A tube fitting failure on a North Sea platform has highlighted a safety-related problem that could affect thousands of fittings currently in service worldwide
According to Parker Instrumentation, tube fitting problems are now readily avoidable, through a combination of appropriate technologies and good practice.
The recent failure occurred mainly as a result of corrosion on the back ferrule - which had been in service for over 12 years - aggravated by poor installation practice.
The twin ferrule fittings used by a large proportion of industrial organisations - and almost universally by oil and gas companies - rely on compression to make a seal.
In order to grip, the back ferrule bites into the tube.
To do this effectively, it must be harder than the tubing material - which is typically 316 stainless steel.
Some fittings manufacturers use case hardening processes such as nitride infusion or carborising to temper the ferrule's biting edge - but as a side effect, this opens up an avenue for corrosion.
It 'pulls' carbon from within the rest of the stainless steel to the biting edge, effectively transforming adjacent material into mild steel. If the fitting is then used in a harsh operating environment, such as an oil or gas platform or chemical plant, sooner or later it will almost inevitably start to corrode.
This situation is not irretrievable, however.
There are now special corrosion-resistant alternatives specifically designed for harsh environments, and the onset of corrosion in fittings is usually easy to spot and correct, given appropriate monitoring procedures.
"We used to employ nitride hardening, as some fittings manufacturers still do - in fact the recent failure was in one of the last ferrules we produced using this technique, in 1986.
This was the best technology available then", said Steve Mullen, Parker Instrumentation's tube fittings product manager.
"At that time, we addressed the corrosion issue aggressively, with a special hardening technique, and the use of exotic materials - which can withstand just about anything the environment can throw at them." The discovery of the drawbacks associated with nitride hardening, and other hardening treatments the fittings industry had employed, led Parker to introduce the Suparcase ferrule treatment.
This uses a unique form of chemical hardening which penetrates for just a few microns.
It does not change the underlying molecular structure of the stainless steel, which maintains its intrinsic spring quality, allowing joints to be remade.
Parker also developed advice and upgrade options for harsh environment applications.
Users with existing installations need to routinely monitor fittings annually: any evidence of corrosion - which typically manifests itself obviously with rust staining at the back of the nut - demands that the fitting in question is inspected internally, and replaced if necessary. Customers who discover corroded fittings should strongly consider replacing them with fittings that have corrosion resistant ferrules When replacing fittings, it is possible to use types which are easily identifiable, simplifying refurbishment.
The other vital step is, of course, to always make ferrule materials the subject of the equipment specification.
Although this has started to become fairly commonplace in the chemical industry, starting around ten years ago, almost all oil and gas companies still specify just a type of fitting and material.
Since these initiatives, Parker has continued improving the safety aspects of its product by developing a range of devices to aid the correct make-up of tube fittings.
An educational training seminar, known as the Safety at Work programme, was also introduced which has gone a long way in helping reduce the number of incidents attributed to incorrectly-assembled tube fittings.
