|Pump developers are working to maintain initial efficiency levels as the pumps age
Power plants rely on plant-specific bespoke pumps to ensure the successful operation of the plant and its key functions. In most power plants, the three key pump duties are boiler feed, cooling water and condensate extraction pumps. In addition, auxiliary process pumps are utilized for balance of plant functions which can include boiler feed booster, closed circuit cooling water and other auxiliary services.
While all pumps play a key role in the plant process, from a service and maintenance perspective, boiler feed pumps are often considered the most costly and integral units in the plant. They are critical to availability and performance.
Each of these pumps is purpose designed on a plant-specific basis. No two power plants are alike – even in duplicate plants, the pressure, temperature, altitude and cooling water properties will differ and this must be factored into individual pump design.
Drivers for change
Within power generation, pumps may be expected to operate for 40-plus years. Total lifecycle costs are considerable, with initial capital costs comprising as little as 5 per cent of the total. Energy is the biggest cost over the life of a critical pump, hence energy efficiency improvements have the potential to offer substantial savings for the overall plant.
A seminal report on pump performance published by the Finnish Technical Research Centre some years ago found that the average pump operates at less than 40 per cent efficiency in the field, and 10 per cent of pumps operate at less than 10 per cent efficiency. The major factors affecting performance include efficiency of the pump and system components, overall system design, efficient pump control, and appropriate maintenance cycles.
The report identified pump over-sizing and throttled valves as the two key problem instigators, highlighting the importance of specifying and designing pumps that are the appropriate size for the system they serve.
As efficiency declines with age, pump developers are working to maintain initial efficiency levels for longer, extending overhaul intervals and reducing downtime. Areas where operational and maintenance issues in existing pumps may be addressed by adopting newer design solutions include hydraulic passage design, upgraded materials, coatings technology, improved bearing designs, and modern sealing technology.
The outcomes should be increased reliability and mean time between overhauls; optimized energy use; an increase in power generation availability; improved corrosion and erosion resistance; improved vibration, pressure and pulsation; and reduction in noise attenuation issues.
Incorporating developments in materials, metallurgy and coating technologies offers advances for pump manufacturers. Metallic materials capable of operating at ever-increasing temperatures are being developed for power plant and aero-engine applications.
Use of more corrosion-resistant materials will extend the mean time between overhauls. The resistance to erosion, corrosion, and cavitation of silicon carbide polymers can increase the life of some applications, and coatings have the advantage of being easily restorable if damage to the material occurs.
Pump manufacturers are constantly refining their products. Sulzer says that innovation will enable reduced investment and operational costs and shorten the lead time of the new range of custom-built pumps. The company’s latest single stage mixed flow vertical cooling water pumps with semi-open impellers provide total pump efficiency of over 90 per cent, which leads the market. Optional full pull-out construction reduces lifting crane capacity and facilitates easy maintenance.
Operators look for every technique that will delay maintenance and minimize 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 optimize maintenance input. Condition-based monitoring can extend service intervals and ensure that intervention takes place only when required.
Analyst Anand Mugundhu Gnanamoorthy, industry manager, Industrial Automation and Process Control at Frost & Sullivan, says that as pumps may be used for decades buyers are increasingly moving towards considering total lifecycle costs.
“Operators have to find the BEP (best efficiency point). In pump operation there is a very narrow band in which to operate at a high efficiency. If it is run at any other speed or any other load, there is a loss of efficiency,” Gnanamoorthy says.
Martin Unterkreutzer, senior sales manager at Andritz, says: “The main drivers for development in pump technology are the reduction in maintenance intervals driven by improvement of materials, but also by incorporating features like condition monitoring. Efficiency will surely become more and more important as energy prices increase. Right now, unfortunately, many customers do not yet think in that way since electricity is fairly cheap.”
|Metallic materials capable of operating at ever-increasing temperatures are being developed
Pumps in thermal power plant
In the past, pumps were primarily designed for continuous operation in coal-fired plants. Pumps are not stopped often (a few times per year on average) and may need up to four or five hours from cold start to reach optimum operational conditions.
The shift to new and different fuels, such as gas and biomass, will see plants running on altered operating regimes often requiring new pump designs with a variety of new characteristics.
