Taking the stress out of fast cycling

An ageing chemical manufacturing process vessel was used for analysis by engineers from TàƒÅ“V NORD because such vessels have similar operating conditions to the main steam-line components of a fast-cycling power plant
An ageing chemical manufacturing process vessel was used for analysis by engineers from TàƒÅ“V NORD because such vessels have similar operating conditions to the main steam-line components of a fast-cycling power plant

Demand for fast-cycling power plants is increasing, resulting in temperature shifts that place greater stresses on the plant and more importantly its components, potentially leading to fatigue and fracture. Can realistic simulation play a role in preventing this, and thereby help to optimise plant cycling and operations?

Axel Schulz, TàƒÅ“V NORD SysTec GmbH, Germany

Renewables are playing an ever-more important role in our power mix, and undoubtedly their greater integration onto the grid is a key part of achieving a low-carbon future. However, their intermittent nature is forcing a growing number of energy producers to use their conventional coal and gas fired power fleet as back-up, constantly having to toggle them from ‘off’ to ‘on’ and back again.

Although this start-and-stop pattern is quickly becoming the norm for power generation companies, their conventional power plants were built to produce continuous or baseload power, rather than to accommodate the production peaks and troughs of today’s energy landscape. Operating under these conditions places greater stresses on the entire power plant, and especially on the thick-walled, main steam-line components through which steam is transported from the pressure vessel to the turbine and where the greatest temperature shifts occur.

In extreme circumstances, plant start-ups can involve a temperature increase in these components of almost 500 à‚°C in a single hour. The temperature change during shutdown is almost identical, but in reverse. Nonetheless, to ensure that our overall power supply continues to meet varying demand, the number of start-stop cycles are expected to rapidly increase in the future. With frequent steep temperature cycling, system components ” such as the vessel, valves and header ” are subject to fatigue and fracture. As a result, their lifespan may be shortened, with an impact operational efficiency and system maintenance, and ultimately electricity production.

Evaluating and solving the engineering challenges of temperature cycling for the power industry is a primary focus of the engineers on the stress calculation and design team at TàƒÅ“V NORD. These projects have taken on even more importance following Germany’s introduction of a new energy programme last year, in which it essentially plans to replace its existing nuclear power base with wind and solar power by 2022. To compensate for the fluctuating power production that such renewable energy resources will create, it is realistic to assume that Germany will soon have a critical need for fast-cycling power plants. And this trend in all probability will be repeated across the globe as the energy industry gradually moves towards lower carbon power generation sources.

Improving plant operations

In response to such changing conditions, TàƒÅ“V NORD created a Cycle Optimised Operation Programme, or COOP, which is designed to improve facility standards, performance and management.

When evaluation and testing more closely reflect reality, plant operation and cycling can be made much more efficient and costs controlled. Realistic simulation with Abaqus Finite Element Analysis (FEA) software from Dassault Systàƒ¨mes’ SIMULIA brand was chosen as the core solution for the COOP concept. Abaqus was chosen because it has very sophisticated capabilities for analysing fluid structure interaction, fracture and fatigue, as well as advanced tools for calculating crack propagation.

German regulations for power plant operation are not only strict, but they are also extremely conservative and based on over-simplified assumptions. Currently, start-up and shutdown processes are calculated as approximately quasi-static operating procedures, and heat transfer coefficients are not used ” the steam temperature is set equal to the wall temperature. Because of these simplifications, fatigue and damage to parts and components can be significantly overestimated.

In the first module of the COOP’s two-part process, TàƒÅ“V NORD engineers use Abaqus fluid structure interaction (FSI) capabilities to simulate real-world heat transfer conditions within plant components. This produces a highly realistic assessment of component stresses and less conservative design codes.

In the COOP’s second module, the Abaqus FSI simulations enable optimisation of the plant components’ interior shape to reduce pressure losses, stresses and noise emissions. This module also uses extended finite element methods (XFEM) and crack propagation analyses ” both part of Abaqus ” to calculate crack geometry and growth. These analyses allow the engineering team to determine new safety standards and more accurately evaluate service life.

COOP in action

To evaluate the COOP methodology, the TàƒÅ“V NORD team carried out an analysis of two ageing chemical process vessels ” components used in chemical manufacturing that also undergo frequent cycling and extreme temperature shifts. The company chose to study these vessels because such process vessels have operating conditions that are a good match for main steam-line components in fast-cycling power plants. The vessels selected for the study had also developed some cracks, so the engineering team was able to compare FEA results with physical measurements.

The process vessels were constructed with skirt support expansion joints manufactured out of creep-resistant steel and designed to minimise thermal loading during cycling. During start-up, a 490 à‚°C medium was added to the vessel, elevating the temperature 135 à‚°C in just 15 minutes. During shutdown, cold water at 50 à‚°C was fed in, cooling the vessel’s contents by 250 à‚°C in 45 minutes. The vessel was subjected to the stresses resulting from these extreme temperature changes approximately 200 times per year, or once every 33 hours (see Figure 1).

