Page 149 - Fluid mechanics, heat transfer, and mass transfer

PUMPS 127
TABLE 5.3 Symptoms and Possible Causes for Centrifugal Pump Problems

Symptom Possible Causes
Pump does not deliver rated capacity. Excessive discharge head, not enough NPSH A , suction line not filled, air or vapor pocket in suction
line, plugged impeller or piping, worn or damaged impeller or wearing rings, inadequate foot
valve size, foot valve clogged with debris, internal leakage due to defective gaskets, wrong
direction of rotation, viscosity of liquid more than that for which pump is designed, inlet to
suction line not sufficiently submerged, and pump not up to rated speed.
Failure to deliver liquid. Wrong direction of rotation, pump not primed, available NPSH not sufficient, suction line not filled
with liquid, air or vapor pocket in suction line, inlet to suction pipe not sufficiently submerged,
pump not up to rated speed, and total head required greater than head which pump is capable of
delivering.
No discharge pressure. Pump improperly primed, not enough NPSH A , plugged impeller or piping, inlet to suction line not
sufficiently submerged, inadequate speed, suction line not filled, air ingress into suction line,
total head required is greater than head pump is capable of delivering, incorrect direction of
rotation, and closed discharge valve.
Pressure surge. Not enough NPSH A , air ingress into suction line, entrained air, plugged impeller.
Inadequate discharge pressure. Not enough velocity, air/gases in pumped liquid, too small impeller diameter, pump not up to rated
speed, worn or damaged impeller or wearing rings, incorrect rotation, discharge pressure more
than pressure for which pump is designed, viscosity of liquid more than that for which pump is
designed, and internal leakage due to defective gaskets.
Pump loses liquid after starting. Suction line not filled with liquid, air leaks in suction line or stuffing boxes, two-phase flow, slug
flow, inlet to suction line not sufficiently submerged, not enough NPSH A , liquid seal piping to
lantern ring plugged, and lantern ring not properly located in stuffing box.
Excessive power consumption. Head too small, excessive recirculation, high sp. gr. or high m liquid, misalignment, bent shaft,
bound shaft, packing too tight, rotating element dragging, and incorrect rotation.
Vibration and noise. Cavitation, excessive suction lift, starved suction, not enough NPSH A , air ingress, inlet to suction
line not sufficiently submerged, worn or loose bearings, bent shaft, misalignment, bound rotor,
improper location of control valve in discharge line, and foundation not rigid.
Stuffing boxes overheat. Packing too tight and/or not lubricated, wrong grade of packing, box insufficiently packed, and
insufficient cooling water to jackets.
Bearings overheat. Oil level too low, improper or poor grade oil, dirty bearings and oil, moisture in oil, oil cooler
clogged or scaled, oiling system failure, insufficient cooling water circulation, insufficient
cooling air, bearings too tight, oil seals too close fit on shaft, and misalignment.
Bearing wear rapidly. Misalignment, bent shaft, vibration, bearings improperly installed, dirt in bearings, moisture in oil,
excessive cooling of bearings, excessive thrust resulting from mechanical failure inside the
pump, and lack of lubrication.
Courtesy: GPSA Engineering Data Book, 12th ed.

. Can the suction size of a centrifugal pump be larger than . “Positive displacement pumps are usually self-primed
the pipeline? pumps.” True/False?
& The piping leading to a centrifugal pump must be at & True.
least one size larger than the inlet flange on the pump. . What is meant by Simplex, duplex, triplex, and mul-
Similarly, the discharge piping leading from the tiplex in pump terminology?
pump should be at least one size larger than the & Simplex refers to single-piston type of reciprocal
flange size on the discharge of the pump.
piston pump. A Duplex pump has two pistons; one
is on suction mode while the other is on delivery
5.1.4 Positive Displacement Pumps mode. Similarly, triplex and multiplex mean three
and multiple number of pistons, respectively, in
. “Positive displacement pumps generally operate at low different modes of operation.
inlet velocities.” True/False? . Illustrate valve action for a double-acting reciprocating
& True. In these cases, generally velocity head portion piston pump and discharge curves for different flow
is ignored. configurations.