In service application equipped with multistage pump the required differential
pressure may decrease (or increase) in the future for some reasons: the system
pressure requirements may change, the pumped liquid properties may change,
there may be a booster pump added to boost the NPSHA to the multistage pump
thus reducing its share of the total head, etc. In such event, the pump would
waste a lot of energy if the differential pressure were not reduced accordingly.

(In this article the term pressure is used, instead of head, and the term
multistage includes two-stage pumps, for simplicity.)

Differential pressure refers to the pressure developed by the pump, and is the
difference between its discharge pressure and suction pressure. In multistage
pump its differential pressure can be reduced in different ways: by reducing the
impeller diameter, by reducing its speed if it were driven by variable-speed
driver or through a gearbox, by throttling the excess head with a discharge
control valve, or by destaging the pump.

Destaging is a method of reducing the differential pressure of multistage pump
by deactivating one, or more, of its stages. Stage deactivation is done by taking
out an impeller and replacing it with destaging parts.  (Upstaging is the reversal
of destaging whereby the destaging parts on a destaged pump are removed
and replaced by an impeller on the deactivated stage.)

Although destaging is usually done in re-rating existing pump, it may also be
done on new equipment where a pump is originally built destaged to meet a
current pressure requirement with the intent of upstaging the pump to meet a
requirement for higher pressure in the future. The driver size is selected to meet
the higher horsepower requirement in the upstaged configuration.

In many hydraulic re-rates involving a reduction in differential pressure
destaging may be preferable to trimming the impellers for many reasons:
efficiency loss due to increased volute “B-gap” is avoided; potential increase in
discharge flow recirculation is avoided; the location of the rated flow relative to
the pump BEP on the performance curve is maintained; the destaged impeller
becomes available for future use if the pump were to be upstaged.

Some disadvantages of destaging over trimming the impellers are: additional
destaging pieces are required, there may be a need to re-balance the axial
thrust load of the pump, it may result in rotor instability that needs to be
addressed – this is a potential issue with two-stage pumps, in multistage pumps
where several stages have to be deactivated, or in pumps with long slender
shaft.

On the other hand, trimming the impellers may offer some advantages over
destaging in some situations: it may help reduce the rotor vibration if the pump
were initially operating with maximum impeller diameter with tight volute B-gap, it
may actually result in improved efficiency at the rated flow, if the rated flow were
initially located further to the left of BEP, it retains the distribution of radial loads
on the shaft, etc.

Destaging a vertical multistage pump is relatively easier than destaging a
horizontal pump. In a vertical pump destaging is usually done by removing a
complete stage bowl assembly and replacing it with a short adaptor column.
There is no need for special destaging tubes to bypass the crossover volute or
diffuser on a deactivated bowl because the complete bowl assembly is removed.
Rotor instability is not much of a concern because the rotor weight does not
contribute to the shaft static deflection. Re-balancing the pump’s axial thrust
load is also relatively easier.

Destaging a horizontal multistage pump can be more complicated depending on
the arrangement of the different stages, more so if the first stage impeller were
of double suction design. In some multistage pumps the impellers are stacked in
series in one direction, in some the impellers are grouped in two – each group in
opposed direction, whereas in others the impellers are paired in back-to-back
arrangement. The manner of impeller arrangement, or pairing, will require a
unique method of destaging the pump. The same is true with two-stage pumps.

In a full version of this article the author discusses the following important
additional information:

  • General guidelines on the proper method of destaging multistage pumps.

  • Destaging parts and thrust balancing pieces – proper selection and
    design.

  • Pitfalls and errors to avoid when destaging and their potential adverse
    effects on pump performance as shown in many actual performance tests
    of destaged pumps.

  • An evaluation of an actual destaging done on multistage horizontal pumps
    where the designer committed three critical design errors that will have
    adverse effect on pump performance.

  • Answers to some commonly asked questions on destaging.
CENTRIFUGALPUMP.COM
Destaging of multistage pumps
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