The Steamboating Forum

I have been running my current boiler with one, and I have bought one to put on my new boiler..

I'm curious if any have considered this or even know about it...

Safety/Relief valves are mechanical devices that can stick and fail to do their job. A burst disc has no moving parts. It will never fail to do it's job. But it's a one time use only. The one on my current boiler and my new boiler are 50 PSI above where the safety/relief valve should kick in. They should never burst. The concept is, no one conscious around can deal with the boiler and the pressure keeps rising and the safety/relief is stuck...

This may seem paranoid, but paranoia/caution around steam power boilers is a healthy pursuit .....

-CB

Not convinced this is an entirely good idea- if you have managed to let the fire raise steam 50psi over the blow off pressure to start with I would be adressing your firing method! As soon as you noticed the pressure going above SWP your first action would be to manage the fire and stop it raising pressure.

The other thing is that while on huge industrial boilers with massive water content and automatic boiler water level control that device would be very safe- however on a launch boiler it is going to empty your boiler in double quick time and perhaps put you in an even worse situation.

Safety valves don't stick- buy a decent one, look after it, test it at the beginning of each trip and you won't have any problems.

Greg

gondolier88 wrote:Safety valves don't stick- buy a decent one, look after it, test it at the beginning of each trip and you won't have any problems.

Anything that moves will fail eventually.

If you think a safety/relief valve can't stick... Hmm. I've seen it myself with a brand new ASME steam safety valve under steam. Worked before, 125 PSI. Pressure built up to almost 140 PSI, slight tap and it released.

I'm not suggesting that my extra safety is for everyone. But I'd rather be alive with a dry boiler then dead because the boiler exploded.

Again - The burst disc should never break unless it's 50PSI above MAWP in my case.

-CB

I agree with gondolier that this is more than likely a bad idea.

I believe that you are overthinking this and putting yourself in a situation where a rare failure of the safety valve would put you into a uncontrollable situation.

Think about it like this, When the burst disk opens due to an over pressure situation the water in the boiler is at a temperature at which it will turn to steam as the pressure in the boiler rapidly decreased due to the now open burst disk. You have a condition in which you are going to empty the boiler rapidly with now way of stopping it AND still have a fire in the firebox. You will damage the boiler as you no longer have water cooling the metal. In the chaos of the noise and steam from the burst disk you're less likely to think in a sound manner about getting the fire pulled and getting the situation under control.

On the other hand, should a safety stick, (which you will know as soon as it happens as you have been paying attention to your plant) it is a condition that you need to take care of immediately but the boiler isn't going to explode with a minor excursion above the MAWP. You have a margin of safety that will allow you to pull the fire and lower the pressure in a controlled manner. A low water situation is more critical as the boiler is less structurally sound due to weakening of the material at high temp but even then you should be able to get the fire pulled and pressure released unless you have let the situation go for too long.

A second safety would be a much better choice as a failure of two properly maintained safeties would be exceedingly rare.

On a firetube boiler fired with wood this is not a very likely scenario. The fuel would likely be depleted before the boiler can reach any sort of range of failure. Raising a larger boiler 50 psi requires a considerable amount of fuel. It would have to be almost intentional.

Paying attention to the plant which is paramount, should never allow this. Leaving the plant unattended is planned for and preceded by closed damper, fuel reduction/removal before the stop, water added to settle everything down. I typically just close the damper and leave the Southworth steampump to run pulling steam/water out out and replacing with feedwater. It works and it looks cool while tied up

As Ken wrote above, low water is the most dangerous situation. If a boiler is ever in a situation where the amount of water is not known, it's best to shut down and start over. Never add feedwater to an empty fired boiler. That was the cause of almost all boiler explosions on record. Water flash boiling (exploding) on contact with the hot steel.

As those state above, a one time safety like this is not such a good idea. Two safety valves, one at WP and one just above it is better. I've never seen or heard of a Kunkle safety valve failing to operate, but as you wrote, anything mechanical can fail and I agree.

The best safety item is the operator.

-Ron

Question to CB:

How will you know that your rupture disc will open at the specified pressure?
Testing might be a bit difficult ....



Anyway the standard for a steam boiler is two type approved safety valves,
same like waterlevel gauge glass, feedwater supplyand main steam shut off.

Some codes forbid rupture discs and fusible plugs.


