received by the International Bureau on 01 July 2015 (01 .07.2015)

1. A system for reducing pressure in a gas flow for a gas let-down system comprising an expander driven by gas at a first pressure expanding to a second pressure, and a compressor for compressing the gas from the second pressure to a third pressure, whereby the third pressure is lower than the first pressure and the third pressure is higher than the second pressure.

2. A system according to Claim 1 , wherein the expander drives the compressor, preferably directly.

3. A system according to Claim 2 wherein the expander drives the compressor by way of a common shaft. 4. A system according to any preceding claim, further comprising a heat exchanger for heating the gas at the second pressure.

5. A system according to Claim 4, wherein the heat exchanger is arranged to provide heat exchange to ambient air.

6. A system according to Claim 4 or 5, wherein the heat exchanger is arranged to provide cooling to a refrigeration load.

7. A system according to any of Claims 4 to 6, wherein the heat exchanger is arranged to provide heat exchange to ground, or water, or an ambient heat source, or a waste heat source.

8. A system according to any of Claims 4 to 7, further comprising a plurality of heat exchangers, with each of the heat exchangers arranged to provide heat from a different heat source.

9. A system according to any of Claims 4 to 8, wherein a secondary circuit Is arranged to transfer heat from a heat source to the or a heat exchanger. 25

10. A system according to any preceding claim, wherein the expander further drives an electric generator.

1 . A system according to any preceding claim, wherein the system is arranged to conduct a portion of gas to the inlet of the expander, and to conduct a further portion of gas to the outlet of the compressor, the further portion of gas bypassing the expander and the compressor.

12. A system according to Claim 1 1 , wherein the further portion of gas drives a further expander that optionally further drives an electric generator.

13. A system according to any of Claims 1 to 10, wherein the system is arranged to conduct a portion of gas to the inlet of the expander, and to conduct a further portion to the compressor, the further portion bypassing the expander.

14. A system according to Claim 13, wherein the further portion drives the compressor.

15. A system according to Claim 14, wherein the further portion drives the compressor by means of a tip turbine.

16. A system according to any of Claims 1 to 10, wherein the system is arranged to conduct a portion of gas to the inlet of the expander, and to conduct a further portion of gas to an inlet of a further expander.

17. A system according to Claim 16, wherein the further expander and the expander both drive the compressor.

18. A system according to Claim 16 or 17, wherein the system is arranged to conduct the further portion of gas from the outlet of the further expander to the outlet of the compressor.

19. A system according to any preceding claim, further comprising a recuperator for transferring heat from one portion of the gas to another. 26

20. A system according to Claim 19 when dependent on any of Claims 4 to 18, wherein the recuperator is arranged to transfer heat from gas upstream of the expander to gas downstream of the heat exchanger. 21. A system according to Claim 19, wherein the recuperator is arranged to transfer heat from gas downstream of the compressor to gas upstream of the expander.

22. A system according to any preceding claim, wherein the system is arranged to conduct a portion of gas from the outlet of the compressor to the inlet of the heat exchanger.

23. A system according to Claim 22, wherein the system is arranged to conduct a portion of gas from the outlet of the compressor to the inlet of the compressor.

24. A system according to any preceding claim further comprising a sealable vessel containing system rotative components. 25. A system according to Claim 24, wherein the system rotative components include an output drive shaft of the expander and an input drive shaft of the compressor, and preferably the or a common shaft by way of which the expander drives the compressor. 26. A system according to Claim 24 or 25 further comprising a gas bearing supporting the output drive shaft of the expander and/or the input drive shaft of the compressor.

27. A system according to Claim 24 or 25 further comprising a magnetic bearing supporting the output drive shaft of the expander and/or the input drive shaft of the compressor.

28. A system according to any preceding claim further comprising a controller for activating the system when a system inlet gas temperature is below a pre- defined threshold. 27

29. A system according to any preceding claim, wherein the expander comprises a turbine.

30. A system for reducing pressure in a gas flow for a gas let-down system substantially as herein described with reference to Figures 1 to 6.

31. A gas let-down station comprising a system according to any of Claims 1 to 30. 32. A gas distribution network comprising a system according to any of Claims 1 to 30.