OCTOBER 19-20, 2012 - YMCA University of Science & Technology

Proceedings of the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
PERFORMANCE IMPROVEMENT OF A CONTROL VALVE USING
COMPUTATIONAL FLUID DYNAMICS
K Thanigavelmurugan # , N.V. Mahalakshmi # , S. Mohan Das*, D. Venkatesh*
# Department of Mechanical Engineering, Anna University, Chennai – 600 025
*Circor Flow Technologies,Coimbatore,India
thanigavel_murugan@yahoo.co.in
Abstract
This article describes the design and performance improvements of a high pressure turbine bypass valve Zick
Twist trim (multi stage, multi path). For effective control of velocity, pressure and temperature, a trim designed
to have a tortuous path was designed. Computational fluid dynamics and FEM analyses were used in the design
process. The valve, which was installed with the designed trim, was tested. To evaluate its performance in the
field, the valve was installed at a 225MW combined power plant system for two months. The results showed that
the pressure letdown was successfully controlled by the designed trim, and the noise level was reduced below
85dB. The main objective of the work is to find the pressure drop, velocity variation, temperature distribution in
the different stages of the turbine bypass valve using computational fluid dynamics. This is done to increase the
performance of the valve.
Keywords: Zick Twist trim (multi stage, multi path, tortuous path trim), turbine bypass valve, pressure
control, velocity control, temperature control, disc stacks, computational fluid dynamics
1. Introduction
Power plant system facilities are experiencing increasingly higher temperature and pressure conditions aimed at
improving energy efficiency. Various valves are used to control flow in the power plant system. Valves used at a
power plant are under high temperature, high pressure, and high differential pressure conditions. Therefore,
erosion, hammering, vibration,noise, and damage may arise due to cavitation, flushing, and seat leakage. Turbine
bypass valves plays a very major roll in power plant applications. A high-pressure turbine bypass valve is one of
these valves to bypass the steam during the starting and stopping mode of a turbine and reduction period of
electric or heating load required at a power plant and to control the pressure of the steam for the turbine
expansion process. A trim is an internal component of the valve that controls pressure and velocity of the steam
by energy loss caused by the flow resistance of the flow path. Control valves for power plant systems have been
previously studied [1–6]. Amano and Draxler [1] presented a study of steam flow behaviour througha highpressure
turbine bypass valve when it suffered a high-pressure reduction in an electric power plantcogenerator
system. Logaret al. [2] developed the advanced steam turbine bypass valve to integrate thecontrol function and
the trip function with a singlestem design. The flow field in a steam turbine mainstop valve bypass valve was
investigated by means of computational fluid dynamic (CFD) simulation, and design recommendations for some
of the most important geometric parameters were presented [3]. Excessive high velocity at the valve trim causes
many problems, such as vibration, noise, and reduced valve life. To prevent this situation, a tortuous path trim
was developed to simultaneously control pressure and flow velocity effectively [4–6].
To consider aspects of fluid dynamics and confirm structural safety in design processes, CFD and FEM
techniques are used to predict flow fields and to check stress and strain information on the valve [7, 8]. However,
little has been reported about the design processor performance tests in the field for valves used at power plants.
In this article, the design and performance testing of a 10˝ turbine bypass valve trim for a 230MWcombined
power plant system are described. The previous valve, which needed to be replaced, had a trim only and used for
reducing the pressure and velocity alone. A high pressure drop occurred simultaneous to a high velocity increase,
after the steam was passed through the tortuous path trim. Owing to the high pressure drop and the velocity
increase, many problems, such as plug damage, vibration, and noise, could be caused by cavitation and flushing.
In light of the previous valve problems, a tortuous path disc trim was designed for a new turbine bypass valve to
control pressure, velocity and temperature by using Laval jet nozzle. In order to design a valve trim, CFD and
FEM analyses were used to consider fluid dynamics and structural safety. Seat leakage, pressure rating, and mass
flow rate tests were carried out in a laboratory experimental steam flow line. Finally, to evaluate its performance
in the field, the valve was installed in a 230MW combined power plant system for two months. The valve
location in the power plant for the field test and the schematics of the valve and tortuous path trim are illustrated
in Fig. 1.
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