For hoist doors the best appliance is an ordinary spring lock opened only by a key, the doors being provided with springs to close them. Various automatic doors, revolving shutters, and other devices have also been tried. A simple and effectual protection is a continuous open-work screen wound upon a roller at top and bottom of lift, and attached to the top and bottom of cage and rising and falling with it, so that the doors into lift are all covered at all times except the one at which the cage happens to stand.
Safety valves (see [Section 89]). Various automatic alarm signals are applied to boilers to warn against low water or excessive pressure.
Automatic valves and other devices are applied to pumping and steam engines to prevent running away. See [note] to [Section 41].
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Section 82.—STEAM TRAPS.
To collect and discharge condensed steam from pipes, &c.
[1499]. Trap operated by a modified form of ball cock, which rises as the box fills with condensed water and opens the discharge valve.
[1500]. Effects the same object by a floating basin. The condensed water enters the box outside the basin, fills it, and lifts the basin which closes the discharge outlet; when the box is full the water overflows into the basin and sinks it, thus opening the outlet valve.
[1501]. Trap operated by expansion of a bent spring which closes the valve, on the principle that live steam is hotter than the water condensed from it.