Does the safety valve have to be above the water level in the boiler ?

[Brian JohnParticipant @brianjohn93961]

[Brian JohnParticipant @brianjohn93961]

My Mamod safety valves arrived today ; one short (28mm) and one long (38mm). I intend to install one of them on the vertical boiler I am building which is 60mm in height.

1. Does the safety valve have to be above the water level ? I had never really considered this point before. If it does have to be above the water for correct operation then I will have to use the short safety valve.

2. To test the safety valve I was thinking of plugging the steam outlet (I have a blanking plug) and watching from a safe distance to check that the valve does eventually blow. The Mamod valves are set to about 15 PSI.

NOTE : I do not have a water feed pump.

[julian atkinsParticipant @julianatkins58923]

hi brian,

the safety valve must to be above the water line otherwise it shoots up a column of uncompressible water rather than steam.

it would be dangerous to check the safety valve with anything other than a cold hydraulic (ie boiler filled totally with water) test on a proper test rig, and bemused how you would do it with a miniature gauge. i am not au fait with small boilers of low pressure but i dont see why the same precautions ought not to be observed.

cheers,

julian

[BazyleParticipant @bazyle]

Not sure what you are thinking you are testing. As a commercial product Mamod will have made pretty damn sure it goes off close enough to spec and only goes down with age. Obviously you check manually that the rubber washer hasn't glued it together.
What is more significant is to test that the heat source isn't so powerfull that it generates steam faster than the safety valve can release it and that the fuel runs out before the water.

[nigel jones 5Participant @nigeljones5]

so you are building a vertical boiler 6 cm high… I wouldn't worry about safety valves! 60mm??????

[HopperParticipant @hopper]

Absolutely it must be above water level, well above. The highest point in the steam space usually.

Otherwise, when it "goes off" it will shoot out superheated water that when it is released into the lower atmoshperic pressure, will flash off into steam, which expands in volume very greatly (1600 times) and rapidly (ie instantly) and is the main danger in boiler explosions. So fitting a safety valve below the water line could be VERY dangerous.

Test of safety valve should be done with cold water pressure. Depending on the blow off pressure of the valve, you can often hook them up to a garden tap with a pressure gauge and observe the blow off pressure. Mains water is usually 50-80psi.at the tap.

[Brian JohnParticipant @brianjohn93961]

If I fit the short valve then that only extends down 17mm into the boiler leaving plenty of room for water so it should work.

Yes, I know it is not necessary but I would like to try fitting one. I may end up not doing so because the top of the boiler is getting a bit crowded : safety valve, steam outlet, water filler plug and the chimney stack through the middle.

[FMESParticipant @fmes]

BJ,

Basically you are altering an approved design to your own specification., this boiler has a flexible hose connection that is designed to blow off if pressure is exceeded.

When used on the boat that it was designed for it would hardly be a problem if its halfway across the pond when it decides to let go.

Don't be misled by the size of the boiler either – 1lb of water heated to boiling point would produce over 26 cubic feet of steam if allowed to expand freely to atmosphere.

Look what happens when Bazyles suggestion of too much heat (and uneducated children) happens (Fizzy take note also) : **LINK**

What you do in your own kitchen / workshop is up to you, any accident would be put down to misadventure, but take this to a place where others can be harmed and you become liable, additionally you cannot carry out a test for insurance purposes on a boiler that you have either made or own.

[Brian JohnParticipant @brianjohn93961]

Lofty : That is am impressive video however I am already aware of the dangers. I hope they were wearing eye protection ! I still intend to connect the boiler to the engine using silicone tubing but I did not really think that was adequate as a safety feature which is why I wanted to install the safety valve. If you think the Mamod safety valve is over kill then I will leave it out.

[Ian S CParticipant @iansc]

Brian, the cylinder on an oscillating engine is another safety valve, as the pressure increases the cylinder lifts off the vale face.

Ian S C

[Brian JohnParticipant @brianjohn93961]

Quite true. I had not thought of that at all…..and it does not take much pressure to lift it off. I have replaced the spring that came in the kit with something a bit weaker. The one supplied was far too strong.

