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Model Steam Safety Valves

Relief Valves

Model steam safety valves, or relief valves as they are otherwise known, are needed on live steam boilers to provide a safe working pressure within the boiler. As the pressure builds inside the boiler through the added heat input, the valve (ball) lifts from its seating and allows surplus pressure to escape to the atmosphere in a safe manner, reseating once again as the excess pressure is reduced to where it should be.

Safety valve for steam engine pressure controlThese safety valves for live steam models are generally stainless steel balls sitting against a softer metal, usually brass, using either gravity or spring pressure to keep the seal intact.

The internal spring needs to hold the ball in place until the pressure in the boiler rises and pushes against the spring pressure and lifts the ball.

For the gravity type, there tends to be a small weight sitting on the end of an arm to force a rod down on the top of the ball, with the boiler trying to lift the ball from below. Obviously this type needs to be sat in an upright position to work properly and the pressure can be adjusted by changing the weight or moving the distance of the weight, either inwards for less or outwards for more, from the push rod sitting on top of the ball.

In both cases a stainless steel rod with a flange on it can replace the ball much like a motor car tyre valve.

In the case of adjusting the spring type of relief valve, usually there is a screw adjustment at the top of the safety valve allowing the spring to be either compressed to exert more pressure on the ball and hence inside the boiler, or released to allow the boiler pressure to be maintained at a lower level.

If you find you have a low pressure boiler and the relief valve does not blow off at the wanted pressure it is possible to install a shorter spring to achieve the desired pressure or alternatively by boring the steam passage out a little and using a bigger ball, that way allowing the ball to be lifted more readily by the lesser steam pressure.

Do not be tempted to install any old spring of the right dimension as these can rust or become softened and relaxed owing to the steam and changeable heating of the boiler, that way making the relief valve blow off at very little pressure or possibly not allowing any pressure to be held within the boiler owing to there being no seal between the ball and its seating.

To make a steam relief valve steam tight it is simply a matter of removing the cap or screw adjustment, take out the springRelief valve and tap the stainless ball with a softer metal to remake the seat. After re-assembly all that is needed is to reset the pressure rating for the safety valve by adjusting the pressure on the spring.

However, do not go in for one that has a much higher pressure than needed, as the springs inside are much tougher and adjustment for lower pressures will not be achievable as you will need weaker springs installed to achieve anything like what you want.

In the same way a weaker spring can be replaced to produce a greater pressure relief where necessary.

Moving onto actual usage, it is wisest to test the working ability of your safety valve at each steam up, making sure it does not allow the pressure to rise more than 10% above what it is meant to do, and also that it reseals to allow the pressure to build up to its correct level. This is to check that the valve is not stuck and could possibly do some damage to yourself or the surroundings.

This 10% margin is what is demanded in the present day steam regulations, as with the PYRTE traction engine with a working pressure of 50 lbs  per square inch, a pressure of 55 lbs is required, but it must go no higher than that.

What normally happens nowadays is that two safety valves are installed so that, as in the case of PYRTE, one lifts at 50 and the second at 55 lbs (or slightly less - the 10%) to provide a safety margin should the first one fail to lift.

Safe steaming.


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