Blowbacks & dropped fusible plugs etc.

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

There isn't a one size fits all answer and a lot of the decision making has got to be related to the actual circumstances. It is probably a footplate crews worst nightmare and the only real answer is to never let it get that far down the line. Keeping a good level of water and not mortgaging it gives you time to react to injector failure or other problem, such as a stuck clack valve.

It was probably invented with an intended purpose of extinguishing the fire and considered to be a good idea at the time. I don't think fusible plugs were universally applied throughout the world. In the UK, Section 32(1)c of the Factories Act required that every steam boiler "shall, unless externally fired, be provided with a suitable fusible plug or an efficient low-water alarm device." On a steam locomotive, conventional low water devices are impractical.

 

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

As the 'Black Book' says: "If the water level in the boiler drops too low and uncovers the [fusible] plug, the lead [core] melts and allows steam to escape into the firebox, which should act as a warning to the Enginemen. Should this occur both injectors should be immediately put on and steps taken to remove or deaden the fire." Usually this would mean either by dropping the fire into the ashpan or shovelling lots of earth or ballast over the top.

No expectation in the text that a fusible plug (or even the several there may be, on large firebox locomotives) is going to release the kind of steam amount that would either reduce or extinguish the fire. Nor that such a release of steam would significantly reduce the boiler pressure. If the top of the plug (ie within the water/ steam space of the boiler) became uncovered, there clearly wouldn't be enough actual water coming through to dampen/ extinguish the fire.

The existence of a myth to that effect is perhaps partly attributable to a need to counter the understandable fears of people who are constantly reminded by the equally uninformed that boilers are liable to explode at any moment.

Now there are reasons why a fusible plug may fail, which are not directly the result of poor boiler managment by the enginemen, and they're nearly always cited as a kind of litany, in defence of crews when things go wrong. But I'm pepared to stick my neck out and state that if you get to the stage where you haven't got enough water in the boiler, and you haven't realised it and done something about it, it's your fault if the fusible goes.

And if you can tell something's gone wrong, the gauge glasses are empty and the fire's roaring away and you may therefore reasonably expect a fusible to melt, if you don't keep alert for the signs of that happening, then to be frank I'd rather not have you anywhere around a footplate near me. And if you can't tell something's gone wrong, then the latter clause counts double.

Confidently expecting a rain of brickbats, I remain
unrepentantly yours

Roger

 

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

If a plug is dropped, the driver has got to have a pretty good excuse for not carrying the full can. A stuck clack valve is one possibility. That can empty the boiler faster than you can kill the fire.

 

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

They are what would be termed in computer speak - a legacy feature.

Once upon a time locomotives were small enough for them to have a physical effect. That wasn't much after Stephenson's Rocket. The problem with them is that they can't be scaled up, you can only add more. So they are an anachronism, but who is going to stick their neck on the block and remove them as a requirement? The answer is no-one. Maybe years ago, but it didn't happen. In this crazy Health & Safety age when soldiers in battle zones are required to wear high visibility jackets when out on patrol in the dark so that they can see each other, there is virtually no chance the requirement to have them will ever be removed. It would be too easy, in the event of a fatal accident, to point the finger at the absence of fusible plugs. Which could then mean the person removing the requirement might then face a manslaughter charge. M'learned friends would see pound signs big time.

 

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

Here is an interesting MoT report on the firebox crown failure of Grange 6859 on Gresford Bank near Wrexham in December 1952 due to low water level. It was caused by a faulty water gauge which was indicating that the boiler was full. One or more of the fusible plugs melted. The crew heard the escaping steam but did not suspect the water level was low - they thought it was a tube failure. The crown of the firebox split soon afterwards.

So this was a case where the escaping steam from the moletn fusible plug gave a clear audible warning despite the fact that the engine was presumably working hard on the bank.
http://www.railwaysarchive.co.uk/documents/MoT_Wheatsheaf1952.pdf

David

 

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

Not entirely. The stuck clack will prevent water going into the boiler, and in some (not all) installations will throw steam out of the injectors' discharge, but that wouldn't empty the boiler of itself. A stuck blowdown valve fully open will empty the boiler pretty fast, but no engineman could be oblivious to that happening.

But it's absolutely true that it takes time to kill or drop a fire. All the more reason why a sensible crew will have it in mind as a last resort from the moment the clack starts to stick. There's no mileage in struggling with stubborn injectors, while continuing to work the locomotive without easing up, and watching the boiler water level dropping away from simple unreduced steam demand. Then it's a false comfort to be telling yourself you're following traditional practice by 'mortgaging the boiler'.

And if a plug does drop, then the railway's insurance company is going to be interested. It isn't simply a matter of handing a Form 1 to the footplate crew.

Roger

 

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

So that's all right then?

It sounds like you're rather missing the point - that they are a far from foolproof way of dealing with the issue of low boiler water levels.

If you read the report you will see that the train in question was rather lightly loaded, and presumably NOT working particularly hard.

