Sentinel 8843 - A different sound of steam

Coming off the rails for a posting, the more I discover about Sentinels, the more interesting they get. In answer to a request earlier in the year, I was invited to spend Saturday 16th July 2011 at the Langport, Somerset, steam rally paying particular attention to John Goold's beautifully restored Sentinel Steam Waggon, works' number 8843.

Who's the happy-chappy in the passenger seat?

Built in 1933, it is a S4 type (meaning shaft-drive, 4-wheeler) with three-way tipping gear (left, right & centre).

Unlike Sentinel 7109, 8843 has a four cylinder, single acting steam engine (2-stroke) driving the rear wheels via a shaft and axle-mounted differential. Sentinel Super waggons before the 'S' type had two-cylinder, double-acting engines and were chain driven with the differential as an integral part of the engine's crank-shaft.

I was able to have good explore of 8843 during the day but what caught my attention was the engine ticking over in neutral gear. I'd always thought that steam engines were permanently linked to their driving wheels but here was not the case. I was also interested by the sound of the engine; having my camera with me, I used it to record the engine ticking over. Initially it is in late cut-off (starting), then shortly after it starts, the middle (intermediate speed) cut-off setting is selected then finally the late (fast) cut-off setting. Note how the sound changes before it finally stops. (Also on YouTube).

Listen rather than watch (the paint is already dry!)

Even more interesting is that this engine sounds far more like a diesel engine than a steam engine yet there is no internal combustion here!

Whilst out on an evening trip on the roads around Langport, I was sitting just behind the cab and able to catch the 'bark' from the chimney. This next video clip is of 8843 starting from rest and changing gear (cut-off) as it accelerates. (Also on YouTube).

Listen rather than watch (the paint is still dry!)

What also impressed me was the turn of speed. Although 8843 struggled a bit on the hills, on the flat it had no trouble getting to 40mph or so and apparently it can do quite a lot more! I always thought vehicle were pretty slow in the 1930s; clearly Sentinel had produced an advanced, modern vehicle but still propelled by steam!

Trailing Engine's Boiler Feed Pump

Whilst I had first run Sentinel 7109's engines on compressed air on 26th July 2009 and on subsequent occasions, I had not checked whether the Boiler Feed Pump (BFP) attached to each engine was in working order. Having renovated the Worthington-Simpson BFP, I was now much better prepared as to what I would find.

Engine BFP in-situ

Both leading and trailing engine BFPs look the same.

Part cut-away drawing

The cut-away shows that the pump is a reciprocating type driven from the crankshaft (or geared from the crankshaft).

Inlet (RH) and outlet (Left centre)valves

Access to the pump piston was not attempted as it would involve breaking and damaging one or more gaskets. From the Worthington-Simpson BFP piston design, I expect a similar multi-ringed piston forced on to the cylinder wall by the pressure of the water being pumped. I may be proved wrong! I was more interested in the condition of the inlet and outlet valves and how they were constructed.

Inlet valve to the right
Outlet valve below the 'bulb'

Inlet Valve

Cap on inlet valve chamber

Cap removed to show inner valve cap

Inner valve cap removed to show the valve itself on its seat

Valve removed to show its seat

Valve itself showing a clean surface

Valve seat in reasonable condition but would benefit from regrinding

Outlet Valve

'Bulb' instead of cap on valve body

The bulb is initially filled with air and provides a cushion to the pulsations of the reciprocating pump. It is similar in effect to a smoothing capacitor in an electronic circuit power supply.

Base of 'bulb' (just about) showing valve internals

Valve on seat with washer on top

I'm not sure of the purpose of the washer; however, it must prevent the valve from fully lifting and thus restricting the maximum flow. To me, it seems as if this might put an extra load on the engine for no apparent good reason - unless somebody knows better? It might also prevent the valve from coming out of its location but Sentinel were better valve designers than that weren't they?

Valve on seat with washer removed

Valve removed with washer in place on top

Valve seat slightly corroded (green)
and needing improvement by regrinding

Cab repaint - A bit of a blow!

Before the boiler is put back into the cab, the surrounding paintwork needs improvement particularly behind the boiler where it will eventually be inaccessible. This was expected to be a quick job but soon took on surprising proportions.

Initially, angle grinders and radial wire brushes were used to good effect on the cab floor. Also on YouTube

Bit of a grind

Then the cab wall revealed a strange effect. A compressed air jet was being used to disperse the settled dust but was found to be also particularly effective at blowing off the paint from the cab walls! (Also on YouTube).

Bit of a blow!

Whether this was due to the particular paint used previously or poor preparation of the underlying surface at the time is hard to say. However, the paint was clearly keener to come off than stay on!

Looking better already - all four boiler support areas are visible

By the end of the day, although not completed, the cab floor was beginning to look a lot better than at the start. To be continued.

Sentinel 7109's whistle valve and how does it work?

A peculiar looking valve had been hanging around Sentinel 7109's stores looking for a use. Eventually, I was advised that it was a whistle valve.

Grubby looking thing!

