Civil Engineering Reference
In-Depth Information
However, when the pump is placed in operation, the actual system curve forces the
pump to operate far to the right on the performance curve. The efficiency is poor, and
the pump is operating in an unstable range that may subject it to damage due to
cavitation and vibration. To achieve the desired flow rate of 3,500 gpm (220 L / s), a
discharge valve must be throttled to reduce flow. Excess head is unnecessarily ''burned
up,'' and an inefficient pumping system results (as measured by volume per unit of
energy). The required input to the pump at 3,500 gpm is 199 hp (149 kW).
The designer would have made a better choice by selecting a pump based on the
most probable conditions within the first 5 to 10 years after start-up of the facility or
installing a VFD. Figure 30-15 also shows how a smaller 20.5-in. (0.52-m) impeller
can provide excellent efficiency at a lower operating head for initial operation. If this
impeller is used, the required power input is only 169 hp (127 kW) at 3,500 gpm (220
L / s). An inexpensive change to a 22-in. (0.56-m) impeller at a future date will allow
relatively efficient pumping even after the distribution system ages and head loss in-
creases. In this case, a motor changeout is avoided by sizing the motor for future
conditions.
Design engineers are obligated to ensure that pumping stations will deliver desired
flow in worst-case situations. However, all too frequently actual operating system
curves are different. Thoughtful pump design calculations show best-case, worst-case,
and most probable system curves plotted together with the performance curve of the
prospective pump. When this step is taken, an efficient and stable pump can easily be
selected.
Pump Testing
Routine testing is an essential requirement for efficient pump operation. Pump test
data can be compared to manufacturers' performance sheets to identify methods of
improving efficiency.
Maintenance: Wear Rings, Impellers, and Packing
Proper maintenance is required to ensure efficient pump operation. Wear on impellers,
casings, and wear rings can result in considerable internal recirculation of flow, which
lowers efficiency. Installation or replacement of wear rings or adjustment of the im-
peller may enable a pump to regain its original efficiency. Improper adjustment to
worn or damaged packing can result in binding of the pump shaft and loss of efficiency.
Rather than continually tightening packing on a problem pump to stop leakage, op-
erators can consider replacing the packing on a routine basis or changing to a
mechanical seal.
LIGHTING
Lighting can account for 30 to 50 percent of a building's electrical energy consump-
tion.* Therefore, lighting energy conservation can be important even though lighting
* This section draws from information found in publications by the Illuminating Engineering Society of North
America (IESNA) (see References 5 and 6) and EPRI (see Reference 7).
