(disclaimer) I have none of the steam credentials of the previous posters, but I've poked around a bit with steam tech and locos.

OK, now this is something I've wondered about, and wondered why no one does it (or does anyplace do it?)---

It would seem that most of the intial stresses come during the time the boiler moves from ambient temperature to 212 degrees. I've thought that placing a boiler on an loop connected to a stationary boiler in the enginehouse/shop/adjacent shed, would allow the water temperature (and thus the metal temperature) to be slowly raised. This is the equivalent of "house steam" from the old days, and could be easily accomplished with quick-connect couplings at blowoff connections, etc. a locomotive could be kept on this system for days, with the boiler all nice and expanded and happy and warm.

Another advantage is with no fire and water temp carefully kept at 210 degrees, one is not using up 1472 service days-- no fire, no pressure.

Boiler could be oil or gas or "solid fuel" fired; it could also be used to provide shop heat with a hot water coil/blower or in-floor radiant loops.

It seems to me that with the expense of boiler work and repairs, the cost of keeping a locomotive boiler warm would be well worth the operating expense--- a few extra days each side of an operating period.

Comments?

Howard P.

Howard, these ideas have been around for a while and were almost in practice at Golden Spike NHS - until a problem with the designof the stationary boiler plant came to light when it became waterlogged due to being lower than the locomotive boilers they were supposed to be supporting, while the locomotives ran dry. Gravity matters. The modern rack engines from Sulzer have a "preheating" option. and can also maintain temperatures during shutdowns. I was told you could even telephone in and remotely start the preheater so she'd be warm the next day.......almost not like railroading at all.

We commonly use preheating pumps on diesel engines lubricating and cooling circuits, so installing something that would circulate hot water through a boiler shouldn't be much different.

OK, I'm confused here: has there ever been (among those of us that work on these things) a consensus that flexional stresses have not played a major part in staybolt failures? I take that as a gvien - if it were otherwise, stays would fail from getting longer and thinner until they give, which isn't the case. Are we simply now wondering why when we design and calculate we only seem to consider tensional stress as the basis for calculations? If so, good question..........no idea.

Probably the slower application of delta T allows for more transfer between parts, so they less unequally change shape, which results in less differential stress. Locomotive boilers don't do things slowly while in service. We turn them on, then turn them off, run hard up a hill then shut down. I don't know if there's a way to accomodate this without accepting staybolts as a renewable commodity, or massively overbuilding (back to the size matters thread). Unless we could maybe figure out how to design prestressed boilers that would gradually loose flexional stresses as they were fired?

dave

I know gravity matters; have been in the HVAC biz for 30+ years and have seen plenty of pumps, boilers, heat exchangers, coils, etc. I have no engineering degree, but that's why I'd advocate a hot water system, not a system designed for producing steam. What were the problems with the Golden Spike NHS setup, they tried to use a gravity system with no pumps?

We use a stand-by oil-fired hot water boiler mounted right inside one of our diesel locos, and it does a great job of maintaining 150 degrees on the cooling system. Once again, keeping all that metal expanded and happy is a good thing. Keeps the lube oil warm, too.

So, besides G.S. NHS, a stand-by hot water boiler has never been tried by any of the steam operations?

Howard P.

I'm told it was located on a lower floor level, and the water gradually migrated down over night. I have no idea as to the exact specs. The SLM / Sulzer version was a stand-alone preheater that couples with quick connects as you suggest, but the last contact I had with them was several years ago and there may have been improvements since.

dave

Dave- DLM apparently can still provide the self-contained preheating units. You can download a brochure at this page:

http://www.dlm-ag.ch/en/pamphlets/64-el ... heizgeraet

While warming the boiler up with an external source of hot water certainly seems like a good way to reduce unnecessary stresses on the components, it's worth mentioning that there may be new issues introduced with this practice. David Wardale noted that broken stay bolts were extremely commonplace in China even though they normally pre-heated boilers with house steam prior to firing up. Wardale noted that this practice might have actually contributed to the problems with broken stays due to the "reverse" heat flow that this practice induces. Normally, the fire side of the firebox sheets and tubes heats up first and gets the hottest. When steam or hot water is fed into a boiler, the reverse happens and the stresses induced can be very different from those the boiler normally sees. The graph posted by Matt Austin in an earlier thread showing the temperatures from the inside of the firebox across the firebox sheets to inside the boiler gives an idea of how different the temperature profile could be with "internal" heating of the boiler.

I'm sure this problem could be avoided or at least minimized by controlling the warm-up of the boiler: start with boiler filled with cold water and gradually bring the whole thing up to temperature by circulating the water through an external heat source. It's just worth noting that this could conceivably make things worse if not properly handled.

That was precisely my thought-- set the loop up with ambient temperature water, then start the boiler and pumps, let things circulate and warm up over a day or two (depending on the burner size), bring up the temp slowly.

The whole point to my mind is being gradual, not fast. House steam and central steam plants back in the day were mostly about "efficiency in dispatchment" and "smoke abatement", not saving boiler work. Dump the fire on the inbound ashpit tracks, move loco into house on stored steam, perform whatever work was needed (including washouts using hot water from the central plant), hold loco on house steam and 2/3 glass of water, with fire materials laid on the grates but not lit off. If some bolts were broken by a more rapid steaming, well, the boilermakers will find them next trip. After all, we have lots of boilermakers.

