Your questions answered: Right-sizing pumps with variable speed drives
In November, CFE Media & Technology hosted a webinar on right-sizing pumps with variable speed drives (VSDs). This webinar discussed how system curves interact with the pump curve to indicate the efficacy of applying variable speed drives (VSDs) to centrifugal pumps.
The webinar invoked numerous questions from the audience. Here are some of the most interesting, which were answered by Eugene Vogel, a pump and vibration specialist at EASA, which is a CFE Media content partner.
1. Can you explain the basics of pump sizing? How does pump sizing differ between a variable speed drive and a constant speed drive? What electrical aspects should mechanical engineers take into consideration when selecting pumps with variable frequency drives (VFDs)?
Sizing a a pump is one of many considerations in pump selection. Focusing just on the sizing aspect, the two primary considerations are reliability and cost. Choosing a pump sized to operate close to its best efficiency point (BEP) addresses both concerns. When an application requires variable flow or variable head, both variable speed and multiple pumps should be considered. In some cases, a combination of both may produce the most efficient solution. In any case, the total life cycle cost of the pumps should be considered.
The two primary mechanical concerns when applying variable frequency drives (VFD) are the likelihood of encountering resonant vibration, and the potential for bearing damage from shaft currents.
2. What is the span of the VFD’s operating speeds? 60 to 100%? What is the lowest speed a pump can keep pumping?
The operating frequency range (speed range) for a VFD depends on several factors. First, as frequency increases, the voltage must increase. The available line voltage is a limiting factor and the maximum motor Voltage is also a limiting factor. The motor may also have speed limitations, (a topic in itself). Aside from those limitations, VFD can output any frequency within the manufacturer design. Commonly that’s up to 120 Hz, but that varies by manufacturer.
The minimum pump speed to maintain flow depends on the system curve, especially the amount of static head. Obviously, that will vary by specific application.
3. In using drives on a pump, what efficiency curve does the pump follow as speed is increased or reduced?
This is a fascinating question. A specific rotodynamic pump has an efficiency curve for any specific speed; the curve defines efficiency at various flow rates. But the efficiency curve changes with the speed. And for any specific pump at any specific speed, the system curve determines the flow rate. So, for any pump, efficiency is a function of rotating speed and the system curve. So, an efficiency curve exists for a pump at one specific speed, or for a pump and system at various speeds, but not for pump alone at various speeds.
4. How does cutting the impeller compare? Energy savings would be same correct?
For a specific system, at a specific target flow rate, trimming the impeller OD or reducing the speed to achieve that target flow rate would have very similar power requirements.
5. VFD costs more. What’s the target time for return on investment (ROI)?
Many stakeholders expect a return on investment (ROI) in 18 months. Large projects like power generation and pipelines may approve an ROI in several years.
6. What software is shown to model pump curves?
The webinar demonstrated H2 Optimize, provided by Engineered Software, Inc. and configured for Fairbanks Morse and for Aurora pumps.
7. Do the energy savings typically compensate for the VFD price?
When a system requires flow control (operation at various flow rates), variable speed options can usually be justified, (VFD is only one of several variable speed options). When a system has a high static head component, it is much more difficult to justify variable speed options.
8. When retrofitting an existing pump motor with a VFD, is it important to use a grounding ring?
Grounding rings or brushes are one approach to limiting shaft currents, which are an issue with VFD drives. The subject of shaft currents, however, is not as simple as “put a grounding ring on it”.
9. Can you speak to the fact that, as you slow the pump, you also slow the motor, which is usually cooled by a fan fixed to the motor shaft.
You said it perfectly. Many VFD application require auxiliary cooling for the motor when operated at lower speeds. For some rotor-dynamic pumps, the power required is reduced along with the speed, so some application can operate at lower speed without auxiliary cooling. Each individual application must evaluate motor cooling requirements.
10. Can I add a VFD to any existing motor? Will motors overheat at low speed?
Many VFD application require auxiliary cooling for the motor when operated at lower speeds. For some rotor-dynamic pumps, the power required is reduced along with the speed, so some application can operate at lower speed without auxiliary cooling. Each individual application must evaluate motor cooling requirements.
11. Can you please go over the financial calculations once again?
Determine the power reduction; convert to kilowatts, multiply the hours of operation per period, multiply by the utility rate in kilowatt hours. The result is the savings per period. Periods may be weeks, months or years.