Page 103 - Introduction to Marine Engineering

90 Boilers

whose pressure is set by the compression nut at the top. The spring
pressure, once set, is fixed and sealed by a Surveyor. When the steam
exceeds this pressure the valve is opened and the spring compressed.
The escaping steam is then led through a waste pipe up the funnel and
out to atmosphere. The compression of the spring by the initial valve
opening results in more pressure being necessary to compress the spring
and open the valve further. To some extent this is countered by a lip
arrangement on the valve lid which gives a greater area for the steam to
act on once the valve is open. A manually operated easing gear enables
the valve to be opened in an emergency. Various refinements to the
ordinary spring-loaded safety valve have been designed to give a higher
lift to the valve.
The improved high-lift safety valve has a modified arrangement
around the lower spring carrier, as shown in Figure 4.14. The lower

Valve stem
L carrier
Lower spring

toon ring
(cylinder)

Steam from boiler
Figure 4.14 Improved high-lift safety valve

spring carrier is arranged as a piston for the steam to act on its
underside. A loose ring around the piston acts as a containing cylinder
for the steam. Steam ports or access holes are provided in the guide
plate. Waste steam released as the valve opens acts on the piston
underside to give increased force against the spring, causing the valve to
open further. Once the overpressure has been relieved, the spring force
will quickly close the valve. The valve seats are usually shaped to trap
some steam to 'cushion' the closing of the valve.
A drain pipe is fitted on the outlet side of the safety valve to remove