by John Parker
Sue and Pam’s day followed their well-established format of a pinch of culture (easily satisfied by Salisbury’s treasures), a purposeful assault on the retail outlets and, favourite by quite a long way, the pub lunch. This long savoured finale was this day enacted at The County Hotel, adjacent to the River Avon, which Sue liked as much for its ambiance, as the food it served. It had, she thought, something of Trollope’s Barchester about it and she could imagine the likes of Mr Harding, Mr Slope and Mrs Proudie sitting in this dining room, though obviously not all at the same table. This thought epitomised her feelings of the differences between her new home and Birmingham. She preferred the old fashioned, genteel air; the slower pace of life that pervaded Bath and the small market towns locally. She still felt herself a Brummie and was at home amongst her humorous, good natured, fellow citizens, but the life in the city was so hectic by comparison.
The meal was enjoyable. They drove home slowly, succumbing to that companionable mood that characterised their relationship.
“Keep your fingers crossed for me. I may escape Cummings’ beating,” said Pam as they drew up at the huge doorway, “but on the other hand, I may be lucky,” she added with a twinkle in her eye. Sue drove the short trip home smiling. Dear Pam, she was priceless.
*
This was the third meeting of the sub-committee and Dave felt a keen anticipation now that he had a meaningful contribution to make. In addition to being able to report upon the on-site test rig, he had information from the Non Destructive Testing Group’s site investigations.
After Henry and Pauline had combined to bring everyone up to date, Henry asked Dorinda McCann to report upon their work and, in particular, to introduce the memo that she and James Collingwood had prepared. Dorinda did this with, Dave thought, an unnecessary amount of formality. In essence though, it was really the tidying up of the information that they had presented at the last meeting, with a few more test details.
It was evident that they were firmly decided that the disc failure was the result of stress corrosion cracking caused by sodium hydroxide, which had been carried over with the steam from the boiler, into the turbine. Dave, though he accepted this was the most likely explanation, believed that other, admittedly less likely, possibilities should not be discounted. Though not having the university background of the others, he felt that his open-minded approach was more appropriate at this early stage.
Joe was next. He reported that he had produced two sets of pre-cracked specimens which he had loaded up to two separate values to obtain, at least a rough idea of the effect of stress intensity. He had also supplied Dave with similar specimens. His own tests were underway using laboratory produced ‘pure’ steam.
Henry expressed delight at the progress being made. He, of course, felt himself, as independent chairman, to be responsible for the overall work of the sub-committee. It was to him that the Technical Committee, overseeing the whole investigation and ultimately the board of the Strategic Supplies Authority, would look to provide answers.
Dave was pleased to report that his on-site test rig had been manufactured and installed on Number 5 turbine and that it was currently being steamed out to clean the system. The pre-cracked specimens he had received from Joe would be placed into the test vessel within days, together with his own specimens. These were un-cracked specimens in the form of plain, small diameter bars, made from LP disc steel, under stress – in effect just long bolts. Dorinda interrupted him, saying that it was ludicrous for anyone to imagine that alloy steels, of the strength level used for the manufacture of turbine discs, could possibly suffer from stress corrosion cracking in high purity steam. She had been studying the phenomenon since her post graduate days, which was longer than she cared to remember. She had contributed to and attended virtually every major conference on the topic. She could say, with confidence, that the small number of chemical solutions that could cause stress corrosion cracking of these alloy steels could be counted on the fingers of one hand and she added, warming to her argument, a pretty small hand at that. James Collingwood added the final point in their argument which was that the credibility of the whole sub-committee would be undermined if it became known that they were pursuing any odd fancy, when there was an urgent job to be completed. Dave was chastened. He was not in a position to refute Dorinda’s superior knowledge, yet he still thought, though did not express his view, that it was good science to include the less likely possibilities, even if only for the purposes of elimination. He sat awaiting Henry’s final judgement.
Before Henry could make his, probably damning, remarks, Joe came to Dave’s rescue in a way that surprised and pleased him. He said that he accepted much of what Dorinda had said but, on second thoughts, it could be that Dave may have a valid point in this instance. Everyone looked across to him for elucidation. After a pause, Joe went on to say that, at this time, everyone seemed to agree that the most likely cause of cracking of the Winford disc and therefore the turbine failure was contaminated steam. Further, it was generally agreed that cracking in normal ‘high purity’ steam was an extremely remote…
“Impossible in my view.”
Following Dorinda’s interruption, Joe continued by saying that this being the case, there remained the slight possibility, though remote, that contamination might still be present at Winford. If this was accepted, then the inclusion of some un-cracked specimens in the trial could be justified. If the chemical analysis of the steam during these trials, which Dave was monitoring, found evidence of contamination, it would give added weight to Dorinda’s hypothesis if the plain specimens did crack.
Some seconds of silence followed Joe’s comments. You’re a real star, was Dave’s unspoken thought. Henry, clearly relieved by Joe’s observation, was pleased to be able to satisfy, to some extent, all parties.
