By Greg Herr, Flowserve Corp.
Typically, when talk turns to power plant pumps, boiler feed and cooling water pumps immediately dominate the discussion. And that’s as it should be: these pumps are critical to a plant’s thermal efficiency and availability. They are also the most costly to produce, operate and maintain, so optimizing their efficiency is imperative. However, in terms of reliability and profitability for the entire plant, it is also prudent (and economically rewarding) to optimize pump system performance throughout the entire station.
The process of identifying, understanding and effectively eliminating unnecessary losses while reducing energy consumption, improving reliability and minimizing cost of ownership over the economic life of the pumping system is commonly referred to as systems optimization. (See “Optimizing Pumping Systems,” published by the Hydraulic Institute, 2008.) This philosophy of asset management as promulgated jointly by Europump and the Hydraulic Institute is based on the principles and rewards of life cycle cost (LCC) programs. (See “Pump Life Cycle Costs: A Guide to LCC Analysis for Pumping Systems,” 2001.) These practices apply to both critical service and balance-of-plant (BOP) pumps.
Stated more clearly, experience has proven that a system approach, and not just a component evaluation, is required to determine the true root cause for degraded unit performance. While improvements will be realized when upgrading individual components, these efforts should be part of a larger scope to improve overall unit availability and efficiency. This is the best path to maximum return on investment.
Comparing Life Cycle Costs
The Europump-Hydraulic Institute LCC model defines two major classifications of cost categories: CAPEX, or capital expenditures, and OPEX, or operating expenditures. The former includes initial purchase and installation costs. The latter includes lost production, energy, maintenance and other costs borne over the lifetime operation of the equipment. For both critical service and BOP pumps, CAPEX is relatively insignificant when compared with OPEX for an expected 40-year equipment lifespan.
Looking first at feed water and cooling water pumps, it is understandable that station operators focus on efficiency and availability. These pumps’ energy and lost production costs account for more than 85 percent of their OPEX (see Fig. 1). But with balance-of-plant pumps such as overhung and between-bearing, radially and axially split single-stage pumps, opportunities also exist for significant savings in maintenance, energy and operating expense reductions. Optimizing the total pumping system will have a positive affect on overall station performance by assuring maximum efficiency and availability.
Application of the LCC model to feed water and cooling water power plant pumps makes it possible to study the important factors contributing to total cost of ownership. Take, for example, a 100 percent boiler feed pump in a 900 MW fossil power plant with the following data (see Figure 2):
- CAPEX $18 million (initial purchase and installation)
- Lifetime 40 years
- Running hours 8,000 hours/year
- Maintenance cost based upon MTBR (mean time between repair) of eight years
- Cost of energy due to production losses $45 per MW hour
- Cost of energy to drive boiler feed pump $25 per MW hour.
The total 40-year cost for this boiler feed pump reveals:
- The original CAPEX is low compared to total LCC, approximately 5 percent to 8 percent.
- Availability has an enormous impact on pump LCC. A 0.1 percent availability difference generates a $12.9 million cost differential.
- Energy cost is the highest pump LCC element and a 7 percent efficiency gain represents a $12.9 million cost saving.
This evaluation shows that upgrading the feed water pumps for energy efficiency and better availability will have a significantly positive financial impact. Today, bad-acting boiler feed pumps and cooling water pumps can be re-rated and upgraded to improve their reliability, with payback periods typically in two years or less.
For BOP pumps (commonly referred to as auxiliary pumps), the general rule for industry cost-effectiveness is to initiate repairs or pursue proactive measures when pump performance degrades more than 7 percent. Given the age of many fossil power plants and the systematic neglect of so-called auxiliary pumps, it is almost certain that many of these pumps are no longer operating at their original service conditions. Typically they are now running back on their performance curves. These pumps are excellent candidates for increasing efficiency and reducing maintenance costs.
The goals of upgrading balance-of-plant pumps are the same as they are for feed water and cooling water pumps:
- Energy optimization
- Increasing reliability to increase the availability of power generation capability/capacity
- Improving corrosion and erosion resistance
- Solving vibration, pressure, pulsation and noise problems for increased MTBR and more reliability
- Performance optimization to increase station output.
Another goal that is often overlooked is standardizing spare parts inventory, which has the combined benefits of reducing inventory carrying cost and simplifying maintenance tasks.
