Go with the flow15 September 2016
The movement of fluids in a nuclear power plant is controlled by dozens of pumps, some off the shelf, but most bespoke. Basic pump design has changed little in decades, but pump experts have been looking to increase efficiency by incorporating improvements to materials, manufacturing and maintenance requirements as Penny Hitchin reports.
Power plants rely on plant-specific bespoke pumps to ensure their successful operation. Key roles include boiler feed, cooling water and condensate extraction pumps. Balance of plant functions can include boiler feed booster, closed circuit cooling water and other auxiliary services. Pump internals are generally purpose designed for each specific plant. Even in twin stations, the pressure, temperature, altitude and cooling water properties may differ and this must be factored into individual pump specifications. Pumps are intended to last the lifetime of a power plant. As nuclear power plant life expectancy has increased from 25 to 40 and now to 60 years, pumps must be designed and manufactured to operate for increasing durations. If operational conditions change, (for example if a reactor power output is uprated) then pumps may need modifying to accommodate this.
The major factors affecting pump performance include efficiency of the pump and system components, overall system design, efficient pump control, and appropriate maintenance cycles. Pumps have a narrow band in which they operate at a high efficiency. At any other speed or load there is likely to be a loss of efficiency. Thus operators seek to identify the best efficiency point (BEP) and run their pumps accordingly.
Initial capital costs of pumps may be as little as five percent of the total power plant but over the life of a critical pump energy and maintenance costs can be considerable. Improvements in energy efficiency and maintenance programmes have the potential to offer substantial operational savings for the overall plant operation.
Pump efficiency declines with age, thus pump developers are working to maintain initial efficiency levels for longer, extending overhaul intervals and reducing downtime. Approaches to increasing efficiency may include improved hydraulic passage design, upgrading coatings and sealing technology and improving bearing designs. Outcomes can include increased reliability and mean time between overhauls, optimised energy use, improved corrosion and erosion resistance and improvements in vibration, pressure, pulsation and noise attenuation issues.
Sophisticated remote controls raise cyber risks
Operators look for every technique that will delay maintenance and minimise downtime and digital sensors and controls provide operational information to support this. Increasingly sophisticated instruments make it possible to measure and monitor processes and this data can be used to control operations and optimise maintenance input. Condition-based monitoring can extend service intervals and predictive maintenance can ensure that intervention takes place only when required. However, in a safety-critical, risk-averse environment such as the nuclear environment, introducing new digital controls may be problematic due to concerns around regulatory approval and cyber security.
Cyber security is an issue of increasing significance for all operators of critical national infrastructure. A 2015 report by UK think tank Chatham House says that there have been around 50 control system cyber attacks on nuclear plants and the risk of serious cyber attacks on nuclear power plants is growing.
Analyst Anand Mugundhu Gnanamoorthy, industry manager, Industrial Automation and Process Control at Frost & Sullivan observes that there is a reluctance to employ state-of-the-art IT within the nuclear industry because of security concern and the range of unknown outcomes from linking Operational Technology (OT) to IT systems. Thus while many industries deploy pumps which incorporate sophisticated sensors and monitoring systems, the nuclear industry is more conservative about adopting them.
In 2003 the Slammer worm crashed the network at Ohio’s Davis-Besse nuclear power plant and disabled a safety monitoring system for nearly five hours. Fortunately the plant was offline at the time.
In August 2006, the Browns Ferry nuclear plant in Alabama experienced a malfunction of two components: the reactor recirculation pumps and the condensate demineraliser controller. The plant’s Unit 3 then had to be manually shutdown. In this instance the problem was caused when microprocessors sending and receiving data over an Ethernet network in the two malfunctioning devices failed due to excess network traffic.
Innovation and advanced manufacturing
Advanced manufacturing techniques (the integration of technology based systems and processes in the production of products) are playing a significant role in advances in pump technology.
Improved manufacturing techniques and developments in materials, metallurgy and coating technologies offer potential advances for pump manufacturers. New alloys offer increased durability. The resistance to erosion, corrosion, and cavitation of silicon carbide polymers can increase the life of some applications and increased corrosion resistance can extend mean time between overhauls. New coatings offer the advantage of being more easily restorable if damage to the material occurs. Prototyping can be carried out using 3D printing techniques.
Dr Colin Elcoate, VP business development, SPX FLOW, Inc. stresses the need for pump reliability. He says the drive is to improve pump efficiency through improving surface technology for hydraulic passage. On the manufacturing side he highlights better casting and finishing processes and the ability to manufacture more complex shapes.
“The more efficient the pump, the less power needed. This drives efficiency of the generating plant.”
Increased computing capacity contributes to improved computational fluid dynamics. Elcoate says that validation of analysis is critical. This is achieved by full 3D analysis and use of prototyping to measure hydraulic performance and flow.
What characterises pumps for nuclear applications is specifics around the integrity of design and complexity of specification and requirements and the function of some of the specifics. However as Elcoate points out: “Different industries use the same pumps and we build on that body of knowledge. We want to make sure engineers/technologists share knowledge across all of our industries.”
Post Fukushima safety measures
The catastrophic 2011 major earthquake led to a 15-metre tsunami flooded and disabled the power supply at Japan’s Fukushima Daiichi nuclear power station. With no electricity or diesel to power the pumps, three reactors were unable to maintain proper reactor cooling and water circulation functions, with disastrous results. Post-Fukushima reviews have led to the introduction of new safety enhancements globally.
