Slide & Piston Valves

[melmerby]

Hi all

What are the advantages and disadvantages of slide & piston valves on steam locos (not poppet valves!)

 

The Classic American 4-4-0 used this arrangement and it was common until around the start of the 20th Century, typically with inside Stephenson's gear and rocker shafts

Examples of imported to the UK locos with this arrangement were the 2 Cooke & Co 0-8-2Ts for the Port Talbot and the 2-6-0 kits imported by the Midland.

 

GWR locos with 2 outside cylinders usually had inside Stephensons gear but operating outside piston valves.

[Compound2632]

24 minutes ago, melmerby said:

What are the advantages and disadvantages of slide & piston valves on steam locos (not poppet valves!)

 

As I understand it, piston valves reduce the resistance in the steam circuit.

 

S.W. Johnson on the Midland Railway was an early British adopter, largely at the prompting of his friend W.M. Smith, Wilson Worsdell's Chief Draughtsman on the North Eastern. Johnson applied them in lieu of slide valves to his existing designs of 4-2-2 and 4-4-0 express passenger engines, from 1893 onwards. That a freer-running engine resulted is witnessed by the 90 mph maximum speed recorded with 4-2-2 No. 118, the joint highest speed recorded with a British locomotive in the 19th century.

 

Slide valves usually relied to some extent on the moisture in the steam to provide lubrication and hence were unsatisfactory with superheated steam, so the general adoption of piston valves in Britain goes hand in hand with superheating. When mounted below the cylinders, slide valves could drop away from the fixed surface when steam was shut off, reducing friction when coasting. I think (from memory) Stroudley and Drummond were fond of this arrangement.

 

I'm sure others will be along to enlarge on the technical details. 

Edited September 14, 2022 by Compound2632

[Morello Cherry]

This might be useful

 

https://ngtrains.com/blog/k27-history/

 

There is a long section on why piston valves.

 

I was trying to find out why Blanche on the FR was fitted with piston valves (Linda has the original slide valves) but I can't find the explanation.

[LMS2968]

Slide valves are simpler, the valve - usually - sits in a buckle which moves its across the ports to admit and release exhaust steam. Saturated steam containing minute water droplets provides a measure of lubrication. When coasting, the valve does not seal the ports thereby allowing bypass of the pressures from one end of the cylinder to the other. With steam applied, the pressure presses the valve against the ports so giving the seal. But - this pressure on the sliding faces gives a lot of friction and absorbs power.

 

Superheated steam should not contain water droplets - that's the point, so no lubrication value. Piston valves are therefore preferred (but there were superheated engines fitted with slide valves, at least some successfully. But piston valves need rings to seal them, and added complication, and the valves need more certain lubrication. On the other hand, resistance to movement is lower so the power absorption less.

[ngtrains.com]

1 hour ago, Morello Cherry said:

 

I was trying to find out why Blanche on the FR was fitted with piston valves (Linda has the original slide valves) but I can't find the explanation.

Because at the time of the rebuild into a superheated 2-4-0 her slide valve cylinders were in poor shape.

 

Preservation at the time wasn’t up for making new slide valve cylinders, plus BREL Crewe offered to fabricate new piston valve cylinders and for the lubrication issues already mentioned and the reduced load on the valve gear piston valves were thought best.

 

The valve gear load/wear is a significant issue. Linda has a much more butch reverser and you need two hands to notch up. Linda’s valve gear typically only does two seasons before it needs new bushes and pins. Blanche can be notched one handed and is just finishing her fourth season since a valve gear overhaul.  (It does rattle a lot at the moment and is getting done this winter).

 

Would we convert Linda? Unlikely.  The degree of superheat isn’t that high and oil technology has come on a bit. Blanche got the best pair of the fabricated cylinders and they haven’t exactly got streamlined steam passages. Wear in the valve faces on Linda has been addressed with replaceable and hardened valve faces but now patterns and casting new cylinders wouldn’t be thought hard - expensive, but not hard.

 

It does give them different characters. I’d call Linda a racehorse, on a good day it goes really well, accelerates quickly and steams like a witch but is temperamental enough to have off days when it can’t be bothered. Blanche is more of a cart horse, just plods on and gets the job done.

 

(it’s my blog that has the K27 stuff on it, I’m an FR driver and get to drive both of them and led Blanche’s overhaul for their hundredth birthday bash. Blanche is known as “the other woman” in this household)

[MarkC]

On 14/09/2022 at 19:18, Compound2632 said:

That a freer-running engine resulted is witnessed by the 90 mph maximum speed recorded with 4-2-2 No. 118, the joint highest speed recorded with a British locomotive in the 19th century.

