Design of Quadruple Offset Butterfly Valve Used in Power Plants

1. Introduction

The eccentric butterfly valve has been used in industrial transportation systems, engine exhaust systems, turbines, and power plants to control the flow rate at the required temperature and pressure of working fluid [ 1 4 ] for a long time. The flow rate is controlled by a disc that rotates from 0 degrees (closed position) to 90 degrees (opened position). Such rotation values depend upon torque value [ 5 7 ]. The unreasonable torque value, corrosion, and high temperature would significantly affect valve sealing performance. The seal must maintain for their integrity, which is critical for high-risk energy production and oil processing applications. Therefore, the seal part must be considered by different working environments.

The different types of butterfly eccentric valves use different seals; at present, three eccentric butterfly valve types are accepted, single offset, double offset, and triple offset [ 8 9 ]. The single and double offset butterfly valves use a soft seal (rubber seal) at low temperature working environments, although the rubber seal has interference with the disc for good sealing and the needless interference leads to more wear [ 10 11 ]. The triple offset butterfly valve (TOBV) uses a metal seal and can work in high temperature working environments. Almost no contact happens between the disc and seat (seal). However, the disc of the TOBV has an ellipse section, which leads to nonuniform contact pressures distribution in the seat (seal) compared with the circular section, and the manufacturing cost is improved. On the other hand, the disc section area is less than a single or double offset butterfly valve in the same contacted pipe size, which means the flow rate is reduced. To solve the disadvantage of the TOBV, the quadruple offset butterfly valve (QOBV) was developed as a new concept, even though it is not accepted in standards such as API-609 standard. Additionally, its sealing problem would be considered at same time.

In previous studies, Kim reports a numerical study of the flow behavior through TOBV. The effect of the turbulence model was examined with consequent flow behavior analysis around the valve with turbulence variables such as turbulence kinetic energy and Reynolds stress [ 12 ]. Wang studied the high-pressure and high-temperature steam-induced vibration in a triple-butterfly valve. The steam pressure fluctuation characteristics of the disc were studied based on the numerical simulation results. The natural frequency of the disc was tested through the experimental modal analysis method to determine the vibration frequency characteristics of various orders [ 13 ]. Naragund found three important torques, seating/unseating, bearing friction, and hydrodynamic torque, for discs with double offset by carrying out the experiment on a butterfly valve, and the total torque obtained was compared using CFD simulation software and was validated. Further, the double offset disc was changed to single, triple, bi-lattice, and tri-lattice. A correlation is established between the experimental value and the CFD values [ 14 ]. Zhang developed an experimental test system for cryogenic high-speed hydrodynamic non-contact mechanical seals. The performances of seals under different working conditions were studied. The main performances of the seals were obtained, and the relationships of the performances with the inlet fluid pressure, the closing force, and the rotational speed were discussed [ 15 ]. Yang improved the structure of radial metal seal of the second-generation ICV, and the related theoretical model of its contact mechanical behaviors was also established. Theoretical and numerical analyses were carried out, which provides methods and means for the selection of application types and the improvement of structural parameters in intelligent completion operation [ 16 ]. Inose investigated changes in the sealability of a greased metal-to-metal seal due to elevated temperature by fundamental gas tightness tests. Changes in grease property at high temperature were investigated. It was found that the causes of the loss of the sealability of the metal-to-metal seal at a high temperature was the decrease in grease viscous resistance against high pressure due to the thermal degradation of grease [ 17 ].

In this present work, to improve the above disadvantages of TOBV, an elliptical disc, an elliptic cone function was established to describe the quadruple butterfly valve (QOBV) disc shape which has a circular section disc. On the other hand, the laminated seal used in the QOBV is suggested for a good sealing, and some designs of laminated seal had a fixed entire thickness (7 mm). Additionally, sealing performance simulations were carried out using thermal-structural coupling analysis. As the results show, the best design of the QOBV shows that the maximum contact pressure is 37.2% lower than the TOBV using 7-way laminated seal layers which have a thickness of A240–316 stainless 1.3 mm and graphite 0.6 mm, and shown in Figure 1