Heat Differential Across A Firebox Water Leg

M Austin · **Joined:** Sat Feb 05, 2005 1:05 am **Posts:** 488

The debate on damage to a locomotive boiler due to fire ups, fast or slow, has raged on for about 150 years now. The big railroads dealt with the issue by filling cold boilers with 150 psi water/steam so the entire boiler inside and out grew uniformly before putting a match to the firebox then rolling them out of the roundhouse stall.

A simple equally thermodynamic analogy is this:
1. You buy a new car that idles at 500 rpm, redlines at 5000 rpm.
2. Every Friday morning you start it up and let it idle all day and night at 500 rpm for the races on saturday and sunday.
3. All day saturday and sunday at the races the engine runs at 5000 rpm on the track and back to idle 500 rpm in the pit.
The operating conditions are 10% design power output or 100% power output.

WHAT CAUSES THE MOST THERMAL FATIGUE DAMAGE TO THE COMPONENT PARTS?

Does it matter that you warmed the engine at 500 rpm idle for 24 hours before you play full range yo-yo with the power/fuel/heat throughput all day saturday and sunday?

The Flannery engine cited below generates about 50,000 pounds of steam per hour at full power. Sitting at a station, let's assume generating about 5,000 lbs/hr steam. Is not the near instantaneous 10 times increase in power/fuel/heat throughput to get a train up to speed a 10 times greater thermal shock to the boiler? At the top of the hill the throttle shuts off and we are back down to the 5,000 lb/hr demand until the bottom of the next hill....ad infinitum....

fjf5766 · **Posted:** Sat Feb 19, 2011 11:41 am

I'm sorry but your posts are difficult to read. Could you please use a bigger font?
Thanks for your consideration.

Fred F

Kelly Anderson · **Joined:** Tue Sep 14, 2004 7:52 am **Posts:** 2477

M Austin · **Joined:** Sat Feb 05, 2005 1:05 am **Posts:** 488

Sure Guys,
This is one to one of my 3rd or fourth generation copy.

Matt

whodom · **Posted:** Sat Feb 19, 2011 2:38 pm

The picture seems to back up something written by David Wardale in "the Red Devil...". He mentions that mainline Chinese locomotives suffered many broken stays, and he said their practice of pre-heating the boilers with hot water and steam might be a contributing factor.

Given what Tross said about "shrinkage" and how the firebox distorts after it's been put in service, combined with the graphic above, I can see how this could be true. Dumping hot water in a cold boiler reverses the heat flow across the firebox plates, resulting in the boiler side of the firebox plates being hottest and the fireside being coldest, and it's easy to imagine this could result in some very different-than-normal stresses in the firebox which could lead to problems.

wilkinsd · **Posted:** Sat Feb 19, 2011 10:03 pm

Mr. Austin and others,

Interesting. Even though I do not have an engineering background, I find these posts informative.

I was under the impression the "big railroads" used house steam to start boilers was more because of a time factor. They wanted to get the locomotives ready for service and out the door, and the house steam was the best way to do it in a relatively quick time.

When I was involved at a museum with a steam locomotive, the fire up process was always done very slowly, usually starting at 3:30am on a Saturday. The boiler wouldn't reach operating pressure until 10 or 10:30am. No house steam was used, just a nice and steady warm up to pressure. Of course, I was younger and very much the "grease monkey" but I remember we had very few, if any broken staybolts.