Fred Starr recollects: rescued by the backroom

Materials World magazine
,
1 Aug 2017

Without Alan Fawcett’s visit to our gas terminal at St Fergus in Scotland – and, if you don’t mind me saying, me – there was a fair chance in 1986 that much of British industry would have shut down. When gas supplies run short, it is easier to cut off factories and process plants than domestic consumers. If the local distribution network runs low, air will get in, creating an explosive mixture.

Our X-ray fluorescence expert at London Research Station, Alan, had been posted to St Fergus, which took gas from the northern North Sea, ridding it of sulphur compounds and heavier hydrocarbons, before sending it south. For the long pipeline trek down to civilisation, the gas had to be recompressed to 70 bar, with a new compressor station every 40 miles. The compressors are driven by aeroderived gas turbines. Jet engines, really, whose second most vital component is a multibladed axial compressor that sucks in the combustion air.  

Alan was up at St Fergus on something of a summertime jolly, but wanted to make himself useful. An obvious action was to take samples of the washings from the gas turbines, a compressor wash having to be done every few days at St Fergus, a coastal location bedevilled with sea mists and storms. Mist or raindrop-borne salt would deposit on the compressor blading, reducing power and causing turbine temperatures to rise. When these reached critical levels, deionised water was sprayed into an engine intake. Alan had taken the samples from the spent washings as they poured out of the back end of the engine, expecting to find nothing more than seawater salts. 

When Alan got back to London and checked the samples, he found sodium and chlorine, but along with these tens of parts per million of nickel and chromium. ‘What do you think, Fred?’ he asked. I did an on-the-spot calculation, saying St Fergus must have a problem. Grams of metal must be coming from the turbine blades. Would they like some help?

By the time Norman Wood, our oxidation specialist, and I got to St Fergus to meet up with its mechanical engineer, Bob Robertson, we had already had a look at a couple of blades. They were from an RB211, which had a somewhat notorious history, its development bankrupting Rolls-Royce, the Government having to step in, nationalising the company with taxpayer’s money. Quite probably, as an existing nationalised industry, British Gas had been ‘encouraged’ to be first to place an order for aeroderived RB211s. But, after six years of operation of his three machines at St Fergus, Bob was becoming quietly desperate. Turbine blades were being replaced every year, and the stock of spares was running out.  

I had a hunch about what had gone wrong before getting to the site. The blades were corroded, but mostly at the blade tips, in the shroud region. Usually, when ‘hot salt attack’ occurs, for this is what the corrosion was, it is about halfway along the blade, where the metal temperature is highest. In some manner, the salt deposits were being centrifuged outwards.

So it was. The washing procedure, as detailed by Rolls-Royce, was to do it soon after an engine had been shut down, with everything still fairly hot. As a result, wet sticky deposits, full of salt, were being washed off the compressor and then being carried over into the turbine. Sod’s law being what it is, they were still liquid enough to be centrifuged onto the blade ends. 

A more corrosion-resistant turbine blade would have helped, but the alloy in question was Nimonic 108, one of the last of the wrought superalloys, where to get a few more degrees of creep resistance, chromium had been cut to a mere 18%, permitting an increase in more refractory elements. The blades had been aluminised to offset the inherently low oxidation resistance of the basic alloy, but this isn’t the best answer to hot corrosion. In my view, it was another aspect of a common failing, where the customer is used to test out a new product. Under financial and political pressure, even Rolls-Royce was culpable. Sadly, we too, as consumers, have had this experience, but won’t, unfortunately, have the backroom girls and boys of R&D to run to our aid.