Stainless in the fire

[fredrosse]

Daniel wrote:
"I choose not to make rust in my boiler at all! Rust in my boiler will cost me big money long term. Rust in my pipework will cause issues with my check valves if I'm not careful, and carried through to my boiler will make it look like I don't maintain my boiler properly. As a result, the barest minimum of my feed pipework is steel, the rest being copper (and rubber hoses). As for keeping the rust out of the boiler, thats called using feed water treatment. Long term result should be minimal boiler maintenance bills."

Daniel, what treatment do you use to avoid rust in the boiler? Perhaps we should start another thread about boiler maintenance and water treatment. It is my understanding that oxygen is dissolved in most water that is fed to the boiler, so removing it somehow, or preventing it from forming rust in the boiler is the goal.

In my condensate and feedwater arrangements I always use copper/brass up to the discharge piping of the pumps, with a strainer screen before the pumps. The small check valves in the pumps can easily be fouled by rust or other debris, I know I have taken Hypro Feed Pumps apart many times, trying to get the check valves to settle down. Downstream of the pumps copper/brass/stainless is OK, but steel would be my preference.

[daysaver1]

Daniel mentioned his preference for using boiler treatment, which assuming one is not feeding the system from the lake/river being a commendable idea. However since I draw 100% of my feed water from the lake/river and pump my condensate over the side (unless I'm in brackish water of course) it would be pretty impractical for me to consider boiler treatment. Most of the time I dump my boiler and feed water system completely on the way home from steaming so boiler treatment has no place in my decisions.

Also to add to this, I blow down every morning after the boat has sat in the water over night. My blow down port is a few inches below water line so I can see what sort of crap I'm discharging from my boiler and knowing when to shut the blow down valve. By keeping my boiler free from crud in the mud ring and venting oxygen when firing up I feel I'm doing much to keep my boiler safe and clean.

[fredrosse]

I think it would be a good idea to put some tri-sodium phosphate into your boiler, even without a condenser. The tri-sodium phosphate increases the ph of the boiler water, which inhibits corrosion, and it keeps the impurities in suspension so that they can be carried away with the blowdown. About 1 teaspoon per 15 gallons (120 pounds) of water. The TSP can be added to the boiler before you fire up, or added with the feed from the lake/river. This is crude, but effective, water treatment.

[DetroiTug]

Fred,

I'm going to start using the TSP, and just found some at the local Home Depot. The Sea strainer on the raw water intake should be a good place to put some and let it dissolve.

Just wondering, would it change the boiler performance is regard to steam generation? i.e. lower boiling point?

Thanks, Ron

[fredrosse]

When adding TSP, consider that the dissolved TSP stays in the boiler water, unless it is taken out by blowdown. Thus if your boiler holds 15 gallons of water, the initial amount of TSP is about a teaspoon. If you blowdown half of your boiler water, then you only have to replace half of the TSP the next time you fill the boiler. ph of the boiler water should be maintained in the 10 - 11 range, adjust the TSP dosing accordingly. A pharmacy, pool supply or aquarium supply house has ph test paper, or you can buy an electronic ph meter (but they are needing constant maintenance).

After I blowdown, I let the boiler cool, and it sucks in a full volume of water from my feed tank when a vacuum forms. I try to deaerate (remove dissolved Oxygen) and add TSP to this tank water before it is sucked into the boiler. Having the boiler totally full with deaerated (no Air/Oxygen) water is good insurance against corrosion. This is the best layup proceedure for idle periods of a day or a few weeks. Of course this layup proceedure is only good if freezing will not occur.

If you leave the boiler not full of water, then air will almost certianly find its way to the wet boiler internal surfaces, and wetness + Oxygen = corrosion. Assuring a fully dry boiler internals is difficult, drying with a fire can damage a nearly dry boiler, better to but a good vacuum pump and fully dry the boiler by holding 29 inches vacuum until all traces of water are gone. The vacuum pump method works well, but tight valves (or double block & bleed valving) is necessary to assure that water is not sucked into the dry boiler.

As far as impacting the boiling temperature due to the addition of TSP, you would be hard pressed to find a thermometer that could indicate this, much less than 1 F change in boiling temperature.