The start-stop cycle requirements of a combined-cycle plant require a completely different operating philosophy. Pumps must have cold-start capability, which may be required more than once per day. Thus, pumps in combined-cycle plants are designed to withstand thermal shocks much better than pumps designed for coal-powered stations. The pumps work at different speeds and require design changes in rotor dynamics, hydraulics and impeller design. Some of the changes may be quite small: for example, minimal changes in design of the impeller or the stiffness of the shaft can make significantly increase efficiency over the life of the pump. Pump manufacturers have ongoing development work underway to improve the design and efficiency of the pumps used in such plant.
The natural gas boom in the US means that gas is likely to fuel over half of the country’s new generation capacity over the next 20 years. In Europe, combined cycle gas turbine (CCGT) plant will play a key role in the transition to a greener energy mix, backing up intermittent renewable energy supplies and providing a lower-carbon alternative to coal.
The ‘clean coal’ technology of ultra-supercritical (USC) steam plants operates at very high pressures, promising higher efficiency and a relative decrease in emissions.
The boiler feed pumps that generate very high pressure need hydraulics, metallurgy and bearings which are able to operate under such harsh conditions. The hydraulics must be capable of generating pressure of 4500 psi and above. Suitable metallurgy must be employed, especially for the barrel enclosure which is designed to handle 12 000 psi.
The boiler feed pumps are very large, with a thick wall construction surrounded by substantial insulation.
Renewable energy leads to new pump designs
The growth in renewable energy is bringing new pump designs to market. Biomass generation, which generally involves a lower output than traditional coal and gas generation, requires low- and medium-pressure boiler feed pumps, which require pump size reduction and potential redesign.
While concentrating solar power (CSP) plants use conventional steam generation equipment, specialized pumps are needed to move the high-temperature molten salt used to store the heat of the sun. Large quantities of molten salt stored in an insulated tank reach temperatures as high as 600°C. A highly specialized pump is needed to pump the salt to the heat exchanger, enabling electricity generation to take place around the clock.
Fred Grondhuis of Flowserve talked to PEi about the specialized vertical turbine pump his company designed for this application.
“Operating at 600°C means using specialized high-temperature alloy materials for construction, as normal materials would not be suitable. The pump is submerged in the molten salt and it reaches all the way down to bottom of the tank to capture the last amount of energy from the tank. The molten salt also actually lubricates internal components of the pump,” he explained.
Development of the pump took a little over two years followed by high-temperature testing at a US government lab. The first pumps were installed five years ago in Spain, and further installations have taken place in the US and South Africa. The pump is designed for a 30-year operating life.
Fukushima disaster drives nuclear safety changes
In March 2011 a major earthquake off the coast of Japan caused a tsunami to strike the Fukushima Daiichi nuclear power plant. With the power supply disabled, the reactor coolant pumps ceased operating, which led to a major nuclear accident. The disaster has led to a raft of safety enhancements at nuclear sites globally.
The conventional generation side of a nuclear plant has pumps similar to those used in a standard thermal plant. Nuclear operators are now taking steps such as putting in an extra pump that will run even if there is no power, to ensure that reactor cooling will continue. Additional redundancy is being added to plant to increase safety levels. Pumps are being subjected to submergence tests to make sure that they will work underwater. 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. Since Fukushima a lot of attention has focused on loss-of-coolant accident, posing the question: if the pump stops, will the shaft seal still work?
To prevent this, pump and seal specialist Flowserve has upgraded its design to include a passive shutdown abeyance seal built into a cartridge. The interchangeable cartridge is suitable for Flowserve’s and other OEMs’ reactor coolant pumps, and the cartridge design means no component assembly is required in containment. The upgrade design has been introduced into the market in the last two years.
Flowserve’s Grondhuis says, “The Flowserve N-Seal with the abeyance seal design is generally accepted as one of the best methodologies to prevent against leaks. It has been installed in the US and we are talking to customers globally to do conversions.”
|Total Pumps Market: Revenue Forecasts for Power Generation, 2010-2020, Global
Credit: Frost & Sullivan
Looking to the future
Pumps have come a long way since the shadoof, the earliest known pump, was developed 4000 years ago by the ancient Egyptians who used a suspended rod with a bucket on one end and a weight on the other to draw water from wells.
Working with CCS involves the need to pump either CO2 gas or liquidized CO2. Carbon dioxide requires a robust sealing system, and as CCS is an inherently cost-negative activity it is important to reduce the failure rate and avoid extra expenditure.
