RE: [PipingDesign] ASME Inspectors

From: <Steve>
Date: Thu Jan 16 2003 - 16:26:00 EST

In Britain in the winter of 1894-5 there were a number of severe frosts which caused in a large number of domestic and industrial boiler explosions and resultant fatalities/injuries. The mechanism leading to the failures was thought to be freezing of feedwater pipes resulting in thermal shock fracture caused by sudden admission of water to a hot boiler. To prove this, in 1895, a test vertical boiler was obtained. It was provided with water supply at 13psi from an elevated header tank. The boiler had a safety valve set to 24psi.
The empty boiler was heated to red hot and then water admitted. The safety valve blew and some water was forced by excess pressure back to the header tank. A little later, more water ran into the boiler and then more was spat out. These cycles continued until the boiler sheel had cooled to black. The whole procedure was repeated 3 times and although the boiler distorted, it did not burst. A second independent test in Scotland gave similar results.

Adding water to a red hot boiler could produce instantaneous evaporation rates far in excess of the boiler rating. As safety valves are sized in proportion to the boiler rating they may not be capable of keeping the pressure within safe limits under these circumstances. Modern boiler design aims for maximum efficient steam production per kg of boiler steel. This tends to result in an increase in the ratio of steel to enclosed volume, which would tend to increase the peak pressure potential under these circumstances.
This is because on cold water admissions, the evaporation energy is provided by the hot steel. As the boiler volume is smaller the pressure must rise faster. Once the steel has cooled to normal, the evaporation rate is again controlled by the firing rate.
The point of all this is to ponder whether or not the current,more cost efficient, design of boilers requires a rethink in safety valve sizing requirements. Or perhaps this has already been done.

Cheers

Steve McKenzie

-----Original Message-----
From: Christopher Wright [mailto:chrisw@skypoint.com] Sent: Friday, January 17, 2003 8:09 AM
To: ?
Subject: Re: [PipingDesign] ASME Inspectors

>Most results seem to indicate "Low-Water Condition" coupled with "Operator
>Error or Poor Maintenance" as being the major contributors.
You might be able to get the National Board bulletin dealing with locomotive boiler explosions, ordinarily resulting from water level neglect. Very spectacular--
Water level in firetube boiler drops below firebox crown sheet. Firebox crownsheet overheats and firebox starts to collapse. Collapsed firebox vents the boiler a
Superheated water inside flashes to steam. Steam kills the engine crew pretty much instantly Flashing water blows almost everything clear of the locomotive chassis

An interesting thing brought out in the article is that if the crew realizes what's happening and hits the hot firebox with water from the tender, the thermal shock initiates the rupture. Otherwise they may have a short additional time to dump the fire and try to abandon the locomotive.

The article has some interesting pictures of the aftermath. I wasn't able to locate the article at the national board site, but I didn't take a lot of time at it.

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw@skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw

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Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ Received on Thu Jan 16 16:26:00 2003

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