The boiler of a steam locomotive has a maximum operating pressure, which is 250 pounds per square inch (psi) in our case. It is very important that this pressure is not exceeded, as a boiler explosion would have disastrous effects. Therefore, we have safety valves on our boiler to release any excess steam pressure. The correct operation of the safety valves is so important for the safety of the boiler that the regulations state that we need at least two. We have three to ensure that excess steam can always be released quicker than it can be generated.

The three safety valves are positioned, side by side, on top of the boiler barrel, just behind the regulator dome. The middle one points directly upwards, and the other two, because of the curvature of the barrel, point slightly outwards. We set the middle one to lift first, at 250 psi, because a vertical plume of white water vapour looks better. We have the one on the fireman's side set to lift next, very slightly above 250 psi, so that the fireman can see it. The one on the driver's side is set to lift last, as that is more likely to dislodge debris from bridges and tunnels, obscurring the driver's view and damaging the loco.

Our safety valves are of the Ross "pop" type. They are designed to open suddenly at the set point, which allows the fireman to keep the boiler pressure quite close to 250 psi without wasting much steam through "feathering". They also close suddenly, making a "pop" sound - hence the name.

Our spare safety valve

The valve is held down on to its seat by a coil spring. This, in turn, is held down by the main casing of the safety valve, which is screwed down on to the body. The valve has a guide to keep it in position, and there is an outer lip, known as the Adams Lip.

The underneath of the valve, showing the Adams Lip (with the small holes in it)

The inside of the valve body, showing the valve seat

When the valve starts to lift, and steam escapes, the surface area subject to the pressurised steam is larger, because of the Adams Lip, increasing the force operating on the spring. This makes the transition from shut to feathering quite definite.

A spindle runs through the centre of the coil spring, and there is a large nut screwed on to the top of it.

The insides of the safety valve, showing the valve, the spring and the spindle

Under the nut is a separate two part cap, which has a number of holes in it.

The two part cap and the top nut

As the pressure from the feathering valve builds inside the chamber, it exerts an upward pressure on the cap. When the pressure has increased enough, the cap rises and lifts the nut slightly. This decreases the downward force on the valve, causing it to rise. As it rises, more steam escapes, the pressure in the chamber increases, and the downward force on the valve decreases even more. All this happens almost instantaneously, and this is why the safety valve lifts very suddenly.

When the pressure has dropped enough, the reverse happens, and the valve closes very quickly - making the "pop" sound.

The safety valves have to be set. There are two adjustments. The pressure at which the valve lifts is set by screwing or unscrewing the outer casing, which adjusts the compression of the spring. The number of holes in the top cap can be adjusted, which affects the threshold between the valve opening and it closing again.

The safety valves are roughly set beforehand, but the only way to set them accurately is with the engine in steam. Setting them usually requires four people. There are two on top of the firebox, setting the valves, and there are two on the footplate. One of those on the footplate is controlling the pressure, and the other is relaying the readings to those on the firebox. The people working on the safety valves have to have a lot of trust in the people on the footplate.

The boiler pressure is brought up up to see where each valve lifts, and it is then brought down again so that any adjustments can be made. This involves partially dismantling the valve. The top nut and the cap are removed first, using a screwdriver and pliers. The parts are too hot to touch, and you mustn't put your hand over the valve, just in case it lifts unexpectedly. We then use a specially designed large spanner to turn the outer casing, and adjust the compression of the spring.

The spanner for adjusting the safety valves

The top cap and nut are replaced, and the pressure is brought back up again. We do this for all three valves.

The centre valve has lifted, showing the top cap raised against the nut

Finally, the split pins are inserted, and the valves are sealed with lead seals.