Rotary ball valve assembly

1. A rotary ball valve assembly comprising a rotary closure element which is rotatable about a rotational axis and which has a through-bore defining a flow inlet and a flow outlet; and an impedance assembly which is positioned, at least in part, within the bore of the closure element, the impedance assembly including a plurality of flow passages through the impedance assembly, an inlet and/or outlet of two or more of the flow passages being elongate and arcuate so that, in use, as the closure element rotates, the arcuate elongate inlet and/or outlet of each said flow passage which is exposed is fully or substantially fully opened before the next passage is opened.

2. A rotary ball valve assembly as claimed in claim 1 , wherein a radius of the inlet and/or outlet of each said flow passage corresponds or substantially corresponds to the radius of the bore of the closure element.

3. A rotary ball valve assembly as claimed in claim 1 , wherein each flow passage is independent of the other flow passages.

4. A rotary ball valve assembly as claimed in claim 1 , wherein the impedance assembly includes a plurality of plates arranged to define the flow passages, each plate having at least one groove or channel so that the respective flow passage defines a tortuous flow path.

5. A rotary ball valve assembly as claimed in claim 4 , wherein one or more of the plates have a plurality of the said grooves or channels.

6. A rotary ball valve assembly as claimed in claim 5 , wherein at least one of the grooves or channels is formed on each major surface of the or each said plate.

7. A rotary ball valve assembly as claimed in claim 4 , wherein each plate is non-planar.

8. A rotary ball valve assembly as claimed in claim 7 , wherein each plate is arcuately formed so that the plates can be arranged in parallel or substantially in parallel with each other.

9. A rotary ball valve assembly as claimed in claim 4 , wherein the plates are stacked in a direction which is transverse to both the rotational axis of the closure element and the axis of the bore.

10. A rotary ball valve assembly as claimed in claim 1 , wherein the impedance assembly includes a plurality of plates having a plurality of apertures, the flow passages being defined, at least in part, by the apertures in the plates.

11. A rotary ball valve assembly as claimed in claim 10 , wherein each aperture in each plate has a length which is different to the lengths of the other apertures in the said plate.

12. A rotary ball valve assembly as claimed in claim 10 , wherein adjacent apertures in each plate extend in parallel or substantially in parallel with each other.

13. A rotary ball valve assembly as claimed in claim 10 , wherein adjacent apertures which define a single flow passage are offset relative to each other.

14. A rotary ball valve assembly as claimed in claim 13 , wherein, due to the relative aperture offset, each flow passage defines a tortuous flow path.

15. A rotary ball valve assembly as claimed in claim 10 , wherein each plate is planar or substantially planar.

16. A rotary ball valve assembly as claimed in claim 10 , wherein the plates of the impedance assembly are stacked in a direction which is parallel or substantially parallel with the axis of the bore.

17. A rotary ball valve assembly as claimed in claim 10 , wherein the impedance assembly includes a cap having through-holes forming part of the flow passages.

18. A rotary ball valve assembly as claimed in claim 1 , wherein the impedance assembly is positioned at the flow outlet of the closure element.

19. A rotary ball valve assembly as claimed in claim 1 , wherein the impedance assembly is positioned at the flow inlet of the closure element.

20. A rotary ball valve comprising a valve housing in which is housed a rotary ball valve assembly as claimed in claim 1 .