The methods to achieve peak efficiency for balance-of-plant pumps mirror those applied to the feed water and cooling water pumps. In-depth root cause analyses for “bad-acting” pumps identifies those for priority corrective action, thereby effecting immediate and positive bottom line results. Mechanical, hydraulic and performance audits are conducted using sophisticated analytical and modeling techniques. Mechanical seal design is reviewed along with pump and seal materials to identify improvement opportunities. Historical maintenance records are studied to determine performance trends and recurring problems. If required, powerful software tools can be utilized to ensure peak performance of BOP pumps in conjunction with feed water and cooling water pump systems. These include software to model the entire steam-water cycle thermodynamics and software to model static and dynamic structural analysis of all feed water piping and supports.
Balance-of-plant Pump Upgrade
Following a main power turbine re-rate, a power station experienced a de-rate of more than 50 percent when running on a single feed water train. While the boiler feed water pumps were assumed to be the culprit, a complete system study was conducted investigating the feed, booster and condensate pumps as well as the low pressure (LP) heaters and heater drip pumps. Performance testing was conducted on the systems for eight days following a yearly maintenance outage. Using the Flowserve ISG portfolio of capabilities, including wireless monitoring and proprietary software interface for data logging, a high degree of data accuracy and resolution was achieved. Testing enabled viewing of the interaction between the pumps, control valves and heaters as unit load was varied. Deficiencies were noted as some pumps were performing well below their published curves. A hydraulic model was generated to analyze the reaction of the LP heaters within the system. (Modeling software can develop system head loss curves and illustrate areas of excessive pressure drop, control inadequacies or hydraulic deficiencies such as low net positive suction head available, or NPSHA.)
The following BOP pumps were evaluated:
- Heater drip pump overhung, single stage pump with no installed spare
- Boiler feed seal water booster pumps three-pump configuration taking suction from the condensate header
- Make-up (MU) demineralizer pump taking suction from the condensate storage tank and discharging into the MU deaerator
- MU deaerator pump taking suction from the MU deaerator and discharging into the discharge header for the condensate pumps.
Findings for the heater drip pump revealed it was underperforming compared to the published original equipment manufacturer (OEM) curve by 15 percent. NPSHA calculations, vibration data and field observation also indicated the pump had experienced cavitation. Pump hydraulic instability and imbalance resulted from the poor piping arrangements for both the suction and discharge.
After thoroughly reviewing the condensate system (including dump valves, drip tanks, tube losses, piping and so on), a new hydraulic design point for the LP heater drip pump was determined. The pumps were up-rated based on the output of the hydraulic analysis that correctly sized the heater drip valve.
The station operator was offered several solutions to remedy the performance problem based on LCC calculations. The short-term recommendation was to rebuild the existing overhung process pump to meet the increased system performance requirements. Recalibration of the control valves and additional minor pump modifications were recommended. This, however, did not address systemic shortcomings in the LP heater design and degradation.
The long-term and more favorable recommendation was to replace the existing LP heater drip pump with a small double-case vertical pump with variable frequency drive (VFD). This would eliminate NPSHA concerns, provide precise level control for heaters and ensure flow is always pumped forward during all unit operating modes. At the same time, several control valves would be eliminated and the VFD would reduce energy usage.
LP Heater Field Testing Method
To evaluate LP heater system performance, both historical and current operating data were collected and reviewed. Historical data included:
- Compilation and review of equipment data sheets, maintenance and operational history
- Performance of thorough field inspection of existing pumps/motor installations
- Field inspection of remaining major system components, piping and valves
- Evaluation of system design for any factors that would lead to excessive pressure drop or flow-induced piping/pump vibration
- Review of instrumentation and controls for the feed water system
- Review of historical process control system (PCS) from previous transients, full and partial load, startup and shutdowns for significance.
To determine current system performance, the equipment was tested under actual power plant conditions with a field-proven wireless data logger system and supporting instrumentation. Heater drain pump operating data were collected in real time at three second intervals and at varying unit level loads or during a normal power ramp. Additional instrumentation was applied to record control valve position (six valves), tank level, pressure and temperature. This data was required to determine a root cause for de-rating and to recover feed water flow.
This methodology provided a very accurate test of overall performance. Validation of the collected data and plotting of pump performance was performed in real time against the published curves to achieve a high level of accuracy. It produced curves correlating pump performance, system head loss and MW output. Pressures and temperatures at key operational points were also recorded to document the system thermal performance in relation to varying feed water flow rates.
Life Cycle Cost Programs and Partnerships
With the growing adoption of the LCC model of asset management, forward-thinking station operators are implementing innovative programs and partnerships to exploit the broader potential of this business philosophy. It has become evident that a systems approach is required to determine the true root cause for degraded station performance or maximizing existing equipment. While benefits will be realized when upgrading individual components, these efforts should be part of a larger overall scope to improve the station availability and efficiency.
