Steam Chest Pressure

[JonL]

On another forum they are talking about efficient driving of steam locomotives and boiler design etc. One thing I saw mentioned perked up an interest I have had for a little while with regards to the fitting on full size engines of a Steam Chest pressure gauge.

What data can you derive from knowing the steam chest pressure? Would it help you to become a more efficient driver? If the engine is working hard would you see the full working pressure of the boiler? (working hard rather than working fast). If working fast with relatively little regulator would the pressure drop very low? Without a snifter I imagine it could even go negative?

I've plenty of room on my footplate of my simple locomotive, would there be any benefit to me as my driving progresses? Or is it pointless at 3.5 gauge? (I wouldn't ruling out fitting one just for the bling of another gauge on the footplate of course, anything that helps me pretend I'm doing it for real).

Thank you steam wizards, I appreciate your knowledge and your willingness to share it with the less educated.

[JonL]

I forgot to say, I read this post but I suspect there is more to it than discussed there.

modeleng.proboards.com/thread/10433/steam-chest-pressure-gauge

[chris vine]

Hi,

Yes, it is just going to tell you the pressure in the steam chest. So that is going to be less than the boiler pressure for several reasons.
1, restriction through regulator - even more pressure drop if not fully open
2, restriction in superheater and steam pipes

Then, if the engine is not using much steam, the steam chest pressure will be higher.
If the cylinders are using lots of steam then the SC pressure will be lower.

If you think about it, even if the regulator is just cracked open and there are also narrow steam pipes, IE lots of restrictions, BUT, the engine is using no (or very little) steam, then the SC gauge will show full boiler presssure.

The interesting part is when you are driving and the valve gear is well notched/linked up and the regulator wide open. This would be most efficient as you are using the steam expansively. In this case there should be a high SC pressure reading.

If you are driving on the regulator in full gear, then there will be quite a low SC pressure.

Once, when driving on the RHDR, we left the station - full gear and regulator just cracked open. Then as speed built up, the engine was linked up to use the steam expansively and the regulator opened more and more. The steam chest pressure gauge was much higher at speed than when we left the station.

If you find that the SC pressure is much lower than the boiler, even when linked up, then you would have to suspect that the regulator/superheater/steam pipes were not large enough.

Hope this helps
Chris.

[andrewtoplis]

I drive standard gauge engines and the Ivatt tanks have these fitted. They are useful when you are trying to coast with a breath of steam in the cylinders, so you can use the gauge to just let 5-10 pounds in. Otherwise though they can be a oddly hypnotising and I was told the LMS removed them for this very reason!

The other locos don't have them and I don't think we miss them.

Edited to add, you can't get a negative steam chest pressure as they aren't a closed system, you would suck air in down the blastpipe (and potentially ash and other rubbish at the same time). Snifting valves are to prevent this.

[jo479]

Slightly different circumstance, I have a 30 psi pressure gauge fitted to the low pressure steam chest on my my 3 1/2" LNWR 2-2-2-0 compound, it runs at about 14.5 psi, the boiler runs at 110 psi. It's interesting to watch the needle fluctuate at each end of the stroke.

[suctionhose]

In general terms, steam chest pressure is a useful indication on the throttle setting and available power etc. Train driving is fairly repetitive so if you know your engine and the conditions outside then it is a useful confirmation that you have the right amount of power set for the climb.

I suspect a gauge in 3.5"g would be not much more than a curiosity - reason enough to have one - because light weight models will slip wheels at a fairly unspectacular pressures.

My loco is a 5"g narrow gauge which has large spaces filled with lead and steel so the adhesive weight is sufficient to transmit all available power to the rails without slipping. That is one of the appeals of the engine - to work it flat out. With a two ton train and about 60% cutoff, you can set the throttle for 60 psi steam chest as you roll into the grade and know it will get there in much the same manner time after time.

On occasions of having to make a standing start with the load on a tight curve and 1 in 60, it's full throttle, full gear and 85psi in the steam chest. Choof...Choof...Choof...
Good fun!

BTW, my gauge - a standard 1" mini pressure gauge - is connected to the steam side of the snifter with about 2ft of 1/8" pipe.

Another piece of trivia is around 1930, some loco's of the NSWGR received a "drifting gauge". (This happened around the time of superheated boilers being installed though not a direct consequence of it if I read the history correctly.)

The gauge indicated pressure and vacuum and therefore acted in the same manner as a steam chest pressure gauge though the limited scale only provided the driver with a means to crack the regulator to keep zero pressure when running down hill thereby keeping a little steam in to cool the superheater elements as well as preventing a vacuum drawing ash down the blast pipe.



Yet more trivia is the Exhaust Pressure Gauge fitted to some engines (36 class?) which gave the driver information on the exhaust back pressure which, if it exceeded 8psi(?) ran the risk of pulling the fire... Someone with closer connection to operating those engines will correct me...

