The Steamboating Forum

Code only requires one relief valve unless it's an electric boiler with power input more than 1100kW or heating + bare tube surface exceeding 500^2 ft and steam generation of more than 4000lb/hr. That's not to say that two isn't a bad idea. Two relief valve have a infinitesimally small chance of failing at the same time.

But I'm not comfortable with 15-20 PSI above when they should go off.

Why? Boilers have a safety margin designed in. Being a small amount over the MAWP is a condition that needs to be corrected immediately, you do have a significant buffer before you are in danger of a failure.

It is - but I don't think anyone has gotten the purpose of this. It's a passive non moving fail safe that should never be needed when the operator is incapacitated and no one is around or no one is around

But it creates a secondary problem which guarantees the vessel to be incapacitated. If, in the highly remote chance that two safeties stick at the same time while the the operator is incapacitated it means that your number has been called.

Cheers

Ken

There are other codes beside ASME.
TRD (Technische Regeln Dampf) and GL (Germanischer Lloyd) demands two safeties, same as two water level indicators, regardless of boiler size.

Scotty

What about running the discharge from the rupture disc into the firebox? As long as the disc and piping are small enough that the steam volume wouldn't be another major hazard in itself, this might assist in quenching/smothering the fire as it empties the boiler, perhaps preventing overheat damage to the pressure vessel.

My biggest worry with a rupture disc would be the uncontrolled emptying of the boiler while it's being fired (whether I was incapacitated or not). At least if it put the fire out as part of the process, that worry would be less.

Cheers,
Scott

What we found on the two Navy ships that I was on was trouble with safety valves sticking open or not reseating properly. Safety valves after comming back from the shop were required to be tested and set after installation. Each boiler had three valves on the drum and one on the superheater. A full set of valves for one boiler was carried in spare storage. Safety valves were tested & adjusted after the boiler had been open and put back in service, after work on the valves, or if an over pressure event was observed in which valve failed to function. There has never been a fatality as a result of a Navy boiler explorion. Valves and steam lines have failed with sometimes fatal results, but not the boilers.

I'd opt for two safety valves over the rupturing disc for this reason, if the disc blows with a ship bearing down on you, you may be dead in the water in more ways than one!

Dave

To test the logic of your argument; next time you have a safety valve blowing off at MWP and a good fire on, open the boiler blowdown valve and see how long you want to be in the vicinity of your boiler. Not long I would think! That is essentially the scenario that you would be putting the boat and potentially any passengers under if you as engineer were incapacitated!!!

First rule of steaming any boiler- THERE MUST BE WATER IN IT IF THERE IS A FIRE IN IT- if you envision being incapacitated and are happy to have your boiler empty while in such state then I'm glad your not on any water near me to be honest!

You have had numerous people, including those that work with/have worked with steam on a daily basis with responsibility for the safety of public life and commercial property- all of whom agree that this really is not a very good idea.

Do what you will, but I would urge you to reconsider.

Greg

Alright gentlemen, I wasn't necessarily recommending this. I was asking if any have heard of this application. I think I'm the only one who has used a burst disc on a small boiler.

I do not recommend for anyone to have this arrangement with a burst disc on a boiler.

I'm going to start another thread tonight or tomorrow about Safety/Relief valves because some interesting points of discussion that were raised in this thread. But I don't want to have a burst discs in that discussion, hence a new thread.

A little Background:
So my current boiler that I've had this device on was a 16" diam dry leg VFT non-code 125MAWP that I've been learning basic operation of steam, only at home in a stationary configuration. Being dry leg VFT, it's a little to inefficient for a practical steam launch.

My new boiler (which is almost complete), is an ASME code stamped wet leg VFT - 24" diam in the firebox, 18" diam above the firebox (Manning VFT inspired). Bigger and more efficient with the wet legs ---- That's the boiler that will go into my first steam launch.

After the discussion here - I doubt I will put a burst disc on the new boiler. I may also remove the one in my smaller boiler also.

-CB

Your second boiler sounds like a beauty- and given your attentiveness to getting things right you shouldn't have any problems. Attack any lack of confidence in your plant at source- get plenty of fires lit and get some steaming in, raise your confidence and learn on the job.

The only daft question is the one you don't ask, so on that basis you may well have answered the question for someone who otherwise wouldn't have asked it.

Happy steaming!!

Greg

We have a pair of safety valves...

I can see the idea behind a burst disk, but also Tue argument they could let all the water out making it worse, or no better.

Keep meaning to see what would happen if with a full fire and boiler pressure I shut the air down to nothing and waited. But i am sure if you left it with air open it would dry out the lower tube plate (vft) with the fire that was in the box.



Daniel