Page 139 - Introduction to Marine Engineering

Pumps and pumping systems 125

and held in by hangers or pipe clips in such a way as to minimise
vibration. Steam pipes or pipes in systems with considerable tempera-
ture variation may be supported on spring hangers which permit a
degree of movement. An alternative to spring hangers is the use of
expansion loops of piping or an expansion joint.

Valves
Valves are provided in a piping system to regulate or stop the liquid
flow. Various types exist with their associated particular function or
advantages.

Cock

A cock is used in small-bore pipework and is joined to adjacent pipework
by a compression coupling. A cock can restrict or close an internal
passage by moving a central plug, usually by an external lever. An
example of a straight-through cock is given in Figure 6.11.

Figure 6.11 Cock

Globe valve
A globe valve has a somewhat spherical body enclosing the valve seat and
valve disc (Figure 6.12). Flanges are provided at either side for
connecting to adjacent pipework, and internal passages guide the liquid
flow through the valve seat. Liquid flow is always arranged to come from
below the valve seat so that the upper chamber is not pressurised when
the valve is closed. A screw lift valve arrangement is shown where the
spindle is joined to the valve disc. A gland with appropriate packing
surrounds the spindle where it leaves the valve bonnet. The upper part
of the spindle is threaded and passes through a similarly threaded
bridge piece. A circular handwheel is used to turn the spindle and raise
or lower the valve disc. The valve disc and seat are a perfect match and
may be flat or, more commonly, mitred. The material for both is often
provided with a very hard stellite coating. Globe valves also exist in a
right-angled form where the inlet and exit flanges are at 90° to each
other.