The Steamboating Forum

The purpose of blowing down is to get rid of water that has had impurities concentrated inside the boiler, wash out scale and rust particles parcipited "mud", etc.

Having as much turbulence as possible within the boiler during blowdown helps mix up and allow removal of solids that are laying in the lower regions of the boiler. One way to get the extra turbulence is with a fairly rapid drop in boiler pressure, as pressure reduces, the water inside the boiler "flashes" to steam throughout the boiler internals, creating turbulence to mix everything up. Then when boiler pressure is a fraction of the initial pressure, open the blowdown valves, and the well mixed up stuff is blown out.

When I blowdown at the end if a steaming day, I have the boiler at about 80 PSI, (fire off) , open the steam blow valve and dump steam into the feedwater tank. This heats the feedwater tank to about 210F, and drives the Oxygen (Oxygen is disolved in the water, but has virtually zero concentration if the water is brought up to the boiling point) out of the water in the tank. When boiler pressure has dropped to around 20-30 psi, I then open the blowdown valves and dump about half the water inventory of the boiler.

Overnight the boiler cools down, creats a vacuum, and sucks the deaerated water from the tank thru the feed pump, into the boiler. In the morning the boiler is completely full of deaerated water, and ready for weeks of wet layup, or another day's steaming.

The boiler holds about 80 pounds of water when completely full (water right up to the top of the pressure vessel), so I have to have at least 10 gallons of water (84 pounds) in the tank before this process is initiated. If there is less water available, the boiler vacuum may suck the tank dry, then suck air into the boiler, don't want that to happen. I also put about half a tablespoon of Tri-Sodium Phosphate into the feedwater tank before it goes into the boiler, to keep the water at a high Ph, preventing corrosion.

fredrosse wrote:... When I blowdown at the end if a steaming day, I have the boiler at about 80 PSI, (fire off) , open the steam blow valve and dump steam into the feedwater tank. This heats the feedwater tank to about 210F, and drives the Oxygen (Oxygen is disolved in the water, but has virtually zero concentration if the water is brought up to the boiling point) out of the water in the tank. When boiler pressure has dropped to around 20-30 psi, I then open the blowdown valves and dump about half the water inventory of the boiler.

Overnight the boiler cools down, creats a vacuum, and sucks the deaerated water from the tank thru the feed pump, into the boiler. In the morning the boiler is completely full of deaerated water, and ready for weeks of wet layup, or another day's steaming...

Well, that all sounds very nice, but...

Water starts to absorb "air" at slightly above freezing (32F Imperial measure). It continues to do so until about 100F. At about 120F it starts to release "air". 150F is about the best you can do without creating problems with feed pump suction - but most air is out by then. All of this assumes that the "hot well" is open to the atmosphere (which it needs to be to allow the hotwell to de-aerate). But as the hotwell cools over the course of the evening/night the temperature of the water in the hotwell gets cooler and it ABSORBS MORE AIR, thus reaerating the water going to the boiler.

When I lay up the boiler on Otter, I fill it with water directly from the hose after putting a charge of TSP into the boiler. This minimizes the amount of entrained air. Since Otter lives on a trailer, this is a normal part of preparing the boat to go back under it's shelter in the side yard.

- Bart

Huh! What a variety of methods.

When I shut down for more than a few days, I do a blow down at fairly high pressure to get most of gunk out. Then let things cool down to twenty pounds or so and then open the blow down and leave it open. That way the boiler is stone dry.

I have already closed all of my feed water valves so there is no siphoning going on. If I think I'm done for the season, I drain and blow out everything, put a 60 watt bulb in the firebox, close up everything to keep the mice out and the boiler is ready to sit for years.

Oh, yes, I disconnect the cylinder cocks and spray some WD-40 in there while I turn the engine over and leave it at top or bottom center in case I get a stuck ring so that I'll have good leverage at the flywheel to get it loose. I go out and spin the prop when I think of it to reduce the chances of sticking. If I'm feeling particularly virtuous I remove the cylinder heads and spray around in there.

I suppose it is possible to store water in a mild steel container if the dissolved oxygen is low and the pH is right. But I'm far too fallible for that. I would sit around on cold Winter nights thinking about rust blisters and freeze cracking of fittings.

"But as the hotwell cools over the course of the evening/night the temperature of the water in the hotwell gets cooler and it ABSORBS MORE AIR, thus reaerating the water going to the boiler."

That is generally correct that water that has been well deaerated will re-absorb air as temerature falls, however the re-absorption process takes time to come to equilibrium conditions. Deaerated water that is brought to, say 100F in a couple of hours has far less dissolved oxygen in it than deaerated water that is brought to 100F and left standing exposed to the air for a longer period of time. The time to reach equilibrium conditions is a function of several variables, for example, water sitting still in a capped tank will re-absorb oxygen more slowly than the same quantity of water with air circulation over the water surface. I will get some numbers to quantify the variables typical for this process.

The process I use sucks the deaerated water into the boiler while it is still quite hot, about 180F or higher, so what ends up in the boiler is well deaerated because it has not cooled down too much, plus it has only been below saturation temperature for a couple of hours, in a closed tank with no air circulation.

The feed pump does not have to work at these high inlet temperatures, as atmospheric pressure is forcing the hot deaerated water into the boiler. When the boiler gets down to 180F there is about 7 PSI vacuum formed in the boiler, plenty enough to suck from the tank. The hot water does hovever heat up all the components along the way, so the tank, feed pump check valves, feed pump piston seals, feed suction and discharge piping all have to be OK with hot water. I have an all metal system, except for rubber heater hose and rubber O-ring seals, both of which are good for over 212F service.

fredrosse wrote:... I will get some numbers to quantify the variables typical for this process.

Please do

... The process I use sucks the deaerated water into the boiler while it is still quite hot, about 180F or higher ...

How are you getting 180F temperature for the feedwater in the hotwell?

... in a closed tank with no air circulation ...

How does the "air" get out of the enclosed and capped hotwell, or do you just fit a "loose" cover or "breather pipe"?

Numbers on equilibrium Oxygen concentrations and time to get there: I have to get to the water treatment guys at work, will post data when I get it.

How are you getting 180F temperature for the feedwater in the hotwell?: I carry two 15 gallon reserve feedwater tanks, and at the end of the steaming day I put a steam line into one of the tanks, and heat the water up to the boiling point (212F or close to it) to drive out the dissolved Oxygen.

How does the "air" get out of the enclosed and capped hotwell, or do you just fit a "loose" cover or "breather pipe?

These tanks have ordinary 2 inch fill caps, which have small vents to atmosphere. During the short period of time when the boiler cools down enough to begin forming a vacuum, the hot water in the reserve feed tank also cools down somewhat, but I think that these conditions do not allow much air to re-absorb into the water before it is sucked into the boiler shell. Once this water is inside the boiler, no further aeration is possible. Valves are aligned so that the hot water is sucked out of the hot tank, right thru the manual feed pump, and into the boiler