Page 129 - Introduction to Marine Engineering

1!6 Pumps and pumping systems

the air is compressed in the vessel, and as the pressure falls the air
expands. The peak pressure energy is thus 'stored' in the air and
returned to the system when the pressure falls. Air vessels are not fitted
on reciprocating boiler feed pumps since they may introduce air into the
de-aerated feedwater.
A relief valve is always fitted between the pump suction and discharge
chambers to protect the pump should it be operated with a valve closed
in the discharge line.
Reciprocating displacement pumps are self priming, will accept high
suction lifts, produce the discharge pressure required by the system and
can handle large amounts of vapour or entrained gases. They are,
however, complicated in construction with a number of moving parts
requiring attention and maintenance.
When starting the pump the suction and discharge valves must be
opened. It is important that no valves in the discharge line are closed,
otherwise either the relief valve will lift or damage may occur to the
pump when it is started. The pump is self priming, but where possible to
reduce wear or the risk of seizure it should be flooded with liquid before
starting. An electrically driven pump needs only to be switched on, when
it will run erratically for a short period until liquid is drawn into the
pump. A steam driven pump will require the usual draining and
warming-through procedure before steam is gradually admitted.
Most of the moving parts in the pump will require examination during
overhaul. The pump piston, rings and cylinder liner must also be
thoroughly checked. Ridges will eventually develop at the limits of the
piston ring travel and these must be removed. The suction and
discharge valves must be refaced or ground in as required.
Two different rotary displacement pumps are shown in Figure 6.4,
The action in each case results in the trapping of a quantity of liquid (or
air) in a volume or space which becomes smaller at the discharge or
outlet side. It should be noted that the liquid does not pass between the
screw or gear teeth as they mesh but travels between the casing and the
teeth.
The starting procedure is similar to that for the reciprocating
displacement pump. Again a relief valve will be fitted between suction
and discharge chambers. The particular maintenance problem with this
type of pump is the shaft sealing where the gland and packing
arrangement must be appropriate for the material pumped. The
rotating vane type will suffer wear at a rate depending upon the liquid
pumped and its freedom from abrasive or corrosive substances. The
screw pump must be correcdy timed and if stripped for inspection care
should be taken to assemble the screws correctly.
A special type of rotary displacement pump has a particular
application in steering gear and is described in Chapter 12.