Page 253 - Know and Understand Centrifugal Pumps

Know and Understand Centrifugal Pumps

SUCTION

BEARING

DOUBLE SUCTION

ATTHE
E OF THIS
PUMP
RESS
IN THE BEARINGS
AND WEAR RINGS
MOTOR
Fiaure 16-2

A pipe size increaser can be used in the discharge piping. This will
reduce the fluid velocity and friction losses. An isolation valve with a
low loss characteristic such as a gate valve should be placed after the
increaser and check valve.

Th rott I i ng

A centrifugal pump should never be operated continuously at or near
the shut off head. This normally happens when a tank or vessel is near
the maximum capacity and an operator or level sensor starts closing the
discharge valve while the pump is running. This is similar to activating
your car brakes while the gas pedal is to the metal. All this wasted
energy is transferred to the fluid being pumped. This type of operation
shortens the life of the pump and increases the downtime. This energy
is converted into heat and vibration raising the fluid temperature. Some
pump casings can dissipate the heat. Other casings contain heat
switches that will trip-out and 'shut off the pump.
An intensive radial load is created when operating near the shut-off
head and the shaft deflects at about 60" from the cut-water. This
concept is explained in Chapter 9 'Shaft Deflection'. The pump will be
noisy, will vibrate and maintenance on seals, bearings and shaft sleeves is
expected.
Pumps are usually over-designed. From the initial specification stages,
future needs are taken into consideration, maximum flow is overrated
and operating conditions are uncertain. Design engineers following a