Page 151 - Introduction to Marine Engineering

Auxiliaries 137

and without impurities. A special type of oil-free compressor may be
used to supply control air or it may be treated after delivery from an
ordinary air compressor. This treatment results in the air being filtered
and dried in order to remove virtually all traces of oil, moisture and any
atmospheric impurities.
Maintenance involves the usual checks and overhauls common to
reciprocating machinery, e.g. crankcase oil level, cooling water system,
operating temperatures and pressures, etc. The suction and delivery air
valves for each stage will present the most work in any maintenance
schedule. These valves are automatic, requiring a small pressure
differential to operate.
The constant rapid opening and closing action of the valves may
require the seats to be refaced. Overheating, use of incorrect lubricating
oil, or the presence of dirt may result in sticking or pitting of the
surfaces. The various buffer plates, spring plates, valve plate and seat
which make up a suction or delivery valve can be seen in Figure 7.3. The
valves should be stripped and all parts carefully cleaned and examined,
any worn parts replaced and the valve seat and plate lightly lapped
separately on a flat surface before reassembly to ensure a good seal.

Split pin
Castle nut (or alternative
Nut washer ^Mocking nut)
Valve seat

Lift washer
Valve plate

Damper plate

pring plates

Lift washer
uffer plate

Centre bolt

Figure 7.3 Automatic valve