Equipment ownership costs typically exceed the original purchase price, so decisions made during the acquisition phase can dramatically affect equipment lifetime ownership cost.
By Tom Carsten and Barry Erickson, Flowserve Corp.
Over the past 20 years, equipment users of all types have been changing their purchasing decision criteria from an initial cost perspective to one that considers the total lifetime cost of equipment ownership. This shift is driven by recognition that the overall cost to own equipment is much greater than the initial acquisition cost. Decisions made during the acquisition phase can dramatically influence lifetime ownership cost.
In 2001, the Hydraulic Institute and Europump published a comprehensive guideline for evaluating life cycle costs (LCC) for pumps. They identified the following important elements:
- System design - Engineering to design system and specify equipment
- Acquisition cost - Purchasing expense and equipment
- Installation - Design of installation and labor to install
- Operation - Operators and operation oversight
- Energy - Cost to supply driving power to pump
- Maintenance - Planned maintenance plus unplanned repairs
- Disposal - Cost to decommission and dispose of equipment at the end of its life
Although pump users are beginning to recognize the importance of LCC, many remain unfamiliar with how LCC elements interact. For example, optimizing installation costs could have a negative effect on equipment maintenance expenses. A comprehensive approach tailoring the optimization process to the specific business variables and objectives is required.
The Merriam-Webster dictionary defines holistic as “relating to or concerned with wholes or with complete systems rather than with the analysis of, treatment of, or dissection into parts.” Applying this principle to pump LCC means looking at all components of LCC as a whole and considering their interactions and conflicts.
In a new installation, most pump components affecting LCC are considered as variables to be optimized holistically. For existing installations, however, many of these components cannot practically be changed. This requires an approach that breaks LCC into two components-LCC to date and future LCC. In most cases, only the pump components associated with operation, energy and maintenance are significant and can practically be affected for future LCC.
Factors Affecting Future LCC
The major cost contributors to future pump LCC are operating (energy) expense and maintenance costs. A common misconception, especially among engineering disciplines, is that energy costs are synonymous with pump efficiency. This is not the case in most situations.
Typical pumping systems use a control valve to regulate flow rate. Although it is necessary for a control valve to be partially closed at the design condition to maintain flow control, it is common to find the control valve sized for an excessive pressure drop. Such a pressure reduction is usually the result of engineering practices whereby compounded design safety factors result in unwarranted pressure and flow capacity.
Figure 1 illustrates a pumping system designed to deliver 680 cubic meters per hour. To maintain the design flow rate, a control valve with a 17-meter pressure drop was selected. Additionally, when fully opened the control valve permits 150 percent of the system’s design flow rate.
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This system could operate properly with a control valve sized for a 3-meter pressure drop at design flow. Figure 2 illustrates the operating costs using a control valve sized for a 17-meter pressure drop versus one designed for a 3-meter differential. Pumping system overcapacity and an inappropriate control valve design increased energy costs by more than 50 percent.
Figure 2 also illustrates that optimizing the pumping and control system can reduce energy consumption. To take advantage of this saving, the pump impeller diameter must be reduced. This is illustrated in Figure 3.
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The argument often made for using an undersized control valve is that it allows the system to operate as intended even if the pump deteriorates. While this is true, the effect on energy cost is significant. Additional attention to equipment maintenance would easily pay for the energy cost reduction and dramatically reduce total costs.
Figure 2 also illustrates the effect of a higher efficiency pump (80 percent vs. 75 percent efficiency). The higher efficiency pump decreases energy costs by 7 percent. Although not insignificant, this example illustrates the importance of considering the total pumping system as opposed to a pump efficiency focus only.
Although pump efficiency is vital, it is important to note that optimizing pump efficiency in an existing installation does not significantly reduce energy consumption unless the impeller diameter is machined to more closely suit system requirements. Impeller trimming and balancing is relatively inexpensive and can be done during planned or routine equipment maintenance. In many cases “right sizing” the impeller diameter delivers an added benefit of improved reliability and lower vibration.
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Variable speed drive installation is often proposed to reduce energy consumption. In systems with widely varying flow rate requirements, and where system friction is a significant component of the system head, variable speed drives can reduce LCC. In some applications variable speed drives can also eliminate the need for control valves and their associated maintenance expense.
