Acceptable liquid velocity at pump nozzles


What is the range of acceptable liquid velocity at the nozzles of centrifugal pump?
Should it be, for example, 10 feet per second (FPS) at the suction nozzle and 20
FPS at the discharge nozzle for optimum performance? Should the acceptable
liquid velocity be made part of pump specifications in the same manner that some
engineers would specify the acceptable operating range of a pump?

These are questions that are commonly asked and, understandably, pump
experts have different opinions on these subjects.

Except in few instances [ * ] there is no inherent value in specifying an acceptable
liquid velocity at the pump nozzles; rather, an optimum pump selection
determines the acceptable liquid velocity for a given rated conditions. Specifying
an acceptable liquid velocity can result in the exclusion of equipment that may
otherwise be superior in design, less expensive, and more readily available.

Suction nozzle

The liquid velocities at the nozzles are determined by the nozzle sizes that, in turn,
are not sized arbitrarily. The suction nozzle size is based on the largest impeller
eye area that the casing will take and this is dependent on the pump suction
specific speed. The suction nozzle is sized such as there is an increasing
acceleration of the liquid from the suction flange, through the casing suction bay
area, and into the eye of the impeller. The pump NPSHR is dependent on the
impeller eye area – the larger the eye area, the lower its NPSHR due to the slower
liquid velocity.
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Discharge nozzle

On the other hand, the discharge nozzle is dependent on the pump specific speed
(also sometimes referred to as discharge specific speed). It is sized such as
there is a deceleration of the liquid as it exits the volute throat area, through the
diffusion area, and into the discharge flange. The volute throat area determines
the discharge nozzle size – it is not based on some predetermined value of
acceptable liquid velocity.

The acceptable liquid velocity through a piping structure is a different issue from
that of the liquid velocity at the pump nozzles. The suggestion of 10 to 20 FPS
velocity typically applies to optimizing the diameter of a piping structure where a
delicate balance has to be reached between the power cost needed to overcome
the high friction loss in the system inherent with high liquid velocity, and the high
capital cost of a larger diameter piping structure needed to reduce the friction loss.

Based on current economic conditions, the 10 to 20 FPS range may no longer be
an optimum figure. There are also divergent cost considerations among different
piping structures that will likely yield a far different “optimum” velocity figure. For
example, a higher liquid velocity may be permissible in a power plant boiler piping
system, but may not be acceptable in a crude oil pipeline over considerable long
distance.

Due to these divergent considerations, it is not unusual to see pumps with
suction nozzle reducers or discharge nozzle increasers, as needed, to connect the
pump with its external piping structure.


R: 0111-ACLI
C: design, installation, operation
F: velocity_nozzles


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