safety valves

[kwoodhands]

I have a drawing of a simple safety valve that I plan to make. The valve is spring loaded and variable pop off.
Can the valve be tested at first with compressed air? Pop off is 30 lbs. pressure. Every valve I have searched for is not variable and starts at 50 lbs. pressure. I would like to test it before installing the boiler.
The valve consists of a brass body, 7/16" brass hex, thru hole, size not shown ,probably 1/6" / 1/8" . threaded 5/16" x 27 no taper.
Two plates with 10/32 nuts top and bottom and a spring inside the outside columns. When 30 bs. is attained the bottom plate lifts up and steam escapes. Then the plates are fixed with the nuts. I will test the safety valve again with the boiler installed. If there is a problem with the shop made valve it will be easier to work on out of the boiler.

 

[jirik92]

Yes, it is possibly tehs it with air. But both pressures (up and down = open and close) are a little bit different. Depend of constructio more then 10%. Simply steam is steam ;-).

 

[kwoodhands]

Yes, it is possibly tehs it with air. But both pressures (up and down = open and close) are a little bit different. Depend of constructio more then 10%. Simply steam is steam ;-).

Thanks, I will make the valve in a couple of days . The safety valve I was supposed to buy I cannot find anywhere. Most of the valves that would fit open at 60 lbs pressure. 30 lbs is the pop off pressure I need.
mike

 

[TonyM]

Just one point missing when making your own safety valve.

The safety valve must pass enough steam so that with the boiler on full fire, the pressure does not rise when the valve is open.

 

[Steamchick]

Hi Mike, How big is your fire in the boiler? - If gas, what gas do you use, and what jet size? I have tables for sizes of Safety valves. - And drawings of various sizes. Valves operate a bit differently when on air instead of steam, but most open at about the same pressure. - Close enough we all do it. Usually, you can compromise a higher pressure valve for lower pressure use by reducing the spring rate, or compression. But springs MUST have proper square ends, or you are likely to disrupt the seating and it will never seal properly.
Try a lighter spring until you find something that works?
K2

 

[kwoodhands]

Hi Mike, How big is your fire in the boiler? - If gas, what gas do you use, and what jet size? I have tables for sizes of Safety valves. - And drawings of various sizes. Valves operate a bit differently when on air instead of steam, but most open at about the same pressure. - Close enough we all do it. Usually, you can compromise a higher pressure valve for lower pressure use by reducing the spring rate, or compression. But springs MUST have proper square ends, or you are likely to disrupt the seating and it will never seal properly.
Try a lighter spring until you find something that works?
K2

I haven't built the burners yet . It will run on alcohol . I'll take your advice on the spring and make sure the valve works every time. The boiler has 3 external 3/16" water tubes. Boiler tested at 60 lbs and held for 30 minutes with hydraulic test. Will test again under steam . I do not have drawings in front of me now so I do not know the the jet size yet.
Thanks to you and Tony for valuable advice
mike

 

[Steamchick]

Ok Mike, if you want to let me have the surface area for heating Length of exposed tubes and shell exposed to heat, and the diameters of tube and shell, then I can estimate the steam prooduction and do a check that way. Should be close enough to confirm your size of Safety valve.
Are the ends and top exposed to heat? (DEPENDS on your cleading and lagging ideas). Are there any flue tubes? Is there a heated engine feed-pipe (drier, superheater etc.) after boiler steam is taken off?
You can write directly to me if you wish?
K2

 

[Steamchick]

Safety valve drawings:
I hope the attached open properly. (both pdf and any valves you make!).
Here's a recommendation...
https://stalbansmes.com/wp-content/uploads/2019/01/The-black-art-of-safety-valve-design.pdfAlso:
a recommended Magazine article (You'll need to buy the back issue to avoid copyright issues!).
Safety valves for live steam boilers: By Kozo Hiraoka:
Live Steam and Outdoor Railroading: Jan/Feb 2007: Volume 41 no 1.
Their website has details for ordering back numbers. I you want to make more then 1 safety valve, then this fully explains what, how, and shows designs in full detail.
It includes a table of steam relieving capacity for each valve design shown. Valuable so you get the correct size for the boiler, an if a new design of boiler you'll need this to justify the safety valve design to the certified tester for the first (Hydraulic) test and related steam test. BEFORE you are allowed to do the steam test. - If the safety valve size is already stated on the drawings of an approved design, then all you need are those drawings. (But it sounds like that is not the case here?).
K2

