Haig Pit

[MBrockway]

I was in Whitehaven this week and noticed that, on the seafront, there are still large brick built chimneys for ventilation purposes. Presumably the better the ventilation the less chance of methane build up. I`m surprised to hear that flour dust could cause an explosion. There must be many types of ship`s cargo that could do likewise and probably many ships have been lost that way over the centuries. I imagine that RN coal burners would have an established procedure for avoiding dust?

I think those are from the old Wellington Pit.

[Rockdoc]

Horizontal excavations are called roads (mines in Scotland), not shafts, but the problem in any mine is ventilation. If you can keep the air moving you will disperse at least some of the methane (firedamp) and carbon dioxide (chokedamp) as well as keeping the air fresh enough to allow the miners to work hard but that becomes increasingly difficult as the extent of the mining increases, meaning you have a natural limit. Having an air circuit so that the circulation could be controlled is quite an old concept. You separate two spine roadways so that fresh air comes along one and the used air goes out along the other. It would be inefficient for workers to have to go all the way round so cross-passages are cut at intervals and doors are used to turn these into airlocks to minimise the loss of circulation.

Many finely-divided powders burn fiercely. 45+ years ago I went on a school trip to Nottingham University to see a lecture on explosives by a Colonel Shaw. These lectures were famous and his successors still give it as a tribute to him. He told us he had been used as a consultant in an inquiry into an accident that occurred while filming a Billy Cotton Band Show. They were filming a sketch in a park somewhere and Alan Breeze was playing an upright piano. The idea was that the piano would explode when the song reached a certain point and Cotton, Breeze and the rest would emerge from the smoke like the Black and White Minstrels. The crew hadn't brought enough smoke pots and someone had the bright idea of sprinkling several bags of flour inside the piano to compensate for what turned out to be the last take.

The piano exploded on cue, the flour was lifted into the air and caught fire and Alan Breeze spent several weeks in hospital with extensive burns.

A mine is an enclosed space so the percussion from an explosion is contained and maintains its force. As MBrockway said, it's hard to avoid getting coal dust everywhere and the blast of air lifts the dust so it will burn if there is the slightest flame. They get round this in modern mines by using an inert powder - stonedust - to interrupt any flame progression. The dust is liberally scattered (especially if a Mines Inspector is due!!) but they also have troughs of the dust that are barely balanced up in the roof of the main roadways so that a blast will knock them over and put a large amount of the inert stonedust into the air.

Keith

[MBrockway]

Re-establishing the air circulation was one of the main things my great grandfather was occupied with in the rescue at the Haig Pit in 1922.

The 'airlocks' were blown in by the blast as were the canvas screens also used to manage airflow. Until these had all been repaired/replaced, the air in the mine was too foul for the rescue to continue.

As well as the obvious products of combustion, an explosion also removes oxygen.

Several of the men who first entered the mine to find out what had happened were overcome by the CO and lack of oxygen and had themselves to be rescued.

Certainly puts a different meaning on 'a bad day at the office'.

[MBrockway]

Forgot to say that the explosion itself was caused by the miners blasting into a firedamp 'blower' - ventilation in the mine was otherwise adequate before the explosion.

Because the pit was 'wet' the coal dust in the passageways further from the site of the explosion was not ignited and the death toll was relatively low compared to the very serious loss of life in some other pit disasters where coal dust has been the major killer.

[Andrew Upton]

Many finely-divided powders burn fiercely. 45+ years ago I went on a school trip to Nottingham University to see a lecture on explosives by a Colonel Shaw. These lectures were famous and his successors still give it as a tribute to him. He told us he had been used as a consultant in an inquiry into an accident that occurred while filming a Billy Cotton Band Show. They were filming a sketch in a park somewhere and Alan Breeze was playing an upright piano. The idea was that the piano would explode when the song reached a certain point and Cotton, Breeze and the rest would emerge from the smoke like the Black and White Minstrels. The crew hadn't brought enough smoke pots and someone had the bright idea of sprinkling several bags of flour inside the piano to compensate for what turned out to be the last take.

The piano exploded on cue, the flour was lifted into the air and caught fire and Alan Breeze spent several weeks in hospital with extensive burns...

I recall seeing a demonstration showing how even custard powder could be explosive - as demonstrated below:

http://www.youtube.com/watch?v=uereyYIVk3k

[Rockdoc]

Gas pockets are a serious problem, even today, and they bore holes in front of the face to drain the gas away in seams where this is a known problem. The weather also plays a hand. A deep depression, like we have arriving tonight, lowers the air pressure and the leakage rate of the gas into the roadways will increase as a result. The gas has to be kept below a proportion so that it isn't at a combustible level so a big storm can force mining to cease to achieve that.

