Dry Pipe CutThursday, August 8, 2002 by Martin Adams
#21 is fitted with a Ruston Balanced Throttle which is located in the steam dome. This is the large dome in the middle which is easily seen in all the photos. It is operated by a long rod which passes through a steam tight packing gland on the upper portion of the backhead and is in turn connected to the throttle lever.
The throttle is placed high in a tall dome to allow only dry steam to the cylinders and prevent water that is sloshing around in the boiler from inadvertently entering the cylinders. Hence the pipe that leads from the throttle valve to the cylinders is called the "dry pipe". A previous boiler inspection report from the 70's called our attention to some pitting on the dry pipe that would require some welding to bring up to code. Upon our inspection, we found the pitting that was referred to. This type of throttle requires regular maintenance to keep it from leaking and we anticipated several hours of rehabilitation work to bring it up to working order. We could have done this work with the throttle still mounted in the dome, a very awkward space at best, or remove it and perform the work on a bench.
The fact that the firebox will be removed for a while was the deciding factor. With it out of the way, it will be easy to replace the dry pipe in one piece. If the firebox wasn't removed, a new dry pipe would have to be installed in two pieces, lowered in through the dome. If this were the case, we would have opted to weld the pits. The decision: rebuild the throttle on the bench and install a new dry pipe.
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