Tuesday, 18 March 2014

Nut & Bolt - Big Time!

Years ago, my mother recounted the story of her first driving lesson in which, once she had got the car moving, her main concern was how on earth to stop it!

Health and safety aside, the principle has stood the test of time. Sentinel 7109's original handbrake screw thread was clearly beyond use when it first arrived at Midsomer Norton. I never took a photo of it but the square shaped 'Acme' thread bore a strong resemblance to the 60 degree angles of a metric thread profile! As the last resort for stopping, it did not provide much in the way of confidence!

The way the handbrake works is quite simple. It's a big vertical bolt with a handle and a nut that's linked to a crank. When the handle is turned, it pulls on the crank which pulls on the brake. (There's some more about it here: Steam Brake Cylinder).
Handbrake Nut and Linkage (below cab floor)
Handbrake Handle (above cab floor)
Best efforts were tried to repair the damaged thread by machining it away and welding new metal in place to allow a new thread to be cut.

It worked initially as in the video below (Also on YouTube).

However, after a while, the cutting tool decided it wanted to dig a little deeper and pulled off some of the metal causing a complete write-off of the original shaft.

Mendip Steam Restorations were tasked with the job and completed both the finishing of a new nut (with posh phosphor bronze 'Top-Hat' insert) and a new handle shaft.
New Handbrake Shaft and Nut
The original handle assembly has been attached to the top of the new shaft. I wasn't sure about a plain thrust bearing and decided that, for minimal cost, a roller thrust bearing could be used as shown below. Whether it will prove strong enough in practice, I don't know but at least there is the fall-back to the original construction.
Roller Thrust bearing below the handle
New nut & insert from below
New nut & insert from above
So, with any luck, we'll now be able to stop 7109 after we've made it go!

One feature which I do intend to add is a shield of some sort to prevent grit getting into the thread; hopefully this will prevent the damage that happened to the old one.

Saturday, 18 January 2014

Vacuum Braking (6) Testing (1)

I have to admit to being mildly excited! I'd previously been unable to include an article on Sentinel 7109's Jet Pump (Vacuum Exhauster) device itself and only realised today when I'd completed a highly sophisticated (i.e. garage based!) compressed air test of it.

So here is the little sucker:
Penberthy GL-1 Jet Pump
It's not very big as can be seen from the picture (hopefully it will be big enough!). Steam (or air) enters from the right and exits on the left. It draws a vacuum through the sideways port.

For quite some time, I've had to be satisfied with the calculations I'd done in selecting this particular Jet Pump; however, it's not a technology I've been previously familiar with and have also been at variance from others who have fitted vacuum exhausters to Sentinels before. So my R&D has not been without some risk as to its success.

My first attempt at testing the jet pump was to connect it to an air compressor. However, this produced a poor result which I put down to the narrow bore of the air feed pipe being less than the bore of the jet pump itself. Thus it was never going to be able to feed sufficient air at the required pressure (around 60 psi).

By a stroke of luck, I found a scrap air cylinder at Midsomer Norton Station that had a 1/2" bore outlet and a valve. I considered that if I could pump this up with the compressor, I could then use it to feed the jet pump from its 1/2" bore outlet, at least for a short time anyway.

So here's the test rig in all its sophistication:
Jet Pump Test Rig
The air cylinder is on the left with the compressed air inlet at the top via the orange pipe fitting. There is also a non-return valve on the inlet. The valve controlled outlet is by the black air pressure gauge. The jet pump is above with an Ex-BR vacuum gauge on its suction port. The jet pump outlet is skywards.

This time I was much more successful (which led to my excitement that what I'd calculated actually worked!).
Vacuum gauge showing 21" Hg with about 66 psi air pressure
(It's easier to read on its side!)
I'm not concerned at the 66 rather than 60 psi. Neither gauge has been calibrated so the figures cannot be relied upon absolutely; however, I'm satisfied they are good enough to show the principle.

Here it is in video form: (also on YouTube).

Jet Pump Test

Some years ago, I visited the USA when printed 'T' shirts were becoming popular. I bought one with the message: "Engineer - Individual who turns abstractions into malfunctions". Hopefully not always!