 

[kwoodhands]

Ok Mike, if you want to let me have the surface area for heating Length of exposed tubes and shell exposed to heat, and the diameters of tube and shell, then I can estimate the steam prooduction and do a check that way. Should be close enough to confirm your size of Safety valve.
Are the ends and top exposed to heat? (DEPENDS on your cleading and lagging ideas). Are there any flue tubes? Is there a heated engine feed-pipe (drier, superheater etc.) after boiler steam is taken off?
You can write directly to me if you wish?
K2

The boiler is 8" long and 1-1/2" ID. Tubes are 3/16" 5-3/8" long
Ends and top not exposed to heat and are external.
There is a 3/16" stay turned down to a 10/32 thread at each end with bronze round nuts hard soldered.
I made a safety valve today but I am not satisfied with it.
Will pop off at 30lbs only intermittently. Also too large for space allowed under the steam dome. I would make another steam dome if the valve worked perfectly.
I called Coles Power Models that is supposed to supply the exact valve I need. They have been out of business for years I'm told. PM Research has a valve that pops off at 60lbs. I e-mailed them today to find out if the pressure can be lowered.
Worse thing is I will do another hydraulic test at 120 lbs.
The boiler is supposed to test at 60 lbs and run less than 30 lbs. I have two more suppliers that I haven't.gotten around to yet.
What do you think about the 120 lb hydraulic test if a valve cannot be made or procured?

 

[Steamchick]

Hi kwoodhands.
Don't fret, at least not in your head. (A monika kWOOD hands can fret as much wood as you like! - Is that a "saw" point?),
It is quite simple, really, just a matter of sizing the valve correctly. I'll try and do that a bit later.
Meanwhile, a question. Why are you trying to fit a safety valve beneath a Steam Dome? Maybe just your choice of words?
A STEAM dome is a small chamber on top of the boiler, usually a bolted-on cylinder with domed top, beneath which lies the steam intake to the pipe that feeds the Regulator. It isn't a cover over a Safety Valve. The Safety valve MUST vent directly to atmosphere.
See this brief video for a view of a steam dome:

A natural feature of "scaling" models, is that not all physical aspects (such as heat, strength of parts, Area, Volume, cross-section, etc.) do NOT scale linearly, so things like safety valves can appear "huge" = too large for the scale - when on very small models, compared to the full sized versions.
I suspect your 30lbs Safety Valve will be good with "not a lot" of tuning?
Can you send me a drawing, modified if necessary to any sizes you have found it necessary to change on your real valve, as a lot depends on the internal clearances, as well as spring pressure, length and spring rate.
I have made a few "wrong" valves, that open, vent then close, but not a "Good" pop action. And there is a "black art" that few have mastered to make really good "pop" action valves. But as these are well known for comparison, and modern mathematical modelling can show the "features" in action, I can maybe offer some improvements to your valve, without junking everything? (I once worked on improving high-speed pneumatic valves and Safety valves have some similar features. More complicated than you may think? - especially when the "pop" action is modelled!).
My initial thoughts are that the clearance hole in the valve from boiler to valve head needs to be MIN. 3/16" dia.
Simply:
Empirical tables give this value, based on 40sq.in of heating surface (calculations always rounded-up), at 30psi. (close to the volume of steam of 50sq.in of heating at 25psi... given in the tables). But there are some other factors, so if you have metric drills, you should be at the nearest size over 3/16" inch. A 7/32" ball - or equivalent valve head is required. What is yours?
Cheers,
K2

 

[Steamchick]

Just another comment re:120lb test.
Don't, if the boiler was not designed for it.
tell me ALL the metal thicknesses, and I'll calculate what is the maximum NWP for your boiler, and Hydraulic test pressure.
K2