[suctionhose]

Feb 25, 2021 18:42:14 GMT jo479 said:

Slightly different circumstance, I have a 30 psi pressure gauge fitted to the low pressure steam chest on my my 3 1/2" LNWR 2-2-2-0 compound, it runs at about 14.5 psi, the boiler runs at 110 psi. It's interesting to watch the needle fluctuate at each end of the stroke.

Have you measured the HP side and compared piston areas to assess the distribution of power? I did with my compound traction engine over a range of conditions - remember in your case the HP would have 14.5psi back pressure!

[JonL]

I don't need much of an excuse to add a gadget or tinkering device (one look at the festooned footplate of a 1:1 scale Locomotive compared to my footplate with a single lonely gauge might be reason enough to fit one!) But I suspect it's information that wouldn't gain me much. It's a small engine and I don't really pull passengers around, that's for the bigger boys.

I'll have to have a think. It's an interesting thing to have, but I would be more motivated if it would help to educate me in my driving.

[suctionhose]

Nobby you don't know what information can do for you unless you have it 🙂

[JonL]

I guess then you would typically select a scale that maxes out at your boilers max working pressure, unless I'm reading things wrong?

I may not do anything with this info, but I suspect if the lockdown gets extended I'll want more tinkering...

[suctionhose]

0 to 80 psi would do it

[Deleted]

Hi Ross

Talking of cylinder pressure reading which I will most certainly do on 4472, where would you suggest to take the pipe feed from?

Regards

Pete

[suctionhose]

Any convenient place on the main steam pipe or from a steam chest itself. Pressure is the same at any point between regulator and port opening.

[flyingfox]

Greeting, in order to get the maximin analytical benefit from this gauge, it is best sited as near to the steam chest as possible, difficult on a three cylinder engine perhaps, but perhaps 3 gauges...? I have not yet fitted one to my A3, but it's on the "to do" list.
Then, if superheaters or regulator were restricting flow, the gauge would give an indication.
On my inside cylinder locomotives, I could connect directly to the common steam chest. On opening the regulator, the gauge would briefly show boiler pressure, then fall to as little as 20 psi when running steadily, notched up.
Very interesting fitting.
Regards
Brian B

[JonL]

Feb 26, 2021 22:33:25 GMT suctionhose said:

0 to 80 psi would do it

My safeties lift a 90, I assume I'd need a bit higher.

With my very exposed steam chests I think I could easily tap into one of them. Famous last words!

[don9f]

Feb 26, 2021 22:43:09 GMT @greenglade said:

Hi Ross

Talking of cylinder pressure reading which I will most certainly do on 4472, where would you suggest to take the pipe feed from?

Regards

Pete

Pete, I don’t know the arrangement on Flying Scotsman, but on the BR Standards, there is a pipe from one of the steam chests taken to the exhaust injector....this operates an internal changeover valve when the regulator is opened. The pipe to the steam chest pressure gauge is “teed” into this pipe near the injector itself. A similar arrangement regards the steam chest pipe/injector is also on some LMS engines, but not necessarily with the pressure gauge.
Maybe your exhaust injector would have such a connection to one of the steamchests?

Cheers   Don

[suctionhose]

Feb 27, 2021 15:55:17 GMT JonL said:

Feb 26, 2021 22:33:25 GMT suctionhose said:

0 to 80 psi would do it

My safeties lift a 90, I assume I'd need a bit higher.

With my very exposed steam chests I think I could easily tap into one of them. Famous last words!

I'd expect your wheels slip at 30

[JonL]

Good point!

[Deleted]

Feb 27, 2021 18:05:11 GMT don9f said:

Feb 26, 2021 22:43:09 GMT @greenglade said:

Hi Ross

Talking of cylinder pressure reading which I will most certainly do on 4472, where would you suggest to take the pipe feed from?

Regards

Pete

Pete, I don’t know the arrangement on Flying Scotsman, but on the BR Standards, there is a pipe from one of the steam chests taken to the exhaust injector....this operates an internal changeover valve when the regulator is opened. The pipe to the steam chest pressure gauge is “teed” into this pipe near the injector itself. A similar arrangement regards the steam chest pipe/injector is also on some LMS engines, but not necessarily with the pressure gauge.
Maybe your exhaust injector would have such a connection to one of the steamchests?

Cheers   Don

Thanks for your reply Don, alas I don't know 4472's setup but have asked a man who might..
One thought occured to me that perhaps it's somewhere that shares a feed to all 3 cylinders. My reasoning being that there is only one pressure gauge?

Regards

Pete

[don9f]

In theory, the pressure in each steamchest should average out the same, hence the pipe for instance on a 9F or Britannia being taken from the RH steamchest. The equivalent boss on the LH side just not being drilled/tapped.

Cheers Don