Spring design for Safety Valves

[Gary LodgeParticipant @garylodge74937]

I wish to design a spring for a Safety Valve on a model boiler to blow off at 80psi. I have Tubal Cain's ( Tom Warshaw) book WPS No 19 Spring Design and Manufacture. Tom refers to spring design as force in lbf but makes no reference as to how to design a spring to blow off at a given pressure. Can anyone help out ( as one is force and the other pressure) so that I can use Tom's book to design the spring?

[Gary LodgeParticipant @garylodge74937]

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Gary ,

Need to know the force needed to hold the valve closed against steam pressure in order to design suitable spring .

Think about the ‘ piston ‘ area of the valve – in most cases its the area inside the seat diameter but can be more complex in some cases .

A secondary consideration is whether you want a sharp or soft opening action for valve .

If you want to then sketch or describe valve in question and I’ll work it all out for you .

Regards ,

MikeW

[Gary LodgeParticipant @garylodge74937]

Hi Mike,

Thanks for the prompt reply and your offer to design the spring for me.

The valves ( I intend to use two) will have a 3/16" ball and a 9/64" seat as per Martin Evans design in his book 'The Model Steam Locomotive' page 157. I intend to use a soft opening as a Pop action could result in one being burnt when a valve releases suddenly, but I am open to your sugestions.

I will need to know the: Free spring length.

Number of coils i.e. pitch.

Diameter of the mandrel allowing for spring-back.

Wire gauge in mm and the type of wire

.( I have 0.71, & 0.80mm in stainless, and 0.77 & 0.85mm in piano wire and 0.82, 0.91, & 01.0mm in steel wire.

Hope this helps and thanks Mike, Kind regards, Gary Lodge.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Gary ,

I don’t have that book so need a bit more information –

either: A scan of the valve drawing .

or : Type of seat – flat end or countersunk . Need angle if countersunk .
Diameter of push rod / pintle .
An idea of how much room is available for spring .

Regards ,

MikeW

[Gary LodgeParticipant @garylodge74937]

Hi Mike,

The seat is flat.

The push rod can be between 3/32" and 1/8" in diameter.

Spring length, I will make valve to suit say between 1/2" and 5/8" long and say 6 to 8 coils.

The diameter of the steam way i.e. the inside dia of the tube above the seat is 29/64" so there is plenty of space for the steam to exit, much more than the 1.1 times the seat area which is I beleive the minimum.

Kind regards, Gary.

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