Understanding torque behavior of the double offset butterfly valve

Tuesday, January 15, 2019 | Comments

Control valves are used over a broad range of applications in process industries to control flow. When it comes to the safety of other components like boilers, reactors, pumps etc., valves play an important role. In many shut-off applications, butterfly valves are the primary selection because of its quarter turn operation, compact design, and simplicity. There are several subtypes of butterfly valves, but based on the offset, there are mainly three designs:

  1. Zero offset or concentric butterfly valve
  2. Double offset butterfly valve
  3. Triple offset butterfly valve
Butterfly valve types
Zero, double and triple offset butterfly valve

In this article, we will only discuss double offset butterfly valve and its hydrodynamic torque behavior. Double offset valve is also known for "high-performance" valve in fluid flow industry. This valve is generally used in power generation, HVAC, oil and gas generation applications.

What is double offset valve?

The double offset valve has two offsets. The first offset is a shift of position shaft behind the of valve disk, and a second offset is a shift of shaft position behind the valve disk slightly one-sided to pipe centerline. Due to this offset, disk moves as cam action movement, which limits contact of the disk with the seat. This enhances the life of the valve and gives the capability to work at high-pressure rating than the simple concentric butterfly valve.

Actuators are the crucial component in the control valve. Its main function is controlling the fluid flow is keeping valve disk at a certain angle of position against a flow of fluid. From the manufacturer's point of view, of the total cost of control valve, the cost of an actuator is up to 30 to 40%. The cost of actuator mainly depends on "how much it has to resist the torque of fluid flow?". And so, the cost of an actuator is directly proportional to as hydrodynamic torque. Control Valve Performer app by simulationHub is now equipped to with this feature of generating characteristic curve "the coefficient of hydrodynamic torque (Cdt)" with respect to several angular positions of a valve. To know more details about this feature and to see the feature live in action, visit this product update.

What is the Coefficient of hydrodynamic torque?

Hydrodynamic torque is torque developed on valve disk due to hydrodynamic forces of flow of fluid. The hydrodynamic torque coefficient Cdt is a factor, which is independent of the size of the valve. For a given valve and valve opening, it is easy to calculate the hydrodynamic flow torque by using Cdt times the differential pressure drops.

Vavle coefficient of hydrodynamic torque equation

The characteristic curve generated for all openings for specific control valve is helpful for a manufacturer and user to predict hydrodynamic torque to sustain valve at a certain angle of position by avoiding any accidents and failures in valve components.

Cdt of double offset valve

For a valve, the variation of Cdt is subject to the flow area available. The velocity at the leading edge of the disc is less than the trailing edge of the disc. Hence the force on the leading-edge side of the disc is more as compared to the force on trailing side of disc. This differential force is the cause of the torsional moment acting on the disc.

For symmetric or concentric butterfly valve, Ccd curve behavior is independent of flow direction and it is fixed for that valve. But in the case of double offset butterfly valve, due to geometry, drastic change in behavior is observed by reversing the flow direction from seat side flow to shaft side flow.

Seat side flow for butterfly valve
Seat side flow for butterfly valve hydrodynamic torque
Seat side flow and hydrodynamic torque behavior
Shaft side flow for butterfly valve
Shaft side flow for butterfly valve hydrodynamic torque
Shaft side flow and hydrodynamic torque behavior

In comparison, the Cdt value for double offset valve for shaft sided direction is higher than for seat sided direction. By observation, it can be seen that, for the higher degrees of opening angle, in case of shaft sided flow direction, Cdt is high. This higher value of hydrodynamic torque will try to close the valve which gives an ability to shut-off the valve, as they can quickly and reliably be moved to a position that will completely stop the flow and halt operation. This has a good application where safety is the first priority, for example, fuel systems in ship’s engine rooms.

In comparison, seat sided flow direction, valve hydrodynamic torque is negative for higher angle of degrees. This will reduce total torque required for an actuator to retain the valve in open condition. The reduced torque value requires small capacity actuator, and thus reduces the cost of the required actuator.

Now obtaining Cdt characteristic is a click away

simulationHub has developed an app to easily and quickly evaluate the flow performance of valve. The Control Valve Performer (also known as CVP app), is easy to use and extremely fast in predicting the hydrodynamic torque of a valve. Along with Cdt, it also calculates other flow performance parameters like CV, KV, and valve pressure drop. The app is based on well-validated flow performance test procedures and computational fluid dynamics. The app process just requires a 3D CAD model and valve opening positions to be defined. The rest of the process is fully automated and calculates the flow performance in less than an hour. The app also generates a details PDF report including flow performance curves and CFD results. All this happens in less than $100 for one valve. This does not only save time and money but also adds great value for being in the approved manufacturer list. Visit the app details page to know more details about the app. You can also see the live 3D projects of valve performance calculations. You can also schedule an exclusive demo for you and your team.

About Author

Blog Author - Aaditya Ruiker

Aaditya Ruiker

Aaditya Ruiker is a CFD support Engineer at Centre for Computational Technologies Private Limited (CCTech), Pune. He loves to work in fields physics and mathematics. Skilled in OpenFOAM, Fluent, C, MATLAB, CAD Modelling. He has completed his M.Tech in Thermal and Fluids Engineering from (Dr. BATU), Lonere, Raigad. His areas of interest are Heat Transfer, Fluid Mechanics, Computational Fluid Dynamics, Numerical Methods, Operation Research modeling. Driving and traveling, playing cricket and chess are his hobbies and he likes to explore historical places.
Blog Author - Prathamesh Badadal

Prathamesh Badadal

Prathamesh is currently an Intern in the simulationHub team at CCTech. At CCTech he is keenly interested in learning the upcoming new technologies in the field of Computational Fluid Dynamics, Machine Learning. His areas of interest are Computational Fluid Dynamics, Numerical Methods, Heat Transfer and Fluid Mechanics. He holds a Master in Heat Power Engineering from Savitribai Phule Pune University. His hobbies are playing Chess and Badminton.

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