Is this fact (which, to my chagrin, I didn't know - every day's a school day 😎 ) the reason for 118's preservation? (Some may know her as 673...)

[Compound2632]

2 hours ago, MarkC said:

Is this fact (which, to my chagrin, I didn't know - every day's a school day 😎 ) the reason for 118's preservation? (Some may know her as 673...)

 

No, because to my chagrin in turn, on checking, I find that it was No. 117 that Rous Marten recorded at 90 mph! No. 118 / 673 was the last in service, withdrawn in 1928.

[rogerzilla]

Slide valves are more foolproof as they will lift if hydrolocking occurs.  They do not cope well with superheated steam, as lubrication becomes difficult, and that's the main reason they died out.

[MarkC]

15 hours ago, Compound2632 said:

 

No, because to my chagrin in turn, on checking, I find that it was No. 117 that Rous Marten recorded at 90 mph! No. 118 / 673 was the last in service, withdrawn in 1928.

Last in service - makes sense too. Thank goodness that she didn't suffer the same fate as the set aside locos which were in Derby Works when W.A.S. paid a visit there, saw them & ordered their destruction 😪

[Wickham Green too]

... and a few veterans that survived at Eastleigh until WW2. ( Not to mention Ben Alder ...... whoever he was ! )

[Compound2632]

1 hour ago, MarkC said:

Last in service - makes sense too. Thank goodness that she didn't suffer the same fate as the set aside locos which were in Derby Works when W.A.S. paid a visit there, saw them & ordered their destruction 😪

 

No. 118 was one of the locomotives set aside for preservation there. Stanier had the others scrapped as he was informed that the paint shop was overcrowded. I think the following list is correct: outside-framed 0-6-0 No. 2385 (Kitson, 1856); 2-4-0 No. 156 (pre-1907 number) (Derby, 1866); the first of Johnson's 0-4-4Ts, No. 1226; and a NLR 4-4-0T, No. 6445. Fortunately a representative of the 156 Class survives, No. 158A, withdrawn in 1947.

 

I read that Stanier was also responsible for the scrapping of North Star at Swindon.

 

Of course his whole policy on the LMS was scrap-and-build (continuing existing policy) so you could call him responsible for the destruction of many fine pre-grouping classes.

[Phil Bullock]

Had an interesting discussion with the crew of 5786 on the footplate on Saturday about valve and cylinder  lubrication.

 

If I have it correct on the GWR …

 

Saturated locos eg 57xx …. Slide valves, hydrostatic lubricator in cab, atomisation off a valve on the regulator.

 

Low superheat engines eg Halls, Early Castles …. Piston valves,  Hydrostatic lubricator in cab , not sure about atomisation in these

 

High superheat engines eg last 20 modified halls, Late Castles …. Piston valves, mechanical lubricator on framing, atomisers on framing.

 

Probably over simplified but perhaps illustrates the above points….

[Compound2632]

12 minutes ago, Phil Bullock said:

If I have it correct on the GWR …

 

What were the first piston valve engines built for the Great Western? And also, the last slide valve engines?

[Blandford1969]

5 hours ago, Phil Bullock said:

Had an interesting discussion with the crew of 5786 on the footplate on Saturday about valve and cylinder  lubrication.

 

If I have it correct on the GWR …

 

Saturated locos eg 57xx …. Slide valves, hydrostatic lubricator in cab, atomisation off a valve on the regulator.

 

Low superheat engines eg Halls, Early Castles …. Piston valves,  Hydrostatic lubricator in cab , not sure about atomisation in these

 

High superheat engines eg last 20 modified halls, Late Castles …. Piston valves, mechanical lubricator on framing, atomisers on framing.

 

Probably over simplified but perhaps illustrates the above points….

Yes, pretty much. 

 

One thing on the difference between them is accelerating away from a stop is less work with a piston valve compared with a slide valve.  

With a slide valve engine the valve, which is D shaped is held down by the steam being admitted. To try to change the position of the reverser with steam on, certainly more than just into first valve risks the reverser flying being impossible to control. Therefore to change cut off its 1) turn on blower, 2) shut off, 3) move the reverser, 4) open regulator and finally turn off blower, (Western region ones are very loud and are not needed with steam on) In first valve it is possible on a well run in engine to move the reverser, BUT you have to be very careful. All of this opening and shutting slows acceleration. 

 

Whereas with a piston valve engine the pressure of the steam is even all the way around the valve and so it is not necessary to shut off to move the reverser. 

 

On something like a pacific it would make getting away if slipping very, very difficult. Whereas as they are piston you can open the regulator to the required amount in the steam chest and then drive off the reverser. By that I mean it is not locked and if the engine goes to slip you very quickly wind towards mid gear then carefully wind out again. 

