Wear & Tear (3)

A quick update on Wear & Tear (2).

I've replaced all three joints from the steam end of the Sentinel 7109's cab mounted boiler feed pump; these are the ones above and below the valve chest and the failed one sealing the end cover. The valve chest ones were undamaged but I replaced them anyway to improve long term reliability.

Light green layers replaced by the dull grey graphite of PSM150/AS

When the pump's end cover joint failed, for unknown reasons steam got into the water end of the pump. This has happened once before and I've never worked out the cause. I can speculate about the check valve to the boiler getting wedged open to allow hot water back into the feed pipe but I don't actually know.

Perhaps the check valve could stay open if the feed water and boiler water were at identical pressures? This is not likely as there is a spring to help the check valve to close. This mystery remains on the 'too difficult' pile. I checked the cleanliness of all three check valves for peace of mind and found no cause for concern.

Not only did the steam get back into the pump's water cylinders but also further upstream in the pipework towards the water tank feed. I mention this because I'd just painstakingly replaced it all (click here) after the frost damage to the original water valve. Being a water feed, I again used 'Heldite' as the thread sealant as it was well in spec for cold water and low pressure.

However, it's not designed for high pressure steam! The result was that the pipe joints were emptied of the Heldite and no longer doing their job.

I've rebuilt the pipework again; this time I've used Rocol's 'Steamseal' as the thread sealant. The name gives it away so next time it should withstand a steam ingress where it's not wanted.

The water cylinder piston rings are made of a fibrous non-metallic material. (click here for more detail). They survived steam ingress the previous time and seemingly this time also as the pump worked well when tested with compressed air.

Wear & Tear (2)

After three years of reliable service, Sentinel 7109's cab boiler feed pump decided it was going to play games and started spitting steam out of the steam cylinder end cover joint.

This early photo shows the light green layer between cylinders and the end cover.

June 2011 photo long before putting the pump to work

The material I used for the joint was Klinger's Klingersil C-4400. I'd bought this in 2010. Its spec shows that it can withstand about 23bar pressure at 230degC. All seemed satisfactory as water boils at approximately 230degC when at 19bar pressure.

The next photo shows the effect of three years' use (and possibly the recent heavy frost?).

Comprehensive disintegration of the joint material

There are splits in many places which makes me wonder if there may have been some frost involved but that is speculation.

In retrospect, what is more to the point is that, while the spec shows the joint material is good for 230 degC and 19bar, the steam supply to the pump has passed through the superheater and is likely to be somewhat hotter than the figure I had originally assumed. I'm thus rather surprised it had lasted as this long!

Later in the restoration process, for all steam joints, I'd switched to a higher spec joint material, namely Klinger's Kingergraphite PSM/AS. More details here.

So that's what will be used for the repair.

I can say I now have more (hard-earned) experience!

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!

Nut Screws Washers & Bolts!

I've been struggling for some time to place an order for the various nuts, screws, washers and bolts plus engineering studs for holding Sentinel 7109 together. The struggle has been to discover the correct type of steel required in each case.

For run-of-the-mill purposes, the material from which these items are made does not matter; however, when the items being fixed are steam fittings supporting 275psi pressure, I (and the boiler inspector) have to get this right.

Boiler shell fixing studs (EN3B)

As an example, the photo above shows some of the specially made EN3B studs required for holding the boiler together. EN3B is a bright mild steel with properties that enable tighter tolerance threads to be cut and hence provide a stronger engagement between the stud and nut threads. It is not particularly strong in a tensile manner. I'd assumed earlier that it was the tensile strength which was to be important but it seems not. More information on EN3B properties can be found by clicking here.

Outer shell in place with fittings & blanking plates
(for hydraulic testing)

Some of the boiler fittings and blanking plates used during the first hydraulic test are shown above. From this experience, I assumedthat EN3B fixings would be needed everywhere that a high pressure steam joint had to be made. However, on trying to purchase EN3B studs, nuts, screws & bolts, it was apparent that these were not commonly available and that there was some inconsistency in recommendations depending who was being asked!

After some considerable amount of searching, I came to the conclusion that the fixings' material required for steam fittings is more to do with what should be avoided rather than what precisely should be used. The material to be avoided is 'leaded' mild steel such as EN1A. Whilst EN1A is easily machined and welded, it is not for this application. This therefore answers why some suppliers of engineering studs do not specify the precise mild steel their stock is made from; hopefully it is safe to assume that they are fit for purpose and will not be made from EN1A!

Nut and Bolt Inscriptions

Restoration of Sentinel 7109 has not failed to expose gaps in my knowledge. One gap that is not so empty now is the meaning of the inscriptions on nuts and bolt heads. I hadn't until recently noticed that the inscriptions could vary from bolt to bolt; however, they do and they indicate the tensile strength capability. I'm not going to attempt to include a dissertation on the subject here but there are some good references such as this one from 'Bolt Science'.

The '8.8' bolt head and compatible '8' nut pictured above are a medium grade metric type suitable for pulling together steam pipe joints; they are easily available off the shelf.

I think I've got to the bottom of these fixing requirements now and have at last placed an order for 11kg of them. So shortly reconstruction of Sentinel 7109 will be able to commence!