Safety Valves (2)

My last look at Sentinel 7109's safety valves was about two years ago. Then I'd initially mounted them on the first version of a manifold constructed from heavy duty steel pipe fittings.

First attempt

All seemed good at the time but a number of factors began to come to light that meant some changes had to be made.

1. I wasn't sure that the metal of the threaded nipples connecting the steel fittings was strong enough and may have been made from 'Blue band' rather than 'Red band' grade pipe. (Red band pipe has a thicker wall and is rated suitably for the Sentinel boiler's wet steam conditions. Blue band is not good enough).
   I replaced the connecting nipples with heavy duty hex nipples which are much stronger.
   
   
2. I was concerned that when I removed and refitted the safety valves for the boiler's next and any subsequent hydraulic tests, the thread would not make as tight a joint as the first time. They would also need to be removed to fit the boiler top cover.
   I inserted a 3000 psi rated union and mating nipple between the manifold and each safety valve body. The union is designed to be undone and also enables the safety valve to be correctly orientated so its exhaust pipe would align with its hole in the cab roof when refitted.
   This does make the assembly taller but practical!
   
   
3. I had assumed that I could obtain a steam supply for the vacuum brakes, the whistle and the pressure gauge from the four-way manifold used to feed the superheater. However, with all the other pipework in place, it was too inaccessible particularly to reach an isolating valve in that location.
   I thus decided that the safety valve manifold would have to include the steam supplies for the vacuum, whistle and pressure gauge.
   
   
4. At least one of the safety valve outlet pipes would have to pass through cab roof support girders. Not good!

Here's the result. Hex nipples hold it all together. The 3000 psi unions sit below the safety valve housings. The two 'L' shaped fittings are now 'T' shaped so the extra outlets can be taken from each of the new 'arms'. The vacuum supply is downwards from the left 'T' and the whistle and gauge supply horizontally from the right 'T' to an isolating valve.

The exhaust outlets now straddle the roof support girder!

Not cricket wickets!

There's more to do to prevent the pipes rattling in the cab roof holes and to ensure it all lines up if taken apart and reassembled.

Safe Safety Valves (1)

Sentinel 7109's new safety valve system is taking shape as shown below.

The Flange and Supporting pipework

I introduced the new pair of Bailey-Birkett type 716SSL safety valves a little while back. I've now obtained the supporting steel pipe fittings and constructed the new support manifold.

Whilst on the surface this construction seems fairly straight forward, it is a safety system and therefore requires rather more formal engineering processes to ensure it is fit for purpose. (It's also my safety I'm concerned about!).

Before I retired, I spent many years on railway signalling and control centre research and development projects. Most were computer based and hence involved formal safety related system/software development project 'V' life cycles. ('V' implies: Define what it has to do; design it; implement it; test the implementation; test it meets the design; test/assess whether it does what it was defined to do originally. And don't forget the traceability!).

Safety valve pipework is somewhat more tangible than software so it would be over the top to do all that but some allusion to the principle is worthwhile.

Requirements:

Firstly, what are the requirements (i.e. WHAT does it have to do)?

The pipework shall:
1. Withstand 275psi.
2. Withstand 230DegC.
3. Support the safety valves.
4. Support the exhaust outlet pipework.
5. Allow the full output of the boiler to pass to the safety valves (4600 lbs/hour).
6. Connect to the original Sentinel boiler flange mounting.
7. Prevent condensation accumulating in the exhaust (i.e. to prevent showering nearby onlookers when the safety valves blow off!).
8. Enable easy removal of the Safety Valves for hydraulic boiler testing.

Design:

Requirements 1-8 are ultimately tested or observed; however, they all need to be taken into account during the design process.

Requirements 1 and 2 (275psi & 230DegC) are satisfied by choosing to use mild steel pipe and fittings and avoiding the weaker malleable iron which is not up to the job. It also requires that a flange to connect to the existing Sentinel flange has to conform to BS 10 (1962) Table F or better.