It wouldn't surprise me that the air doors were either opened and/or damaged by the blast. The screens, called brattice cloths in mining, are used when tunnelling new roads (aka heading) to give air circulation in a blind stub. You effectively divide the roadway in half so the air goes in on one side and out along the other. As they're temporary, brattice cloths would end up thrown around like bed-sheets.

I'm not really surprised that some people were overcome when they were trying to rescue their colleagues. The intense loyalty of mines to each other is the reason why nobody from a pit where there's been an accident is allowed underground as part of the rescue. An odd man might go as a guide for the Mines Rescue people but no more.

Keith

[PhilB]

What special precautions would the RN have had for dust in coal burning ships?

[Frank_East]

Flammable gases,liquids and combustible materials at a given particle size are all capable of causing explosions when mixed with the appropriate concentrations of air and a ignition source of sufficient energy is present.

The ratio of the gas,liquid and combustible material particles to air will determine if there is a risk of an explosion and this ratio will lie between the LEL (Lower Explosive Limit) and the UEL (Upper Explosive Limit).Gases ,liquids and combustible materials have their indIvidual LEL and UEL indexes.Handling of these gases,liquids and combustible materials follows practices and safety procedures to ensure that the ratios lie outside of the LEL and UEL during designed operation.Operate outside this safety envelope either intentionally or through incident and providing a ignition source is present,a explosion will occur.

Take the gas in discussion,methane.Methane has a LEL of 5% to a UEL of 15% which means that a lean mixture below 5% and a rich mixture above 15% will not result in an explosion.However for the purposes of air quality in a mine, the methane level would never be allowed to be at a level above the human being threshold.

Another gas considered to be dangerous is hydrogen which has been used for electrical generator cooling for nearly 60 years where hydrogen is used for cooling the generator windings and rotor.The LEL is 4% and the UEL is 75%.The operating envelope ensures that the hydrogen purity is controlled with a minimum purity of 85%.Even so the management of the cooling system ensures that hydrogen purity runs at about 96%.

As said any combustible material at the right particle size mixed with the appropriate concentration of air and given a ignition source will cause an explosion..flour,grain dust even sweepings up can cause explosions.There was a extensive loss of life in Galveston,Texas about 30 years ago when grain dust exploded when loading was taking place at a grain elevator resulting in the roof of the elevator being blown off.

A major problem with coal is that when it is burnt as pulverised fuel, particle size akin to talcum powder (which will also act as an explosive medium) is capable of causing explosions if its operating envelope stays within the LEL and UEL range and is accompanied by a temperature sufficient to ignite the pulverised particle.Start up and shutdown sequences of coal milling plants are such that the lean ratios (a prerequisite condition for a PF explosion) are passed through.The prevailing temperature is strictly controlled to ensure while a lean mixture exists,the temperature is not allowed to be at a value where it would provide,an ignition source.

Regarding gases being released during periods of low atmospheric pressure,there was the tragedy at the Abbeystead Water Treatment Plant,Lancashire about 30 years ago when a methane explosion led to the loss of the lives of a number of visitors.What transpired was that the plant had not been in frequent use for operational reasons,I believe and that during a period of low atmospheric pressure,methane was released out of the rock strata of unlined tunnels connected to the WTP. The WTP was situated below ground level and not ideal for propagating explosions and was the venue for a number of visitors that were being shown round. When the plant function was demonstrated for the visitors,there was an immediate explosion which resulted in the deaths of a number of the visitors from the ignition of the accumulated methane by an ignition source provided by the switchgear which was not designed as intrinsically safe.

[Rockdoc]

Pulverised coal offers a number of problems and the answers aren't all simple ones. The minimum transport velocity of pulverised coal in air is lower than the flame-propagation speed. The normal way to shut down a mill is to stop feeding coal in but keep the fans running until the PF has been purged. In an emergency shut-down this isn't possible so there would be a chance of a flash-back along the pipework to the mill and the strong likelihood of an explosion. To prevent that a flap-damper is placed just behind the burner, with a balance weight set so that it closes fully before the air speed drops below the flame speed.

Keith

[Frank_East]

Firing pf for electricity generation steam raising has come along way since the decision to design the progressive large generating units from the mid 1950s. for pf firing.PF operation has benefited and honed from the early experiences,home and abroad and varies as different coals are fired.Best practices have led to PF Codes of Practice which have been the mainstay for ensuring that plant does not constitute a danger while to those who operate it, or who might be in the vicinity of the plant.

