Floating ball valves are widely used across most manufacturing plants in the Oil and Gas, Petrochemicals and Refining industry. They are cheaper in cost and offer a high quality sealing between the ball and seating surface. Based on the process medium, temperature, pressure of the application the seat rings could be of metal or PTFE construction.The floating ball valves work on the principle of self sealing mechanism wherein the higher the medium pressure the higher the sealing between the ball and seat ring on the downstream side of the valve.
Why the Trunion mounted ball valve scores over floating ball valves.
Floating
ball valves are widely used across most manufacturing plants in the Oil and
Gas, Petrochemicals and Refining industry. They are cheaper in cost and offer a
high quality sealing between the ball and seating surface. Based on the process
medium, temperature, pressure of the application the seat rings could be of
metal or PTFE construction.The floating ball valves work on the principle of
self sealing mechanism wherein the higher the medium pressure the higher the
sealing between the ball and seat ring on the downstream side of the valve. To learn more on the basic guidelines to selection criteria of valves read "5 Questions you must ask before you buy Industrial Valves" [Click Here] to read my earlier post on the subject
However
there are a few disadvantages if the
valve is being cycled frequently. The friction between the sealing surfaces can
be quite large resulting in frequent scratches and deep scoring on the surface
of the ball. This often calls for frequent maintenance and repairs which are
expensive. The other challenge being high operating torque which can be a
disadvantage for frequent quick shut off applications. Mounting a gear box
operator manual/motorised can be rather expensive in place of the lever handle
operator whose dimensions are limited by ISO14313 and API6D standards.
To know more on "How to Select the right Actuator for your valve?" [Click Here ]
To know more on "How to Select the right Actuator for your valve?" [Click Here ]
Some of the
challenges with regards to high operating torque can be addressed by reducing
the co-efficient of friction between the sealing surfaces. Using PTFE as a
material of construction (MOC) for seat rings instead of metal seat rings can
reduce the friction between the ball and the seat ring, however it may not be
enough to reduce the operating torque significantly. Further compromising on the
design of trim parts to reduce operating torque viz; increasing the stem
diameter may lead to alteration of dimensions of other valve parts resulting in
an overall increase in cost of the valve. Also using PTFE seat rings may not
always be ideal for the chosen process medium, temperature and pressure of the
process application. This could have a bearing on the life time value of the
valve as a whole. I have illustrated in the drawing below the thrust and load factors on a trunion ball valve vs a floating ball valve.
Many valve
design engineers have tried to work on the design of the floating ball valve to
overcome the above challenge. As the seats are fixed and the ball is floating,
there is a larger movement of the floating ball along with the flow medium
pressure. The higher the flow pressure the higher the tilt of the floating ball
and higher the sealing against the downstream seat of the valve. As a result of
this tilting action the flow pressure is applied on the whole circular area of
the ball minus the area of the ball covered by the downstream seat. The higher
closing torque or Force required to operate this valve is largely a result of
(F = p X A)
This was a technical article written for one of our customers M/s AS Schneider, Taurus brand of trunion mounted ball valves.
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