Requirement 3 is satisfied, firstly, by ensuring that there is a vertical 3/4" male BSP thread to connect to each safety valve and, secondly, by using sufficiently heavy gauge material.

Requirement 4 is satisfied again by using the heavy gauge material. Additional support from the cab roof may also be included later to support the exhaust pipework.

Requirement 5 is satisfied by ensuring that there are no parts of the pipework that are of a smaller diameter than the 3/4" inlet to the safety valves.

Requirement 6 is satisfied by using a compatible flange. Easy you would think but when Sentinel made their safety valve mounting, there were no standard flange sizes and ratings! So the nearest standard type needed to be chosen that involved minimal adaptation.

This resulted in a Carbon Steel flange as follows:
A screwed flange of nominal bore 1.5" (to allow a male 1.5" BSP thread to be attached).
A BS10 Table 'F' type capable of 300psi at 232.2DegC.
5.5" Diameter.
Four holes 11/16" diameter on a pcd of 4.125" to fit 5/8" studs.
0.5" thick.

The four mounting holes had to be elongated slightly to fit the Sentinel flange.

The flange dictated that 1.5" pipework had to be used which conveniently led to the considerable strength of the final structure and avoidance of any diameter less than the 3/4" inlet to the safety valves (requirement 5).

Requirement 7 is satisfied by including narrow bore draining pipes in the exhaust pipework.

Requirement 8 is satisfied by incorporating unions into the exhaust pipe elbows so that the pipe can be detached easily. The safety valves can then be unscrewed without having to take the exhaust pipework apart. A cap is screwed onto the 3/4" male thread to seal for hydraulic boiler testing.

Implementation:

Below is an early mock-up I did at South West Engineering Supplies to get the hang of the idea. It was not the first attempt; I started off using 3/4" pipe bends from the 'T' piece to the base of the valves but did not believe it was a strong enough structure to carry the weight.

Early Mock-up

The implementation evolved to the final version by using shorter 'running' nipples having no plain centre section to keep the links as short as possible and by using single piece reducers to convert from 1.5" to 3/4" diameter fittings. Thus I believe I've minimised the number of thread interfaces and maximised the strength of the structure.

The Final Version

The black 'stuff' on the threads is a jointing compound from Rocol called 'Steamseal'. It is more technically called 'Foliac Graphite and Manganese' and is specified to be able to withstand steam pressures up to 2800psi at temperatures up to 600 DegC. Good stuff! Easily exceeding requirements 1 & 2.

At this stage, requirements 1-6 have been embodied into the design and implementation.

Requirement 7's condensate draining pipe is shown below.

Condensate Draining Pipe (bottom left)

To prevent a condensate pool accumulating, the small brass fitting had to be machined so that it would not protrude into the large pipe.

Flush Drainage hole

I hope I've demonstrated how setting out the requirements before design and implementation drives towards a compliant solution. Often, it's easy to assume that what has been done before will do but that approach tends to only just get what you want if you are lucky and doesn't cater for doing something that hasn't been done before.

Of course, if you haven't got ALL the requirements identified at the start, you may still not get what you want!

Next to do is the upward exhaust pipework.

A Nice (Replacement) Pair!

Those of you who have been following the 'blog for a while will know that the restoration started out with a number of original bits missing. One by one, I've been acquiring or making replacements, sometimes authentic to the originals but not always. One example of the latter is the safety valves.

A typical Sentinel safety valve assembly is shown in the picture below which belongs to Isebrook at Quainton Road. It is very much an integrated assembly with the lower casting having the valve seats. Unless all the parts are present, this is not going to be an easy reconstruction - particularly as it needs to be accurately calibrated for the release pressure.

Isebrook's (6515) Safety Valves

Long ago, I concluded that this approach belonged in the 'too difficult pile' and that modern items should be used instead. (Later, if suitable authentic items become available, who knows what may happen but at least I know that the safety system will be reliable and accurately calibrated).

