WIK USER INSTRUCTIONS ENGLISH BO2662 5/09
Page 38 of 76
pressure head in the diffuser passages. Leaving the
diffuser, the liquid enters the intermediate cover
[1460] where it is directed into the inlet of the next
impeller. The flow follows this same sequence from
the first to the last stage. The liquid from the
discharge diffuser enters the pump case, and then
leaves through the discharge nozzle in the case.
5.10.2 Balance
In a multi-stage, high-pressure pump, radial and
axial balance is a requirement for satisfactory
operation. Unbalanced radial forces cause
excessive vibration, unnecessary wear, and noise.
Unbalanced axial forces must be controlled to
prevent damage to bearings and to the internal parts
of the pump.
5.10.2.1
Radial balance
Two separate radial forces must be balanced, one
dynamic, the other hydraulic. Dynamic unbalance is
caused by variations in the distribution of weight in
castings for impellers. The impellers in your pump
have been balanced at the factory in special
dynamic balance machines. Radial hydraulic
unbalance is caused by unequal pressure in the
liquid around the periphery of the impellers. This will
create a radial thrust, and if not balanced, will cause
serious shaft deflection. Radial hydraulic balance is
obtained with diffusers fitted with equally spaced
multiple vanes. Such diffusers will equalize the
pressure at all points about the periphery of the
impellers, thereby eliminating radial thrust and in
turn shaft deflection.
5.10.2.2
Axial balance
In centrifugal pumps where the impellers are in a
line, the differential pressure across each impeller
will tend to generate a thrust towards the suction
end of the pump. To compensate for this build up of
thrust, your pump incorporates, at the outboard end
of the shaft and inside the head, a balance drum that
rotates within a balance drum bushing. The
clearance between the drum and the bushing is
close, so the drum acts as a piston with its inner face
acted on by discharge pressure and with its outer
face open to suction pressure. Thus the resultant
force toward the outboard end of the pump counters
the thrust acting toward the inboard end.
5.10.2.3
Balance drum piping
The balance drum piping affords an unrestricted
passage between the area immediately outboard of
the balance drum and the suction end of the pump,
serving to reduce pressure on the outboard shaft
seal as well as the balance drum. It also serves to
transfer leakage past the drum to the suction system.
5.10.3 Minimum flow
Another phenomenon of the multi-stage pump is for
the temperature of the liquid to rise progressively as
it is pushed through the pump. If flow is reduced
below the minimum required, then overheating and
flashing may occur, causing severe damage to the
pump. The plant's minimum flow system should
ensure that the flow is kept at or above minimum,
bypassing to suction the flow necessary to prevent
overheating and flashing.
5.10.4 Minimum flow orifice
The minimum flow orifice is furnished with the pump
for integration in the plant's piping. Design of the
orifice is based on the differential of the pump minus
the line loss in the plant's piping. Thus, the orifice is
peculiar to each pump as well as the system in
which it functions.
5.10.5 Net Positive Suction Head
Centrifugal pumps also require Net Positive Suction
Head (NPSH) for proper operation. This is reflected
in the pump's characteristic curve. When changing
plant conditions that affect NPSH, inattention to
these curves is likely to cause damage to the pump;
in fact, this is a major cause of failure. For example:
Throttling inlet pressure back to a point on a curve
that is below the required NPSH will cause cavitation
in the inlet eye resulting in immediate and major
damage to the internal assembly of the pump.
5.10.6 Shaft seal
In your pump, a mechanical seal provides shaft
sealing. A mechanical seal is a precision device in
which the face of a mating ring on the shaft sleeve
rotates against the face of a primary ring on the seal
housing. The sealing faces are highly polished and
run with a very thin film of cooling liquid. The seal is
loaded by spring, hydraulic forces pushing against
the mating ring. A seal flush system is employed to
keep the mechanical seal at a nominal temperature.
5.10.7 Cooling water system
A plant-equipped cooling water system circulates
water through the oil cooler, seal flush, stuffing box,
and/or pump pedestal, helping to maintain the
temperature of these components at the acceptable
level for satisfactory operation.
5.10.8 Lube oil system
The lube oil system is an integral part of your pump.
A pump mounted on the pump shaft or on the
baseplate pumps lube oil from the baseplate-
mounted reservoir, through supply lines and the oil
cooler, to the pump bearings; the hot oil then drains
back into the reservoir. The system may also include
