Unlike the USA, for example, Europe has not previously made much use of the ball valve for control purposes despite the fact that this type of valve has many advantages over the short-lift globe valve that is much more widely used for such applications. The reason, as far as the European situation is concerned, has been the inadequate control stability that could be obtained from conventional ball valves.
Distorted characteristic curves
In order to achieve a high degree of control stability it is essential for a control device to have an equal-percentage characteristic that will provide a linear relationship between heat output and the amount of opening of the control valve (otherwise known as the system characteristic). Under standard test conditions a conventional ball valve exhibits an S-characteristic. However, once installed in a plant this characteristic becomes severely distorted due to the fact that ball valves have an extremely high flow characteristic (kvs value) in relation to their nominal diameter.
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