DMU failures and revving up

[Tomnick]

Would an isolation of the CGIC have been sufficient, negating the need to isolate a whole car or any brakes. Sounds like like was the case as it ran at normal line speed afterwards. And like has been said, you can’t use the compressor speed up whilst moving so after the driver has used what presious little air he/she had to stop at the station, they needed to “Rev” it to build up air to get going again. Then the normal revs of the movement would have been sufficient until there was a complete failure of the air line.

Assuming 150s are broadly similar to other 15x classes: as I understand it, the CGIC just bypasses the unloader valve, so wouldn't have any effect in this case (unless the air leak happened to be there!). Is the main reservoir separated from the main res pipe by a non-return valve, though? If so, or if the main reservoir is separated from the compressor by a non-return valve (which it must be, surely?), I can't see how any leak on the supply side would cause loss of main res pressure as the compressor on the other vehicle should keep the main res pressure up, unless there was a problem there too?

The Res presumably will vary slightly on each vehicle, depending on compressor efficiency, condition of the overall air system and any leakage, and so on? Just as opening of dump valves happens at different times on each vehicle; they don't all go off in unison. That's assuming each gauge takes an idependent reading from the tank on that specific vehicle?

Again assuming no huge differences from 15x classes, the main res pipe between vehicles should keep the pressure fairly constant between vehicles over time? I've seen quite a big difference (between two units coupled) before when charging the system from near zero, with one unit building pressure quickly and the other not so quickly, or when coupling to a completely dead unit, which showed that the rate of flow through the air connection on the coupler isn't great, but I'd have thought that it'd keep itself pretty well equalised once it's reached a level and is just keeping it there.

 

[TheEdge]

Assuming 150s are broadly similar to other 15x classes: as I understand it, the CGIC just bypasses the unloader valve, so wouldn't have any effect in this case (unless the air leak happened to be there!). Is the main reservoir separated from the main res pipe by a non-return valve, though? If so, or if the main reservoir is separated from the compressor by a non-return valve (which it must be, surely?), I can't see how any leak on the supply side would cause loss of main res pressure as the compressor on the other vehicle should keep the main res pressure up, unless there was a problem there too?

Unless we are mixing up our acronyms the CGIC doesn't bypass the unloader valve, it bypasses the compressor governor, so the compressor just runs and runs charging the system until the first (or hopefully not second!) unloader valves dump the air to back around 9bar. There could have just been a leak somewhere within the air system that was just beyond the ability of the compressors, I've been on a 156 where the driver needed to run the compressor speed up at each station as there was a leak which the compressors couldn't keep up with at normal speeds but could with speed up. There are non return valves in there somewhere, no idea where!

Again assuming no huge differences from 15x classes, the main res pipe between vehicles should keep the pressure fairly constant between vehicles over time? I've seen quite a big difference (between two units coupled) before when charging the system from near zero, with one unit building pressure quickly and the other not so quickly, or when coupling to a completely dead unit, which showed that the rate of flow through the air connection on the coupler isn't great, but I'd have thought that it'd keep itself pretty well equalised once it's reached a level and is just keeping it there.

I've never really noticed there to be any great difference between vehicles or units once coupled and charged, the main res is, in theory one unbroken pipe/tank the full length of the train, so should be the same pressure its entire length, same goes for units coupled together and dead, never really noticed the two building at different rates. Although I did once couple a 153 to a totally flat 156 and watched as the main res just plummeted the moment I pressed the couple button and the 156 stole all the air for itself.

 

[Tomnick]

Unless we are mixing up our acronyms the CGIC doesn't bypass the unloader valve, it bypasses the compressor governor, so the compressor just runs and runs charging the system until the first (or hopefully not second!) unloader valves dump the air to back around 9bar. There could have just been a leak somewhere within the air system that was just beyond the ability of the compressors, I've been on a 156 where the driver needed to run the compressor speed up at each station as there was a leak which the compressors couldn't keep up with at normal speeds but could with speed up. There are non return valves in there somewhere, no idea where!

I think we’re both talking about the right acronym, just the wrong terminology somewhere! As I understand the terminology, the compressor governor controls the unloaded valve (opening it, to vent the compressor output to atmosphere, when the main res pressure reaches 7.7(-ish)bar and closing it, so that the compressor’s filling the main reservoir again, when it falls below 6.6bar. That’s what the CGIC bypasses, so that if the unloader valve is stuck open for any reason, the compressor can still be used to keep the main res up (but will keep putting more and more air in until the first safety valve lifts at 9bar). I think we both agree on that! Just making the point that the CGIC doesn’t really isolate any part of the air system, it just bypasses it, and in any case should be separated from the main res pipe by at least one non-return valve! That’s what puzzled me about the claim that the air leak was on the compressor pipework somewhere.

