtime or reduce pressures on scheduled shutdowns.  So too is on-line valve testing technology, which can afford significant time and cost savings by enabling valves to be tested on-line at normal operating pressures and temperatures prior to shutdown, so that only those needing attention need be removed from the line during the scheduled outage. Now, however, a recent project has taken this technology a stage further, with an exciting breakthrough demonstrating that controlled application of this on-line valve testing technique can be applied to achieve examination and approval of relief streams that will satisfy the Pressure Systems Regulations requirements (which put significant emphasis on the performance of such protective devices).

The benefits are considerable. It is true that some pressure systems have twinned relief systems enabling relief valves to be removed for overhaul without shutting equipment down, but this is the exception rather than the rule, and moreover it introduces the need for isolation valves in the relief stream. Traditionally, the relief stream would be inspected by removing the relief valve for overhaul and set point adjustment in a workshop (which is both costly and time-consuming), and visual inspection of the pipework. Now, however, this inspection process can be achieved on-line, under normal operating conditions, with no disruption to production - the answer industry has been waiting for in terms of extending intervals between shutdowns.

Where the relief valve examinations were required to be more frequent than for the corresponding equipment, the on-line testing option enables the respective examination intervals to be more closely synchronised, bringing the important advantage of reducing the need for shutdown, and the associated operating and fixed costs. The full benefit to be gleaned from methodologies that allow longer intervals between shutdowns can thus be enjoyed.  Furthermore, on-line testing can also serve to remove a task from a tightly-subscribed shutdown schedule.

The findings come as a result of an investigation and pilot scheme by Furmanite International in conjunction with Huntsman and ABB Eutech, its inspection authority, to trial this on-line valve testing system, using the Furmanite-developed and patented Trevitest technology.

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