 

 

[Phil Bullock]

10 minutes ago, Blandford1969 said:

Yes, pretty much. 

 

One thing on the difference between them is accelerating away from a stop is less work with a piston valve compared with a slide valve.  

With a slide valve engine the valve, which is D shaped is held down by the steam being admitted. To try to change the position of the reverser with steam on, certainly more than just into first valve risks the reverser flying being impossible to control. Therefore to change cut off its 1) turn on blower, 2) shut off, 3) move the reverser, 4) open regulator and finally turn off blower, (Western region ones are very loud and are not needed with steam on) In first valve it is possible on a well run in engine to move the reverser, BUT you have to be very careful. All of this opening and shutting slows acceleration. 

 

Whereas with a piston valve engine the pressure of the steam is even all the way around the valve and so it is not necessary to shut off to move the reverser. 

 

On something like a pacific it would make getting away if slipping very, very difficult. Whereas as they are piston you can open the regulator to the required amount in the steam chest and then drive off the reverser. By that I mean it is not locked and if the engine goes to slip you very quickly wind towards mid gear then carefully wind out again. 

 

 

Cheers Was the technique used on 5786 at weekend….

[Blandford1969]

55 minutes ago, Phil Bullock said:

Cheers Was the technique used on 5786 at weekend….

One of the best things on a Pannier of course is that on the level you can pull them up almost to mid gear and due to the increasing lead with Stephenson's they just fly (well at 25mph anyway) .

[Phil Bullock]

1 minute ago, Blandford1969 said:

One of the best things on a Pannier of course is that on the level you can pull them up almost to mid gear and due to the increasing lead with Stephenson's they just fly (well at 25mph anyway) .

Totally master of a 4 coach train on SDR …. Never got out of first valve , fireman had an easy trip! 

[Michael Hodgson]

Aren't piston valves also a bit more difficult (=costly) to manufacture/maintain?

[Phil Bullock]

30 minutes ago, Michael Hodgson said:

Aren't piston valves also a bit more difficult (=costly) to manufacture/maintain?

Yes there’s a comment back up the thread regarding the need for valve rings which means machining grooves in valve heads …. No such frippery in slide valves 

 

7 hours ago, Compound2632 said:

 

What were the first piston valve engines built for the Great Western? And also, the last slide valve engines?

Last slide valve locos built to GW design (but not by GWR!) 16xx and 94xx panniers built by BR in the 1950s.

 

Oh hang on did 94xx have piston valves? 
 

First piston valve engine is more of a challenge …. No 100 of 1902 perhaps? 

Edited September 19, 2022 by Phil Bullock
Query re 94xx

[melmerby]

10 hours ago, Phil Bullock said:

 

First piston valve engine is more of a challenge …. No 100 of 1902 perhaps? 

The Krugers (1899) had piston valves, No.36 a sort of prototype didn't.

[Wickham Green too]

Can't find any documentary evidence for the 9400s  -  but the tail rods are very close together so they must be slide valve !

[62613]

Other advantages of piston valves vis - a - vis slide valves were:-

 

1: The valve heads could be widely spaced, so that the steam/exhaust passages could be made straighter (and therefore with a greater area), thus reducing the resistance to flow in them, resulting in a more free - running engine

 

2: With inside admission piston valves (that is, with the high pressure steam contained between the valve heads), the valve rod gland and the valve chest cover, don't need sealing against high pressure steam.

[LMS2968]

The slide valve components of 7714 laid out at Bridgnorth, where it is having a P&V Exam. The valves themselves are on the right, the buckles, with spindles each side, are middle. The valve chest cover is between them.

 

[2251]

On 20/09/2022 at 09:21, melmerby said:

The Krugers (1899) had piston valves, No.36 a sort of prototype didn't.

 

More precisely, 2601 (which, unlike the other Krugers, was a 4-6-0).

[Compound2632]

24 minutes ago, 2251 said:

More precisely, 2601 (which, unlike the other Krugers, was a 4-6-0).

 

To be clear, are you saying 2601 had piston valves but the other nine Krugers had slide valves?

 

Which was the first class all built with piston valves? Presumably the 41xx 4-4-0s had slide valves, since they pre-date No. 2601; indeed Wikipedia says so for the Badmintons and Atbaras leaving one to infer that the later Flowers were the same. As far as I can make out the Cities were also slide valve engines though later rebuilt with piston valves but the Counties had piston valves from new. The Saints had piston valves - was the example of de Glehn's atlantics to tipping point?

 

I find it curious that despite Churchward's reputation for innovation and absorption of best practice, Swindon should be lagging a decade behind Derby on this point!

Edited September 23, 2022 by Compound2632