I've never really noticed there to be any great difference between vehicles or units once coupled and charged, the main res is, in theory one unbroken pipe/tank the full length of the train, so should be the same pressure its entire length, same goes for units coupled together and dead, never really noticed the two building at different rates. Although I did once couple a 153 to a totally flat 156 and watched as the main res just plummeted the moment I pressed the couple button and the 156 stole all the air for itself.

When the system’s being charged, I guess it depends how free the air is to flow through the air connection. Whenever I’ve (carefully!) coupled to a completely dead unit, it’s taken a good couple of minutes for the air pressures between the two units to equalise (pressing the ‘couple’ button shouldn’t make any difference though - most times, I couldn’t press it as electrically coupling them would have scuppered the next part of the move!). Once both are up to pressure, though, I’d have thought that they’d stay pretty much equal - unless the MRPICs were isolated, perhaps.

 

[brel york]

The main compressor pipe failed

 

[Tomnick]

The main compressor pipe failed

Shouldn't that be separated from the main res pipe, if not the main reservoir itself, by a non-return valve though? That'd mean that the failure shouldn't cause any loss of air from the main res and the compressor on the other vehicle should be able to keep the main res pressure up - or was there already something wrong there?

 

[brel york]

All I heard was it should have been checked on exam on Sunday but it didn’t happen

 

[MG11]

Well Class 222 engines seem to shake and vibrate when the train stops in a station, you can feel yourself shaking in your seat as soon as it arrives in the platform. It might be to do with the eco-mode applied to them though.

 

[Tomnick]

Well Class 222 engines seem to shake and vibrate when the train stops in a station, you can feel yourself shaking in your seat as soon as it arrives in the platform. It might be to do with the eco-mode applied to them though.

Certainly nothing to do with building air pressure as per the topic, and it wouldn’t be anything to do with eco-mode at a short station stop either. Probably just the engine revs dropping to a level that matches the resonant frequency of something.

 

[Raul_Duke]

Well Class 222 engines seem to shake and vibrate when the train stops in a station, you can feel yourself shaking in your seat as soon as it arrives in the platform. It might be to do with the eco-mode applied to them though.

222 compressors are electric and only one should be working at a given time anyway unless you’ve selected all for some reason. It’s different to some of the 15x’s where the compressor is directly coupled to the engine.

 

[sw1ller]

So a 222 has more than one compressor? That sounds like forward thinking to me! Who’s responsible for such a crazy idea? (175’s have only 1, even a 3 car). Anyone know how many a 221 has?

 

[sw1ller]

Shouldn't that be separated from the main res pipe, if not the main reservoir itself, by a non-return valve though? That'd mean that the failure shouldn't cause any loss of air from the main res and the compressor on the other vehicle should be able to keep the main res pressure up - or was there already something wrong there?

Yeah, my earlier comment about the CGIC was pointless, I apologise if any confusion was caused. If the main “hole” or break was in the compressor pipe somewhere then the only isolation Cock that could enable the train to carry on at line speed would be the ICMR (not MRPIC). That way the BCU would still be getting air from the other unit.

However, this doesn’t tally up as for some reason it needed rescuing by a 150 & 156 (one would have done but I assume they were already formed).

Only thing that makes sense to me is the train has gone over something in the run up to all this (possibly days before) and caused damage in more than one area and the single compressor couldn’t keep up with demand, as it were.

I’m just speculating here of corse. I just like to have a think about these things.

 

[brel york]

It came back from works attention on sat afternoon
Failed in traffic on Sunday

 

[edwin_m]

Well Class 222 engines seem to shake and vibrate when the train stops in a station, you can feel yourself shaking in your seat as soon as it arrives in the platform. It might be to do with the eco-mode applied to them though.

I think that's some kind of resonance which is set up when the engine is idling, possibly the engine mountings themselves. Rather like how some panels of a train interior (not to mention many components of an Enviro bus) rattle when the engine revs are low but are quieter when the engine is working harder.

 

[Intercity Exec]

So a 222 has more than one compressor? That sounds like forward thinking to me! Who’s responsible for such a crazy idea? (175’s have only 1, even a 3 car). Anyone know how many a 221 has?

