Your questions answered: How to Size a Centrifugal Pump Driver

Kevin Anderson, senior technical training specialist at Grundfos Pumps Corp. in Olathe, Kan., tackled unanswered questions from the April 12, 2016, webcast on sizing centrifugal pump drivers.

04/25/2016


Kevin Anderson, senior technical training specialist at Grundfos Pumps Corp. Courtesy: GrundfosKevin Anderson, senior technical training specialist at Grundfos Pumps Corp. in Olathe, Kan., tackled unanswered questions from the April 12, 2016, webcast on "How to Size A Pump Driver." 

Q: Will a 200-hp motor still consume the horsepower at a 200-hp rate?

Kevin Anderson: The motor will use only the amount of horsepower required by the pump. The break horsepower (BHP) required (delivered) is determined by the formula mentioned in the webcast. It is the (gpm x TDH x S.G.) / (3,960 x pump efficiency). If the motor is rated at 200 hp, but the head, flow, and efficiency dictate a lower power usage, the motor will only delver the horsepower required. Note that the horsepower curve generally is lower at low flow, and higher at high flows. It does not deliver the same horsepower across the entire pump curve.

Q: Please define hydraulic power.

Anderson: Hydraulic horsepower is the same thing as water horsepower. It is the (gpm x TDH x S.G.) / 3,960. It is the first step of the BHP calculation, but does not include the pump efficiency part of the calculation.

Q: What software was used for the calculations?

Anderson: Use the Grundfos Express Suite software that was used to develop the webcast slides.

Q: Can PSAT provide the same set of answers to different scenarios presented?

Anderson: PSAT can compare various operating points and energy usage once a pump has been picked.

Q: Suggestion rather than question: For the software, it would be very helpful to specifying engineers to add a field that shows the relative cost differential for each motor selection bump (of course this would be rule-of-thumb approximate costs as market conditions change).

Anderson: Very good point. The PSAT software and many others will actually do those types of calculations based on pump efficiency, motor efficiency, operating hours, energy costs, etc.

Q: Is there a driver sizing webcast or other virtual classroom available that would address sizing for pumps with vertical line shafts?

Anderson: Grundfos will be hosting a webcast on May 18, 2016, titled "Vertical turbine pumps-wire-to-water," which will address many of these issues. Go to the Grundfos Training website to register.

Q: What about duty cycle for non-continuously operating pumps as in the case of a pump farm with multiple pumps on a manifold?

Anderson: Duty cycle, and/or load profile should always be considered when selecting pumps and motors. Parallel pump systems can often be an advantage for energy savings, as well as an opportunity to use smaller parallel pumps together. View our recorded webcast titled: "Efficient pump selection and control." It goes in to some of the benefits of selecting for parallel applications.

Q: I was curious about your comment regarding the pump imposing the load on the motor. if my pump has a 56.17 duty point, and I use a 100 hp versus a 75 hp motor, are you saying they'll both output the same 56.17 hp, and pump the system at the designed flow and head? Also, if I happen to have a spare 100-hp pump lying around, what would compel me to use a properly sized pump versus the 100 hp pump?

Anderson: Yes, they will both output only the required duty load based on head, flow, and pump efficiency. In this case, it was a design point of 56.17. However, your total wire-to-water efficiency may be different depending on the efficiency of the motor. The wire-to-water efficiency is determined by (gpm x TDH x S.G.) / (3,960 x pump efficiency x motor efficiency). It would depend on which of the two motors would be the most efficient at the operating point. The 100-hp motor at that duty point (roughly 50% of total load) or the 75-hp motor (roughly 66% of total load) would determine the overall efficiency.

Q: If a motor has a VFD, does that affect how a motor is selected? For example, is a non-overloading motor necessary if a VFD is used?

Anderson: I'm sorry for the confusion. The selection of a VFD would not have much effect on the size of motor selected. The selection of a VFD would just have to be added in a total wire-to-water calculation (gpm x TDH x S.G.) / (3,960 x pump efficiency x motor efficiency x VFD efficiency). However, the selection of a drive could have an effect on where the various motors would be operating on their prospective motor curves (full load or partial load).

Q: What is the difference between margin over specification and service factor?

