Steam & Excursion > How to know when a boiler blowdown is needed.

So I see photos showing steamy sludge-water blowing out of a pipe behind the firebox. How would the engineer on a steam locomotive know when such a blowdown is needed? Is it just something that needs to be done every so often, or is it done after the crew notices the engine isn't developing pressure properly, or what?

Thanks. Such things are mysteries to me.

- Kit

Kit Courter
Menefee, CA
LunarLight Photography

During the "regular steam days", the operating crews KNEW their territories and the quality of water they had to use, thus blowing down while in route was based on their individual operating district, and practices developed over decades. Plus, if the locomotive in question is equipped with the Wilson Blow Down Separator system, the Engineer could actually SEE the condition of the sludgy water being discharged from the chute located under his cab. On the Southern Pacific, for example, Both the Fireman AND Engineer could see the condition of the blow down water exiting those large blow down mufflers, located down by the forward edge of the trailing trucks.

An engine is required to be blown down at least once daily, and as often as necessary.

Failing to blow down often enough causes accumulation of minerals and mud in the boiler. This can happen quickly, meaning within hours of taking on bad water, and has both immediate results and long term results.

The short term effect is that the water will lift, foam, and/or prime much more easily than if the water were clean. This creates problems with getting water into the superheater and cylinders, and with giving false readings of the waterglass. It can cause the glass to show half a foot or more water than is actually in the boiler.

Long term effects are buildup of scale (which reduces heat transmission) and accumulation of mud and scale in the waterlegs of the firebox, and resultant overheating.

"Bobbing" the waterglass is shown in this video

http://www.trainorders.com/discussion/read.php?10,3186924,3186926#msg-3186926

As explained in the referenced thread, there is a constant flow of condensed steam flowing from the top of the glass, accumulating in the visible portion, and draining back into the boiler through the bottom valve. Thus, the liquid seen in the glass is normally perfectly clear distilled water. Bobbing the glass refills the glass with water directly from the boiler, undistilled, and you can immediately see the condition of the water in the boiler. It takes about a minute for fresh water to replace the dirty water in the glass.

The dirty water may appear muddy or milky. Neither is good.

A boiler is required to have one blowdown. I believe SOO 1003 has only one. Most engines have two, one on each side. Some have three, the third one being located in the bottom front of the throat sheet, and is connected to a trough in the belly of the boiler to help reduce accumulation there. NP engines usually had three.

On the NSSR in Duluth, MN we only blow down SOO 2719 once a day, as we have extremely clean water. At then end of the a months use of the engine, bobbing the glass will not show any signs of being discolored!

OTOH, I've run engines in central Wisconsin where the water literally looked like milk, and would rise 7" in the waterglass when the throttle was opened. Not good.

-John Stein



Edited 1 time(s). Last edit at 10/23/13 10:22 by LarryDoyle.

I remember looking at a picture of a failed boiler. The blow down on the fireman's side had created a small trail from the valve, up the water leg to where it ended in the clear area of the boiler. The dirt, scum, etc., completely blocked the water leg except the small trail steam had cut through the muck. That's what happens when you don't wash the boiler on a regular basis.

Nudge

Wolsztyn Poland, one of the few places where engines are run hard on a daily basis. Two 100 mile round trips each day with multiple stops and speeds exceeding 60 mph. Excess of 1000 miles a week. Some Polish crews have you do a blow down about every other station stop. (Actually after departing station and away from crossings) Other crews maybe once or twice during a leg of the trip. The water quality appears good, but looks can be deceiving. They do add water conditioners. Their engines have blow downs on both fireman and engineer side and usually when one does it the other will follow. Their blow down is a big stiff manual lever under the seat box. It takes a fair amount of strength to lift and close.

On many railroads, especially those with known poor
water quality to begin with, the water service people
took a sample of the boiler water from every inbound
locomotive at every roundhouse. This sample was tested
for total dissolved solids, and if the numbers were
too high, both engineer and fireman were subject to
discipline.

However, blowing down removes heated water from the boiler, so you have to add more from the tender to replace it. And you have to heat that replacement water all over again. So blowing down too often becomes a waste of fuel. I suppost there is a balance struck between maintaining clean water in the boiler through blowing down when needed and blowing down too often, resulting in a boost to the fuel tab.

- Kit

Kit Courter
Menefee, CA
LunarLight Photography

Some rough calculations

At 200 psi a 2" orifice will blow off 95 pounds of steam in 10 seconds, releasing roughly 114,000 BTU of energy.

With a reasonably good coal at 12,000 BTU per pound, that would be about 9 1/2 pounds, which is 2/3 of a shovelful.

If your coal costs $200 a ton, that's a cost of about $1.00 for a ten second blowdown, inefficiencies aside.

CimaScrambler Wrote:
-------------------------------------------------------
> However, blowing down removes heated water from
> the boiler, so you have to add more from the
> tender to replace it. And you have to heat that
> replacement water all over again. So blowing down
> too often becomes a waste of fuel. I suppost
> there is a balance struck between maintaining
> clean water in the boiler through blowing down
> when needed and blowing down too often, resulting
> in a boost to the fuel tab.
>
> - Kit

There was a thread fairly recently on the RYPN interchange board regarding boiler water treatment. The subject is closely related to the frequency of blow downs. Without claiming to be on top of the subject, an engine that operates on the same railroad as is more common today on tourist lines, the necessary number of blow downs can be somewhat predicted based on mileage run and the results of water testing. Besides wasting water and fuel, excessive blow downs can also waste expensive water chemical treatment.

I think the calculations of how much steam can move through a given orifice at a given pressure drop are misleading us here? It seems to me that the the 2" diameter blowdown stream is WATER, not STEAM? It expands greatly upon exiting the valve.

The cloud of steam I see when a 10 second blow down is performed could not be generated by just a partial scoop of coal, I think?

A 10 second blow down seems awfully long.

Tominde Wrote:
-------------------------------------------------------
> A 10 second blow down seems awfully long.

Not at all. You blow down as often and for as
long as needed. That is determined by experience.

You can pay for a little extra fuel and water
treatment, or you can pay for more frequent boiler
washes, which also use fuel. And labor.

"Pay me now, or pay me later."