pressure releif valves

[macmodels]

My pressure releif valves have 3 or4 holes just above the base. When the boiler comes up to pressure, the steam sputters out of these holes. On u-tube I have seen videos of locos, when they blow off, they send a plume of steam streight into the air. I've done searches on this type to no success. What are these called, and where could I purchase two likethese
thank you Paul

[gearedloco]

They are called "Safety Valves." Numerous designs have been developed. One type is discussed at some length int "Building the New Shay" by Kozo, available from Village Press, the parent company of Live Steam and Outdoor Railroading. Other designs have been presented on LS&OR and it's previous incarnation, Live Steam.

Hope this helps!

-bill

[Harlock]

In the world of safety valves, there are some really obnoxious ones, and there's some nice quiet ones that come on gently and make a quiet stream of steam. The worst ones sound like flatulence because they are opening and closing rapidly, no damping.

unfortunately I have no information on what brands or kinds are better.

[Dan Rowe]

A pressure relief valve does not have any differential blow down. These are commonly used for hot water heaters and other applications that need over pressure emergency protection.

A safety valve has a differential blow down feature. This can be machined in as Kozo's design or a more complex adjustable type is available with an adjustment ring.

The way this works is the escaping steam acts on a larger surface after the valve opens so it adds lift to the valve. The pressure in the boiler has to drop below the pop off pressure before the valve will seat. This keeps the valve from chattering at the pop off pressure.

The other thing to consider is the relieving capacity of the valve should match the boiler. As most model safety valves do not come with relieving capacity information this is not as simple as stating the fact.

Here is a link to some safety valves but no relieving capacity data.
www.americanmodeleng.com/id6.html

Kozo covers this subject in his article on safety valves. It is also covered in the AMBSC boiler code part 1 and 2 for copper and steel boilers. (Australian Miniature Boiler Safety Committee)
Dan