Unfortunately, variable speed drives often involve significant installation and set-up costs that may offset any savings. Control systems must be adapted to the drive; sometimes the power supply system must be conditioned to minimize the effects of high-frequency harmonics.
Because of the potential costs involved, a thorough analysis of the operating profile, associated savings and the life-cycle time frame become important in the decision process. A short future life-cycle time window may result in an unacceptable payback.
Note the differences between a flow rate regulated system and a flow quantity regulated system. Examples of flow rate regulated systems include systems in which the flow rate must be controlled to maintain proper system functionality. Such systems will always incorporate a device to regulate the flow rate such as a flow control valve or a variable speed drive.
A municipal potable water system that supplies stand tanks and waste treatment lift pumps is a good example of a flow quantity regulated system. In such an application, control means are often simply on-off based on a vessel or pit level, or set fully open. In a flow quantity regulated system, energy consumption is primarily affected by pump and motor efficiency because changing the existing piping systems usually is impractical.
Reliability and Energy-The Competitive Edge
Maintenance costs and energy costs are often interrelated. Hydraulic experts have published numerous papers demonstrating the relationship between pump selection and reliability, explaining how pump flow rate, speed, impeller diameter and suction energy all affect reliability (Figures 4a - 4d). In most cases, reducing energy consumption lowers maintenance expense; however this is not always the case. For example, Figure 4c illustrates that reducing impeller diameter can result in damaging suction recirculation.
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Although reducing energy consumption in pumping systems may seem straightforward, energy savings calculations require a thorough evaluation and understanding of the existing system and an equally thorough evaluation of the potential savings of each option. Simplified analyses often greatly exaggerate savings. Additionally, physical and economic constraints in existing systems must be balanced against actual energy savings.
An example of a complex pump system is shown in Figure 5. The flow goes directly to the deaerator tank which supplies up to four separate boilers 90 percent of the operating time. The rest of the time the flow goes to an ion exchange regeneration system. Four identical pumps with poor reliability (1.1-year mean time between repairs) along with a desire to optimize operation of the system led to an investigation and engineering analysis.
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Pump inspection revealed extensive suction and discharge recirculation damage to the impeller. Further investigation of the pump design revealed a very high suction energy ratio and a relatively low NPSH (net positive suction head) margin reliability ratio (Figure 4d).
The field testing also revealed significant pump deterioration and a wide variation in performance capabilities. Because these pumps operate in parallel, a high likelihood exists that one or more pumps operate near shutoff, as shown in Figure 6. The net result is accelerated damage and a short mean time between repairs.
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A pumping system approach and a total cost of ownership (TCO) perspective resulted in a variety of solutions for consideration and evaluation. Table 1 (page 36) summarizes the financial and reliability aspects for each option.
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Option 1: Replace the existing pumps with larger impeller pumps with upgraded materials. This option was rejected as a result of an even higher suction energy ratio.
Option 2: Replace all four pumps with one larger pump equipped with a variable speed drive. This option provided the most energy effective and reliable solution, however the high equipment and installation costs resulted in an unacceptable payback period.
Option 3: Continue with the existing pumps under a revised operating scheme. Although there would be no capital costs for this option, reliability would continue to be a problem and energy savings minimal. This is essentially a do nothing option.
Option 4: Replace the existing 4 x 3 x 10 pumps with 6 x 4 x 10 pumps. Due to differences in hydraulics of these pumps they would produce significant energy and reliability savings. The operator rejected this option because the performance curve of the pump is very flat creating the risk of running one or more of the pumps at shutoff during various operating scenarios.
Option 5: Replace the existing pumps with identical size pumps having improved hydraulic characteristics and upgraded materials. This option ultimately was selected.
The above example demonstrates that a thorough knowledge and understanding of pumping systems is required to effectively undertake pumping system analyses.
Tom Carsten has worked in the pump industry for 32 years with Flowserve and its heritage companies and is currently the Director of Alliance Development for Flowserve Pumps and Flowserve Flow Solutions.
Barry Erickson, PhD, is a key account manager with Flowserve and has worked in the pump industry for 34 years with Flowserve, Durco, Goulds and Allis Chalmers pump companies.