TM 3_Pump Selection_10_14_2016 DRAFT Technical Memorandum
(Starleaf Pump Station Study)
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To: Michael Garza PE, Asst. Director of Public Works
Jamie Brierton, CIP Coordinator Jerry Davis, Water Operations Supervisor From: Bryant Caswell, PE (Texas PE #91721)
Dorian French, PE, RPLS, DWRE (Texas PE #40453)
Jennifer Roath, EIT (Texas EIT #48676)
Date: October 14, 2016
Subject: Starleaf Pump Station – Pump Type Selection
This technical memorandum reports on the evaluation of pump type options for horizontal split
case (HSC) and vertical turbine (VT) pumps and the evaluation of motor drives for variable
speed or constant speed. The pump type evaluation considered multiple qualitative factors and workshop discussion. The drive type analysis was based on hydraulic modeling of numerous candidate pumps and their output flow and efficiency curves
HYDRAULIC MODELING
Pump Sizing
For the hydraulic study in Task 2, three 5,000 gpm pumps were modeled at the design peak
demand of 14 MGD. The study for this task conducted a more detailed evaluation of demands
for multiple seasons with numerous candidate pumps, and for constant speed and variable frequency (speed) pump drives (VFD). Modelling of low winter demands revealed that 5,000 gpm pumps would have an inefficient operating range when used with VFD’s, indicating that the pumps were oversized for that condition. Therefore, the pump size was reduced and the number
of pumps increased from three to four. This creates a pumping system that when modelled
operates closer to peak efficiency in multiple demand scenarios, operates efficiently with VFD’s, and has slightly fewer starts per day. The apparent best fit for the SPS would four 3,500 gpm pumps.
Pump performance and system curves for a three 5,000 gpm pump station and for a four 3,500
gpm pump station are provided in the supplemental information section at the end of this
memorandum. Figure 5 illustrates how the 5,000 gpm pumps would operate too far to the left on the efficiency curve for winter demand.
Motor Drive Type Cost Comparison
In addition to the size and type of pump, the hydraulic model was used to perform a comparative estimation of power consumption for variable frequency drive (VFD) pumps versus constant speed pumps. The energy cost of low and high demand scenarios is summarized in Table 1
below.
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Table 1 – Energy Cost Summary
Low Demand
(3 MGD)
High Demand
(14 MGD)
Constant Speed Pumps $95.34 $629.80
Variable Frequency Drive $59.29 $658.15
The model uses assumed operating controls. For the VFD scenarios, the pumps throttle on and off as necessary to try and keep the elevated tanks at 80% full. In the constant speed scenarios the pumps turn on/off at 655/665, 653/663, 651/661, 650/660 for pumps 1-4, respectively. In addition to the controls, the energy coast was assumed at $0.05 per kWh. During high demand
energy costs for constant speed and variable speed pump drives were similar. However, during low demand the VFD pumps consume less energy. On average, savings are approximately $5,000 annually for a VFD.
There are three approaches to starting a motor: across the line (ATL), reduced voltage solid-state starter (RVSS), and VFD as mentioned above. For an ATL, any induction motor will typically
draw approximately six times rated full load amperage (FLA) for as long as several seconds when starting with full voltage. This inrush current can be a strain on the power supply system as well as incur significant costs as most electrical providers add charges based on peak current draw. An RVSS can be used to start the motor while limiting the inrush current to typically two to three times FLA, albeit over a longer period of time, thereby reducing the strain on the power
supply system. A VFD which can be used to speed up or slow down a pump to vary the pumping rate as the system requires, can actually limit the inrush current to the motor during start up.
In relative terms, the cost of a VFD is approximately twice that of a RVSS. The difference in price includes the drive itself, cooling requirements, and building square footage. Table 2 compares the price and size between two manufacturers. Prices can vary widely depending on
the features that are added to the components, but VFD is consistently more expensive.
Table 2 – Cost and Size Comparison of RVSS and VFD Units
Parameter Brand X Brand Y
RVSS VFD RVSS VFD
Cost $136,138 $254,314 $21,000 $43,000
Dimensions Unknown 101”x102”x24” 75”x37”x24” 90”x 60”x25”
While the height and depth will be similar, the overall width of a VFD is likewise approximately twice that of a RVSS. For example, on average an additional 3ft is required for the electrical
components. This equates to an additional 20 sqft of building space, or approximately $7,000
assuming $350 per sqft.
Due to the limited heat generated by the RVSS units, the RVSS pump system will require approximately two tons of cooling. Dual two-ton indoor DX fan-coil units could be suspended from the roof structure, thus not requiring floor space. It is recommended that, regardless of the
Technical Memorandum (Starleaf Pump Station Study)
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size of the units, two units be used for a relative redundant system. The use of two such units in this case would provide 100% redundancy.
Due to the heat generated by the VFDs, the VFD pump option will require approximately
106,800BTU/Hr cooling (10 ton installed). This could be provided by using two 5-ton units. These units would require floor space in the electrical room. To maintain the electric room space at 85°F on a hot, full-loaded day, both units will need to be in operation. However, if one unit were to fail, the remaining unit will have the capacity to keep the space below 104°F.
