Page 141 - Introduction to Marine Engineering

Pumps and pumping systems 127

can not be manually closed (Figure 6.13). A free lifting valve may be
used or a hinged flap.

Gate valve
A gate valve should be fully open or closed; it is not suitable for flow
control. When open it provides a clear full-bore internal passage for the
liquid since the valve or gate is raised clear (Figure 6.14). The spindle is

^Handwheel
Spindle

Valve cover

Spindle nut
Valve seat
Wedge face
Wedge gate
Valve body

Figure 6.14 Gate valve

threaded over its lower portion and when turned causes the gate to raise
or lower. The gate may be parallel or wedge-shaped in section fitting
against a matching seat. Larger valves have replaceable seat rings and
gate facings.

Relief valves
Excess pressure is avoided in pipe systems by the use of relief valves. The
valve disc is held closed by a spring arrangement on the stem (Figure
6.15). The spring compression can be adjusted to enable the valve to
open at the appropriate pressure. Boiler safety valves are a special case
of relief valve and are described in Chapter 4.