Various studies have shown that many process pumps are oversized. Optimizing oversized pumping systems can save large amounts of energy, as well as make an impact on pump reliability by reducing pump vibration and extending bearing, seal and impeller life. Oversized pumps will operate to the left of the best efficiency point (BEP) on the pump curve, creating significant internal recirculation, low-flow cavitation, and high shaft loads.
There are several potential ways to address an oversized pump and system condition, including the following:
- Install a different pump.
- Modify the control strategy to include a flow recirculation line.
- Trim the impeller.
- Install a variable frequency drive (VFD).
- Reduce pump speed.
1. Install a different pump
Installing a different pump will typically be the most expensive option and may only be feasible if considerable energy savings or reliability savings are available from the current installed condition. Many pump upgrades will require a new baseplate and piping modifications on the suction and discharge. Given this fact in tandem with the upfront cost of the pump itself, this would likely be a last resort if none of the other possible solutions presented hereafter are deemed to be viable. If a different pump is chosen, then other reliability improvements should also be evaluated with the pump setup, such as fixing any piping issues, baseplate issues, or pump upgrades that may not have been available when the original pump was installed.
2. Add a flow recirculation line
Adding a flow recirculation line on the pump will improve the pump efficiency as the pump operation moves to the right on the curve, but the total energy consumption will increase with a flow bypass control strategy. This can be a viable option if the pump is small with low-flow conditions without significant consequences for wasted energy. The new operating point for adding a flow recirculation line can be seen in Figure 1.
3. Trim impeller
One of the most common actions for an oversized pump is to trim the impeller. Trimming an impeller reduces energy consumption due to excessive head from an oversized pump; however it does little for over capacity of the pump sizing, as shown in Figure 1. Trimming an impeller can, in some cases, lead to lower pump efficiency.
While trimming the impeller has drastic effects on the pump, determining how much to trim the impeller is typically dictated by the system and/or control valve operation. The impeller should be trimmed to get the control valve in the system to operate in a reliable position, which is typically 40 to 60 percent open. If the control valve differential pressure (?P) is in a more closed position (e.g., 25 percent open), then the impeller should be trimmed to obtain a more reliable control valve position (55 percent open). The pressure drop can be estimated by the control valve equation by getting the Cv for each valve position.
Figure 1 shows a pump operating at 1,000 GPM at 65 ft. with a control valve typically running 25 percent open. The calculated pressure drop difference to get the control valve at 60 percent open was 15 ft. The impeller was trimmed from .25×12.5" to .25×10.62" diameter to accomplish a 15 ft. head reduction.
Figure 1. Impeller reduction and recirculation line operating points
4. Install variable frequency drive
Variable frequency drive (VFD) applications on pump systems can be a good solution to remove wasted energy by removing a control valve. By removing waste head from the control valve pressure drop, the pump can slow down. This reduces the pump brake horsepower (BHP) needed due to the lower head requirement.
Most of the time when a VFD is installed, the original motor is left in place and motor efficiency losses are small and neglected. During a conversion to a VFD, the original motor may go from 90 percent loaded with 0.87 power factor and 94.5 percent efficiency to 50 percent loaded with energy reduction, which may lower the motor power factor to 0.6 and lower the efficiency to 93 percent. On the other hand, the pump efficiency could increase 5 to 10 percent, depending on the original operating point.
VFDs are only viable for high-friction head systems, not high-static head systems. A VFD is typically only justified when 50 percent of the total head is due to dynamic head in the system. VFDs are also not generally feasible if the process system feeds a lot of different control valve systems. In these types of systems, header pressure control is required for the VFD control to meet all of the system pressures required to manage all feed sources. This typically results in a fairly high system pressure, just slightly lower than the fixed-speed design, as the highest pressure source becomes the driving factor in the pressure setpoint. With multiple sources, it is important to carefully evaluate each control valve operation in the system. As such, it may be difficult to justify a VFD on these type of control systems.
5. Reduce pump speed
Reducing pump speed may be the best option for optimizing an oversized pump if the process conditions and pump hydraulics will support it. Lowering pump speed will improve the hydraulic cavitation characteristics of the pump by lowering net positive suction head required (NPHSr), improving bearing life, and saving energy.
Consider the pump curve in Figure 2, which was operating at 1200 RPM. It originally had a 300-horsepower motor. Suction energy adjustments to NPSH resulted in an NRSPr of 28 ft. The net positive suction head available (NPSHa) in the system was 15 to 19 ft. Insufficient NPSH caused pump cavitation, which elevated typical vibration levels as high as .4 in/s. The control valves in the system operated 25 to 35 percent open, which presented the opportunity to reduce pump head.
Figure 2. 1200 RPM pump curve original operating point
This pump operating at a slower speed, 900 RPM, shown in Figure 3, presented a good alternative to the original oversized condition. Since the pump was slowing down and could only slightly reduce the head, a larger impeller had to be installed. This pump would need a 20.75" impeller operating at 6,000 GPM at 85 ft. Due to shifting the operating point, the pump efficiency improved to 78 percent. Due to system head reduction, this pump would only require 165 BHP, which also meant a smaller 200-horsepower motor. This would be a critical factor, as moving to a smaller motor horsepower would allow the 900 RPM frame to fit on the existing 300-horsepower baseplate, which made this an even more attractive solution.
Figure 3. Pump curve for same pump operatng at 900 RPM
When suction energy and NPSH margin were calculated, the NPSHr for this pump at a lower speed was 12 ft. This would drastically reduce the pump’s cavitation potential. As a result, vibration was reduced to 0.11 in/s. This was not a huge surprise as suction energy is a function of the square of the speed. The pump system control valves were now operating in the 50 to 70 percent open range.
Not every pump system will work out where all the factors align to allow a modification along the line of what is described above; however, looking at all options will help produce the best possible result. No matter what your budget, there is usually some measure of incremental improvement that can be employed to improve the efficiency and reliability of an oversized pump system.
Randy Riddell, CMRP, CLS is the Reliability Manager for SCA at the Barton Mill in Alabama. He has over 25 years of industrial experience with a career focus on equipment reliability. Mr. Riddell has a BSME from Mississippi State University and is a Certified Maintenance & Reliability Professional from the Society of Maintenance & Reliability Professionals. He can be reached at [email protected] or 256 370-8105.