So these are the new devices: Bailey Birkett type 716SSL, size DN20 with the inlet below (3/4" BSP) and outlet to the side (1 1/4" BSP). One is set at 275 psi (18.97 bar) and the other at 280 psi (19.31 bar). The SSL part of the type indicates that the valve seats are made of stainless steel to handle the elevated boiling point found at 275 psi. Many thanks to South West Engineering Supplies of Chippenham for obtaining these.

Joyce's (7109) Brand New Safety Valves (1)

Each has the capacity to release the entire boiler steam output alone (4600 lbs/hours or 2087 kgs/hour).

Joyce's (7109) Brand New Safety Valves (2)

Now for the plumbing! Again, as the pressure is 275 psi, malleable iron pipe fittings are just able to withstand the working pressure but without a good enough safety margin. As a result, mild steel fittings and pipe work will be used.

The Perfect Joint (1)

Sentinel 7109 works at 275psi, a higher pressure than most steam locomotives, waggons and traction engines. Many steam industry suppliers have off-the-shelf items for up to around 180 psi but, for Sentinels, more specialised items need to be sourced, in particular jointing material.

For the steam fittings to the boiler and piping to the engines, I've chosen a robust 1.5mm jointing sheet made by Klinger and sourced from Seddon & Black (now Craftmaster Steam Supplies). Identified as PSM/AS, it is a tanged stainless steel sheet sandwich with exfoliated graphite either side (instead of bread!). It is certainly up to the task but it presents a challenge of its own.

Safety valve flange adaptor (1)

The tanged stainless steel sheet makes the material difficult to cut. Tin-snips are fine for outer edges where the curvature is slight; however, inside edges of smaller radius cannot be done this way. Small holes can be cut with hole punches so long as there is a solid backing of just the right hardness for the punch to cut through to. The best combination I've found is to use a chunk of heavy paving stone and a length of old oak from an ancient desk.

In the photo above, I used hole punches for the stud holes, tin-snips around the outside and a chisel for the centre hole - which is clearly not too tidy!

Safety valve flange adaptor (2)

Wandering off-subject for a moment, this is what it looked like before it was removed from the boiler. EN3B studs have been used to replace the originals.

Before removal from the boiler!

The standard type of Sentinel safety valve manifold, similar the one pictured below, would be fastened to the four-hole flange.

Sentinel Safety Valve manifold

I had to buy half a square metre of the PSM/AS sheeting which was more than I wanted just for the boiler fittings' joints. As it had seemed pricey to use large copper washers for the steam pipe joints, I pondered whether there was a way of making the washers using PSM/AS. This would of course mean cracking the problem of cutting holes with diameters of around an inch or two and only a quarter inch total material width.

There are gasket punch sets available but, as they come at a price and don't claim to be able to cut the tanged sheet sandwich materials, I'm looking to find a better alternative.

Hole Cutters

A hole cutter looks possible but, without care, could easily tear the material surface away from the inner tanged sheet.

I've an idea how to do this effectively but some R&D is needed to perfect the technique and the outcome of that will have to wait for the next article!

Sentinel 7109 - Bits Missing

When Sentinel 7109 arrived at Midsomer Norton Station in December 2004, not everything was present. It was missing the following items:

• Four 'Spectacle' glasses, hinges and frames for the cab windows
• Two cast "JOYCE" nameplates



Maker's Plate (LHS) and Nameplate (RHS)

• Two cast Sentinel 7109 maker's plates
• A Cab mounted Boiler Feed Pump (replacement obtained Nov. 2010)
• A suitable whistle (replacement obtained July 2011)
• A boiler blow-down valve (replacement obtained June 2011)
• An ashpan (replacement fabricated early in 2012)
• Safety valves (replacements purchased March 2013)
• Loads of pipe-work
• Inter-axle chain oiler pot (fabricated February 2013)
• Linseed oil water treatment device (not required)
• Steam lance
• Two water gauge glass protectors (replacements purchased April 2013)

Although many items have been sourced, any offers of suitable replacements for the missing items will be welcomed with open arms!