Sunday, 6 October 2013

Vacuum Braking (1) Requirements

Ejector Construction (from Penberthy literature)

As an industrial shunter, Sentinel 7109 never had train braking equipment fitted. However, at Midsomer Norton (MSN), 7109 will be towing passenger trains on occasions and on pretty steep gradients. If I recall correctly, through train braking has been a legal requirement for passenger carrying since the 1870s so it would severely curtail 7109's capabilities if it was omitted!

Heritage passenger rolling stock generally has vacuum rather than air braking and, at MSN, vacuum has been chosen to be the norm. As such, some time ago I committed myself to the Research & Development needed to satisfy this requirement - little did I know what I was getting into!

I'm not going to dwell on how vacuum train braking systems work; it's covered in many places on the internet such as here.

What isn't generally covered is the requirements and design of the locomotive equipment which is what I've been figuring out for the last few weeks (if you were wondering why I haven't published so much 'blog material lately!).

Requirements - what does it have to do?

1. It shall be able to evacuate four Mk1 carriages' braking systems to 21" Hg in 30 seconds.

2. It shall be able to maintain a vacuum level in a leaky system.

3. It shall be able to raise 21" Hg vacuum level at any boiler pressure above 100psi.

4. It shall be able to:
    a. Release the train brakes (Running).
    b. Apply the train brakes (On).
    c. Hold the train brake vacuum level (Lap).

Implications of the requirements

1. 30 seconds is a reasonable time to be able to evacuate a train and pull its brakes off. This is a figure for when there is no remaining vacuum in the carriages. After a brake application, part of the system is still evacuated so the time will be shorter.

21" Hg is the gauge vacuum level for the system.

A Mk1 carriage has a vacuum system volume of about 4 cu ft. Therefore four carriages have about 16 cu ft to be evacuated in 30 seconds. Fewer carriages take less than 30 seconds!

2. Vacuum brake train pipes tend to leak so that, having developed a vacuum level, the level will often reduce allowing the brakes to come on unintentionally (it's a fail safe system). Thus a means of continually sucking a vacuum is helpful.

GWR locos use a piston vacuum pump which can be heard giving a 'Phut' for each rotation of the driving wheels. Other locos use a large ejector to get the brakes off quickly and a smaller ejector to maintain the vacuum.

It is also feasible to use only a single large ejector to take the brakes off and to maintain the vacuum by only activating it when needed. Being large, it does not have to be used for long periods so it will react fast but won't consume any steam when it is inactive. This is the solution I am minded to use but with the possibility of adding a smaller ejector in parallel if the single ejector isn't satisfactory.

3. If boiler pressure is falling, the last thing wanted is for the brakes to also start to coming on. Thus, it is wise to design the ejector system to operate at a controlled lower pressure, e.g. 80-100 psi. The boiler pressure can then fall to around 80-100 psi before the brakes are affected.

4. These functions are provided by a brake valve having 'Running', 'Lap' and 'On' settings. The ejector system will only be linked to the train pipe when in the released position.

Design Investigation

I began by contacting others in the Sentinel loco community who had worked on similar systems. To create the vacuum, it seemed that all had used Penberthy water lifting ejectors of various sizes and with a greater or lesser success. The amount of design information on these did not seem sufficient to make informed design decisions. In one case, the realisation that the ejectors were designed for water lifting had led to an internal redesign of the nozzle geometry - with some improvement. (See the diagram above for the general shape of an ejector).

Using the internet, I searched for Penberthy and found that they are an American company very much in business supporting other vacuum applications such as vacuum packaging and process industries. 'Ejector' now seems to be re-branded under the name 'Jet Pump' for possibly obvious reasons.

Jet Pumps come in a number of varieties and capacities. For 7109's purposes, it needs to be a steam pumping gas (air) type as opposed to liquid pumping of liquids, solids or gases - they're clever little devices (and some not so little)! I found a very useful application guide to Jet Pumps here.

It would seem that either a type GH or GL will be suitable.

More (lots) to follow.

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