After lunch, Dave passed around a summary of the results of non-destructive testing. A number of apparently sound discs had been removed from the LP rotors of the failed turbine. These had been tested for surface cracking around their bores and keyways, using a magnetic technique. Widespread cracking had been found in some of the discs. It was the pattern of the cracking which Dave thought instructive. The discs that had been positioned near the inlet of the LP steam were the most badly cracked, whilst those nearest the exit from the turbine contained no cracks. In general, cracking of the remainder diminished moving along the turbine rows towards the exit.
It was known that the steam conditions varied along the LP turbine, being just slightly wet near the inlet and becoming increasingly wet along the machine, until the final discs were operating in a continuous shower of water. Furthermore, the fine balance of the steam conditions near the inlet end, was such that it could be either just wet, or just dry, depending upon operating conditions.
Dave summarised, the disc cracking was worst where the steam cycled between being wet and dry and it was absent where the discs were completely wet. It could be postulated therefore that cracking occurred predominantly when the disc surfaces were alternately wet and dry which, he had to admit, fitted in with Dorinda’s idea of contamination. One could imagine the gradual build-up of salt/impurity concentration as the wet/dry cycle took place, rather like the formation of a salt tidemark on a part of a beach, where the sea water evaporated as the tide went in and out. Henry thanked Dave for his contribution.
Dave was pleased with the eventual outcome of the meeting, but the residual tension caused by the argument about his work, with the implied suggestion that he was an idiot, made him weary and this feeling persisted during his journey home. He was less than pleased to find the place a bit of a mess, as well as being very noisy – the Potters’ kids were there again. He was glad that Jo and Katy had friends, but these two were particularly boisterous and this infected his daughters, which meant that when the four were together, he knew that he would have to resign himself to two or three hours of disruption.
Sue seemed to take it all in her stride. Of course, the bilateral agreement wit
h Betty Potter suited her. This arrangement, plus the car, had led to a great improvement in her quality of life and she was no longer dependent upon Pam, who had been away quite often lately anyway.
*
During idle moments Sue had wondered about Pam and her relationship with her husband. Although she had not met Lord Marden, she had seen his photograph in the local paper in a variety of roles, including cutting the ribbon across the front of a new block of flats and pink-clad on horseback at the local meet. He appeared to be older than Pam, but still an attractive, imposing figure. On occasions, Sue also contemplated Pam’s precise relationship with Charles. Pam had, to Sue’s knowledge, had several meetings with him, including at least one in her London flat. She wondered? Her thoughts then turned to Betty Potter and what her reasons for wanting to be away from home so regularly might be. A man?
Oh! Susan, do stop it. Inexplicably, Peter then came into her mind. What would people be thinking about her? Had anyone from the village seen them out together, say in the Crystal Palace in Bath? What would they be saying? She shook her head dismissively. It was nonsense. Peter was nice, charming and good company and that was it. She didn’t care what other people made of it.
“Just how long are those bloody kids going to be here?” Sue was shaken out of her reverie by Dave’s outburst. “They’re becoming a blessed nuisance,” he continued.
“For goodness sake, Dave, they’re kids, they’re enjoying themselves. It seems you’ve forgotten what that was like since you’ve been down here.” Having said that, Sue went into the kitchen to prepare the evening meal. She was annoyed, partly with herself for her reaction to her husband’s irritability. Her mood had been altered in just the few minutes that he’d been home. In recent times, there had been periods where an uncomfortable atmosphere simmered just below the surface between them and this was the case for the remainder of that evening. He was so completely focussed upon his investigations.
A couple of days later, Peter called to say that another London trip was planned shortly, did she want to take advantage of it? Sue responded enthusiastically saying that she would be delighted but she’d have to check with Mrs Potter. The arrangements were subsequently confirmed.
13
Dave called Ian into his office to learn how he was progressing with the extra set of specimens for the Winford rig. Ian reported that they were all loaded into the test frame and ready to go.
Tony Richards phoned to remind Dave that it was their turn to present an item for the departmental monthly seminar. He wanted him to make a presentation on the latest developments in the Winford investigation. He suggested that the main theme should be a general introduction to stress corrosion and its particular relevance in the case of the Winford LP disc cracking.
*
As Gritty was involved with an urgent job for the West Bay water treatment plant, Dave elected to go to Winford alone the following day. He was keen to get his on-site tests started. His first job on arrival was to climb the temporary scaffolding up to the Number 5 turbine LP steam inlet pipe, where the manifold valve arrangement had been fitted. He turned off the steam supply. When the vessel had cooled he removed the internal stainless steel trays and after loading his specimens, he replaced them in the vessel. At last, he thought, he could begin the test in earnest. As on the previous occasion, he only opened the steam valves partially to allow the specimens to warm up slowly. Later he turned the valves fully open, checked the temperature and pressure and was pleased to note in his pocket book:
TEST COMMENCED 1415 12th March 1970.
Now that really did feel good. He had also been pleased to note that Bunsen’s equipment was working; the various monitoring instruments clicking away, as the ink markers plotted their readings onto paper charts. At last he felt part of the investigation, such a major investigation too. He couldn’t have imagined it a year ago – no, six months ago, even. He was in his element, doing real research of great practical value, not just an academic exercise.
