Safety valves

I have a Martin Evans design B1. The boiler has passed it's initial hydraulic test and I am now carrying out static tests in preparation for it's full certificate. With a good fire and the blower on, the pressure gauge is showing about 120 pounds (working pressure 100psi) though the safety valves don't allow it to go any higher. Do I suspect the gauge or the safety valves, and if the latter, can anyone tell me how to pass more steam through the safety valves? As this is the first time I have got to this stage with a steam loco, any help would be appreciated.

Pete Falla

Sounds like you are confusing volume with pressure....but not sure! Is it blowing at 100 then rising to 120 or blowing at 120? If its pressure then adjust, if its volume then you need a bigger orifice/valve or 2 valves?

Are you using a decent accurate pressure gauge or the miniature one that you intend to fit to the boiler for use when running? The miniature ones can be very innaccurate and you should never set the safety valves using one of those.

As Fizzy asks, are the safeties starting to lift at 100psi and then rising to 120? You are allowed a 10% increase in the accumulation part of the steam test which means a maximum pressure of 110psi. If the pressure is increasing by 20% then the safeties are not adequate.

John

Peter,

Your pressure gauge should be checked against a calibrated gauge (preferably traceable to national standards). Ideally you should have your max pressure (ie 100 psi) in the middle third of the scale where the gauge is generally most accurate. If your pressure gauge has a full scale deflection of 120 psi, then I think you sailing pretty close to the wind, and would recommend having a gauge with 150 psi fsd.

If you don't mind spending a bit of dosh, then I would recommend the soft pop safety valves sold by Polly Engineering. I have two fitted on my loco set 5 psi apart, but only one valve ever blows off, so good they are at shifting steam that the pressure hardly rises above max. Also as the pressure drops, they shut off cleanly, so more banging the safeties with the shovel.

Regards Luigi

Good afternoon Pete,

As Luigi has said, soft pop valves are the way forward and if you are operating the locomotive at 100 psi then a 10% is the maximum permissible rise allowable. Both valves must be set to the same release pressure, there's no advantage in setting one valve higher than the other, you may as well leave it in your tool box! Please remember, these are the most important components on your locomotive and should also be carefully maintained.

It is useful to have a reasonably sized pressure gauge, 3" diameter or so, to say 200 -250 psi either certified or checked against your local clubs master for setting the valves when the locomotive is in steam, also it is a requirement to have a red line on the face of the dial (not as I've seen sometimes, on the glass) indicating the maximum permissible working pressure.

On a recent overhaul of a 5" gauge locomotive I took the safety valves apart to check the components and to my surprise one valve was partially scaled over, no doubt due to lack of use.

Polly Engineering can supply both a drawing and suitable springs should you wish to make up your own soft pop valves (usual disclaimer, only a satisfied user)

Please see below the partially blocked safety valve: -

Regards,

Bob Youldon

An interesting two part article wiritten by Bob Branson on prototypical and model safety valves may be found in Engineering in Minature Page 142 Vol 34 No.4 and Page 181 Vol 34 No 6.

Regards Luigi

Many thanks to you all. The valves started lifting at 100psi and the gauge stopped going up at 120. This is the model gauge so I will try again with a proper pressure gauge. In any case it would seem that Polly safety valves would be a better option so Jayne may be getting an order some time soon.

Pete