Various descriptions of safety valves are in use on board steam vessels, some of which are of a very improper construction, being liable to be impeded in their action by design or by accident. For a safety valve to fulfil the purpose for which it is designed, it should be so arranged that the load upon it cannot from any cause exceed that which the boilers have been calculated to sustain, and whilst the engineer should have the means of raising the valve, so as to ascertain from time to time that it is not set fast, he should have no means of holding it down, or preventing its rising. In many vessels, however, the valves are placed above deck, and are loaded by weights, either placed directly on the spindle, or suspended from a steelyard lever acting upon the spindle; in either case the load may be increased and too often is to a most dangerous extent. In others, a more judicious arrangement prevails, the valves being so situated as to be inaccessible to the engineer, who therefore cannot increase the load beyond that which they were originally intended to carry.
The annexed cut represents a section of a safety valve of this description.
a is the valve-box, b the valve, upon which is placed a cylindrical weight c, which nearly fills the box, there being merely sufficient space round at the circumference to prevent rubbing, and sufficient space between the top of the weight and the cover to allow the valve to rise to the requisite height, and the cover is bolted down to the box. dis a bell-crank lever within the boiler, acting upon the lower end of the valve spindle; e is a rod connected to the lower arm of the lever, and passing through a stuffing-box in the front of the boiler. Upon pulling out the rod e, the valve is raised by the lever, and the steam escapes at the waste pipe f, and upon thrusting the rod in, the valve falls by its weight. In order to guide the valve, and at the same time to afford the means of turning it in its seat, the upper end of the spindle, which passes through the weight c, is made square, and enters a corresponding cavity in the key ff, which turns in a stuffing-box in the cover, the cavity being of sufficient depth to allow the valve to rise.