As previously mentioned, the critical service pumpsboiler feed, condensate and circulating waterreceive most management attention. Quite often, successful re-rates and upgrades of these units provide the segue to adopting a more comprehensive program. As important as these pumps are, it is more important to note that balance-of-plant pumps may account for as much as 85 percent of the station’s pumping units. Poorly performing auxiliary cooling water, condensate transfer, filter backwash and other auxiliary pumps can have a direct and negative impact on station efficiency and availability.
The most successful programs to date are proactive, metrics-based partnerships specifically targeting life cycle cost reductions and equipment availability improvements. The station operator and pump manufacturer jointly establish precisely defined performance goals and metrics based upon economic justifications such as payback, return on investment, internal rates of return and so on. A contract is written with compensation based upon attainment of key performance indicators.
Among the services offered through these contracts are energy upgrades, performance upgrades, reliability upgrades and “bad actor” assessments. The first three of these have been discussed earlier in this article. A closer look at bad actor assessments will reveal some compelling reasons to pursue this program. One customer reported a 15 percent to 20 percent reduction in life cycle costs per bad actor pump.
Moreover, the successful completion of a bad actor assessment often is the “proof statement” that makes the broader adoption of a LCC program feasible.
Regardless of plant size, empirical evidence reveals that 5 percent to 10 percent of a power station’s installed base will become bad actors or chronically problematic equipment. A detailed audit can identify those pumps that are most negatively affecting life cycle costs and plant availability. From this audit:
- A list of bad actor pumps and seals is generated
- Highest energy usage pumps are identified
- Pumps causing the highest incidences of unscheduled outages are identified
- Maintenance methodologies (run-to-failure, preventative, predictive, etc.) are reviewed
- Other items such as life cycle cost database, parts inventory, operational issues and so on are addressed.
Upon approval of recommendations, corrective actions are implemented to remove the first bad actor from the list. Once improved performance is confirmed, the next bad actor asset is addressed and so on, until all are restored to expected performance.
Beyond Pump Performance
It would be easy to draw the conclusion that LCC and optimization programs are focused solely on equipment upgrades, re-rates, energy usage and similar issues relating to pump performance. However, there are several other types of programs that can help deliver lower total cost of ownership.
Typically, these programs focus on inventory optimization, operations and maintenance, technical solutions, safety, health and environmental issues, training and consultation as well as equipment reliability and energy efficiency.
Inventory Optimization: Inventory optimization programs can systematically minimize parts inventory and inventory-related costs, reduce capital costs, improve asset utilization and simplify maintenance. This can be achieved through standardization programs that employ comprehensive equipment audits, inventory analysis and cost tracking to mitigate obsolescence and shrinkage. In some cases, a fourfold decrease in inventory cost has been achieved.
Strategic Procurement: Strategic procurement programs facilitate transaction efficiency to reduce administrative, equipment, parts and maintenance costs. A dramatic example of this is the power and exchange program for API and ANSI/ISO process pumps. This program can result in significant cost savings and reliability improvement as worn power ends are exchanged for ones which incorporate the latest industry standards and technologies, such as API 682 seal chambers, stiffer rotors, improved bearing isolators, and more. This is particularly attractive to power station operators as the majority of their pump inventory comprises these workhorse process pumps.
IT Solutions: The deployment of sophisticated IT solutions for advanced data analyses and asset management is literally revolutionizing the life cycle cost model. The maturation of onboard intelligent sensors, intelligent pumping algorithms that predict pump behavior and prevent failure, advanced communications and data convergence solutions, wireless networking systems and other advancements are having a profoundly positive impact on pumping systems optimization.
Workforce Development: Read any trade magazine or attend any technical conference regardless of the industry segment, and the “graying” of the workforce is a topic of highest concern. Where will the replacements for the experienced process engineers, reliability engineers and skilled tradesmen come from? The answer lies in customized training and specialized mentoring programs administered through expert industry educators. Training and mentoring the next generation of station operators and maintenance craftsmen are critical components of the LCC model.
Reduce life cycle costs, manage assets more efficiently, increase plant availability and output and enhance workforce skills: these concepts are increasingly taking hold as station operators intensify their efforts to maximize plant profitability. More and more, the LCC asset management business philosophy is being adopted as the means to that end. Successful implementation is contingent upon a truly integrated systems approach. A relentless pursuit of this business model in all aspects of station operations will significantly enhance the attainment of maximum profitability.
Author: Greg Herr is manager of Technical Services-Power for Flowserve Corp.