Thursday, 2 January 2020

Steam Heating (4)

Following on from Steam Heating (3), Sentinel 7109 had a brief steam test a few days before the start of the 2019 Santa specials. (I'll write about the reason for the brevity in due course).

To the casual observer, the carriage heating performed well although it also showed that the carriages themselves have a number of leaks and cold areas to be attended to. The leaks certainly add to the atmosphere but not always where you want it.
Carriage Warming up and running (Photo: Roger Burfitt)
To the not-so-casual observer, i.e. me, all was not quite as perfect as I'd hoped.
Spirax-Sarco PRV on the left
The Blue Spirax-Sarco Pressure Reducing Valve (PRV) can be adjusted to give the 20-40 psi output pressure range required but it is faced with a very variable load. (For carriage warming, steam is squirted down the steam heating pipe and it exits as steam or water through a series of drip valves after passing through various heat exchangers. There is thus a back pressure from the carriage pipework and hence a 'load' on the steam generator. On initial starting-up, the far end of the pipework is opened to atmosphere to allow the steam through rapidly with much reduced back pressure).

When initially started from cold, the steam is fed into a large cold space. It immediately condenses and this takes place progressively along the two carriages until all is warmed through and steam is exuding from the hose at the far end of the train.

The loading thus initially varies considerably and then will vary yet again depending on how many carriage heaters are switched on and off.

Whilst the PRV can be set initially so that it supplies the right pressure, the changing load then plays with the setting. The PRV also tends to oscillate with a repetitive 'kerr-dunk' noise which I doubt is particularly kind to it.

Setting the PRV to be either on the high side or low side of the pressure range seems to calm down the oscillation. It can also be calmed by limiting the inlet flow using the isolation valve.

I've put a reducing-flow orifice in the inlet steam feed to the PRV which has also helped a little. However, I suspect that if I did the same on the PRV outlet, I would have more success as it will smooth out the load from the carriages. The inlet orifice is easy to fit but the outlet one requires taking it all apart again under the buffer beam.

Adjusting the PRV outlet pressure is also a bind as the PRV is under the buffer beam and not easily accessible.

The PRV does its job well but depends on the loading being more constant than here. More tweaking is required to get this right and make it simple to operate but, according to the Santa Special passengers, it seems to be entirely satisfactory!
Beginning the climb, steam everywhere! (Photo: Roger Burfitt)
In Steam Heating (3), I anticipated that the carriages would be too cold, too hot or somewhere in between. Oddly, I was right so without too many reservations, I declare Joyce's new Steam Heating apparatus to be a success!😊

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