For pulverised fuel firing,the minimum transport velocity is designed to be 20 metres/sec.Increase this speed beyond the optimum and there is danger of the burner fire hitting the back wall.Decrease it and there is a danger first of all that the flame ignition point will retard into the burner mouth and cause progressive damage.A serious fall below the minimum transport velocity will result in the flame ignition point occurring in the pf pipework However as soon as the primary air/ pf mixture delivery velocity decreases,the pressure maintaining the reflux dampers open will be insufficient to keep the reflux dampers open. The reflux dampers will then shut to isolate the milling equipment from the burners and the combustion chamber.One of the dangers of poor design or maloperation pf systems is if the burner ignition takes place back from the burner mouth and is left in this condition when the associated milling plant is shutdown is that on restart,there is an ignition source present which gives a high risk of an explosion occurring.The installation of carbon monoxide monitoring on the milling plant equipment (includes PF pipework) gives a good indication of the present of an incipient fire,thus drawing the need to take appropriate action.

Other protection is fitted on boiler draught plant and milling plant to ensure that any undervoltage situation cannot adversely effect the boiler supply of combustion air,removal of combustion gases and pf product minimum transport velocity....protection relays operate to trip the plant at a reduced voltage (80%) after a set time delay....typical 3 secs for milling plant and 6 secs for draught plant.In addition,certain parts of the milling plant most susceptible to the effects of explosions are usually reinforced by material with mechanical strength which is designed to a factor of safety of 3 or so.

Sequencial tripping/interlocks are also incorporated in the control design such that the tripping of one essential part of the milling plant will initiate a total trip of the milling plant group so affected.

Taking a mill out of service involves cease feeding,applying cold (attemperating) air to cool the mill (will also reduce the pf product temperature) as the mill empties and purging off the pf pipework.The danger when taking a mill out of service or establishing a mill is the fact that the primary air /pf product ratio will pass through a lean mixture where there is the greatest danger of an explosion occurring.This is prevented by ensuring that the temperature of the mill and primary air/pf product is reduced during the shutdown sequence.On establishing a mill,the mill conditions have to pass through a lean mixture and this done as quickly as possible with the temperature of the mill increased as the coal throughput increases and milling is established.

See that coal is generating the bulk of the system demand,twice the nuclear output. The CCGT gas plants are being two shifted on account of the gas prices and are not matching coal even at the peak.

[Rockdoc]

You must have worked on coal-fired power-stations, Frank! I did an apprenticeship with International Combustion Ltd and spent time after graduation working on our mills, boilers and incinerators. Corner fired boilers fed by LoPulCo LM45/3P mills for me every time!

Keith

[Frank_East]

Keith,

I remember ICL and their Lopulco mills well with their separators and boiler corner firing with tilting burners. Served as a junior engineer at Willington over 50 years ago.The explosion dampers on the boilers to cope with ignitions problems.The oil burners,FFI 80s,air operated which on initiation swung into the combustion chamber with mechanical clatter accompanied by the noise of air operation.Ignition instability would throw out the burners so we usually "locked in the burners" as to keep the oil burner in service and maintain ignition for the coal burners.The boilers with explosion dampers,one at each side of the superheater a would relieve over pressurisation incidents and issue dust when they opened at times of combustion chamber instability.

Senior engineers,the majority were veterans of wartime service experience and appeared to be capable of handling any emergency. Experienced the big shutdown of 1962/63...often thought how the industry would cope in another emergency of that type since there has been a down dumbing in power plant engineering during the last 20 years.

Lopulco mills,vulnerable to coal moisture,the struggle to attain a good throughput with wet coal.Had a test Loculpo for the 500mw units which would give 80mws on a 100mw unit...blazed the way and impressive when the beast was performing well but groans from system control when we lost it.

Then on probably the largest pf boiler ever commissioned by ICL...pressurised pf system,we used to run the mills at over 70 degrees C pf product temperature...any loss of the primary air fan would cause a "puff" in the Lopulco mill and the explosion doors would lift.In the early days of operation,the lifting of the explosion doors scorched the bunker platework which remained as timely reminder that things can go wrong in the process.

From my experience, I think the ball mill on a suction system can outstrip the performance of the Lopulco mill.The advantage is that being a suction sytem,boost gas can be taken from the combustion chamber into the mill at in excess of 400 degrees C and very effective in drying out wet coal during the milling to give a good pf product temperature...could get 180mws out of these mills which means with favourable coals we could run with 3 mills to steam the generator at 500mws.Hardly dropped load during adverse weather when the coal stock used to be snowed over.

[Rockdoc]

There speaks the voice of experience! I do know what you mean about experienced people staying calm. In the department I went into we had a number of ex-Navy and Merchant Navy Chiefs who never got ruffled. I can remember our senior Performance Engineer, Roland Dyson, standing at the control panel for the ICL boiler at Thorpe Marsh, chin in hand and making tiny adjustments to get it humming exactly how he wanted it.

Keith

[Frank_East]

Keith,

Similar backgrounds.