 

[BobsModels]

Mike

A photo of the boiler would really help understand what is being attempted. What is the boiler powering? You are describing a water tube boiler with 3 water tubes 3/16" x 5 3/8" long connected to a 1 1/2" tube 8" long. Total volume of the tube is 7.83 oz and volume of the tubes is about .08 oz. Given you will only fill the boiler to about 2/3 its volume the total water to heat is 5.22 + .08 = 5.3 Oz if water. Mighty small boiler. Virtually no heating area. Not high steam producing boiler. I have a suggestion for the safety valve , but I need to see what you are doing. Depending on your application you really may not need a real sophisticated safety valve. Just one that works.

Please let us see what you are doing and what it looks like. I think that will let us help you with a practical solution.

Bob

 

[Steamchick]

Hi Bob, interesting approach. The total boiler volume of about 8fl.oz. water x pressure of 2 bar gives about 0.4 bar- litre. Regs for small boilers below 3 bar litres only require a relief valve I think? This is equivalent to a Mamod or similar boiler, so a Mamod relief valve should suffice, I guess?
But my analysis follows a text book method based on the volume of steam generation that the boiler can achieve. This is based on some empirical tables using the total available heating area and the method of firing (meths). Hence the volume of steam that can be developed on full firing that the relief valve must be capable of relieving.
My estimate determined the bore of the relief valve needs to be at least 3/16" dia. (IF my estimation is correct). I don't know how this compares to your proposal, but I am interested to find out? Always glad to learn!
Thanks,
K2

 

[BobsModels]

K2

You guessed exactly what I was going to propose but really would like to see his application. Wilesco makes a real nice valve that doubles as the filler hole for it largest boiler, a D22. It appears it is set for about 1.5 bar ie about 22psi if I read the picture of the gauge on their site correctly. My wife has been collecting these boiler / engines for over 35 years, up until last year she had about 150 of them. Sold off over half her collection, just no place to display them all. She runs several of them at threshing shows were I display my models.

My guess is these would more than suffice for his application, probably could play with the spring a bit to get it closer to 30 psi, but not sure why. That size boiler is not going to run very long at 30psi, not matter how little steam you are using. Around 20 or so should work OK. That is why we need to see what he has and how he wants to use it.

Built and ran 1 1/2" steam locomotives for over 30 years, as well has a large 14" gauge engine that weighed over 4000#. steam was a fun hobby at that size. Also had a 1/4 scale traction engine that required state inspection, its safety was based on heating surface area which required a safety valve that almost emptied the boiler if it went off and just stuck a little. I had one of my locomotive safties set about 10 psi less than the big one. Goal of course was to not set it off .

Bob

 

[Steamchick]

Thanks Bob. You should post a picture of your 14" gauge engine!
The Mamod, Wilesco, etc safety valves usually project inside the boiler which reduces the external profile of the boiler. But I don't know if they have a "Pop" action?
Must get my toys out and try them?
K2

 

[BobsModels]

K2

Here is a picture of the 14” gauge taken around 1987. I co-owned it with friend in the 1980’s, we laid about 1200’ of track at a Threshing show grounds. I sold my share, and since then it was sold in the mid 90’s. It was built in late 40’s early 50’s by a gentleman in Illinois and ran at county fair until around 1973 when he completed a 15” gauge C B & Q 3000 Hudson, which is still being run at the fair. The 14” is a 4-4-4 arrangement, Wagner Castings. It got its name, Iron Pony, from the school children of the town.

My guess is there are people on this site that have seen both of these locomotives running.

******************************************
->->Now back to Mike's questions. I have a suggestion that you look at a Jensen safety it has a 5/16-24 thread size. The spring on it is suppose to be easily modified. Here is link - Jensen Manufacturing Company, Inc.. For the price it might be worth a try. My wife should have one of these, I will post a picture later.