Anderson: Margin over specification will cause the motor to be oversized by a percentage over the standard motor sizing specification. If the motor sizing specification was set at "based on duty point" and we had the "margin over specification" set at 15%, it would select a motor that is 15% over the duty point. If the motor sizing specification is set at "non-overloading," and we had the "margin over specification" set at 15%, it would select a motor that is 15% over the maximum load possible for the full range of the pump curve. Using "margin over specification" could cause oversizing of the motor. A service factor of 1.15, on the other hand, allows us to operate a motor at 15% above the rated horsepower for that motor. It could lead to the risk of undersizing the motor depending on the application.

Q: To size a generator, we often see pump motor sizes, but from your presentation, the actual running kW may be much smaller than the motor size. What would be a good guess for the percentage loaded for a common pump?

Anderson: To convert horsepower to kW we must do the following equation: (gpm x TDH x S.G.) / (3,960 x pump efficiency x motor efficiency x VFD efficiency) x 0.746. This will convert EHP or electrical horsepower to kW. I would recommend always sizing a generator to the maximum load of the motor. If it was just sized to the duty point and there was a system head change or a miscalculation, it could risk undersizing the generator.

Q: Excellent presentation. You have not mentioned how the pumping efficiency is changing with the many options of sizing the motor considering service factor, etc. What is the optimum efficiency to run the pump and what is the penalty if we oversize the horsepower for the motor?

Anderson: The pump efficiency changes as the head and flow change in the system. The motor does not have an effect on the pump efficiency. Sizing for efficiency can often be a challenge. It requires knowing the full load profile for the system. View our recorded webcast titled: "Efficient pump selection and control." The penalty for oversizing the motor in most cases is the additional upfront cost of the larger motor. Also, in some applications, drastically oversizing the motor as well as oversizing the pump could cause both to run at very inefficient points.

Q: Other than line shaft losses, what other factors affect the motor size on line shaft turbines?

Anderson: First, you must remember to calculate all TDH and friction losses up to the discharge head of the pump. There are also losses associated with the stuffing box assembly. Additionally, if a gear head is used in conjunction with, for instance, a diesel engine, then losses across that device and associated components must be accounted for. Grundfos will be having a webcast titled "Vertical turbine pumps-wire-to-water," which will address many of these issues on May 18, 2016. Go to the Grundfos Training website.

Q: Given the service factor allowance, are the calculations the same for multiple pump systems? (non-continuous duty, HVAC applications, etc.)

Anderson: The calculations for horsepower are the same whether doing single or parallel pumps. The major difference is that we would need to calculate where each pump is running at the various operating points to determine how much horsepower is being consumed by each pump and motor in the system. I would recommend that you view our recorded webcast titled: "Efficient pump selection and control." 

Q: The selection choices seem to depend on the credibility of the gpm/head to meet, as in measured (high) versus designed, and may be built differently (low). How does paralleling pumps effect this choice? (I.E.: 2 x 100% pump/motors, or even 3 x 50%)

Anderson: Great question. I would recommend that you view our recorded webcast titled: "Efficient pump selection and control."

Q: Please review pump power reserve. I'm having trouble finding anything about it in the literature. Is this a Grundfos measure?

Anderson: "Power reserve" tells us what percentage of the motor horsepower rating is left unused after the pump horsepower load has been imposed on it. Note that the values displayed are percentages. There are three values displayed, as described here, as the power reserve at rated load, maximum load, and at the sizing specification load. The values represent the unused portion of the selected motor's potential output at those points.

Q: How are the power reserve factors calculated?

Anderson: In the row below the power reserve, a 60 hp motor has been selected. At the rated duty point, our actual BHP load imposed by the pump is 6.82% less than the motor rating of 60 hp. However, as indicated by the red font, the load imposed by the pump at the maximum point on the curve (69.44 hp) is actually 13.60% over the motor's 60 hp rating. The reserve based on the sizing specification, which is the rated power at the duty point, is also 6.82% under the motor rating. What this is trying to tell you at a glance is whether or not there is any overloading of the motor at various points on the pump curve. Red fonts are a warning.

Q: Is the pump design software available at the Grundfos website?

Anderson: Grundfos Express Suite software was used to develop the webcast slides.

Q: Why is the driver sizing checklist not to be used on line shaft turbines?

Anderson: There are additional values that must be considered for line shaft turbines that are not a part of this list.



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