Scotty

I'm the only one then unless anyone anyone chimes in.

I understand the risk of gondolier88, marinesteam, and DetroiTug about it resulting in a potential very dangerous low water condition.

ASME steam safety valves can stick, I'm surprised that this isn't known to steam boaters - it is known in the US traction engine community. As I understand they don't stick to failure, they eventually will release. But I'm not comfortable with 15-20 PSI above when they should go off. You can lightly tap them and they will go off if they stick a bit.

DetroiTug wrote:The best safety item is the operator.

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.

Scotty wrote:Question to CB:
How will you know that your rupture disc will open at the specified pressure?
Testing might be a bit difficult ....

Not really. They are engineered to rupture at a specified pressure. I wouldn't build my own.
I bought two and tested one myself to make sure it ruptured at steam pressure on my boiler. When I did that test I had a valve behind it and shut the valve behind it. It broke exactly at the pressure I ordered it to rupture.
http://www.mcmaster.com/#rupture-discs/=uheylf

Scotty wrote: Anyway the standard for a steam boiler is two type approved safety valves,
same like waterlevel gauge glass, feedwater supplyand main steam shut off.

I'm not sure what standard you are referring too... The ASME code doesn't require two safety/relief valves.

Scotty wrote: Some codes forbid rupture discs and fusible plugs.

Some codes require fusible plugs. (Even if fusible plug have their own problems)

-CB

One thing to remember is that we're not using boilers in unattended 24/7 service; we've got a skilled operator right there at hand.

Rupture disks protect against over pressure - a rare event. (Yes, there are under-pressure disks for large tanks that cannot take vacuum). I've seen plenty of boiler failures (unsoldered and failed copper tubes) due to low water - I've never heard of a safety sticking to the point of causing a failure.

If I were to go to the trouble of adding another separate connection (w/ union and plug to facilitate hydro testing), I'd use
another safety.

But I'd really rather add an low water alarm - there's the safety device we're missing. A spark plug would work as the probe... a good project for the big boat.

- Bart

barts wrote:One thing to remember is that we're not using boilers in unattended 24/7 service; we've got a skilled operator right there at hand.

You are missing the point. Skilled operator is out of the picture or incapacitated. Boiler is on it's own.

barts wrote: But I'd really rather add an low water alarm - there's the safety device we're missing. A spark plug would work as the probe... a good project for the big boat.

Has been done. I wouldn't rely on it a launch. Electricity fails or the spark plug builds up water smutz - fails.

-CB

We get the point, but we're not seeing the justification for such a device.

The claim that relief valves are not all that reliable and should not be trusted etc. The companies that make these have their necks out nine miles from a liability standpoint. We are lucky they are even sold to us without some sort of professional credentials, I would venture to say that every relief valve is tested and calibrated before it is shipped. To not do so would be very careless on their part.

A faulty relief valve became that way in service. A simple test would verify its reliability. Its part of the annual inspection. A valve that sticks once should be replaced immediately. Anyone that fails to do so should find another hobby.

I was at a 3-day traction engine show and one of the engines had a relief valve stick open, the valve looked as old as the tractor. Next day same guy, same valve back under steam. Careless. Buy a new valve and replace it if there is ever a doubt about it. On another note.. I'm a fan of Ebay treasures, but I'd never buy a relief valve from there and many are sold there. Ebay is a great outlet for discarded/rejected products from manufacturers - probably retrieved at the recyclers. I bought two new name brand ball valves from Ebay once and they were both defective. A brand new steam rated check valve that I had on the boiler feedwater came from Ebay, it never sealed. No more Ebay valves of any type for me. `

Re: Incapacitation. Boating alone is dangerous on any type of boat or any water activity for that matter especially on a steamboat. Ok for sake of discussion, a steamboater is out alone and becomes incapacitated under way. Result: The throttle is open, the boat continues on until it runs out of fuel/steam or runs in to something. Steamboater becomes incapacitated dockside: There are always people around, no matter where I've had my boat, there are always curious folks whether it be a steam meet or a public ramp. I'm assuming if I was laying in the floor of the engine room seemingly asleep and unresponsive one of them would find that a bit odd (odder than we are normally perceived) and call 911.

Re: No one around. Leaving a fired boiler alone while it is building pressure or taking no precaution to assure that the pressure will not stop rising beforehand would be very irresponsible.

-Ron