The disadvantage of this valve is that it admits of being tempered with, and the effect of a small addition to the weight is magnified considerably in its action on the value.
It consists of a valve resting over a gun metal seat. The valve seat is fixed on a mounting block, fitted over the boiler shell. One end of the level is hinged to a rod of the mounting block, while the other end carries a weight. A short strut is placed over the valve.
The required weight W at the end of the lever for maintaining the pressure P in the boiler is obtained by taking moments about the hinged point.
i.e., PaL1 = WL2
Where a = area of the valve exposed to steam,
L1 = distance of valve centre from the hinged point, and
L2 = distance of the centre of the weight to the hinged point.
The lever safety valve is used in stationary boilers only.
Description of Lever safety valve:
It consists of a valve resting over a gun metal seat. The valve seat is fixed on a mounting block, fitted over the boiler shell. One end of the level is hinged to a rod of the mounting block, while the other end carries a weight. A short strut is placed over the valve.Working of Lever safety valve:
The thrust of the lever with its weight is transmitted to the valve by the strut. When the steam pressure exceeds the safe limit, the upward thrust of steam lifts the valve from its seat and the lever with its weight. The excess steam escapes till the pressure falls back to the normal value. The valve then returns back to its original closed position.The required weight W at the end of the lever for maintaining the pressure P in the boiler is obtained by taking moments about the hinged point.
i.e., PaL1 = WL2
Where a = area of the valve exposed to steam,
L1 = distance of valve centre from the hinged point, and
L2 = distance of the centre of the weight to the hinged point.
The lever safety valve is used in stationary boilers only.
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