Therefore, for cooling VFDs, it is recommended that two split DX 5-ton units be used. If cooling
the RVSS system, it is recommended that dual two-ton split DX units be used. The relative costs for these two approaches are shown in Table 3.
Table 3 – Cost Difference for Cooling
Description Cooling for RVSS Units Cooling for VFD Units A.C. Units Split System DX Air-Cooled $17,000 $84,600
Ductwork, etc. $7,590 $50,180
Total (not including OHP, Contingencies, Bonding, etc.) $24,590 $134,780
When combining all the related costs, added HVAC costs, building square footage, and average drive unit price, a VFD will cost approximately $160,000 more per unit than a constant speed
pump with an RVSS. Assuming two of the four pumping units would be equipped with a VFD, it would be approximately $320,000 added cost to the facility. With the energy savings estimated at $5,000 annually, there is not a strong financial argument for installing VFD equipped pumps.
PUMP TYPE
In the workshop with the Water Operations Supervisor and other key city employees, multiple
criteria was discussed in regard to selecting the pump type. These included pump life, operator
safety, operator familiarity, maintenance access, flexibility & drive type, ease of expansion, and tank operation.
Pump Life
VT pumps can be more susceptible to vibration, which can increase the service intervals to replace bearings. While HSC can also have vibration issues, the pump and motor are above
ground where the operator may be alerted to problems sooner, and more easily able to access the
pump impeller itself. Because of its ease of access, maintenance cost for HSC pumps can be less.
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Operator Safety
This item is related to safety of the operator while servicing pumps, motors, and piping. Considerations include: crane access to pumps, large valves and fittings. With VT pumps in a facility without a bridge crane, operators may be required to operate rooftop access hatches
introducing a safety risk.
Operator Familiarity
The existing pump station at Village Parkway has HSC pumps of various sizes. The City’s Operators have working knowledge and experience with servicing and replacing this pump type. Additional training for VT pumps would be necessary, including new standard operating procedures.
Maintenance Access
HSC pumps are above ground with all components easily accessible. To repair or replace a
component, the section can be valved off and work can be done on the part. VT pumps are
submerged in a well, therefore, in order to repair or replace a part, the entire tube must be lifted out of the shaft for maintenance.
Flexibility & Drive Type
When considering VFD over constant speed pumps, VFD offers more flexibility in how the pump operations are managed. With motors of the size proposed, some type of reduced voltage
motor starter would be desirable to lessen the starting inrush current. In the past 10 years, VFD
costs have become similar to soft start costs. Furthermore, VFD pumps provide an additional level of operational flexibility. HSC are split coupled, while VT pumps are closed coupled. Closed coupled pumped required more effort to maintain than split coupled pumps.
Ease of Expansion & Building Footprint
Larger HSC pumps required a larger building footprint, leaving less space available for control and electrical room or other desirable spaces such as office space. VT pumps require less
footprint and fluctuates less with pump size.
Tank Operation
Because the new site location is relatively flat, the ground storage tank and the pump station will sit at similar elevations. This provides an advantage for vertical turbine pumps. Short of raising the floor of the tank, horizontal split case pumps will require a reduced operating range to ensure sufficient suction pressure.
To increase tank operating range for a HSC pump, attention should be given to the net positive
suction head required (NPSHr) of potential pumps relative to the head losses from the tank to the pumps.
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CONCLUSIONS AND RECOMMENDATIONS
The City has decided to move forward with four 3,500 gpm pumps based on the improved efficiency of operation which will result in lower energy costs for the life of the facility. The decision was also made to select HSC type pumps due to operator familiarity and ease of
construction and maintenance access over VT pumps. Concerns regarding NPSHr and tank operation can be addressed with proper pump selection and suction piping design. Having not identified a compelling operational advantage for VFDs, it is recommended that the pump motors be constant speed but equipped with RVSS units. This can be revisited during the design phase, when a detailed cost analysis is performed in coordination with pump manufacturers.
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SUPPLEMENTAL INFORMATION
Figure 1 – Constant Speed 5,000 gpm under 3 MGD Demand
Figure 2 – Constant Speed 5,000 gpm under 14 MGD Demand
Efficiency Curve
Pump Curve
Operating
Points
Efficiency Curve
Pump Curve
Operating
Points
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Figure 3 – Constant Speed 3,500 gpm under 3 MGD Demand
Figure 4 – Constant Speed 3,500 gpm under 14 MGD Demand
Efficiency Curve
Pump
Curve
Operating
Points
Efficiency Curve
Pump Curve
Operating Points
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Figure 5 – VFD 5,000 gpm under 3 MGD Demand
Figure 6 – VFD 5,000 gpm under 14 MGD Demand
Efficiency
Curves
Pump Curves
Operating
Points
Efficiency Curves
3 Pumps On
2 Pumps On
1 Pump On
Operating
Points
Technical Memorandum (Starleaf Pump Station Study)
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Figure 7 – VFD 3,500 gpm under 3 MGD Demand
Figure 8 – VFD 3,500 gpm under 14 MGD Demand
Efficiency Curves
Pump Curves
Operating Points
Efficiency Curves
4 Pumps On 3 Pumps On
2 Pumps On
Operating
Points