Steam Safety Valve
Safety valve prevents the rise in pressure to valves above the design pressure. If the pressure inside the boiler exceeds the design limit, the safety valve opens and discharges steam to atmosphere till the pressure reaches normal working pressure.
These are available in three designs :
(a) Dead weight,
(b) Lever loaded, and
(c) Spring loaded.
Dead Weight Safety Valve
- Figure1 shows as a line diagram the arrangement present in a dead weight safety valve.
- The dead weight is calculated based on the working pressure (p) and the area of cross-section of the valve (a) on which the pressure acts.
- The dead weight (W) and the carrier weight (w) together exert a downward force and the steam pressure and valve area produce is the upward force.
- If the steam pressure exceeds a limiting value then valve lifts of its seat and allows steam from boiler to be vented to atmosphere till the limiting value of ‘p’ is reached.
- Dead weight may be reduced for a given pressure by reducing the area ‘a’.
- But this area should be large enough to release the steam quickly and bring back the pressure to normal level in the event of rapid build-up of excess pressure.
Figure 1: Dead Weight Safety Valve
Lever Loaded Safety Valve
Figure2 shows the principle of operation of this type of valve. In this valve
W (M N) = p . a . M
where W is the dead weight, a is the area of the valve seat, p is the steam pressure, M is the distance of valve centre from fulcrum and N is the distance of dead weight from the valve centre. Compared to dead weight safety valve in this valve by increasing lever length (M N) it is possible for us to use lower dead weights for a given operating pressure.
Figure 2 : Lever Loaded Safety Valve
Spring Loaded Safety Valve Steam Generators
- This valve is shown as a line diagram in Figure 3.
- It has two steam passages and two valves.
- These valves close the passages under the action of a central spring.
- The limiting pressure can be set by varying the tension in the spring.
- The extended lever is operated occasionally to check the functioning of the valves.
- ll high pressureboilers have spring loaded safety valves.
- Slight increasing in mounting does not affect the performance.
Figure 3 : Spring Loaded Safety Valve