Pumps & Systems, March 2008
Few would dispute that variable frequency drives (VFDs) save energy, but the exact amount depends on the system. Hydraulically speaking, the main difference between a variable frequency (speed) drive and a discharge valve is that a VFD only changes a pump curve, while a valve only changes a system curve. A pump operates at the intersection between its H-Q curve and a system curve, and a change in either moves the operating point to a new intersection.
Consider a centrifugal pump operating at 4,000-gpm and producing 300-ft of head at 1800-rpm. According to its curve (see Figure 1 below), pump efficiency is 82 percent. This happens to be at the best efficiency point (BEP) point, but, in general, the same logic would apply to any operating point.
Suppose we want to reduce the flow by 50 percent, to 2,000-gpm. By closing the discharge valve, we would change a system curve, which would intersect the same pump curve at 370-ft of head, 61 percent efficiency. Such control process is simple, quick and easy. However, it has other issues, which we will review.
Hydraulically, the system in Figure 1 is referred to as predominantly friction (no static head). Such system is ideal for VFD application for flow control. The primary reason is at the heart of the affinity laws, which state that pump flow changes directly with the speed ratio, head as a square and power as cube. Therefore, the relationship between head and flow is H=aQ², a parabola. Since a system curve is also a parabola (H = bQ²), a given point on a pump curve scales down with speed ratio. At the same time, the point slides down along the system friction curve, and coefficients a=b for a given system setting.
Figure 2 below shows how pump flow is reduced by exactly 50 percent with RPM reduced in half, while pump head is reduced as a square of a speed ratio (900/1800)², i.e. 0.25 x 300 = 75-ft. Notice that efficiency does not change as the entire efficiency curve “slides” to the left and remains at a peak of 82 percent.
What would happen if a pump operates against constant (static, or mostly static) head, as in a lift station application? Consider Figure 3.
[[{"type":"media","view_mode":"media_original","fid":"464","attributes":{"alt":"Figure 3. Flow control by VFD, static system head example","class":"media-image","id":"2","style":"margin: 3px; float: left;","typeof":"foaf:Image"}}]]In Figure 3, 300-ft is a constant static head, with friction losses assumed to be negligible in comparison. A range of RPM variation is significantly reduced, so the entire pump H-Q curve does not drop below the contact H=300-ft. By applying affinity laws, we will find RPM=1650, a new pump curve (H-Q) intersects a constant 300-ft head line at desired 2,000-gpm and efficiency at that point is 62 percent. Table 1 compares these three cases.
| Throttling | VFD (friction) | VFD (static) | |
| RPM | 1800 | 900 | 1650 |
| Flow, gpm | 2000 | 2000 | 2000 |
| Head, feet | 370 | 75 | 300 |
| Eff, % | 61% | 82% | 62% |
| Power, hp | 306 | 46 | 244 |
| Energy cost, $/KW-hr | 0.10 | 0.10 | 0.10 |
| Energy used per year, $ | $200,193 | $30,187 | $159,701 |
| BEP Flow, gpm | 4000 | 2000 | 3667 |
| Operation %BEP | 50% | 100% | 55% |
Observations
- For friction-dominated systems (long pipes, flow transfer cases), a VFD saves a substantial amount of energy, and operates pumps reliably due to close proximity to BEP flow (100 percent in example shown)
- For static-dominated systems (injection against constant pressure, lifting against constant head), the energy savings are substantially less, and the pump surprisingly operates substantially off-BEP position, not significantly different from a valved flow control case
- For cases where both systems are present, an in-between scenario would result
VFD drives are expensive devices and require certain knowledge of controls, proper electrical and electronic maintenance and care. Flow control by VFD requires programming and training and is more complex than simple control by throttling a valve. These are some downsides to VFDs, but they offer the convenience of remote control (as do some valves), energy savings when operated in friction-dominated systems and more reliable pump operation. Operating closer to the BEP means less loads, better seal life, lower vibrations and improved suction recirculation conditions.
The user must have a detailed understanding of a system's hydraulics, unique H-Q curves and flow control requirements to make sure the VFD investment is justified. Organizations implementing VFDs must be prepared to deal with the added complexity but also appreciate their usefulness. The anticipated benefits of a VFD hinge on an organization's efforts to train users to operate and maintain such systems. For organizations leaving these issues to chance, the anticipated benefits can quickly become headaches.
As always, a parting quiz! At what RPM will a pump completely stop delivering flow to the system in the case shown in Figure 3? The first three people who answer correctly will receive a free pass to a Pump School session, per the schedule posted online.