Figure 1: A complete 33-hour start-up/shutdown cycle for a chemical process vessel that has extreme temperature shifts of 135 à‚ºC in a 15-minute start-up window and 250 à‚ºC in a 45-minute period during shutdown
Figure 1: A complete 33-hour start-up/shutdown cycle for a chemical process vessel that has extreme temperature shifts of 135 à‚ºC in a 15-minute start-up window and 250 à‚ºC in a 45-minute period during shutdown

As a result of this frequent and extreme thermal cycling, cracks developed in all 82 expansion joints, all at the same position, varying in length and depth depending on time of service. For the simulation, TàƒÅ“V NORD engineers only needed to model a 15-degree sector of the process vessel because of its symmetrical shape and relatively constant operational load.

As part of the COOP’s first module, the team carried out a sequentially coupled temperature-stress analysis (in this special case, an advanced FSI-calculation was not necessary). They set up this simulation using real-world operating values for temperature (heat transfer), mass flow (speed) and pressure, as well as actual component geometry. The results indicated that a stress maximum of about 1150 MPa occurred on both heat up and cool down when the temperature transient was steepest (see Figure 2). Using fatigue curves of test rods ” made of materials similar to the process vessel ” subjected to load cycles, the lifespan of the component was calculated to be 400 cycles, or about two years.

Figure 2: FEA of the stress distribution around the expansion joint of the process vessel skirt support shows equivalent stress (left), circumferential stress (centre) and axial bending stress (right)
Figure 2: FEA of the stress distribution around the expansion joint of the process vessel skirt support shows equivalent stress (left), circumferential stress (centre) and axial bending stress (right)

In the COOP’s second module, the engineering team used the results of the real-world stress calculation as a starting point and performed a shape optimisation by changing the contours of the expansion joints (the same technique can be used on a vessel contour). They determined that minor changes in shape could deliver significant improvements in fatigue strength and that as geometries are modified to improve flow, the resulting pressure losses and local bending stresses can be reduced.

To further understand the interactions between vessel stresses, fatigue, crack geometry and crack propagation, the team used the XFEM capability in Abaqus coupled with contour integral calculations. TàƒÅ“V NORD engineers agree that the XFEM method makes it possible to study crack growth across elements in a way not possible previously. The contour integral calculation enabled the engineers to analyse stress intensity values for a series of crack depths and shapes. The results showed that the greatest stress occurs when the vessel is most rapidly heating up; that the stress decreases with increasing crack depth; and that the crack grows more rapidly on the outside than the inside of the frame connector.

The engineers then compared their simulations with physical testing using strain gauge and crack measurements on the vessels themselves to verify the Abaqus FEA results. With both stress distribution and crack growth, the team found a close correlation between Abaqus results and the measurements. It quickly became apparent that at a high number of cycles crack growth was being overestimated.

A clearer view through simulation

For TàƒÅ“V NORD, realistic simulation has provided a window into the black box of power plant operations. With a better understanding of the stresses of thermal cycling, its engineering team can make better-informed recommendations to energy providers on optimising their power plant cycling processes and system component designs. Improvements include the nuances of ramp-up and ramp-down rates, as well as more precise timing of maintenance, service and inspection cycles.

Thus, in addition to its capability of helping to minimise unplanned maintenance, simulation in combination with a plant’s normal monitoring systems can provide plant operators with an effective condition-based or predictive maintenance approach. Based on the knowledge of the behaviour of the different components gained by monitoring what is happening in the power plant, simulation can be used to predict what will happen, allowing the plant operator to determine when to best schedule maintenance. Or if an anomaly is detected in the system, simulation can be used to help determine whether to keep operating the plant until the next scheduled maintenance outage, run the plant at a lower capacity, or whether to plan for an earlier shutdown.

Also, component designs can be modified for a greater lifespan through simulation. For example, lowering the stress and fatigue level of a main steam-line valve could save upwards of €200 000 ($266 000) in replacement costs. The COOP is designed to minimise and manage unplanned equipment repair and reinvestment. Since early last year, TàƒÅ“V NORD has been employing the COOP process in Germany and will soon be working with energy customers worldwide. By incorporating advanced FEA tools into its methodologies, TàƒÅ“V NORD is confident it will be able to help create more reliable and more flexible power plants.

XFEM: a powerful tool

Simulation has become an integral business practice across a wide spectrum of industries. In the power industry, it is employed by key industry players, ranging from OEMs to consultants and utilities, and in different market sectors, including nuclear and wind power.

Abaqus is well suited for use in the power industry because of its ability to simulate the interrelationships between fluids, temperatures and components, and ultimately predict what will happen to the component.

Fracture mechanics capabilities in FEA are not a new concept, but traditionally these capabilities have been both time consuming and extremely labour intensive, primarily because the model or finite element mesh creation was based on the geometry and location of the crack.

XFEM, in contrast, is independent of the underlying crack geometry, so it can quickly and easily set up fracture and failure studies without time-consuming manual meshing and re-meshing.

The three main benefits of XFEM are:

  • Mesh creation is quick and relatively easy because it is independent of whether a crack is present or is likely to appear;
  • Crack propagation is done automatically taking into account different conditions, i.e. damage propagation criteria, which are independent of the underlying mesh;
  • It provides simulation stability through algorithms to ensure the accurate prediction of how the component crack will grow.
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