All too often an existing pump can become too big, too small, or unfit for its current
service because of changes in its operating conditions. This results in the
machine becoming a “bad actor pump” – to use a commonly used phrase in the
One solution to this “bad” situation is to consider replacing the equipment with a
new unit that is right-sized for the current service. Another solution is to consider a
hydraulic re-rate in lieu of buying a new unit.
Hydraulic re-rate refers to changing the rated operating conditions of a pump by
modifying its hydraulic parts, or components, to effect the change.
The rated conditions can be the machine’s operating capacity, differential head,
and speed; the liquid's specific gravity, viscosity, temperature, suction pressure,
and vapor pressure; or the site's net positive suction head available (NPSHA).
Hydraulic parts are parts that are in contact and in the main path of the pumped
liquid as it enters the suction nozzle and exits the discharge nozzle.
They include the impeller, volute (or diffuser), the suction case or suction bay of
the casing, the cover if it were part of the suction bay, and the suction and
discharge nozzles. The design drawings depicting these parts are aptly referred to
as hydraulic drawings.
In multistage pump, it may include the short crossover, intermediate crossover (if
the first stage impeller were of double suction design), the long crossover, and
the final discharge section. In a barrel or double case pump it may include a
separate suction case, and the barrel.
From hydraulic re-rate standpoint wear rings, shaft sleeves, and balance disks
are not normally considered hydraulic parts because their design has no bearing
in determining or setting the rated conditions although these parts may have to be
changed also during a re-rate for axial thrust balancing. The design drawings
depicting these parts are referred to as machining drawings – they are not
Advantages of hydraulic re-rate
- It results in higher hydraulic efficiency and in reduced energy consumption.
- It improves the reliability and prolongs the useful life of the equipment.
- The re-rate can be done faster and can be scheduled to minimize
disruption to plant operations.
- It minimizes risks and hazards, and improves safety to equipment and
- The decision process to do a re-rate is simpler and faster because it may
not require the review and approval of others in the corporate hierarchy.
- A re-rate typically does not require expensive and time-consuming
modifications to existing base and piping that may be required of a new
- Existing spare parts on stock may still be usable.
- Depending on location or legal jurisdiction, re-rated equipment may not be
subject to the same stringent environmental compliance, such as
emission limits, that typically apply to new equipment.
- Operations and maintenance personnel are already familiar with the
equipment – no additional training, or learning curve, is needed.
Disadvantages of hydraulic re-rate
- The modification required to do the re-rate has to be done within the
physical confines of the existing parts or components. The pump may not
achieve optimum performance or efficiency for the service compared to
new equipment. Compromises in the design may have to be made
because of existing physical or dimensional constraints.
- Its life cycle, although may be extended due to improved reliability, is
expectedly not as long as that of new equipment.
Changes or modifications
Some of the changes or modifications that are typically made to perform a
hydraulic re-rate are:
- Change the impeller with one of lower or higher flow design.
- Change the impeller with one of lower or higher differential head design.
- Change the impeller with one of different NPSHR characteristic due to a
change in site NPSHA.
- Modify the volute to decrease its throat area to hydraulically match a
decrease in rated flow rate.
- Modify the volute to increase its throat area to hydraulically match an
increase in rated flow rate.
- Increase (or reduce) the impeller diameter to increase (or reduce) its
differential head using the Affinity Laws to estimate the new diameter
- Increase (or reduce) the rotor speed to increase (or decrease) its
differential head using the Affinity Laws to estimate the new speed.
- De-stage, or up-stage, a multistage pump to change its differential head.
De-staging refers to the removal of one or more impellers; up-staging is
the reverse of de-staging.
Not all centrifugal pumps are good candidates for hydraulic re-rates. The costs of
replacing small ISO, ANSI, or ASME pumps with new units are probably less
expensive than the combined cost of engineering and replacement parts needed
to do a hydraulic re-rate. In general, most API centrifugal pumps, and bigger sizes
of ISO, ANSI, and ASME pumps are good candidates.
- What are the characteristics of a high flow impeller, relative to an existing
impeller, if one were to perform a high flow hydraulic re-rate? What about a
low flow impeller?
- What is the practical limit in flow rate by which the BEP of a given impeller
can be shifted to the right of its performance curve? What about to shifting
the BEP to the left?
- What precautions should be taken when chipping, or cutting, back the
volute lips to increase its volute throat area?
- Tests have shown that a modification to an impeller casting can be made
to increase its head rise to shut off. What is that modification?
- What is the main advantage of a double suction impeller with staggered
vanes over a typical double suction impeller with non-staggered vanes?
What is its main disadvantage?
- What precautions should be taken when V-notching an impeller? How
does V-notching the impeller outlet vanes help alleviate the vibration at
impeller vane pass frequency?
- What precautions should be taken when chipping or cutting back volute
The answers to these questions are excluded in this beta version and are
discussed in the full version of this article. Read more.
F: hydraulic rerates