All in all it's an excellent report, and I do wonder whether in current conditions such a clear and succinct summary would result had that happened recently. It's compulsive reading for anyone involved with steam boilers.

One can't help thinking how lucky the crew were in this case. So many things went in their favour. There was a French engine on a passenger train about 100 years ago which launched the boiler clean out of the frames and hundreds of metres into an adjacent field after a crown collapse.

 

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

Exactly.

The amount of ducking and diving, and backside covering that goes on these days, would mean that the message becomes diluted and lost. I would expect something like a 500 page report now, much padded with bovine excrement. Also the investigator was clearly someone knew what he was about, which isn't always the case now. The degree of traceability that existed then is quite interesting. When ISO9000 became the in-thing 30 odd years ago it was heralded as a new (ish) concept. But the railways (and the railway inspectorate) had been doing it for decades. Pity most of that has been undone since.

 

The 6869 incident again gives pause for thought - there was a second method of checking the water level (the try cocks), but it wasn't used - the gauge glass was trusted implicitly. These incidents can get very confusing very quickly - things can appear to have the reverse effect to what is expected.

By the way, I think Steve's reference to a stuck clack valve emptying the boiler is if the clack sticks in the open position (presumably due to some foreign object between the valve and seat). Thus you have the boiler pressure pushing the water out - and I daresay an injector is unlikely to pick up in those circumstances!

 

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

With reference to 6895, and other (mainly) GWR locos with only a single gauge glass, I've never felt very comfortable with this arrangement. With two glasses, it's pretty obvious there's something amiss if the two show different levels. With a single glass, if the level looks OK, you would be unlikely to realise you had a problem until it was too late. The cocks might be of use if the glass has blown, but not for much else unless you know you're in trouble due to some other factor. Having said that, thousands of loco's operated like that for decades, so the risk must be small. S160's also had the same arrangement I think, though some have been modified to two gauges in more recent times.

As regards boilers emptying back via the clack and injector feed, doesn't this depend on whether the loco has top feed or not?

 

Re: top feed, I believe you are correct.

What I meant re: the 6895 incident is that there was another method of checking the water level available to them that wasn't used. There were warning signs that something was amiss when they started noticing strange behaviour, such as the water level not dropping and constant blowing off, and then when they heard the fusible plug blowing. It's not a specific criticism of the crew, more a reminder that all available tools should be considered.

 

Re: Reported Incident with 70013 Sun 27th May on the Peak Forrester.

The incident on 35005 at Paddock Wood was attributed to a burst tube near the firebox tubeplate.

35005 never ran on the mainline again after this incident and Steam Dreams subsequently sold the loco to the MHR.

A new firebox tubeplate will need to be fitted to 35005 during its next overhaul and the MHR, have this in their posession.

 

I'd be prepared to wager a Mars bar that 35005's firebox will need a lot more than just a tube plate at is next overhaul!

 

Of course it will need a lot more work than just the tube plate, i suspect the usual areas such as combustion chamber, syphons and doorplate will all need attention/replacement.

But with the amount of money needed to carry out the work expected to cost around £100,000, things can`t be that bad overall, or the MHR would be appealing to raise more or would not even give consideration to starting an overhaul.

As the MHR boilersmith pointed out recently, its not the platework that costs(relatively cheap), its the stays that rack up the bills.

 

My reference to a stuck clack was indeed to one that has stuck in the open position. I've experienced this on a number of occasions but never one that has stuck closed. It depends how open the valve is when it sticks but you are unlikely to get the injector to pick up unless it has got a shut-off valve to let you get the water running. A top feed will generally only allow steam to escape but it can still empty the boiler alarmingly quickly when you are up against it!

With regard to GWR water gauges, it has always amazed me that they persisted with try-cocks and that BR did nothing to rectify this outmoded form of indication. The fact that both gauge glass and try-cocks were on the same pedestal is also surprising as there is always the risk of the common passageways becoming blocked by scale, etc.

 

Apologies for having misunderstood you, Steve. My experience has only been the opposite: top feed clacks failing closed.

As to the GWR practice of reliance on an unduplicated safety-critical device... I have my own opinion but wouldn't want Swindon's many hagiographers dumping on me.

Roger

 

No need to apologise. We obviously didn't come to a clear understanding......

 

Another report, this time from the US, which makes interesting, if sobering, reading concerns the Gettysburg Railroad where a crown sheet failed due to low water in 1995. http://www.ntsb.gov/doclib/safetystudies/sir9605.pdf

This highlights the importance of proper maintenance, and also mentions how that particular design of boiler didn't have fusible plugs, but was designed to fail in stages in a more controlled fashion if water levels became too low. I suspect that we don't have any of that design in this country, or whether they would be allowed unless modified to include a fusible plug.

What a report like this does is remind us that even if you have standards set down, and even enforceable by law, they have to be adhered to to be of any use. Some may grind their teeth at the ever increasing burden of health and safety regulations and record keeping, but they do have their uses too.

Steve B