Alternative view

With mounting flange and whistle-mounting elbow

Whistle valve roof-mounted in Isebrook

Whistle valve roof-mounted in William

Unlike Isebrook and William, Sentinel 7109 has the whistle and valve mounted vertically on the cab front.

Front-mounted whistle (the valve is inside)

So how does a whistle valve work and how does it enable the 'pop' action needed for the whistle?

Functional Parts

The main valve body allows steam in through the top right connection where the air-hose adaptor is shown here. The steam is held back by a conical valve pressed against a valve seat by a spring and also by the steam pressure.

Conical valve (not to scale!)

Valve seat inside end of valve body

Spring and retainer which screws into the end of the valve body

A rod passes through the valve body and rests against the conical valve's base. To sound the whistle, the conical valve is pushed off its seat by the operating lever's movement which pushes the rod against the conical valve's base.

Rod (not much more to say on this!)

Operating lever

The steam can then pass out to the whistle through an aperture in the valve body's flange.
The steam pressure holds the conical valve in place until the lever is used to push it open. This is a sudden action which is also terminated abruptly by the steam forcing the conical valve shut again. Hence the whistle valve enables a quick 'pop' to be achieved as well as the long 'poop'! 

There is one more aperture in the valve body shown with a copper tube attached in the Isebrook valve picture (but, oddly, not on William).

Condensate drain

As a result of the whistle being sounded only occasionally (hopefully), the valve body will generally be cold when steam is admitted. Thus the copper drain tube is added to allow the resulting condensate to be drained away.

When it's all assembled, it looks like this.

Note the elegant MDF cab front substitute and supporting clothes' peg!

Finally, when I first took the valve apart originally for renovation, the conical valve seemed to have a leathery pad between it and the seat. As the pad was in an 'aged' condition, I discarded it wondering where to find a replacement. On subsequent closer examination, it looked as if the conical valve and seat were both in good condition and, after a small amount of regrinding, were clearly perfectly satisfactory. Thus I concluded that either the leathery pad was accumulated muck or was a misguided bodge from the past! Who knows? But it isn't there now!

Clean as a Whistle!

When Sentinel 7109 was originally brought to Midsomer Norton Station in December 2004, its whistle was missing. From early pictures of 7109 in service, a tall thin item was required as a replacement.

Tall thin whistle 1

Tall thin whistle 2

Other Sentinel locos seem to have a variety of whistles such as William's but something more authentic to the original was wanted.

William's proud possession

Early July 2011, the ideal thing came up on Ebay and I was the only bidder! Although its fittings were not quite suitable, it seemed as good as was likely to be found (another was offered for £350 which was declined rapidly!).

On arrival, it was tarnished but in good working order and a definite possibility.

Clearly, it once had a sticky label attached

Gun-metal mounting and steam inlet to the left

It's such a simple, passive device, I'm sure I could have made one!

Cap machined from solid gun-metal

To clean off the tarnish, I used a phosphoric acid gel originally used for derusting car bodywork. Finally it was brightened-up by Brillo pad and a french polishing technique known as grisse d'elbeaux.

Shiny and clean as a whistle!

Some say (where've I heard that before?) that the whistle looks like one from an 08 diesel-electric shunter. I checked this out in Brian Haresnape's British Rail Fleet Survey "7 Diesel Shunters". It appears that 08s did have this type but they also had a number of others and horns too. As Sentinels also had this type and others, I'm sure this whistle will do just fine!

Despite many attempts at testing the whistle with compressed air, my neighbours are still talking to me. However, when I put the video below together, it frightened the hell out of the local cats and the next-door dog had a fit!

(Note: I've sometimes had trouble playing the video repeatedly. If this is the case, reload the window (F5 on PCs) and try again). Alternatively, it's here on YouTube.

Sentinel 7109's Boiler Mountings

Sentinel 7109's boiler is supported in the cab on four brackets around the firebox.

Four Fixing brackets (there's another round the back)

Three fixing holes (rear set)

It is held in place with seven longish and three shortish bolts about one inch in diameter. The longer ones go through the footplate floor; the shorter ones through a thinner supporting bracket towards the cab front. In a pretty dilapidated state, I've cleaned them up using an electric drill and wire brush.

Before (LH) & after (RH) brushing-up

Judging by the state of the top ends, which presumably had the hole through to take a split pin and castellated nut, they required a certain amount of violent percussion to remove them when the boiler was taken out. Despite their state, they are probably adequate to reuse. Anybody seen the nuts?

Water Pump Pumps Water!

Despite the original heading of the last WS BFP episode, I've used that heading here and corrected the previous one (owing to the fact that no water had actually been pumped at the time!).

I concluded that the water pump valves did need regrinding. Advised to use genuine valve grinding paste, I 'Googled' it and found that it is available from various manufacturers in a two part tin: coarse paste at one end and fine at the other. Chippenham is not an easy place to buy valve paste but KM Farms Parts had what I wanted.

Typical Coarse/Fine Valve Grinding Paste container
(Spot the product placement in the form of the Worthington Valve handle!)

I hadn't done anything like this before so it was a bit of an adventure working out the best way to use the paste. The fine type seemed to be all that was needed.

