Many facets of everyday life—including vehicles, home appliances, factories, buildings and infrastructure—are under pressure to become more efficient. The same is true for the power generation industry. As the number of renewable energy generation sites increases, traditional generation methods have to compensate for variable availability and natural demand fluctuations. However, the original design of cogeneration plants, using combined cycle gas turbines (CCGT), includes operating with a fixed power output.
Gas-fired plants are increasingly required to operate “off-peak,” which means the output must vary to match demand. In this situation, the fixed amount of generated steam pressure has to be broken down across valves on the high-pressure feed system. Configuring a variable speed drive (VSD) to the feedwater pumps can provide significant improvements in efficiency, which helps to minimize operating costs.
Innovative Speed Control
A gas-fired cogeneration plant located within a refinery in Germany used a boiler feedwater pump to provide 1,000 cubic meters per hour (m3/h) (4,400 gallons per minute [gpm]) of water, with a head of 1,355 meters (m) (4,450 feet). The pump was set up at a fixed speed, operating at 2,980 revolutions per minute (rpm), and required a 4.1-megawatt (MW) (5,500-horsepower [hp]) motor.
Since the pump’s installation, the user’s production cycle had changed significantly, and the pump needed to run on partial load because of changes in power demand.
To meet the variable system requirement of 500 m3/h (2,200 gpm) to 1,000 m3/h (4,400 gpm), the power plant used a valve at the discharge to break down pressure and throttle the flow. This meant that the energy exerted to develop the pressure was wasted.
To improve the feed pump’s efficiency, the facility had to modify its operating range by configuring a speed control mechanism.
Initially, the end user considered two more conventional options: a variable frequency drive and a hydro-dynamic speed coupling. However, both alternatives had disadvantages including the size, inconvenience and cost of installation for the medium-voltage VSD, as well as the inherent efficiency losses of the coupling.
Despite the disadvantages, the VSD offered good energy efficiency, and the coupling was compact and relatively easy to fit, sitting between the main motor and the pump.
The facility chose an innovative third option, one that was developed for the renewable power industry. This method delivered the benefits of both alternatives but without the negatives. The variable speed electro-mechanical drive offered a compact, convenient solution that could be installed between the motor and pump, and it was extremely energy-efficient—even more so than the large VSD.
For this application, the combination of a VSD and a mechanical geared assembly was the ideal solution.
The electro-mechanical drivetrain allows the main motor to remain mounted in line with the pump, but it uses a planetary gear arrangement driven by a high-power servo motor and VSD system as an override that takes over progressively as the required operating speed drops.
The addition of this technology makes the entire power transmission system supplying motive power to the pump up to 95 percent efficient. The new arrangement provided power savings of 1,090 kilowatts (kW), which translates to an estimated yearly savings of €218,000 ($237,000) to €436,000 ($475,000), depending on the annual operating hours. In addition, the initial installation costs are lower than those for a VSD, and the overall efficiency is higher.
As with all control technologies, there are advantages and constraints that end users must consider when implementing this type of system. The solution’s overall footprint can be a major factor, especially on retrofit situations.
In this case, the electro-mechanical drivetrain enables the main drive motor to be connected directly to the grid, removing the need for bulky speed-control equipment.
However, this solution has limitations and currently can be used only with equipment drawing up to 20 MW (26,800 hp) of power and with a maximum speed of 14,500 rpm. Nevertheless, these constraints still allow many types of equipment to benefit from the latest innovation in speed control for large-scale pumps.
Pumping applications account for a significant percentage of global energy consumption. Creating savings, such as those in this example, will have a major benefit for applications that use medium- and high-voltage drives to power high-performance pumps, such as those used in the oil and gas, petrochemical and mining industries.
As pressure increases on most industries to reduce manufacturing costs and minimize overhead, questioning conventional thinking on motor control becomes increasingly important. There is no single answer for every application, but new solutions should be examined for suitability.
Many applications that experience a change in duty requirements will benefit from an improvement in design. These improvements can be delivered by retrofit experts that offer advice, design proposals and complete installation services based on years of experience.