vapor pockets reach the surface of the impeller, the local high fluid pressure
will collapse them, causing noise, vibration, and possible structural damage
to the pump. Applications which require high NPSH, such as feed pumps, may
require booster pumps.
Discharge Conditions (Pump Selection). The characteristics of a
pumping system express the relationship between flow rate Q and head H. Head
is the sum of the geometric height, friction losses, valve losses and others.
Once the flow rate and head have been determined, the designer can refer to
the manufacturer's performance curve to select the proper curve.
Discharge Control. Discharge volume and pressure can be controlled
by one or more of the following methods:
Modulated discharge control and valve throttling,
Variable speed control,
Use modulated discharge when regulation is required and the
horsepower decreases as the discharge is restricted. When using speed
control, care should be taken that the head capacity curve not have a rising
capacity characteristic at any speed on the performance curve. This could
create a very unstable operating condition. Where process demand conditions
vary widely, multiple pumps sized to meet demand requirements running at
constant speed can be economically installed and operated.
Series and Parallel. Pumps connected in series will develop a head
equal to the sum of the heads developed by each pump at any given flow. Pumps
operating in series are referred to as pressure additive. Each pump must be
selected to operate satisfactorily at the system design flow. Pumps connected
in parallel will develop the equivalent head at each pump discharge and the
system flow is divided among the pumps. Pumps operating in parallel are said
to be flow additive.
Boiler Water Feed Pumps. Safety factor used in selection of boiler
water feed pumps shall be 25 percent in capacity (for pump wear, blowdown
requirements and excess boiler capacity available for short periods) and 15 to
25 percent for pressure. If pump discharges through an orifice, the
additional flow required to prevent flashing shall be added to the pump
capacity over the 25 percent safety factor.
(1) Pumps operating in parallel shall have similar head capacity
curves and equal head at shut-off. Individual curves shall indicate
continuously rising head characteristics from maximum to zero capacity. The
curves at different speeds and those showing the required "net positive
suction heads" (NPSH) are available from the pump manufacturers.