Lubrication drift from Tornado thread.

A quick question from a novice.

What prevents the "upping", of oiling rates for higher speed runs (mainline v heritage running), is their a specific reason not to do this????

I could imagine problems with cylinder rings getting coked up with unused oil residue, more mess to sort eventually etc...

Not knocking what has happened, just a question out of curiosity really.

 

AIUI it’s about capacity. You can chuck in as much as you like, within limits, but you have to balance lubricatiin requirements with capacity.

You don’t want to be sending oil up the lum in vast quantity tho.

 

Mechanical lubricators , or the restrictor type have a limited capacity of oil and by upping the rate of flow, you may prevent one problem but also add others, then you have to look at the type and make up of oils, will you need to specify a different type of oil for higher speeds oil can breakdown over a certain heat range its only a very thin film between metal to metal contacts anyway, much of the lubrication on locos runs through small bore copper piping, it don't take much to block a feed, or even for the operating lever to come adrift

 

This was a problem with mechanical lubricators: it was possible the alter the drive to feed more oil to the steam chest and cylinders, and many drivers did this on the basis of, The more oil, the better. It wasn't. The valves and rings became heavily carbon coated leading the steam blow by, and when the engine was stripped to have the fault rectified, the fitters had a terrible job getting the valves out.

 

If only they could find a lubricant that fractures into very pure graphite !

 

Wasn't there an engineer somewhere who was criticised because he thought it was better to use a lower degree of superheat with a hydrostatic lubricator?

 

If only you could get the superheat high enough...

 

What, Maunsell? There's a 24 element superheater in an S15 boiler, nothing particularly low superheat about that for a contemporary British mixed traffic class.

 

Churchward ?

 

It's worth remembering that 'More' is not always more, so to speak, to much volume can easily lead to less superheat as production of heat is dependent on several factors, not least of which is grate area, in reallity the more efficient a boiler is, the less heat is available for superheating.

 

Bronze piston valve rings anyone ?

 

If I remember correctly, on "Red Devil", Wardale has a system for injecting saturated steam to control the temperature of the valve rings.

 

I may have been ever so slightly deliberately misunderstanding LesterBrown's clear inference regarding Churchward, but the point still stands. Most of the Southern classes of Urie and Maunsell ancestry had higher superheat than their GWR contemporaries and yet still used hydrostatic lubricators perfectly successfully. Eric Langridge, who moved from the LSWR to the Midland, wrote about the comparative freedom from carbon build ups under the valve rings of the LSWR classes superheated by Urie, as compared to that found on the Midland.

I don't see how anyone could reasonably suggest that a 24 element superheater would be 'too much' given the general duties of the Southern 4-6-0s with the N15/S15 boiler, and commentary on this forum from those who crew them certainly doesn't seem to suggest they lack steaming capacity. If superheat is fitted, you can't really take it out of the boiler efficiency equation; if the steam produced has higher specific enthalpy you don't need to produce as much of it to have the same combined boiler/superheater efficiency.

 

First few MNs had hydrostatic lubricators when new.
A huge contraption on the fireman’s side, must have been cursed by a few sore heads!

 

Superheat temperature also affects cylinder efficiency. A cylinder full of highly superheated steam will do the same amount of work as steam at the same pressure but a lower temperature, but due to it's lower density it contains less enthalpy. Less enthalpy is lost up the chimney.

 

I'm not dissagreeing or trying to argue, particularly not on specific cases, merely pointing out that it is possible to 'overdo' things and get into the laws of diminishing returns, if 'Superheating' was such a simple matter then everyone would have done it.

 

In a power station, the superheat temperature is pushed as high as possible, to the limit that the metal can withstand. However a power station boiler is usually run flat out for hours on end (if not longer).
A locomotive is more of a compromise. Sometimes they are not running long enough for the superheater to reach maximum temperature. The superheated Stephenson boiler is not really an ideal heat exchanger; The steam is heated by some of the coolest gas, just before it enters the smokebox, and increasing the superheat temperature will also increase the heat losses of the gases passing through to the smokebox. Finally, as discussed upthread, there are limitations due the lubrication of the valvegear. These don't really apply to a turbine where, (hopefully) there are no sliding surfaces; the bearings are seals are remote from the main steam flow. The turbomotive had a larger superheater than the other Lizzies.

 

Did anyone ever try to superheat with tubes in the firebox ? not perhaps to get really high superheat, but because a boiler with extra tubes instead of superheater flues would be a tad better at producing steam in the first place ?

 

Thermic siphons?

 

Not a superheater though, because they are full of water - by definition, a superheater has to take steam out of equilibrium with the boiling water so that the temperature can be raised over and above the saturation temperature.

My gut feeling about trying to arrange superheating inside the firebox would be that you couldn't sufficiently cool any tubes carrying the steam (by definition they would not be in contact with any water, which has a large with a large heat capacity, but only in contact with steam) so there would be considerable risk of a tube failing, with obviously highly dangerous results.

Ultimately, you have only got a certain amount of energy available. To a limited degree, you can redistribute it between the amount used to boil water; the amount used to further superheat the steam; and the amount wasted up the chimney. But you cannot change how much energy you have available in total, only - to a degree - the balance of what goes where.

Tom