Spring Loaded safety Valve

It is loaded with a spring instead of weights. Hence it is called spring loaded safety valve. It consists of a cast iron body having two branch pipes P1 and P2. Two separate valves are placed over the valve seatings, which are fixed to the top of the branch pipes. A lever is placed over the valves by means of two conical pivots.
The lever is attached to a spring at its middle. The spring pulls the lever in downward direction. The lower end of the spring is attached to the valve body by means of a shackle. Thus the
valves are held tight to their seats by the spring force.

Working of spring loaded safety valve:

When the steam pressure exceeds the normal working pressure , the valves rise up against the action of the spring and allow the steam to escape from the boiler till the pressure in the boiler reaches its working pressure.
The spring loaded safety valve is much lighter and compact compared with other safety valves. For locomotive or marine service, the safety valve should be such that it is unaffected by jerks and vibration likely to occur in such device. Hence spring loaded safety valve is preferred for locomotive and marine services, in addition to stationary boilers.
For locomotives and marine engines both the lever and dead weight types are unsuitable for obvious reasons,
and the valve must be spring loaded, as such valve is unaffected by vibration or deviation from the vertical.
• Disadvantage :
One disadvantage of this valve is that the load on the valve increases as the valve lifts, so that pressure required 
just to lift the valve is less than that required to open it fully.