[artemis]

Water greedily absorbs oxygen from about 34F up to around 100F. At about 130F it starts to release (give off) oxygen. "Hot Well" became the name of the condensate-ready-to-go-to-the-boiler tank when engineers starting regulating the circulating water pump to the condenser to achieve a condensate temperature of about 150F. This ensured that oxygen was being driven OFF and the water taken from the hotwell was as free of oxygen as was easily maintainable. Above 160F the feed pump could cause the water from the hot well to "flash" to steam on the suction stroke. Also the reason that injectors do not draw their water from the hot well.

If you allow your boiler to fill itself from the hotwell on cooling your boiler will be full of de-aeriated water. You can add your boiler water treatment compound to the hotwell. This is another good reason to run condensing. Lowers possible buildup of rust in the boiler by keeping it flooded when not in use.

When you "light off" your boiler after a "filled" layup, DON'T drop the water level on firing. Open a valve that will vent to the atmosphere (stack blower, whistle...). This will allow any re-absorbed oxygen to be driven off from the boiler water to the atmosphere. When a little steam starts from the "exhaust" then lower the water level in the boiler to operating level. This is particularly a better procedure for fire tube boilers as it protects the upper portion of the tubes.

[DetroiTug]

I know I'm swinging off on a tangent here, but what about purging air/oxygen from a boiler then closing it off for layup? Maybe CO2 or some other inert gas?

-Ron

[Aheadslow]

I know I'm swinging off on a tangent here, but what about purging air/oxygen from a boiler then closing it off for layup? Maybe CO2 or some other inert gas?

-Ron

Ron , they often use Argon for the preservation of artifacts in Museums. It is a non corrosive inert gas,the one draw back is that it is heaver that air,so if you used it: it would tend to hang around in the bilges. Unless you pump it out through a hose or something over the side. It is readily available through welding supply shops and is used in MIG welding to prevent corrosion during the welding process. I can see no reason that it couldnt be used to keep the boiler corrosion free during layup. Actually sounds like a good idea . I however could easily be wrong.

[fredrosse]

Nitrogen is the best “blanket gas” to keep Oxygen out of a boiler that is not completely water filled. Nitrogen is much cheaper than Argon, and slightly lighter than Air, so it will not tend to accumulate in low bilges, etc. The Argon remains inert even at welding temperatures, but the Nitrogen can be considered as an inert gas for the temperatures we encounter within our equipment, up to several hundred F or C.

Nitrogen is used to fill our new heat exchangers in shipment, to prohibit entrance of Air and Humidity, which would cause rust. A heat exchanger, maybe 5 feet in diameter and 50 feet long will have a small (20-60 SCF) high pressure bottle of Nitrogen attached, with a regulator set at 1 PSIG to preclude the entrance of air. The Nitrogen is added at the factory, and the small bottle is used to keep the pressure slightly positive during transit and storage.

When filling with Nitrogen, a generous purge flow is needed to wash out all the humid air from a vessel that is initially filled with air. Alternately, if you can draw a high vacuum on the vessel to be protected, then fill with Nitrogen, no purge is required.

For a steaming boiler, all of the air (and Oxygen) should already be purged out, so attaching the small Nitrogen bottle with regulator set at 1 PSIG will need no purge, thus conserving the Nitrogen. As the boiler cools and approaches 1 PSIG conditions, the regulator will start to feed Nitrogen into the boiler as required. A good (Swagelok or similar) check valve should be installed in the Nitrogen feed line to prevent steam pressure from flowing back to the regulator when steam pressure is above 1 PSIG. Tubing of 1/4 inch OD or even smaller is big enough for this job. Alternately, the Nitrogen regulator should be a non-bleed type (if pressure goes above the regulator setpoint, it will not vent off the discharge side of the regulator) to keep the hot steam away from the regulator, which is probably not rated for the steam temperature.

[farmerden]

It's amazing the info we get when a topic is started ! Thanks Ron Fossum for the info on the hotwell-I've never heard it explained so well. My hotwell is cold so I should be reducing the size of the keel condenser even though it specs out correctly in the HP to size of condenser. I had no idea about the oxygen absorbtion of water at different temps! As a matter of fact this is not the only thing I have no idea about!! So thanks ,guys, for continuing my education-I need all the help you can give!! Den