In competitive power markets, the focus is on plant efficiency and cost control. This often means that developers overlook devices that can reduce plant operation and maintenance costs as well as increase efficiency.
Bernd Harnisch, Voith Turbo Pte Ltd., Singapore
Energy generation in today’s market is becoming more and more efficient, with combined cycle power stations on the rise with net efficiencies reaching almost 60 per cent. While there is much focus on efficiency at full load, little or no attention is paid to the impact of part-load (cycling) operation on a plant’s efficiency. This article tries to shed some light on the design of the boiler feedwater system and some common misconceptions ” or myths ” regarding the use of variable speed drives for the high pressure boiler feed pumps.
Part-load vs base-load
Myth No. 1: Combined cycle power stations can be operated as base-load power plants to best utilize the higher efficiencies provided by this technology.
The idea of operating combined cycle power stations at base-load throughout their service life is very appealing. In practice however, gas turbine powered stations are much more suitable than e.g. coal-fired plants to cover peak capacities.
Figure 1. Absorbed power of high-pressure boiler feed pumps (throttle vs. speed control)
Furthermore, there is no guarantee for base-load operation over the lifetime of 25 years. Government regulations and PPAs may change and electricity pool systems might be established obliging operators to bid for every kWh to sell. In times of recession, decreasing or stagnant electricity demand will ultimately result in excess capacities installed thus forcing power plants into part-load operation.
If after five or even ten years the plant has to go into part-load operation, the original design, i.e. as a base-load plant, may not be suitable for the more demanding requirements of a cycling plant. Assuming that no provisions were made to operate the high-pressure boiler feed pumps at variable speed, the operator may face increasing maintenance cost due to higher pressure levels and increased wear and tear. Frequent start-ups may cause problems with the minimum flow control valves while the feedwater control valves have to cope with higher differential pressures due to reduced plant output.
Myth No. 2: If combined cycle power stations are operated as base-load power plants the use of variable speed boiler feed pumps has no economical advantage.
The potential of auxiliary energy savings in a combined cycle power station depends mainly on the following factors:
- Load pattern: The load pattern has a significant impact on the potential savings in auxiliary energy as shown in Figure 1.
- Test point of heat recovery boiler: Depending on regulations, steam generators have to withstand an excess steam flow of up to 105 per cent, an excess feedwater flow of up to 125 per cent and an excess feedwater pressure of up to 110 per cent of design capacity. This leads to a design of a high-pressure boiler feed pump that largely exceeds the requirements of the 100 per cent MCR (maximum continuous rating) operating point. In addition, contract guarantees from the boiler and the pump manufacturer further widen the gap between design and nominal operating conditions. The use of fixed speed pumps inevitably results in high losses in the feedwater control valve even if the plant is exclusively operated at base-load.
- Fixed pressure versus sliding pressure characteristic: If the boiler is operated in sliding pressure mode, the pressure difference between pump characteristic and boiler resistance line increases dramatically towards lower flow rates and with it the power losses of a fixed speed pump compared to a variable speed pump.
A study carried out for a 425 MW combined cycle plant in Singapore supports the above arguments fully. The study ” based on exclusive base-load operation ” shows a power saving of 180 kW per 50 per cent boiler feed pump. As a result an energy saving of some 3 million kWh can be achieved during one year. The theoretical values have been verified and confirmed in real life operation. At 100 per cent MCR the pumps are running at 2580 r/min compared to a design speed of 2877 r/min.
Figure 2. Boiler feedwater pump and boiler characteristics
When related to the total plant output, the plant’s net efficiency at full load can be increased by 0.1 per cent compared to a design using fixed speed pumps. In view of the very moderate capital expenditure for the inclusion of variable speed couplings the cost for other efficiency enhancing measures to achieve the same goal may be far higher. The efficiency advantage remains throughout the plant’s lifetime and can increase to as much as 0.5 per cent if the plant goes into cycling operation.
Myth No. 3: Additional installation of hydrodynamic variable speed couplings reduces reliability and increases maintenance cost.
