Clack Box

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Stefan Andrzejewski
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Clack Box

Post by Stefan Andrzejewski »

Can the Steam experts tell me what a Clack Box is. I noticed this on one of the photos of 3117.
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John Ashworth
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Re: Clack Box

Post by John Ashworth »

It's the thing that allows water into the boiler. Water comes from the injector through a non-return clack valve.
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Steve Appleton
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Re: Clack Box

Post by Steve Appleton »

Indeed, John is right.

Just to add a little more to the answer. The boiler is pressurised to about 1200 kPa (roughly 11 to 13 times atmospheric pressure). The water temperature in the boiler will be around 190 degrees celsius. Whilst the injectors are not in operation there is only normal atmospheric pressure in the boiler feed water pipes. Steam would therefore escape from the boiler back down those feed pipes and out through the injectors if there was no one-way shut-off valve at the boiler end to contain that pressure.

When the injector is operated, high pressure water builds up in the associated boiler feed water pipe. When the static water pressure in the pipe becomes higher than the boiler pressure, it overcomes the boiler pressure to push open the clack or check valve and allow feed water to flow into the boiler. When the injector stops operating, the feed pressure drops and the valve snaps shut quickly to ensure that the boiler steam does not escape back down the pipe -- hence the "clack".

On many locomotives, including the ex-SAR ones, the relatively cooler feed water is delivered into the boiler near the front to safely mix with the hot boiler water there and avoid any possibility of subjecting the somewhat hotter firebox crown sheet and walls to a cold water thermal shock which could cause stress and ultimately failures.
"To train or not to train, that is the question"
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