Cylinder drain cocks (steam)

[feldom76]

Hi,

After a steam engine has been stopped for some time, the driver opens the cylinder cocks to clear condensed water from the cylinders. Why do these cocks remain open for so long after the engine appears to be well underway? How does the driver judge when they can be closed?

Thanks

 

[ThirteenArches]

There's condensate in the cylinders because they've cooled while standing. So the cylinders - big masses of metal - have to have time to warm up above 100°C again before closing the cocks, otherwise you get more water condensed in the cylinders.

 

[Taunton]

Regarding the original question, why open for so long after underway, this is a feature of running steam locos in preservation. For those of us who remember actual mainstream operations with steam locos it was never done like this, and in fact blowing steam through them was decidedly unusual. Old YouTube clips of mainstream steam loco operation will confirm this.

As I understand it, it is done nowadays from "an abundance of caution" to minimise straining of the mechanical parts when starting. And in all fairness, preservationists do not have the backup, or the financial resources, of a major workshop like the main railway had to deal with any mechanical issues. Although it was most unusual to have these happen.

 

[Teaboy1]

There are nearly always [or at least on BR Standards] Spring Loaded Relief valves at each end of cylinder as a final last-chance to prevent massive damage. Not sure what opening setting is but it will be close to max boiler pressure. But as stated it is a bit of over-caution, better to be on safe side, etc

 

[Rescars]

The BTC Handbook for Steam Locomotive Enginemen states that "Cylinder cocks should always be open when the locomotive is standing or at any time when there is an indication of water in the cylinders." The same chapter contains drawings of the "steam operated cylinder cocks" fitted to some of the BR standards.

 

[Pigeon]

Adrian Vaughan sometimes perpetrates nonsenses when he tries to write about some point of engineering which he does not understand (or if he does, he is incredibly bad at communicating it). Usually I can disentangle what he is on about, but not in this case:

In "Grub, Water & Relief" he says that with the Mather Dixon engines in the first days of the GWR, the drivers would not shut off steam to stop at stations; instead they would open the drain cocks so the steam passed through without having any effect. He says they did this because if they did shut off steam, the boiler water level would drop and expose the firebox crown; not only does this make no sense, but on the very next page he contradicts himself, saying that if a station stop lasted more than a few minutes they would have to uncouple the engine and run it back and forth to operate the boiler feed pump - because (of course) with the drain cocks open and the engine still using steam, the boiler water level did drop.

It is of course impossible to find out by means of the internet any more about these engines than a handful of basic dimensions, so I remain baffled.

 

[ThirteenArches]

I don't know the anatomy of Mather Dixon engines, but in the early days of steam, few people had the foggiest idea of what they were doing. So the drivers' superstition isn't a bit surprising. But leaving the cocks open kept the cylinders warm, at fairly small thermal cost (otherwise any surplus steam just went through the safety valves), and you could just take off again without worrying about knocking the ends off the cylinders. Boiler feed is a quite different problem, which wasn't solved until injectors became the norm.

 

[Pigeon]

Having got hold of a copy of MacDermot I find there is a letter by Gooch on the subject, complete with cross-sectional diagram of the boiler, which explains in some detail these engines' tendency to prime; basically there was very little steam space above the surface of the water in the boiler barrel and what there was was mostly filled by the main steam pipe, so when the engine was using a lot of steam and the water was boiling hard, the rush of steam through this restricted space would pick up water and blow it along with it to the steam collector. He then goes on to describe modifications that would increase the steam space, and hopes that when this is done "we will be able to run with a safe level of water in the boiler".

He says the reason this is difficult at present is that if the driver is running fast and then shuts off steam for a station, the water level drops and exposes the firebox crown, so instead the driver has to reverse the motion while keeping steam on. Unfortunately that's all he does say, and it still leaves the puzzle of why shutting off steam should cause the water level to drop.

My best guess so far is that when the engine was running fast, continually using a lot of steam and then replenishing the water as required using the axle-driven feed pump, it was possible to run the quantity of water in the boiler down to below the normal static minimum level while still maintaining an adequate "dynamic" level; the boiler could end up in an equilibrium condition where the rush of steam towards the firebox end was carrying enough rush of water with it to maintain a level above the firebox crown and an indication in the gauge glass before it ran back again into the main part of the boiler, even though the static level of the same quantity of boiler water would be below the firebox crown (but higher in the main part of the boiler). Stopping the flow of steam would stop the rush of water and allow it to settle at an inadequate static level. So they would have to reduce the steam flow gradually while turning on the feed pump in order to return to a statically safe condition.

