## 1 Introduction  
As global warming and the depletion of energy resources have intensified, the growth in worldwide energy demand currently faces the difficulty of installing new power generation facilities as a result of limited funding and the strengthening of environmental regulations. Thus, combined cycle power plants (CCPPs), which consist of gas turbines and steam turbines, are becoming more important as the global demand for electrical power increases [1; 2]. CCPPs manipulate discarded heat energy and store in hot gas into rotational energy, and the gas turbine compresses air and mixes it with fuel that is heated to a high temperature. The hot air-fuel mixture moves through the gas turbine blades to make them spin, and then the fast-spinning turbine drives the generator that converts a portion of the spinning energy into electricity [3; 4; 5]. With the stoichiometric burn, the power output of turbine drops sharply for flow rates above a critical value as a result of aerodynamic losses produced by the rotor blade stalling, and the turbine performs poorly in excessive load and exhaust gas pressure-states, which brings about a decrease in energy production. In order to prevent the above problems, the bypass system facilitates the regulation of the exhaust gas energy available at the turbine wheel, and thus allows the turbocharger to be controlled and to not produce too much boost pressure. In this system, the bypass valve, which is mounted between the chamber and the atmosphere in parallel with the turbine, limits the maximum pressure in the chamber to prevent the instantaneous air flow rate through the turbine from exceeding the values above which aerodynamic stalling at the rotor blades would produce a severe fall in power output [6; 7; 8]. As the modern CCPPs require an increase of exhaust pressure in order to fulfill the restrictions of pollutant emission and to improve energy productivity, the following complementary functions have to be taken into account while designing the bypass valve: resistance to the harsh engine boundary conditions (temperature of 350 \({}^{\circ}\)C and working pressure of 1 MPa); smooth operation of the disc during opening and closing; and maintenance of sealing in closed valve operation to maximize gas energy directed to the turbine [9; 10]. In the damper valve, which had been used in the existing bypass system, thermal deformations and stress concentrations occurred at a high temperature because there is no offset of the disc rotary axis from the seal. Also, a gap between the body and the disc due to absence of seal caused leakage, back flow into the turbine, and damage of the blade, as illustrated in Figure 1.  
The dual-layered seal with polytetrafluoroethylene (PTFE) and metal had been adopted to solve the above-mentioned problems, but PTFE should be frequently replaced owing to its low durability. Metal has a worse sealing performance than PTFE [11; 12], and its mechanical properties are deteriorated at the high temperature [13], so new sealing material, particularly composite material, is needed to improve the sealing ability and durability of other original seals. The existing single and double offset butterfly valves (SOBV and DOVB) have simple geometric structures to be easily manufactured, but rubbing and friction between the seal and the disc cause life shortening and wear of the seal during opening and closing, as shown in Figure 2. Therefore, this study applies the triple offset (radial eccentricity, axis eccentricity, and cone axis angle eccentricity) butterfly valve (TOBV), whose surface of the seal is in full contact when the valve is fully closed. This eliminates galling and minimizing seal wear and rubbing for long life and tight shut-off. In the previous studies related to the TOBV, [PERSON] determined suitable triple eccentricities to improve the seal performance, and checked the static and dynamic interference through motion simulation [14].