Boiler feed bush locations

What is the basis behind the advice not to have cold feed to a boiler at the backhead? I have my own idea but would be interested in the wider forum.

Dave

whatever the answers are, I will bet not one of them has a shred of evidence to support it! Most boilers I can think of have the feed exactly there. Only place I have not put one is on the smokebox end as I thought it would probably get too hot, but again nothing to support this by way of fact.

I don't have evidence that would hold up in court, but I would suggest cold water not be fed directly onto a firebox crownsheet especially at low water! I bet not one answer will argue with that.....

Other than that case, if feed is at centreline of barrel or lower in model boilers, front or back of boiler, you're probably fine.

The intention of the rule of thumb of not feeding to the backhead is that putting cold water onto firebox plates, the hottest plates in the boiler, will cause extra thermal stresses in those plates which is to be avoided if possible as a general rule of boiler design.

Again I can't prove it in court but it kinda makes sense, doesn't it?

JD

On my traction engines both the pump and injector feeds are part way down the boiler barrel, comfortably in front of the firebox and at, or below, the water level.

Andrew

Hi Dave

I think the answer to that is why feed cold water into the hottest part of the boiler and cool it down, its much better to feed it in where the water is much cooler. The reason they tapper the boiler is because little heat is taken from the tubes into the water so it makes sense to feed the cold water in there. This is my opinion others may not agree.

Stan

hi david,

an extremely good question!

the simple answer from fullsize is that it is the worst place to feed water into a boiler, and is only done in miniature out of convenience or bad design.

why 'worst place'? surely if this is the hottest part of the boiler then ideal place to feed water? no, because it upsets the water circulation, plus additionally if below the water line you get considerable or increased scale forming in the check valve which will make the check valves unreliable.

Churchward, Maunsell etc carried out a great deal of experimentation in fullsize and the best place to feed water was the front 1/3rd of the barrel. Churchward used a sloping tray inside the boiler from the top feed points.

ive fitted check valves to the backhead on some of my locos but all are taken via internal pipes to the front 1/3rd of the boiler and above the water line.

in the smaller gauges you might not have much choice where to put them in 2.5 and 3.5"g. most of LBSC's own locos as opposed to his published designs had top feed!

cheers,

julian

Wouldn't that do a wonderful job of condensing the steam?

A bit more on this. Presumably if there is an injector feed, as this is warmer, then it is not as much an issue as other feeds; and the height will be so that the top nut is accessible without taking the cab floor up.

On the theme of backhead bushes, what is the preference for blow down bushes? The height will be just above the foundation ring, but cross-wise, do you prefer in the middle or at one side?

All the best,

Dave