Page 123 - Introduction to Marine Engineering

110 Feed systems

feeder'. This is a multi-stage centrifugal pump driven by a constant-
speed electric motor. The number of stages is determined by the feed
quantity and discharge pressure.
Steam turbine-driven feed pumps are usual with high-pressure
watertube boiler installations. A typical turbo-feed pump is shown in
Figure 5.10. The two-stage horizontal centrifugal pump is driven by an
impulse turbine, the complete assembly being fitted into a common
casing. The turbine is supplied with steam directly from the boiler and
exhausts into a back-pressure line which can be used for feed heating.
The pump bearings are lubricated by filtered water which is tapped off
from the first-stage impeller. The feed discharge pressure is maintained
by a governor, and overspeed protection trips are also provided.

High-pressure feed heater
The high-pressure feed heater is a heat exchanger of the shell and tube
type which further heats the feedwater before entry to the boiler.
Further heat may be added to the feedwater without its becoming steam
since its pressure has now been raised by the feed pump.
The incoming feedwater circulates through U-tubes with the heating
steam passing over the outside of the tubes. Diaphragm plates serve to
support the tubes and direct the steam through the heater. A steam trap
ensures that all the heating steam is condensed before it leaves the
heater. Bled steam from the turbine will be used for heating.

Operation and maintenance
During operation the feed system must maintain a balance between feed
input and steam output, together with a normal water level in the boiler.
The control system used is described in Chapter 15.
The condenser sea water boxes are protected by sacrificial mild steel
plates which must be renewed regularly. The tube plates should be
examined at the same time to ensure no erosion has taken place as a
result of too high a circulating water speed. Any leaking tubes will cause
feedwater contamination, and where this is suspected the condenser
must be tested. The procedure is mentioned in Chapter 7.
Extraction pumps should be checked regularly to ensure that the
sealing arrangements are preventing air from entering the system. It is
usual with most types of glands to permit a slight leakage of water to
ensure lubrication of the shaft and the gland.
Air ejectors will operate inefficiently if the ejector nozzles are coated
or eroded. They should be inspected and cleaned or replaced regularly.
The vacuum retaining valve should be checked for air tightness and also
the ejector casing.