With the significant increase in global energy consumption, the public is paying more and more attention to the nature and scope of our grid. Concerns about fuel supply security, plant safety and emissions, and climate change mean that energy policy has become the subject of public debate. In this environment, the design, construction, and operation of contemporary power plants and next-generation power plants have all the tools they need to demonstrate that they have enabled the plant to meet and exceed the expected efficiency, safety, and emissions targets.
The design content of a new system or redesigned system comes from several aspects: past experience and engineering experience will help to start the process, and the constant calculation rules will bring more detailed equipment size information. However, in order to efficiently design a power plant to operate safely during normal operation or in the event of a planned or unexpected shutdown, it is necessary to have a clear understanding of the transient characteristics of the system.
Mentor's Flowmaster is known around the world for its ability to simulate liquid and gas systems in steady state and transient conditions; it utilizes detailed component models and powerful solutions. Building a virtual prototype of the system in Flowmaster allows the user to determine the best point of operation and run as early as possible in the design process. Empirical data supporting the Flowmaster component model enables users to run simulations early in the design phase and is confident that the results are correct and useful.
This article will discuss how this approach can be applied in the industry to design the best power generation system. The focus of the article is on typical cooling water circuits, especially capacitive reactance. This is just one of the many components of the Flowmaster component library that are closely related to the power generation system. Other applications include, but are not limited to: damping, heat exchangers, steam traps, and turbines.
Example: Cooling water condensation circuit
Cooling water networks are important for the operation of all thermal power plants. The Rankine cycle is often used in all chemical fuels and nuclear power plants, which requires steam that fills the entire turbine to be reliquefied into water before returning to the furnace. This requires a large amount of water to pass through the steam side of the large heat exchanger before returning to the source; this water usually comes from the sea or a large inland river in order to have sufficient production capacity.
These condensers are important components that are expensive, so it is important to understand their performance in the environment. It is important to optimize the cooling water system. It is necessary for the designer to understand how the system responds to an unexpected shutdown caused by a pump trip or a safety valve close.
Figure 1: Flowmaster schematic for a cooling water network
In the network shown in Figure 1, the two condensers are each driven by two centrifugal pumps. Once the two pumps trip, an intermediate control valve will open, allowing the remaining pumps to drive the pair of condensers. Flowmaster's simulation analysis capabilities allow users to:
Confirm that the design process is sufficient to ensure safe operation
Determine the impact of the program on circuit performance
Establish the consequences of an unmanaged shutdown
Determine what steps need to be taken to ensure that equipment integrity and plant safety are not at risk even under such extreme conditions
To take this into account, Flowmaster includes a detailed cooling water condenser model to understand the tank geometry, the installation of the gas injection valve, and the porosity in the tank. To check the effectiveness of the controlled shutdown program design, the time history of the critical point pressures in the circuit and the tank liquid level can be reviewed. The interference to the system is as follows:
Two pumps that power the uppermost condenser are tripped. Note that the relationship between the rate of change of the pump speed (blue line in Figure 2) and the torque applied by the impeller to the drive shaft is inversely proportional to the inertia of the pump and motor.
Two seconds after the pump trips, the valve downstream of the pump begins to close to prevent any damage from backflow.
At the same time, the valve connecting the two circuits starts to open, allowing the remaining two running pumps to drive the two networks.
Figure 2: Unexpected shutdown
To determine if this procedure is sufficient to ensure a controlled shutdown, we can observe if there is a pressure spike or vapor bubble formation in the pipe network. In addition, we can determine if the tank level is affected. The most convenient way is to annotate the Flowmaster schematic with the maximum or minimum value obtained at any given point in the simulation. The output (Figure 3) indicates that the design process does not cause the pressure to reach an alarmingly high level.
Figure 3: Flowmaster simulation results with node pressure deviating from the nominal value
After considering the pressure of the entire circuit, the condenser itself can be inspected in more detail.
Figure 4: Model schematic
The condenser model itself is unique to Flowmaster and has been developed over the years to allow users to accurately predict the transient response of shell-and-tube condensers and their connected circuits. It allows the user to consider changes in tank level, flow rate and pressure, as well as the effects of installation gas injection. This particular model is not intended to describe heat transfer, but can be done with other Flowmaster heat exchanger components.
In this model, the structure of the condenser is: the water tank is 2.6 meters high and is installed at 3.7 meters above the reference water level. The tube bundle extends to 0.35 meters below the top of the tank. In order to avoid the plant from shutting down, it is assumed that the minimum acceptable safety standard is that the liquid level cannot be lower than this point.
Figure 5: Simulation results for circuit failure
During the simulation, the two pumps tripped as before, and the valves connecting the two circuits could not be opened, so the uppermost condenser was completely shut down. This situation of course has a great impact. As the pump decelerates, the valve closes and the pressure in the downstream system gradually decreases, eventually forming a vapor bubble, which increases as it ruptures within 22 seconds (see the red line in Figure 5). The resulting pressure peak reaches 11 bar, which is much higher than the normal working pressure. The vapor bubble increases and ruptures again, and the resulting pressure peak is a typical water hammer phenomenon. This has the same effect on the condenser. The drainage of the two tanks is much lower than ideal. The outlet tank can only recover its water level through the reverse flow. For example, the water flow through the tube bundle is "sucked" back into the condenser and returned through the tube. In this case, if no measures are taken, the equipment is highly likely to suffer considerable damage.
Flowmaster allows users to perform prototype simulations of various designs, from injecting air to the condenser itself to other devices in systems such as pressure regulating tanks and air bags. The simulation process pointed out two major problems that need to be addressed:
The vapor cavitation forms and ruptures directly downstream of the pump. Depending on the pressure rating of the selected pipe, it may cause damage to the equipment.
The water level of the water tank is lowered, and a large amount of vapor vacuoles are formed inside the components. Extremely likely to cause damage to the equipment.
The time history of these two phenomena indicates that steam bubbles in the pipeline have formed before the operation of the condenser is disturbed. The best way to solve this problem is to add two surge towers to the design. 6.
Figure 6: Two pressure regulating towers are placed directly downstream of the pump
Once these devices are in place, Flowmaster re-runs the network. If properly sized, the formation of vapor bubbles and the resulting pressure peaks can be prevented. From the pressure at this position (Figure 7), it can be seen that the problem can be solved. Although the pressure cannot be lowered to a level slightly lower than atmospheric pressure for a short period of time, it does not form vapor bubbles and does not cause water hammer. Flowmaster can be used to optimize the size of these devices, ensuring proper use of the device and avoiding excessive installation costs.
Figure 7: Adding two pressure regulating towers to solve the problem of water hammer and steam vacuole
to sum up
This article briefly describes how to use a virtual prototype in the design process of a power generation system. In this process, all components in the power generation system can be designed and simulated to optimize fluid flow, heat transfer and ensure that the system meets today's safety, efficiency and environmental friendliness. This article also describes how the Flowmaster's condenser components can be used as part of a transient simulation to help understand the nature of controllable and uncontrollable transient events. This virtual prototyping approach can replace the time-consuming and costly physical prototyping and avoid over-engineering issues. For more information on Flowmaster's power generation simulation methods, please visit the Flowmaster China website.
Dongguan Guancheng Precision Plastic Manufacturing Co., Ltd. , https://www.dpowergo.com