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Trimming or replacing impellers on oversized pumps

Trimming or replacing impellers on oversized pumps

April 20, 2009, Posted by Krisna Ditya Utama at 3:11 am

Overview
One of the best examples of wasted energy involves the traditional practice of oversizing centrifugal pumps by specifying the rated conditions and making pump selections with excessively conservative margins. As a result of this approach, pumps are forced to operate at reduced flows which are far from their best efficiency point (BEP). An unnecessarily large impeller will produce more flow than required, wasting energy as the flow rate typically must be regulated with bypass or throttle control. Oversizing a pump represents only 0.5 to 1 % gain in pump efficiency at the BEP whereas an energy saving of up to 15% can be realized by correcting the practice of including too much margin. While it is true that some margin should always be included to compensate for the wear of internal parts and increased clearances – which, in time, will reduce the effective pump capacity– much of this margin can be eliminated when justified by a high level of confidence in the hydraulic calculations and system requirements.

Suggested actions
Consider impeller trimming when any of
the following apply:

  • The head provided by an oversized, throttled pump exceeds process
    requirements.
  • System bypass valves are open, indicating excess flow rate.
  • The pump is operating far from its design point.
  • The operating head and (or) flow rate are greater than process requirements.

Impeller trimming refers to the process of machining the outside diameter of an impeller, which reduces the energy added to the system fluid. As the impeller diameter is reduced, additional clearance between the impeller and the fixed pump casing increases internalflow recirculation and is accompanied by a slight reduction in evciency. However, impeller trimming is often a practical alternative to replacing the entire pump assembly when the impeller diameter reduction is within the manufacturer’s recommended limits. In many cases, a lower flow impeller may be available that would accommodate the system requirements.

Oversized and throttled pumps that produce excess pressure are excellent candi-dates for impeller replacement or trimming, to save energy and reduce costs. In addition to energy savings, impeller trimming also reduces wear on system piping, valves, and other system components. For manufacturing standardization purposes, pump casings and shafts are designed to accommodate impellers in a range of sizes. Many pump manufacturers provide pump performance curves that indicate how various models will perform with different impeller diameters or trims.
diameter less than the minimum diameter shown on the manufacturer’s curve. Excessive trimming can result in a mismatched impeller and casing. It should therefore be limited to about 80% of a pump’s maximum impeller diameter unless other-wise stated. Net positive suction head requirements rates and increase at the higher end of the ?ow rate will normally be greater with a smaller impeller, but engineers shouldconsult with the pump manufacturer to ming the impeller. Manufacturers can often provide trim correction information based on historical test data.

How impeller trimming works

which in turn reduces the amount of energy imparted to the pumped fluid. As a result, the pump’s flow rate and pressure both decrease (see figure 1). A smaller or trimmed impeller can thus be used efficiently in applications in which the current impeller is producing exces-sive head. Pump and system curves can provide the efficiency or shaft power for a trimmed impeller.

If the manufacturer’s curves are not available, affinity rules can be used to estimate the variations in pumping performance with changes in the impeller diameter:
Q2 / Q1 = D2 / D1
H2 / H1 = (D2 / D1)2
kW2 / kW1 = (H2Q2) / (H1Q1) = (D2 / D1)3
where:

Q = pump flow rate, in cubic meters per hour (gpm)

H = head, in meters (feet) (H1 is head for the original impeller; H2, for a trimmed impeller)
kW = kilowatt (brake horsepower)
D = impeller diameter, in centimeters (inches)
In practice, impeller trimming is typically used to avoid throttling losses associated with control valves, and the system flow rate will not be affected.
Note that, in contrast to centrifugal pumps, the operating regions of mixed-flow and axial flow pumps are limited because of flow rate instabilities. facturer before changing the impeller able for a multistage centrifugal pump that is oversized for current operating conditions. When a pump serves a critically important process, it might not be possible to wait for the impeller to be trimmed. In that case, consider ordering another impeller and continuing operation until the new impeller can be installed.
Example
A double-suction centrifugal pump equipped with an impeller 35 cm (13.8 in) in diameter is throttled to provide aprocess cooling water flow rate of 650 m3 operates for 8 000 hours per year with a head of 50 m (164 ft) and pump efficiency (134.1 bhp). Pump and system curves indicate that a trimmed impeller can supply the 650 m3/h required flow rate at a head of 40 m (131.2 ft). From the affinity laws, the diameter of the trimmed impeller is approximately as follows:
(H2Q2) / (H1Q1) = (D2 / D1)3
Holding Q constant,
D2 = D1 x (H2 / H1)1/3
= 35 x (40 / 50)1/3
= 32.5 cm (12.8 in)
Assuming that the pump effciency remains unchanged, installing a 32.5 cm trimmed impeller reduces input power requirement to the following:
kW2 = (H2 x Q2) / (367 x )
= (32.5 x 650) / (367 x 0.8)
= 72 kW (96.6 bhp)
Estimated energy savings, assuming a 94% motor effciency, are as follows:
(kW1 – kW2) x 8 000 hours/year / 0.94 =
238 298 kWh/year
At an electricity cost of 5 cents per kWh, total cost savings are estimated to be  US$ 11 915 per year.

References
Improving Pumping System Performance: A Sourcebook for Industry, DOE and Hydraulic
Institute, 2006 Match Pumps to System Requirements, Hydraulic Institute and World
Pumps, December 2008. Optimizing Pumping Systems: A Guide to Improved Efficiency, Reliability, and Profitability, Hydraulic Institute, 2008.

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