Automation and remote and condition monitoring are playing an increased role in power generation, driving costs down by reducing the workforce needed to operate plant. Frost & Sullivan’s Gnanamoorthy believes that continued development of intelligent pumps will lead to increased efficiency.
“The pump industry has traditionally needed a lot of technicians for maintenance, but end users are looking to move away from this towards the intelligent pump which will report when maintenance required,” he says.
“Remote monitoring and intelligent pump monitoring is evolving. It has been pioneered in agriculture and oil and gas, and introduced in the last five years in North America and Western Europe. We are now starting to see it introduced in power plants.”
Global pump market
Power generation is a relatively small segment of the global pump market. In 2013, 14.3 per cent of global pump revenues came from power generation. That year the global revenue from pump sales to the power generation market was around $5 billion, and the rate of growth was 5.9 per cent, according to Frost & Sullivan. Rates of growth are predicted to increase steadily to around 10 per cent by the end of the decade.
The fastest growing sectors for pump sales are oil and gas and agriculture. In the market for pumps, emerging economies such as the BRIC countries (Brazil, Russia, India and China) have a growing demand for power.
This is reflected in a buoyant market for pumps in these areas. Other regions where population growth, rising standards of living and urbanization are fuelling growth include the Asia Pacific region, North Africa and the Middle East.
As the general economic environment improves in North America and Europe, pump revenues are expected to gradually return to moderate growth rates.
Demand for power in the mature economies of Western Europe is starting to recover from the effects of the recession, during which a reduction in the demand for energy inevitably stalled plans for new capacity. Europe is seeing plans for plant upgrades and additions. The focus is on improving efficiency by replacing inefficient pumps and motors or, increasingly, on adding variable speed drive systems to enable multi-speed functioning, reducing energy use.
Case Study: Adjustable angle impellers: added flexibility for cooling water pumping
This case study highlights a cooling water pump featuring adjustable impeller blades which can be adjusted without affecting operations. The pump is deployed in thermal power stations where the cooling water quantity needs to be regulated.
Ensuring a good fit between pumps and the system is a key in ensuring efficiency. Power plant cooling water pumps may be required to operate with different combinations of delivery rate and head. Impeller angles and speed are two of the variables which affect pump efficiency. Different rates of water flow and head require a range of impellers operating at specific speeds. The impeller design and the initial angle of the blades are selected to meet specific process requirements before a vertical line shaft pump is installed. Thus Andritz cooling water pumps achieve efficiencies of up to 90 per cent and higher at operating point. However, additional flexibility beyond fixed-angle impellers or manually adjusted impellers may be needed – for instance, if there are fluctuations in the cooling water level or differences between day and night operations.
There are typically two ways to achieve this: speed control with a frequency converter, or hydraulic impeller blade adjustment.
A frequency converter has its strength in applications with large fluctuations in head. While the pump can be adjusted with infinitely variable speed control, there may be a higher cost associated with the frequency converter for large cooling water pumps, including cabling and the air-conditioned room required for installation.
The pump division of Austrian engineering company Andritz has incorporated a hydraulic device into its vertical line shaft pumps which can adjust the impeller blade angles to accommodate changing conditions while the pump continues to operate.
The hydraulic adjusting device comes into its own where substantial changes in delivery rate are required. The system enables the impeller blade angle to be moved up to 15° between minimum and maximum. An oil-filled servo-cylinder rotates the impeller blades via sliding blocks and adjustable cranks, an established technique that Andritz has been using in its water turbines for decades.
Using hydraulically adjustable-angle impeller blades enables compensation for changes in operating conditions while the pump is in operation, avoiding downtime and optimizing efficiency. Andritz says the system has a long service life and does not require any electronic spare parts (which may become obsolete over the 40-year life of a power plant system).
Examples of power stations which use the hydraulic impeller adjusting device include:
- A coal-fired power station in the Netherlands where less than half the normal volume of cooling water is required during thermal shock operations;
- A coastal power station where a hydraulic adjusting device is used to cover the difference in cooling water delivery head caused by a big tidal range of 10-24 metres. The lower efficiency of frequency converting plus the higher costs for the frequency converter made the hydraulic impeller blade adjusting device a far more economical option.
Penny Hitchin is a journalist focusing on energy matters.
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