*****************************************

Bob

 

[kwoodhands]

Fellows I got the valve to pop off at 22 lbs. I think if I shorten the spring slightly i might get closer to 30 lbs
If not I'll be satisfied with 22 lbs.
I will call Jensen tomorrow, they have a list of other parts that may come in handy.
Bob, I love the Iron Pony. Years ago I built a 3/4 scale Pennsylvania Switcher from Kozo's book. It is a display model only as I did a poor job of hard soldering and could not stop leaks. Every time I took care of a leak , another popped up.
After a while I decided to run it on air only .
The boiler I just finished is the first one that tested good after 2 tries. This is for Rudy K scale model of a Case tractor.
I actually have driven similar Case tractors but with rubber tires, not grouters as Rudy calls them. This tractor has nine gears. First time gear making . Made every mistake possible until I finally realized what I did wrong. Mistakes were made with Acetal so it wasn't costly. The gears are now made in brass , except for the bull gears which are aluminum. Did not have brass large enough to make the bull gears.
Time to start garden plants in the shop so the tractor will wait a week or two.
Thanks fellas , mike

 

[Steamchick]

Bob - Love the pictures! What a beautiful pair of "models"!
Kwoodhands: I suspect you'll be running with almost continuous water feed on this boiler, as it is quite small? - But it always depends on what load is on the engine as to steam consumption. You need the pressure to develop the engine torque (Make it go!) then adequate heat to boil the water (continuous feed?) and make the steam. - I think this will have adequate surface area for the volume of water, with a feed rate to balance the steam consumption, but how much pressure you'll need depends a lot on the engine versus load, via gearing. (All of which will consume power).
It is a big balancing act!
Glad you are swapping springs, lengths and compression to find what suits 30psi.
Just for my curiosity, what wall thickness are the boiler and water tubes?
I re-read the thread, and if the drawing says "test at 60psi" then you must not steam over 30 psi, according to ASME regulations. (NJ being in the British colony of the USA). (Safety Valve relief not exceeding 32psi on full fire, max. steam, regulator etc. closed).
Hope that is useful?
K2

 

[Steamchick]

I have a drawing of a simple safety valve that I plan to make. The valve is spring loaded and variable pop off.
Can the valve be tested at first with compressed air? Pop off is 30 lbs. pressure. Every valve I have searched for is not variable and starts at 50 lbs. pressure. I would like to test it before installing the boiler.
The valve consists of a brass body, 7/16" brass hex, thru hole, size not shown ,probably 1/6" / 1/8" . threaded 5/16" x 27 no taper.
Two plates with 10/32 nuts top and bottom and a spring inside the outside columns. When 30 bs. is attained the bottom plate lifts up and steam escapes. Then the plates are fixed with the nuts. I will test the safety valve again with the boiler installed.

Hi again Mike.
Just had a quick look at the boiler: Re: your comment post #9: Assuming your 1 1/2" tube is greater than 1mm (0.040") thick wall, with the end plates and stay bolt as you describe the boiler should be good for a MAXIMUM of 50psi NWP - NOT higher. According to ASME standard. The Hydraulic test pressure to prove the design would be at 110psi.
Your comment about the Safety valve: "thru hole, size not shown ,probably 1/6" / 1/8" " means that you possibly have too little hole in the safety valve to prevent overpressure when on FULL FIRE with NO STEAM DEMAND from the engine, etc. My tables suggest you need 3/16". Can you slide a drill up the bore o the valve from what would be the "inside" of the boiler, up to the sealing plate, to see what size hole you actually have in it? The size of hole you have is critical for Safety, and then when you get that right you can better tune the spring rate. If you have a drawing of the Safety valve, then I can be more precise with my suggestions?
Thanks,
K2

 

[kwoodhands]

Steamchick, I made the valve exactly like the one shown with the counterbored recess. Worked well after trying different spring lengths. I got the valve to pop off at 28/29 lbs 6 straight times.
The valve I made was 3/16 " too long so I threaded the 5/16-27 threads that go into the boiler deeper. I was almost done when I snapped the threads off. I made the valve from 1/2" brass . At first I was mad at myself. Then I decided I'll make another one . Now that I know the dimensions and exactly what the valve will look like , probably will take a few hours at most to make another. Then I will test it with steam after I make the firebox burner.