His thoughts moved on to his family and his relationships with them. This part of his life was not going so well at present. The demands of this project had, he admitted, made him increasingly irritable, but Sue was being unreasonable as well. She was the one who had wanted the move and yet she had spent most of the time since complaining about her lot. He was working for the family, for goodness sake, and on the odd occasion when he socialised with his workmates at skittles, or whatever, she seemed to think it was unreasonable. Well, his chance had come, he had to grasp it, and he would. It may be that Sue’s brother-in-law was under the thumb – but no, not for him.
*
The conference room was crowded. He hadn’t thought that this topic would have been so popular. He supposed it could be that people wanted an excuse for a break from their own work, though he assumed that at least some had come along out of genuine curiosity. Although some of the staff, particularly the engineers and chemists, would have heard of stress corrosion, it could be that they had only a vague notion of the process.
The background chatter died down as Sweety rose to open the seminar. He reminded his audience that these meetings were held for good reasons, other than just getting away from desks and labs for forty five minutes’ rest on a Friday – laughter. One reason, he continued, was to bring a project up for discussion when the research officer responsible felt that his work was in need of some fresh impetus, or where the future direction of the research was in some doubt. It had been found that comments from colleagues, from quite different backgrounds, had often given a new perspective, from which the project had later benefited. Today however, this was not the case, he told the audience. David Harrison of the Corrosion Group and his colleagues appeared to be progressing well with their investigations into the cause of the LP turbine disc failure at West Winford power station. He for one was looking forward to learning a little more about the process of stress corrosion cracking, which seemed to be the agreed mechanism responsible for the well-publicised incident. So, over to you David.
Dave began by saying that, as it was a cosmopolitan audience, he wanted to keep things simple. He hoped that those who were familiar with the subject would excuse him if he over compensated as he generalised. He went on to suggest that when the word corrosion was mentioned, most people thought of old cars, corrugated iron structures, seaside piers and so on. In other words things made of iron or steel, exposed to rain or seawater. “Surprisingly, in many cases, these do not present a serious technical problem. Certainly, the discolouration of the surface and an alarming amount of corrosion product – rust – appears to suggest a major loss of metal, but often this is superficial. This does, however, provide early warning, allowing timely action to be taken to avoid failure.”
He continued by asking his audience to consider a different situation where there is a sudden leak of a hazardous substance from a pipe, with no warning and no obvious evidence that corrosion has been taking place. No rusting, no unsightly brown surface deposit – nothing – just bright shiny, metal. This may be a vital part of some equipment, such as a transcontinental gas pipeline which has cracked completely through.
“The process of stress corrosion cracking, although less common than general corrosion, takes this insidious form of attack and is therefore potentially a much greater threat than the usual forms of corrosion, with which people are familiar.” He continued, “Stress corrosion is caused by the combined action of stress and corrosion resulting in the development of fine, usually deeply penetrating, cracks. This is not a new phenomenon, one of the earliest recorded instances being the cracking of brass cartridge cases at the time of the Indian Mutiny in the late 1850s. At the turn of the century, another series of stress corrosion failures occurred in steel steam boilers, due to the presence of sodium hydroxide in the boiler water.” He went on to say that some of the audience might be wondering why stress corrosion is not more common. The reason is that only a few chemical solutions cause this highly localised attack in just a few
alloys. These solutions need to be sufficiently aggressive to cause some corrosion, but not so aggressive as to cause the attack to spread out as happens in the case of iron or steel in damp conditions, which he had mentioned earlier.
“Stress corrosion cracking often occurs in normally corrosion resistant alloys, such as brass or stainless steel, which possess good protective surface oxide films. If this film is damaged and corrosion results, the oxide film usually reforms quickly and prevents the attack from spreading laterally though, in the presence of a stress, it may continue to penetrate into the metal. Common solution/alloy combinations causing stress corrosion cracking include ammonia with brass, chloride solutions with stainless steel and sodium hydroxide with ordinary steel.”
Dave suggested to his audience that they would be relieved to know that he was nearing the matter in hand, at last; the Winford turbine disc failure. The discs were made from a medium strength, low alloy steel and from what he had already said, they could probably guess that the most likely cause of the stress corrosion cracking was sodium hydroxide. The boiler water at Winford, he reminded them, did contain some sodium hydroxide which, ironically, was added to prevent corrosion within the boiler itself. It was thought that at Winford some sodium hydroxide had been inadvertently carried over in the steam and entered the turbine, perhaps during some unusual boiler operation. This contaminated steam would have passed through the HP turbine as normal, as the steam would have been hot and dry at this stage. However, as it cooled and entered the LP part of the turbine, some wetness would have been present and droplets of contaminated moisture could have been deposited in the disc keyways and crevices. Although the concentration would have been very low, this could have gradually increased, as the steam in this area cycled between wet and dry depending upon turbine conditions. As the centre of the discs, that is the bore and especially the keyway, was under stress, all the conditions for cracking were present. Dave concluded by briefly mentioning his Winford test programme.