Classes 220, 221 and 222 have 2 compressors on 4 cars and 3 on 5 cars, they are mounted on all intermediate vehicles. 222 7 cars have 4 compressors, again all on centre cars but one centre vehicle doesn't have one. Compressors are screw type (hence the lack of vibration compared to a reciprocating type compressor).

 

[sw1ller]

Classes 220, 221 and 222 have 2 compressors on 4 cars and 3 on 5 cars, they are mounted on all intermediate vehicles. 222 7 cars have 4 compressors, again all on centre cars but one centre vehicle doesn't have one. Compressors are screw type (hence the lack of vibration compared to a reciprocating type compressor).

Thanks for that. I’ve never been near a 221 so didn’t have a clue.

 

[edwin_m]

Are the compressors on 22x mechanically driven (so will only work if the engine on that car is running) or electric (so can presumably be powered by other engines via train wires)?

 

[Intercity Exec]

Are the compressors on 22x mechanically driven (so will only work if the engine on that car is running) or electric (so can presumably be powered by other engines via train wires)?

22x vehicles have electrically driven compressors. Only 1 compressor will be running at a time and a second is primed for if the run signal is lost from the operating compressor and in these circumstances the primed compressor will start to run. 22x units need to have 2 compressors to enable a brake release to be permitted, so the computer will not allow a 3car formation to operate as it is programmed to refuse a brake release (driving vehicles have no compressor). It makes no difference whether the engine on the vehicle is running or not as a voltage fed into that vehicle will run the compressor (each vehicle can only provide its own electrical power or be cross fed from one other predetermined vehicle). If the engine is not running on the vehicle with the running compressor or the vehicle that can cross feed to it then that compressor has no power available to run, the run signal is lost and another compressor will be called into action by the computer.

 

[sw1ller]

22x vehicles have electrically driven compressors. Only 1 compressor will be running at a time and a second is primed for if the run signal is lost from the operating compressor and in these circumstances the primed compressor will start to run. 22x units need to have 2 compressors to enable a brake release to be permitted, so the computer will not allow a 3car formation to operate as it is programmed to refuse a brake release (driving vehicles have no compressor). It makes no difference whether the engine on the vehicle is running or not as a voltage fed into that vehicle will run the compressor (each vehicle can only provide its own electrical power or be cross fed from one other predetermined vehicle). If the engine is not running on the vehicle with the running compressor or the vehicle that can cross feed to it then that compressor has no power available to run, the run signal is lost and another compressor will be called into action by the computer.

This is going to sound picky now, but it’s a genuine question. If the system is set up to not give break release if only one compressor is detected..... and the run signal is lost on a 4 car 221 to one of them, how does it know it’s actually now on backup mode rather than just running with one compressor?

 

[Intercity Exec]

This is going to sound picky now, but it’s a genuine question. If the system is set up to not give break release if only one compressor is detected..... and the run signal is lost on a 4 car 221 to one of them, how does it know it’s actually now on backup mode rather than just running with one compressor?

The compressor in question is sending a signal to say that it is running, if it were to lose its 400v power supply then it is no longer powered and thus it no longer sends its signal to the computer to say it is running. (The unit still has its control circuits running throughout). The computer still knows it is there, it is still detected, as such the alternative compressor will run and the one that can not will become unavailable, not the backup. A TMS indication will confirm the compressor is not available. TOCs will give serious consideration to removing a unit from traffic immediately if 2 engines of a 4 car shut down, it used to be a rule but is not enforced so strictly now. The unit wouldn't, however, start a journey in that state. This scenario is unlikely to occur, not impossible, but unlikely, as it would only occur if 2 adjacent vehicles (either the 2 centre cars or one centre car and its cross feeding end vehicle) were to shut down. Any other combination wouldn't affect the compressors. A 4 car unit could only be left without a compressor if 3 specific engines were shut down, the remaining end vehicle, which in this scenario still has its engine running is only capable of cross feeding to the other end vehicle and as neither has a compressor thus one can not run. As the cross fed power is degraded a vehicle can not receive cross feed and then pass it on to another vehicle. It is quite difficult to explain, I hope that the explanation above makes sense.

