AUXILIARY BOILER ROOM EQUIPMENT AND SYSTEMS
Scope. This section focuses on the major pieces of equipment and
systems relating to central heating plants and on criteria which help in
selection and operation of such equipment.
Types. The major types of pumps in the process industries are
centrifugal, axial, regenerative turbine, reciprocating, metering, and rotary.
These pumps can be grouped into two categories: dynamic and positive
displacement (refer to Table 22).
a) Dynamic pumps include centrifugal and axial types. These pumps
operate by developing a high liquid velocity which is converted to pressure in
a diffusing flow passage. These pumps operate at high speed, provide high
flow rates and generally require less maintenance than positive displacement
b) Positive displacement pumps include rotary displacement,
reciprocating plunger or reciprocating steam driven types. These pumps
operate by forcing a fixed volume of fluid through gear teeth, sliding vanes,
screws or other principals into the pumps discharge zone. They are self
priming and work with high viscosity fluids. All positive displacement pumps
have "slip" which displaces less fluid volume than the actual displacement of
the piston or rotating element. However, when positive displacement pumps are
in good condition, their efficiencies exceed those of small capacity
centrifugal pumps. Reciprocating type pumps are generally high maintenance
items due to lubrication and packing gland problems. They have been largely
replaced except for special applications. Flow rate changes for rotary
displacement or fixed plunger pumps can be accomplished by speed control or
modulated by-pass. Variable stroke pumps can have the stroke adjusted
automatically. Reciprocating steam driven pumps are easily controlled by
governors on the steam supply.
Selection. When selecting a pump the following items must be
considered: operating pressure, temperature, corrosiveness, abrasiveness,
specific gravity, motor type, suction pipe arrangement, size, location,
mounting, coupling, valve requirements, and overall efficiency. Table 22
summarizes operating parameters for various pumps and is provided to assist in
the selection of pumps.
Inlet Conditions. Net Positive Suction Head (NPSH) -- The net
positive suction head (NPSH) is the total suction head in ft of liquid
absolute determined at the suction nozzle and referred to datum less the vapor
pressure of the liquid in ft absolute. If the NPSH is less than required, the
fluid will cavitate; a good safety margin is 2 to 3 ft of fluid. Cavitation
is the vaporization of fluid in the casing or suction line. If the fluid
pressure is less than the vapor pressure, pockets of vapor will form. As