Figure 3. Load pattern and boiler feedwater pump power consumption
The operating experience of customers worldwide is in strong contrast to the common opinion that an additional device will result in lower reliability or higher maintenance cost of the overall system. To understand the supposed contradiction, one needs to look closer into the characteristic of a hydrodynamic coupling.
Contrary to a fixed gear box, the hydrodynamic variable speed coupling changes the operating characteristic of the boiler feed pump. Torque is transmitted by using the kinetic energy of hydraulic oil flowing from the primary wheel (pump impeller) to the secondary wheel (turbine). A mechanically driven pump maintains the oil flow needed to dissipate the hydraulic losses while the moveable scoop tube adjusts the oil level in the coupling according to the required output speed. A 4-20 mA signal from the control room is all that is needed to control the speed.
The discharge pressure of a speed-controlled pump is lower than that of a fixed-speed pump. Thus the pump can be operated at its optimum operating point and internal efficiency. Controlling the feedwater flow by valves only (throttle control) causes the internal pump efficiency to drop. Feedback from power plant operators with fixed-speed pumps indicates that wear and tear of the feedwater control valves results in increased maintenance cost and even in replacements of complete valves every year. This is even more valid in case of the start-up (minimum flow) valves that have to compensate a differential pressure between boiler (approximately 10 bar) and the boiler feed pump (80-180 bar) during cold start. Wear and tear on these systems is minimal in speed-controlled systems because the lower pressures can be directly provided by the pumps. Besides, the whole starting process becomes easier to manage and problems such as water hammer or cavitation can be easily avoided.
Hydrodynamic variable speed couplings help protect the valves of the feedwater system and provide reliability and availability that is unmatched by any other means of variable speed control.
Myth No. 4: Hydrodynamic variable speed couplings increase the project cost significantly.
The cost for hydrodynamic variable speed couplings are not very high compared to the overall project cost. Typically, a coupling for a 500 MW combined cycle power station costs some $50 000. At the contractor’s side cost for modified foundations and implementation into the control system has to be considered.
On the other hand, some of the additional cost may be saved when designing an optimized system making full use of the benefits of the latest design of hydrodynamic variable speed couplings. The conventional start-up and feedwater valves may be replaced with less sophisticated designs as the operating requirements are less severe. Recently, Voith developed a new, hydraulically supported design for the scoop tube control. This control provides extremely fast reaction times (minimum to maximum flow within 5 s) and high accuracy thus allowing designing feedwater systems without feedwater control valves in the future. Common header systems would still require feedwater control valves to balance any operational variation between two or three boilers.
The integrated oil system of the variable speed coupling can also supply lube oil to motor and pump bearings, whereas a separate lube oil supply unit is needed in case of fixed speed pumps.
Myth No. 5: Hydrodynamic variable speed couplings are not cost-efficient.
Despite the tremendous development of frequency converters and improved filter systems to handle the harmonics feeding back into the mains, the demand for hydrodynamic variable speed drives has not changed over the years. The absorbed power of centrifugal machines decreases with the 3rd power of speed. Thus, the efficiency advantage of electrical systems combined with their usually higher investment cost does not result in shorter payback terms. Because of the utmost importance of reliability and availability in a power plant, the hydrodynamic variable speed coupling will remain first choice for many operators worldwide.
A valuable asset
While suppliers spare no efforts to further improve today’s efficiencies of combined cycle power stations by increasing gas turbine temperatures and selecting ever more exotic turbine materials, the use of hydrodynamic variable speed couplings for the high-pressure boiler feed pumps is still considered a luxury. The concentration on lowest capital expenditures for the power plant makes investors and contractors fail to notice a device which not only is able to increase the plant’s net efficiency by another 0.1-0.5 per cent, but also to protect their capital investment by reducing maintenance and wear and tear of the complete feedwater system. When looking for alternatives at comparable price levels one will easily find out that the hydrodynamic variable speed coupling is an asset all too often wrongly overlooked.