 

[sw1ller]

The compressor in question is sending a signal to say that it is running, if it were to lose its 400v power supply then it is no longer powered and thus it no longer sends its signal to the computer to say it is running. (The unit still has its control circuits running throughout). The computer still knows it is there, it is still detected, as such the alternative compressor will run and the one that can not will become unavailable, not the backup. A TMS indication will confirm the compressor is not available. TOCs will give serious consideration to removing a unit from traffic immediately if 2 engines of a 4 car shut down, it used to be a rule but is not enforced so strictly now. The unit wouldn't, however, start a journey in that state. This scenario is unlikely to occur, not impossible, but unlikely, as it would only occur if 2 adjacent vehicles (either the 2 centre cars or one centre car and its cross feeding end vehicle) were to shut down. Any other combination wouldn't affect the compressors. A 4 car unit could only be left without a compressor if 3 specific engines were shut down, the remaining end vehicle, which in this scenario still has its engine running is only capable of cross feeding to the other end vehicle and as neither has a compressor thus one can not run. As the cross fed power is degraded a vehicle can not receive cross feed and then pass it on to another vehicle. It is quite difficult to explain, I hope that the explanation above makes sense.

That actually made sense. Thanks for that. Interesting.

 

[No Fault Found]

The unloader valve just dumps the air between the compressor and the check valve so that when the compressor starts up again, it's not starting up to a pressurised system. Basically taking the load off of the compressor.

 

[Tomnick]

The unloader valve just dumps the air between the compressor and the check valve so that when the compressor starts up again, it's not starting up to a pressurised system. Basically taking the load off of the compressor.

15x or 222? The compressor’s driven directly off the engine on the former, so runs all the time. The unloader valve just takes the load off it when the air system’s fully charged.

 

[sw1ller]

15x or 222? The compressor’s driven directly off the engine on the former, so runs all the time. The unloader valve just takes the load off it when the air system’s fully charged.

No Fault Found could mean when the engine is started on a 150. They tend to hold air quite well. A 158 on the other hand wouldn’t matter, the second the engine stops they lose air anyway. I’ve seen some of them down t 1-2 bar after 10 minutes.

 

[Tomnick]

Maybe, though I wasn't aware that the unloader valve opened specially until the engine's running. I can't think it'd add a huge load to the engine anyway (whereas it'd make sense on a 222 with an electric compressor).

I sometimes really can't believe how quickly 158s lose their air!!

 

[AC47461]

it seems few people here know their Thomas the Tank engine

Haha, I noticed it straight away, but only read it this morning.

"You sir, give me your bootlaces....!"

 

[sw1ller]

Maybe, though I wasn't aware that the unloader valve opened specially until the engine's running. I can't think it'd add a huge load to the engine anyway (whereas it'd make sense on a 222 with an electric compressor).

I sometimes really can't believe how quickly 158s lose their air!!

I don’t think it would open only when the engine is running, but it wouldn’t need to open on a 15x if the engine was off. I’m pretty sure all unloaders would release if the pressure went above 7.5, 8.3 or 9.1 bar.

I don’t know why the air on a 158 leaks so quickly. All I know is, they’re a pain. The middle doors hold the air longer due to the location of the reservoir, so it’s worth trying them on your way down the train. But they don’t hold it for much longer.

 

[Intercity Exec]

15x or 222? The compressor’s driven directly off the engine on the former, so runs all the time. The unloader valve just takes the load off it when the air system’s fully charged.

No Fault Found describes the operation of the unloader valve perfectly, doesn't matter whether we are talking 15x, 22x or Anything else. It's only job is to bleed air from the delivery pipe from the compressor to the main air pipe on request. This is requested when the compressor is no longer required to charge and either bleeds to atmosphere (15x) or is to be switched off (22x). It is easier to think of the compressor as either charging the main air system or not charging the main air system. On a 15x the compressor is indeed running all the time but a valve within the compressor either sends the compressed air to the main air pipe or bleeds it to the atmosphere, this has nothing to do with the unloader valve which is a separate valve altogether. As it happens, some 150s were never fitted with unloader valves and were later modified (whether they all have them now or not I can not confirm). The idea, as described by No Fault Found is that the compressor has a run up opportunity upon being requested, it starts up by needing to charge a section of uncharged pipe and when this is charged to a level above that of the main air system the pressure overcomes the non return valve and air flow commences into the main air system. When the charging job is complete the compressor is now not charging the main air system, the non return valve closes and the unloader valve bleeds the delivery pipe pressure (the sudden and often loud whoosh of air you hear fairly often) for the whole process to repeat itself again. The pressure within the main air system is controlled by the compressor governor which tells the compressor to charge main air system/bleed what it is generating (15x) or tells the compressor to stop or start (22x). 15x air pressure is regulated to between 6.7 and 7.6 bar and 22x air pressure is regulated to between 8.5 and 10 bar.