 

[unslet]

Talking to 2 ex BR drivers at Grosmont, they both queried the use of drain cocks, stating they were overused and would cause lubrication issues with the cylinders.

 

[Rescars]

Having got hold of a copy of MacDermot I find there is a letter by Gooch on the subject, complete with cross-sectional diagram of the boiler, which explains in some detail these engines' tendency to prime; basically there was very little steam space above the surface of the water in the boiler barrel and what there was was mostly filled by the main steam pipe, so when the engine was using a lot of steam and the water was boiling hard, the rush of steam through this restricted space would pick up water and blow it along with it to the steam collector. He then goes on to describe modifications that would increase the steam space, and hopes that when this is done "we will be able to run with a safe level of water in the boiler".

He says the reason this is difficult at present is that if the driver is running fast and then shuts off steam for a station, the water level drops and exposes the firebox crown, so instead the driver has to reverse the motion while keeping steam on. Unfortunately that's all he does say, and it still leaves the puzzle of why shutting off steam should cause the water level to drop.

My best guess so far is that when the engine was running fast, continually using a lot of steam and then replenishing the water as required using the axle-driven feed pump, it was possible to run the quantity of water in the boiler down to below the normal static minimum level while still maintaining an adequate "dynamic" level; the boiler could end up in an equilibrium condition where the rush of steam towards the firebox end was carrying enough rush of water with it to maintain a level above the firebox crown and an indication in the gauge glass before it ran back again into the main part of the boiler, even though the static level of the same quantity of boiler water would be below the firebox crown (but higher in the main part of the boiler). Stopping the flow of steam would stop the rush of water and allow it to settle at an inadequate static level. So they would have to reduce the steam flow gradually while turning on the feed pump in order to return to a statically safe condition.

Interesting to see that (according to Wikipedia) at least one of the Mather Dixon locos (Planet) survived in use as a stationary boiler after withdrawal from traffic. This suggests that there was nothing too much wrong with the design of the boiler itself. The half dozen Mather Dixon locos were far from identical in many respects. Were the boilers similarly varied in design?

 

[Taunton]

Priming, water being carried along with the steam, is overcome as a side benefit of superheaters, which turn it into all steam before reaching the cylinders.

 

[Pigeon]

Interesting to see that (according to Wikipedia) at least one of the Mather Dixon locos (Planet) survived in use as a stationary boiler after withdrawal from traffic. This suggests that there was nothing too much wrong with the design of the boiler itself.

Confirmed in MacDermot. Indeed there doesn't seem to have been anything wrong with the boiler's ability to raise steam; it was getting it out without carrying the water with it that was the problem. Of course for a stationary boiler the operating conditions are rather different, and it wouldn't surprise me if they made some modification to the steam collecting arrangements as part of the process of converting it.

The half dozen Mather Dixon locos were far from identical in many respects. Were the boilers similarly varied in design?

The complaint seems to be directed at the locos as a group rather than at particular examples, so I would guess that the boilers were similar. Probably they had gone just that bit too far in trying to shrink the size of the boiler to help with meeting Brunel's weight restrictions.

 

[Rescars]

The complaint seems to be directed at the locos as a group rather than at particular examples, so I would guess that the boilers were similar. Probably they had gone just that bit too far in trying to shrink the size of the boiler to help with meeting Brunel's weight restrictions.

The extraordinary 2-2-2 "Ajax", which had solid steel driving wheels 10ft in diameter (so offering maximum wind resistance!), is shown in illustrations with a fair sized steam dome at one end of the boiler and a slightly smaller dome at the other.

 

[Pigeon]

That was actually the one that became a stationary boiler. There was a gap before it was delivered, during which time the problems with the previous two had become apparent, and the arrangement of domes is thought to have been a modification to increase the steam space in response. The ones that followed were probably also modified, but the basic boiler dimensions of all of them seem to have been similarly undersized.