The valve seats are basically a pair of flat discs of which the moving one sits loosely around a supporting stem; as such the valve has to seal both at the outer edge and the inner edge for it to work. If the inner edge leaks, then water seeps back through the gap around the valve stem.

Mating parts of an upper chamber (outlet) valve

By inking the inner and outer seat surfaces and rubbing them together, it was clear that there was incomplete contact and scope for improvement. The outer seat needed to be ground down in all eight cases for the inner seat to come into contact. Coarse paste was needed initially as there was a fair amount of metal to be removed.

It seemed from the initial state of the valve seats that the inner seat could never have made contact unless it had eroded considerably during the pump's original working life. If this is the case, then there is much reliance on the boiler inlet 'clack' valve to prevent leakage back through the pump. Alternatively, the valves may rely on being flexed by back pressure to make contact at the inner seat. I had expected the pump piston rings and valves to need to be a tight fit but I'm not so sure now. Some leakage allows the pump to operate without seizing in one position. Time will tell!

Lower chamber (inlet) valve seat

The lower chamber (inlet) valves are of a different design and have indents between inner and outer seats. Again, both seats need to seal for the valve to work properly. As can be seen, the lower valve seats were not easy to get at but fortunately not impossible!

The valve grinding had clearly paid off as the characteristic 'raspberry' produced before was now much better pronounced. Also the inlet could be felt to suck and the outlet to blow.

Moving swiftly on to testing with water. Eat your heart out William Heath-Robinson, here's the test set-up. Recycling is a strong feature!

Complete with spurt!

The pump self primed at the first attempt with the reservoir (watering can) at the same height as the pump but it needed help when having to suck from a lower level. There is also a leaky weld in the inlet pipe which will need repairing.

And the video you've all been waiting for... (also on YouTube).

I'd noticed that the flow did not seem even; half the time it was steady and half a bit slower. At first I thought one pump cylinder was faulty but then realised that this is bound to happen as each pump cylinder has a piston-rod in one half with a consequent reduction in the cylinder capacity at that end! 'I-think-I-can,'I-think-I-can...' comes to mind as the typical two cylinder steam engine rhythm.

So the pump seems to work. I'm still slightly nervous that the pump valves don't seat well but they are probably better than they were when previously in operation.

The user manual (yes, believe it or not, there is a user manual for these pumps) makes no comment about valve seat geometry; however, it does mention the pump piston rings. The four hollow rings are intended to have 1/16 inch of total axial play which I guess is to allow them to fill out when pumping the water to make a seal. What is not mentioned is that the only way to check the play is to remove the piston and rod completely (including the steam end also) and have a look.

No! This is going to remain one of those assumptions that it was OK originally so it's OK now as taking it all apart again belongs in the 'too difficult pile'!

Sentinel 7109 gets a Duvet!

Having passed its first hydraulic test, Sentinel 7109's boiler is now being prepared for repatriation with the loco. Before this can happen, its lagging and outer cladding sheets have to be fitted as they would be impossible to work on once the boiler is lodged within the bijou confines of the loco's cab.

With the boiler at Mendip Steam Restorations, the plan is to take the various bits and pieces to the boiler and assemble them prior to transporting the complete sub-assembly back to Midsomer Norton Station to be dropped straight into the loco. This enables a single crane hire to complete the job.

An earlier post showed the cladding sheets in preparation. The lagging also needed to be cut to size with apertures (in the right place) to fit around the boiler fittings. Fortunately, the cladding sheets can double as templates for locating the apertures.

The ticket office platform at Midsomer Norton station seemed an ideal large flat surface for the cutting out. However, the first problem to overcome was that the wagon containing the cladding was on the far track and there was a train in the way.

Wagon stuck behind 7109 on the far track

Manhandling large floppy sheets between (stationary) trains isn't the first subject I'd mention to a safety inspector but needs must so a ladder was put diagonally between the wagons and the sheets precariously carried across to the platform.

Midsomer Norton's Circus Tricks

The lagging material was laid out on the platform and cut to length. All edges were sealed over using self-adhesive Aluminium foil tape designed for the purpose.

Preparing the templates

The lagging material is a high-spec (expensive) version of the type used for loft insulation. It is Aluminium foil faced on each side and made from ceramic rather than glass fibres so as to handle the boiler's high temperatures. Its trade name is 'Superwool Ceramic Blanket' manufactured by Morgan Thermal Ceramics and supplied to us by Heritage Steam Supplies (HSS). The Aluminium foil is to help avoid condensation being held in contact with the boiler casing and also to reflect heat inwards to the boiler's outer surface. The Aluminium foil tape also came from HSS.

Apertures were cut in the lagging using the cladding sheet templates and a Stanley knife (Why are sharp knives always called Stanley?).

Cutting around a template aperture...

... and removing the waste

The aperture edges were sealed with foil tape to keep it all in place.

Completed Lagging sheets

A good day's work was made possible by the good weather but could have been easier and safer without the manhandling of cladding sheets across the gap between the wagons.
All that was left to do was put it all away again.

More of Midsomer Norton's Circus Tricks

There are one or two small preparation tasks remaining and then the boiler, lagging and cladding can be assembled ready for repatriation with Sentinel 7109.