[Maurice Cox 1Participant @mauricecox1]

I do not agree that an oscillating engine can be regarded as a safety valve. The cross sectional area of the steam port is usually tiny. The pressure exerted on the port face, tending to lift it against the spring, will diminish in proportion to the diameter of the port. If the port were 1/8" diameter, (most that I have seen are smaller than this) its area would be less than 1/64of a square inch. At a working pressure of 15 p.s.i. the force exerted on the stationary port face would be between 3 and 4 ounces. Even if the spring were light enough to allow the port block to lift, then the other requirement of a safety valve would probably not be met i.e. to release the steam faster than it it being generated. Several inches of 1/8" copper pipe with a block of brass over the end is hardly an easy way out for it!

Maurice

[Steven VineParticipant @stevenvine79904]

Just a thought. If space is tight, is it possible to braze a vertical tube to the top of the boiler end cap, and mount the safety valve in the end of that.

Steve

[BazyleParticipant @bazyle]

Posted by Lofty76 on 19/11/2014 06:28:13:

Basically you are altering an approved design

Curious. I missed the mention of the design – another thread maybe?

[JasonBModerator @jasonb]

Its a commercially available boiler kit so assumed to be suitably designed.

The fact it is from the US does not imply it would pass UK or in Brian's case Austrailian tests or at such a small size if it even needs testing for use in that particular country

 

Edited By JasonB on 20/11/2014 18:58:58

[FMESParticipant @fmes]

Posted by Bazyle on 20/11/2014 18:50:47:

Posted by Lofty76 on 19/11/2014 06:28:13:

Basically you are altering an approved design

Curious. I missed the mention of the design – another thread maybe?

Yes indeed Bazyle, you must have missed it.

[Paul LousickParticipant @paullousick59116]

Hi Brian,

How big is your boiler ?

If it is used in Australia, it may not comply with the requirements set out by the Australian Miniature Safety Committee and will not be approved. Australia has a much higher standard for model boilers than America or GB.

The boiler which I am building has been designed in GB and can be certified for use at 130 psi. In Australia, the plate sizes and boiler stays have to be bigger and I can only have it certified for 100 psi.

Best to check with a boiler inspector from a model engine club first.

Paul.

[Brian JohnParticipant @brianjohn93961]

This is a small pot boiler which will probably generate 10-15 PSI if I am lucky. There are no boiler stays. The boiler is 6cm high and 5cm in diameter.

I live in Cairns at the moment and there are no model engineer clubs here. The situation may improve if I move to Brisbane in the next few months.

NOTE : some of the Saito boilers use the safety valve as the filler plug which is an interesting to solution for small spaces.

[JasonBModerator @jasonb]

Actually brian in some of the photos of this boiler it looks like the boss for the filler cap is threaded internally and externally can you confirm.

Mamod use the saftey to fill the boiler. So you could do the same

Edited By JasonB on 21/11/2014 07:37:02

[Brian JohnParticipant @brianjohn93961]

You are right ; the boss is threaded internally. I am not sure why. I did the try the Mamod safety valve in it. It goes in four turns but does not quite tighten up ; it would reguire one more turn to give a good fit I think. The threads are not an exact match and I did not want to force anything.

[nigel jones 5Participant @nigeljones5]

I thought a vertical boiler 6cm high was a bit odd! Now you have explained it I agree that you could get a very serious scalding from this! Lofty take note

[HopperParticipant @hopper]

If you have a lathe you should be able to make up a new filler plug standing say 20mm tall, threaded to fit the safety valve inside. That way the safety valve would be above the steam space.

Of course, it depends on where the actual valve and seat are located. If it is just the spring and spindle sticking down into the steam/water space and the valve and seat are at the top, clear of the water, it should work ok.

[FMESParticipant @fmes]

Hi BJ,

Mamod also do the bush for the safety valve so you could get one of these and have the correct threads for both parts.

Pic attached for info **LINK**

[Brian JohnParticipant @brianjohn93961]

Yes, I have the bushes for the safety valves however I think the boiler is too small ie. the safety valve if fitted (even the short one) extends down too far into the boiler. I will probably rely on the silicone tubing blowing off under high pressure.

I am still thinking about it.

[JasonBModerator @jasonb]

No more that it would on a Mamod traction engine

[Author]

Posts