# Resetting safety valve liftoff pressure



## Dr. J (Feb 29, 2008)

OK, here's another newbie question..... The pressure gauge on my Roundhouse Forney reads just over 20 psi when the safety valve starts to lift. I removed the safety valve and hooked it directly to my air compressor and got essentially the same result: the valve opens at 25 psi according to the gauge on the compressor. The nominal lifting pressure according to Roundhouse is supposed to be 40 psi. 
Question 1: Is it safe for me to screw in the fitting on the top of the valve to raise the lifting pressure to 40? 
Question 2: Will I see a performance benefit from doing so?
Thanks.
Jim C


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## jfrank (Jan 2, 2008)

The higher the pressure the more power the locomotive has. I adjust most of mine to 50lbs.


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## Kovacjr (Jan 2, 2008)

Yes to adjust the safety you screw in the top fitting on the valve. Do it with the compressor so you have an accurate lift off pressure about 45-50lbs is sufficient. I have all my engines set for 60-65PSI. The benifet is how the engine runs and power al slower speeds. 

FYI the valves adjust themself over time so this is probally why its lifting at 25PSI, You can put a dab of enamal paint on the threads after you set the valve. It acts as loctite but you do not want to use loctite on the safety,


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## Dr. J (Feb 29, 2008)

Does this mean the idea is to raise the steam pressure to "just under" liftoff pressure, and keep it there while running? I'm guessing here - the higher pressure in the steam line (say, 35 psi instead of 20) lets the engine do the same amount of work with less consumption of water than at 20 psi? Essentially a tradeoff between refilling the gas and refilling the water?


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## Steamboil (May 12, 2009)

HI,

I like to run mine at about 40 to 60, and I try to set safety valve one at 65 and the second between 5 and 10 psi more, or about 75 psi total. Forthermore, for my engines that only have one, then I like to have them set around 70 psi. 

The goal is to run the fire hot enough to generate the steam needed for the weight of your train. The safety valves are a back-up, and should really not be going off (popping off) if your firing your engine correctly. However, this can be hard on some engines that do not burn gas.

For extra info, when we run the full scale engines, we set each valve 10 psi apart. And they must be recertified every 30 to 365 days depending on the type and quanity of valves on the boiler. Talerance is 3 psi each too.

Happy steaming,
Tom S.


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## Dr. J (Feb 29, 2008)

Thanks. I've adjusted mine to lift at 35 instead of 20. 

Anybody know where I can get replacement compression springs for a Roundhouse safety valve? I'd like to keep a few on hand.


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## Kovacjr (Jan 2, 2008)

You set the valve on air much safer. The springs can be bought from Roundhouse easily. Just send an email they are fast with a response in my expirences.


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## Dr. J (Feb 29, 2008)

I've written to Roundhouse and received no reply. I've also spoken to a couple of certified Roundhouse dealers, and the word I've been given is that Roundhouse will not sell the springs to anyone. They'll sell you an entire safety valve, but not the springs. The reason (I think) is that they're afraid of liability issues arising with guys like us, who are tweaking things. With the valve, they can always say "It was set properly when it left the factory." 
Jim C (aka Dr. J)


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## Nick Jr (Jan 2, 2008)

The blow off valve on my SR&RL had been stamped with a dimple in the side locking the threads. After I drilled it out I was able to adjust the pressure up to where it belonged. It was also blowing off at 25lbs of pressure. Nick Jr


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## Kovacjr (Jan 2, 2008)

Youn can get new springs from Mcmaster carr. Just need to take all the measurements from the old one.n As for the spring pressure you will have to try it out. maybe a 1-2lb spring??


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## Dr. J (Feb 29, 2008)

Hi Jason 
I've just ordered a digital caliper. When it comes I'll read off the dimensional info and order some springs. "Small Parts" is another company with lots of springs. 

If we want a lfiting pressure of 40 psi, and if the cross-sectional area of the end of the spring were 0.1 sq in, does that mean we want a spring rated at 4 lbs? In general terms: (Desired lifting pressure in PSI) x Surface area of end of spring = Lbs resistance of spring? This doesnt address how many Lbs of resistance over how many fractions of an inch of deflection on the spring, which is fixed by the valve itself. Fortuately, springs are cheap, and I can bench-test the valve using my air compressor, before running on the engine. 
Jim


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## Dr. J (Feb 29, 2008)

Thanks, Nick
Maybe this explains why i wasnt able to tighten my valve. I removed the spring and stretched it out a bit. Probably not the best way to achieve the end result. I'm definitely getting a range of springs and will play around to see what gives me 40 psi.
Jim


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## John Allman (Jan 2, 2008)

If we want a lfiting pressure of 40 psi, and if the cross-sectional area of the end of the spring were 0.1 sq in, does that mean we want a spring rated at 4 lbs? In general terms: (Desired lifting pressure in PSI) x Surface area of end of spring = Lbs resistance of spring? 

Alas, it does not. The actual area of the ball seating in the valve is approximately 1/3 the diameter. But even that is a rough estimate. On top of that, spring rates are linear only in approximation to deflection. And elastic only to a certain limit (which you will hopefully not reach!) 

The best thing to do is to forget all the math, and test it to see what you get with various springs. A spring with the same number of coils using similar diameter wire will result is a spring rate that is close enough.


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## Nick Jr (Jan 2, 2008)

Dr. J, glad I could shed some light on the mystery. Nick Jr


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## Dr. J (Feb 29, 2008)

John
Thanks. I figured that the compression curve for the spring would be non-linear. You're right: I need to get a bunch of springs that will physically go in there, then fiddle with the setting on the valve stem. Actually, ball point pen springs come close.
Jim


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## Nick Jr (Jan 2, 2008)

Don't forget, they should be corrosion resistant. Nick Jr


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## Shay Gear Head (Jan 3, 2008)

I have a friend that makes safety valves and I believe he uses Kadee coupler springs. Of course the internal diameter will determine the size. Isn't there adjustment by screwing in the top of the safety itself?


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## Taperpin (Jan 6, 2008)

Bruce, 
Kadee springs look like copper/phosphor bronxe but are just acopper flashed steel spring..in wet conditions they rust away! 

316L stainless steel is good for safety valve springs. Gordon.


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