I was just thinking after I mentioned the pf explosions we had on the Lopulco mills at T.M.1. The pf explosion that burnt off the bunker paint was investigated and the reason found was that one of the three rollers have seized on the table which provided the ignition for the pf explosion...fortunately the explosion doors on the top of the mill propagated the explosion but projected a flame up to bunker level.As with all engineering problems,engineering never lies and the reason was found to be that the trunnion bearing lub oil seals on one of the rollers allowed the ingress of pf into the bearing causing it to eventually seize.To ensure that was no recurrence,oil samples were taken each day by the O&G on each mill and we then tested a 100cc sample which had been diluted with a thinner, sufficient to flow through filter paper.Any sample on the filter paper considered to be above 4% pf in oil resulted in a trunnion bearing oil change.Our chemists supplied a sample filter paper with 4% pf contamination of trunnion oil for comparison.An interim measure until remedial engineering could be enacted.

Invited back to T.M in March 1994 for a reunion dinner on the last day of commercial operation....the plant had run for 30 odd years and having cross compound sets added complexity to its operation...installed in the days when the road weight restrictions limited the capacity of individual generating units to a much lower capacity than they are now.

Incidentally the US TVA bought the same CAP cross compound turbine as T.M. 1...now we do not own a steam turbine manufacturer.

At T.M the ICL contractor senior man was a chap by the name of Brook(e)s.Did you know him?

[MBrockway]

...often thought how the industry would cope in another emergency of that type since there has been a down dumbing in power plant engineering during the last 20 years ...

A theme I too can relate to, though in my case from the chemical industry and even more so, in commercial computing.

Many friends from my generation still active in chemicals, relate very sobering stories about slimming down of operating procedures being forced on the plants by younger generation business managers in the name of cost control.

Such managers unfortunately belong to a generation that has no experience of the catastrophic consequences of poor procedures and do not understand the rationales that originally gave rise to them. Plant level operations managers who defend them seem nowadays to be perceived as being obstructive and often run the risk of becoming marked men with respect to their onward career progression.

Luckily the possible consequences in the chemical industry - e.g. potential phosgene poisoning of Runcorn/Widnes - are such that a certain amount of sense prevails.

Not so in commercial computing which is why incidents like last year's six day Royal Bank of Scotland payments system shutdown happen.

Then again, maybe our predecessors were making the same lament about us 30 years ago - LOL!

Rant ends.

[Rockdoc]

I worked for a Ken Brookes as a new graduate in the Test & Performance Department at ICL for about three years. Slim, dapper, black hair and a moustache so quite distinctive. He was the main reason I left ICL in 1977 and went to the NCB. Not my favourite gaffer by a l-o-n-g way.

One of our regular jobs was to go to Fiddlers Ferry and check the seal-air passageways were clear on the newly-restored rollers. There should have been a seal-air fan for each roller on every mill so that the bearings were pressurised and pf was less likely to pack the bearings. The repaired doors usually had grease blocking the passages and we'd have to get the site bods to flush hem through with a solvent. We always did FF's mills but nowhere else. Although it was a cost to the CEGB, we could save them money. Many's the time we found a seal-air fan had had the wrong-handed impellers fitted so it sucked instead of blowing!

Keith

[ianw]

A really fine dust has a very large surface area compared to its volume. Hence it burns very quickly and explosively. A number of flour mills have been destroyed in this way.

[MBrockway]

A really fine dust has a very large surface area compared to its volume. Hence it burns very quickly and explosively. A number of flour mills have been destroyed in this way.

Only if the material is combustible - this is why stone dust is used in mines to 'dilute' the coal dust.

[Frank_East]

What special precautions would the RN have had for dust in coal burning ships?

Phil,

Ship boiler rooms would be normally subject to ventilation to expel dust and to maintain adequate air changes.So there would be a slight suction in the boiler rooms to achieve this.During action stations,all exhaust dampers would be shut and the boiler room pressurised with a slight pressure effected by the air delivery fan.

But the danger from coal dust explosions came from the coaling up procedure where dust would penetrate every orifice of those taking part.While loading coal from the quayside or collier on to the ship,into the bunkers with the bunker trimmers ensuring that the coal was evenly distributed. In a dust ladened atmosphere,the risk of an explosion was minimised by the use of Davy safety lamps as the only light source allowed.But there is evidence that oil lamps were also used which would represent a source of ignition in an explosive coal dust atmosphere.

As I see it,there was no personal safety equipment issued to the coaling party but no doubt individuals would use cloths as a face mask.

As a final precaution against dust,the ship was hosed down in the coaling areas and the mess deck cleaned up.Those who were involved referred to this final measure as "the water carnival"

Coaling...carried out accompanied by the coaling hymn to the tune of "Holy,Holy,Holy"

Coaling,Coaling,Coaling

Always...ing well coaling.