Relief Vales and Drain Cocks

An experiment – Steam Operated Combined Drains & Relief

Much earlier in the process I baulked at drilling the cylinder castings for the cylinder drain cocks because they looked hard to drill with out risking damage to some rather expensive castings. Moreover, previous experience with manual cylinder drain cocks on the loco had been poor (leaky, difficult linkages etc.) and on the steam launch most people seem to opt for 4 manually operated cocks which involves a deal of “faffing” in use.

So I decided to try to create some steam-operated cylinder drains and combine them with the relief valves fitted to the cylinder covers. Now we need to add a health warning at this point – the design I am writing about is unproven, I will let you know how they work in practice.

The idea

The standard relief valves are very similar to normal ball-on-seat safety valves, with the spring holding the valve preloaded with a 3/16 bolt (see copyright snippet of the standard drawings below – as usual click for bigger picture).

Standard Drawing from 1970's (c) Camden Books I think.

Standard Drawing from 1970’s (c) Camden Books I think.

As you can see these use small inserts either side of the spring to bear on the ball and allow the top bolt to adjust spring pressure.

I made a set of these exactly as drawn, and then developed a modified version where the bolt/cap setting the spring pressure is replaced by a small piston, cylinder and push rod assembly which are used to load the spring when operating in “relief valve mode” and remove the spring pre-load when operating in cylinder drain mode.

Operating Pressures

A bit of computation showed that the opening pressure exerted on the ball by steam in the cylinder (that is resisted by the spring) was around 10 lbs at full boiler pressure. Of course the relief valve needs to stay shut unless unusual pressures are experienced, but also full boiler pressure (~200psi) is not going to be experienced on the LP cylinder (or on the HP one under normal operation).

So this suggests that the relief valves need to be set at an appropriate pressure for their position, and normal operation.

Also when in relief mode, the open passage through the valves needs to (probably) be smaller than when in “relief mode” to allow the engine to operate with the drains open. This suggests that there needs to be some pre-load on the spring in “drain mode” to limit this opening.

So the net is that the valve needs to have two opening pressures (one for relief mode and one for drain mode) and both of these need to be adjustable at commissioning time. I think the design provides this as follows:

The Finished Articles

The images below show the parts, and only think a few comments are needed:

  • The bore of the steam cylinder was chosen to provide twice the force which can be expected to be needed (~20 lbsf @200psi) – 5/16″
  • The steam cylinder is sealed with a single O-ring (a 008 size ring) – I don’t think this will get too hot to trouble a standard ring.
  • A cover to route exhaust from valves into drain pipes has been added, made from lengths of 1″ O/D stainless “nipped” between O-rings on the valve body. (the unions for the drain pipes are not yet in place)
  • The steam cylinder is screwed into the main body and it’s position (bearing on the spring assembly) defines the “drain mode” spring preload and the effective opening of the valve.
  • The “copper coloured” push rod will further depress the spring assembly when steam is fed to the steam cylinder, and it’s length will set “relief mode” preload and therefore opening pressure.
  • The stroke of the steam cylinder seemed to need to be less than 3/8″, but the effective stroke is set by the pushrod.
  • The lower valve on the HP cylinder will need to have a small adaptor made, as it needs to be angled out to clear the stuffing box and connecting rods – this is a problem with the standard design too.
A full kit of parts for one valve, and three (un-hexagoned ones)

A full kit of parts for one valve, and three (un-hexagoned ones)

Finished (apart from drain pipes)

Finished (apart from drain pipes)

The parts of the steam cylinder

The parts of the steam cylinder

 

 

 

 

Notes on manufacture

Materials

Yet again I have had to re-learn the “make sure what the material you are using is”. Two examples this time: 1) I picked up an off cut of “brass hex” from Metal Supermarkets, which proved to be almost unusable – drilling a 1/4″ hole “burnt out” one bit, and threading it was almost more than I could manage. 2) A bundle of “phosphor bronze rod” I purchased many years ago has just got re-filed as “probably copper”, as it really hates being cut or threaded, tearing and seizing at every turn. grrrrr

Screw-cutting v dies

I purchased a while ago a set of threading inserts for the lathe, and by the time I had finished this job I had concluded that cutting all the threads on the lathe was faster and more reliable then using a tailstock-mounted die holder. You can use a die as a dienut to finish the thread, but the screw cut thread is true and much less likely to be damaged in cutting. I had a couple of examples when cutting with dies, where the thread was stripped as it was cut for the first few threads, and for a short thread (for a union for example) this can wreak a component you have invested a lot of time in.

Lastly the BSB (aka “Brass”) range of threads are good as they are all 26tpi so for those of us without a quick-change gearbox on the lathe the amount of change-wheel fiddling is much reduced.

Hexagons

Lots of the parts needed a hexagon machining on the outside – these were quite quickly produced on the mill using the set-up shown below…

Hexagoning Set-up

Hexagoning Set-up

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