Friday, 17 January 2014

Vacuum Braking (5) Design (4)

One difficulty with Sentinels is that the 275 psi boiler pressure is above the 250 psi rating of many off-the-shelf valves. At 275 psi, water boils at 212 DegC so special materials are needed to withstand these conditions. Any valve that has to isolate a steam supply from a boiler has to be rated as such.

Three new ones are required on Sentinel 7109:

  1. A 1/2 inch type to isolate the vacuum braking supply.
  2. A 1/2 inch type to isolate the supply for a steam cleaning lance.
  3. A 1.25 inch type for the boiler's blow-down valve. (See later).
They were made in Switzerland by Valtaco and sourced from Poynton Valves. This is what they look like:
The 1.25 inch and two 1/2 inch ball valves
These are known as ball valves because the moving part of the valve is ball-shaped (but with a hole through). To begin with, investigation had sent me towards using gate valves as these are intended for isolating supplies rather than regulating a flow. However, gate valves are better suited to liquids at relatively low pressures. Ball valves can be built to be much stronger and are very effective at isolation duties.
Full bore when open
Those of you who have followed my blog for some time will recall that a blow-down valve had been obtained some time ago; however, my wariness about fitness for purpose of high temperature and pressure components has been heightened as the project has progressed.

The original blow-down valve was made from gunmetal with flange fixings. Whilst it looked the proper job, when I came to investigate the safety valve mounting flange requirements, I found that I would need a BS10 Table 'F' rated type made from Carbon steel. The Table 'F' flange is 1/2 inch thick which is somewhat more than the flanges of the blow-down valve. Add in that gunmetal is not as strong as Carbon steel and it becomes clear that the original would not be strong enough. Not having any specification to support its construction means that I really can not be sure it is good enough.

The ball valves pictured above are made from stainless steel with Carbon reinforced PTFE seals and are specified for 212 DegC and 275 psi. I thus have complete confidence that these valves will be strong enough. Also, to coin a phrase, they were 'Reassuringly Expensive!".

Tuesday, 24 December 2013

Pipe Dream Come True

Back in April 2013, in a previous article, I wrote about the curved main steam pipe which was to be remade by Mendip Steam Restorations. It was to replace the original one which had been badly damaged by corrosion and pitting of the outer surface.
Original Main Steam Pipe
The replacement finally arrived on Monday 25th November 2013. It had been delivered earlier but found to not quite fit correctly. In the days when 7109 was constructed, there was not such a thing as flexible pipe. As such, the ends to which the curved steam pipe has to fit are fixed in space very precisely; hence the difficulty with making the item. However, on the brighter side, by igniting the first fire in 7109 for many decades, an oxy-propane torch was able to provide the heat to shape the pipe to fit.

Unfortunately, this was not the end of the story. Being a main steam pipe wrapping around the boiler in the cab, it's very close to the driver. Hence it's not a nice thing to have fail at an inappropriate time!

Problems were found with the end fittings such that they could not be examined by X-ray to check the soundness of the welded end flanges. Complete new ends were machined from solid to avoid the necessity for any weld at the mating faces. The new section then had to be welded to the rest of the pipe to a high standard.
Brand-New Main Steam Pipe
It looks in shape very like the old one - which is lucky really!

The original fittings have been salvaged for the new pipe.
End section and flange machined from solid
Sections have been welded to pressure vessel standards.
One of the very tidy coded welds
My apologies for such a long time since the last blog article - the reason being that I have recently been heavily entrenched in the arrangements and running of the two December steam weekends at Midsomer Norton. We haven't had any steam for some years so it was bound to be a taxing time.
Bristol Harbour Railway's Peckett 0-6-0 'Henbury'
tucked up in Midsomer Norton's Goods' Shed
Much has still been going on behind the scenes with 7109 and I've now received the vacuum brake ejector from the USA (ironically stamped with the word 'China'!). More of that to follow.

Wishing you Seasons' Greetings for Christmas and the new year. 7109 didn't quite steam in 2013 but I have high hopes for 2014.