Pumps are being subjected to submergence tests to make sure that they will work under water and nuclear operators are installing pumps that will ensure reactor cooling will continue to run even if there is no power.
Any pumping system needs power. This is generally an electrical motor which relies on a managed electrical power supply.
Diesel engines require fuel storage and delivery and starting systems which often rely on electrical power. Steam from the reactor or steam generator is a source of available energy, although turbine drives generally rely on auxiliary DC power services for turbine governing and control as well as oil lubrication. However, it is possible to deploy a steam-driven safety pump which does not rely on other external services and can thus improve a nuclear plant’s ability to maintain core cooling regardless of a blackout.
SPX FLOW’s TWLTM steam-powered pump is designed to pump cooling water for core cooling and decay heat removal with no external power, lubrication or other support systems. It can start and operate efficiently for a minimum of eight hours while fully submerged and has been designed and tested to withstand seismic events. The turbine water lubricated (TWL) safety pump is a combined pump and steam turbine on a single shaft in one casing. There are no seals, no leakage path and no oil required. With the self-lubricating pump and turbine on the same shaft, a mechanical system of springs and levers control the pump and with no electronic control system the pump can operate without external inputs.
The compact component was originally designed in the 1960s as a small, low maintenance turbine-driven boiler feed pump. Hundreds were installed in merchant navy vessels. In the following decade demand from the nuclear industry led to adaptation and nuclear qualification and it was deployed in second generation PWRs and BWRs.
Interest in using the TWLTM has risen after Fukushima. The cooling water safety pump has been optimised for use within auxiliary feedwater (AFW) or reactor core isolation cooling (RCIC) systems for both new and upgrade applications. Improved capability has been demonstrated by full submergence tests carried out at the SPX FLOW Glasgow facility and full seismic tests performed at the University of Bristol. Other improvements included replacing the filter with a cyclone separator. The pump has full nuclear quality assurance and has also been approved by the US Nuclear Regulatory Commission for use in RCIC systems on advanced BWR plants.
As many as 90 TWLs have been installed globally. They have been retrofitted to existing plants – for instance on the Slovenian Krsko station. They are also being fitted to new plants. In 2015 TWLs were ordered for TVO’s new EPR reactors at Olkiluoto in Finland to operate alongside the existing reactor cooling sub-systems. In partnership with GE-Hitachi SPX FLOW is delivering TWLTM safety pumps to the Japanese nuclear fleet and they are being fitted as standard to Chinese CPR1000 APRs.
In most nuclear reactors the primary coolant pump operates at around 300°C at relatively low pressure. A mechanical seal
is integrated into the pump. Pump and seal specialist Flowserve has upgraded its design to include a passive shutdown abeyance seal built into a cartridge to ensure that the shaft seal will still work if the pump stops. The interchangeable cartridge is suitable for OEM reactor coolant pumps and the cartridge design means no component assembly is required in containment.
Nuclear pump market
The global market for pumps is a mature market which broadly reflects economic growth trends. Key industries are water and wastewater, oil and gas, power generation and agriculture. Frost & Sullivan’s Mugundhu Gnanamoorthy says that the power industry is probably third of fourth by value. Within this, the nuclear market plays a relatively small role in the sector.
Supplying the nuclear market requires experience, certification and qualifications. Established OEMs supply equipment and servicing for pump lifetimes and with high barriers to entry it is a tough market to enter. OEMs are consolidating and the biggest nuclear pump suppliers continue to acquire heritage brands. SPX FLOW brands includes Pompes Guinard and Clyde Union. Flowserve brands include Byron Jackson and IDP.
Opportunities for OEMs to supply pumps to new nuclear power stations are thin on the ground. However with around 430 reactors operating globally, pump OEMs are on call for maintenance and support. SPX FLOW’s Elcoate points out that it is important to maintain skills and capacity by engaging in markets globally and incorporating knowledge and lesson learned across the board.
Looking forward: drivers for change
Pump manufacturers are constantly refining their products. The development of oil and gas production from deeper wells is driving development of pumps equipped to operate at higher temperatures and pressures. Advances feed through into applications in other industries, enabling reduced investment and operational costs and shortening the lead time of the new range of custom-built pumps.
Operators look for every technique that will delay maintenance and minimise downtime. In many industries digital sensors and controls provide operational information to support this. Increasingly sophisticated instruments make it possible to measure and monitor processes and this data can be used to control operations and optimise maintenance input.
Condition-based monitoring and predictive maintenance can extend service intervals and ensure that intervention takes place only when required. As power plants move towards automation, the intelligent pump replaces experienced technicians.
The safety-critical risk-averse environment of nuclear power faces a dichotomy: operators want pumps that have a long established track record, while at the same time seeking improvements in efficiency, cost and lead times.
Mugundhu Gnanamoorthy believes that the gradual evolution of pumps is likely to continue with the biggest changes coming in the field of automation. He says: “The basic technology is not going to change. The changes will be in how the pump is operated. Predictive maintenance, the ability to help service professionals find out what they need to do will be the most important one.”
Elcoate says: “We see gradual improvements in efficiency. The shift that the industry needs is to drive improvements in cost and lead time. It’s all about economics.” He added: “Operators look to the supply chain for innovative solutions and ideas to reduce operating costs while achieving the same levels of safety and reliability.”