Howard P.

Stephen: Sure.

Hugh: You got it on using house steam. Basically, the hot zones of the firebox (sheets, staybolts) have normalized at the maximum firing rate, with the associated temperature gradients. The firebox and staybolts on the sidesheets other than the hot zones are used to growing up relative to the wrapper sheets, and the sheets and staybolts may have plastically deformed to this regular growth.

When the boiler is heated evenly with house steam or gets a hot washout, the temperatures and gradients are different than what the boiler ever sees in service, and consequently the direction of bending may be reversed. Bending a coathanger wire back and forth is what you do when you want to break it in two pieces.

The answer provided by Dr. Tross is gradual heating and cooling with the source of heat where it usually comes from (firebox) and cold washouts. The later is beneficial in that the scale does not cook to the surface of the sheets.

The reality is that the practice above sacrifices short-term availability of the locomotive for long term availability and service life. Things have to get done sometimes, but I think it is important to understand what external heating does so it is only used as needed. Keeping a small fire in the firebox when it is being externally heated and using heaters to keep the firebox warmer during a washout may be a way to mitigate stresses - it would be interesting to load a boiler up on strain gauges and thermocouples and play around to see what it likes best.

Matt: Yes, in 1908, many thought it was firebox expansion breaking the staybolts but could not explain why the cracks in the staybolts were the wrong way relative to stress in that direction. The more complicated explanation advocated is certainly interesting and reason to have a high margin of design.

I'll add a couple of things for everyone's consideration. The new boiler for Monticello Railway Museum's Southern RY 2-8-0 was built in St. Louis, MO, by Continental Fabricators. The boiler was stress relieved as a complete unit in their furnace, which is very similar to that described by Randy on the first page of this thread, except they use it regularly. The furnace recorder for our boiler shows about a 6 1/2 hour warm up from ambient to 1150 degrees, a two hour soak at 1150, then a slow cool down back to ambient.

In regards to boiler/firebox movement, we set up some dial indicators at the front of the firebox to see which way and how much the boiler was moving between stone cold and hot. We put three under the throat sheet, measuring vertical movement of the bottom of the mud ring (A 4" thick plate mud ring), and horizontal movement both front to back and side to side. We also placed an indicator under the rear waist sheet angle to measure vertical displacement at that point. Waist sheet bolts were removed, and the front furnace bearers were removed so the firebox was free to move vertically at that point in either direction. One of the interesting things was how soon movement appeared on all the indicators, very shortly after lighting the fire, and coinciding with the sound of tube bundle expansion. We've also placed an indicator at the rear of the boiler a few times to look at total lengthwise expansion, on which we've seen ranges between 7/16" and 1/2", and this is a small boiler. In the below spreadsheet, the temp. readings were taken with a non-contact infrared thermometer, measuring the same location on the sidesheet, just above the level of the mud ring.

Kent

I've often wondered about staybolts being changed over the years and installed to different depths having an effect on the sheets. If you put one flexi in with a tight fit and one next to it looser in the sleeve would that eventually rear it's ugly head?
Hope that made sense, I'm getting sleepy.

I'm normally one not to post operating practices as an example for all to review but I will make a couple of comments here. We have been using a "steam blanket" approach to preheating our locomotive boilers and maintaining temperature on our off days for 5 years.
The system uses a valve to control steam flow into the water leg of the boiler and a steam trap copnnented into the boiler at the level of the middle try cock. Yes, the boiler functionally resembles a large radiator. The power boiler providing the steam operates at 100psi. There is no condensate return to the power boiler, all water vented from the locomotive boiler is dumped into the bolwdown system.

It takes less than five minutes to connect the hoses and open the valves. There is a significant amout of steam/water hammer untill the water temperature rises above 180degF. This can be minimised by underfilling the cold boiler A standard preheat is 40 hours long and raises the boiler pressure to 10-15psi. At this point the water level is into the glass and the locomotive is moved outside and the fire lit.

To maintain temperature durring the off days the locomotive boiler is brought into the shop with a dieing fire and 1/4 glass water. The steam lines are re-connected and the valve opened a predetermined amount. Then the stack is capped and the temperature and pressure slowly fall until an equilibrium is reached between the heat loss from the locomotive boiler and the steam from the power boiler. This usually take about 24 hours and the locomotive boiler settles between 25-and 35psi.
The boiler has never been "wired for sound" to record the process but we have used an IR temp gun to moniter the temperature gradient aroud the boiler. Most of the time it is within 10degF.

Have we seen any benifits? I'd like to answer yes. We have had no broken bolts in that time and no delays caused by light off difficulties. It is not uncommon to have the boiler under pressure for six weeks during the summer. We have eliminated the crew time required for a cold fireup every week and some of the worries around special events.

The downside is the water chemistry has to be monitered closely or your could fill the boiler with distilled water. But the cennection to the trap functions like a surface blowdown and should in theory be removing the scum from the water line. The cost anaylsis is a bit difficult. Yes we run the power boiler most of the summer and there are fuel costs associated. We may be able to answer that when the next 1472 comes round.

I have though about adding a shell and tube heat exchanger between the two boilers which would fit some of the water treatment issues but what we have is working fine and would only add complexity and cost.

One final note. Buy good steam hose and fittings. The cheap crap is not worth the hassel.