Thursday, 7 November 2013

Vacuum Braking (4) Design (3)

Having settled on the GL-1 Jet Pump ejector running at 60 psi in Vacuum Braking (3) Design (2), here are the next three challenges:

1. How to connect the Ejector's American NPT threads to UK BSP threads?

2. Selecting a Pressure Reducing Valve to take the 275 psi boiler pressure down to 60 psi for the ejector.

3. Selecting a Vacuum Relief Valve for making sure that the Jet Pump does not draw more than 21" Hg of vacuum.

1. The GL-1 Jet Pump Ejector has a 3/4" steam inlet pipe thread and 1" exhaust and suction pipe threads. The trouble is that the American NPT and British BSP threads are not generally compatible and Sentinel 7109 uses BSP threads.

There are two challenges here: the pipe threads themselves and the need to be able to assemble and disassemble the ejector from the rest of the pipework. Initially, I'd started to look for simple female NPT to female BSP pipe couplings. However, it occurred to me that, if I could find a pipe fitting supplier with both NPT and BSP threaded unions, the thread linking the union halves might be the same. Thus it would be possible to create a union with NPT thread at one end and BSP at the other.
Mixed NPT-BSP Union halves
After a lot of internet searching, I eventually found Nero Pipeline Connections Ltd who seemed to have what I was looking for in stainless steel. I rang Nero and a very helpful Daryl offered to go and actually try a NPT and BSP union together. He rang back proclaiming a success with a proviso that I might have to do a little fitting to ensure a good seal between the dissimilar union halves. This was the sort of service I needed and have been able to obtain what I needed as in the photo above. Having tried the dissimilar halves together, there does not seem to be any need to persuade them to fit with each other.

2. The Pressure Reducing Valve not only has to drop the boiler pressure from 275 to 60 psi to suit the ejector but it also has to be able to let enough steam through for the ejector to do its job (and possibly a bigger ejector if ever needed).

I'd been guided towards Spirax Sarco as a suitable supplier partly because Gervase was already using one successfully but also because another Sentinel had a different type which had the persistent habit of blowing a continuous Raspberry! This was not a particularly attractive feature and one which was worth avoiding if possible (I'll diplomatically not say which Sentinel has this feature!).
Spirax Sarco BRV2S rated at 276 psi and 212 Deg C.
with Orange hat.
The beast is in the photo above. It arrived very quickly from BSS in Gloucester, UK.
Two optional features had to be chosen:
(1) To ensure sufficient steam flow was possible, I chose a 1/2" type easily capable of supporting a GL-1 ejector and even a much bigger GL-2 if found to be needed later.
(2) To be able to set the 60 psi outlet pressure, I chose an 'orange' rated spring allowing a range from 3.5 to 8.6 bar (60 psi = 4.1 bar).

The full specification of the BRV2S can be found here.

3. The Vacuum Relief Valve has to let air into the vacuum pipework when the 21" Hg level is reached. It also has to be able to let more air in than the ejector can pump out so there is a size factor too.

It took me a long time to find a supplier of a suitable device. Eventually, I found Flowstar of Kingston upon Hull, UK, who distribute products made in Hamburg, Germany, by Niezgodka GmbH. The Type 91, size 1 with a Viton seal and 3/4" male thread fitting is the chosen one.
The Niezgodka VRV data sheet has most of the detail while an additional data sheet covers the discharge capacity (2nd column under '18'). Note: 1 cu metre = 35.3 cu feet).
The size 1 type is good for 50 cu metres/hour = 29.6 cu feet/minute. (I enquired about the empty cells in the discharge capacity table and, for the size 1, 50 cu metres/hour also applies at greater vacuum levels than -0.6 bar). 29.6 cu feet/minute is plenty to overcome the suction possible from a GL-1 ejector or a GL-2 should it ever be necessary. So at least I won't have to replace all the parts should I find I need a larger ejector after all!
Niezgodka Type 91, size 1
Note: the Penberthy Technical Data Manual (page 9) shows a graph which indicates that a GL-1.5 ejector running on 60 psi is capable of 13.5 cu feet/minute at a suction pressure of 21" Hg gauge (= 9" Hg Abs.). Therefore it is safe to assume that a GL-1's capacity will be less than 13.5 because it is smaller. A GL-2 is not twice the size of a GL-1.5 so the 29.6 cu feet/minute of the VRV will be more than a GL-2 can remove.

Next, I'll look at the boiler's isolation valve, the reason why a curvaceous syphon pipe is used with a steam pressure gauge and possibly at the driver's brake valve.
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