WO2016120626A1

WO2016120626A1 - A system for reducing pressure flow

Info

Publication number: WO2016120626A1
Application number: PCT/GB2016/050190
Authority: WO
Inventors: Adrian Alford
Original assignee: Corac Energy Technologies Limited
Priority date: 2015-01-28
Filing date: 2016-01-28
Publication date: 2016-08-04
Also published as: GB2536866A

Abstract

A system for reducing pressure in a gas flow for a gas let-down system comprising: an expander driven by a first gas input; a heat exchanger, wherein an output of the expander is input to the heat exchanger; and a compressor driven by a second gas input; wherein a portion of the heat exchanger output is combined with gas having passed through said compressor, to form an outlet flow; such that the pressure in the outlet flow being lower than the pressure of the first gas input.

Description

A system for reducing pressure flow

The present invention relates to an apparatus, system or method for reducing pressure in a gas flow for a gas let-down system.

The present invention relates to a system for reducing pressure in a gas flow for a gas let-down system comprising an expander driven by a first gas input; a heat exchanger, wherein an output of the expander is input to the heat exchanger; and a compressor driven by a second gas input; wherein a portion of the heat exchanger output is combined with gas having passed through said compressor, to form an outlet flow; the pressure in the outlet flow being lower than the pressure of the first gas input. Such an arrangement allows for excess heat to be generated and utilised elsewhere if required. Preferably, the outlet flow comprises a portion of the compressor output. Optionally, the second gas input may comprise a portion of the first gas input. Optionally, the second gas input may comprise a portion of the heat exchanger output. Optionally, the system may further comprise a pressure reducer arranged to receive a portion of an output from the compressor. Preferably, a portion of the first input is directed to an inlet of the pressure reducer. Such a system may be retrofitted to, or replace an existing pressure reducer. Preferably, said portion of the first gas input is directed to bypass the expander and the compressor. Preferably, the pressure reducer is upstream to where the portion of the heat exchanger output is combined with a portion of an output from the compressor.

For defrosting and/or efficiency, preferably, the system is arranged to direct a portion of the compressor output to supply heat to the heat exchanger.

For efficiency, preferably the portion of the compressor output directed to supply heat to the heat exchanger is from a junction upstream to where the portion of the heat exchanger output is mixed with a portion of an output from the compressor. For efficiency, preferably the portion of the compressor output directed to supply heat to the heat exchanger is from a junction upstream of the pressure reducer. Optionally, the pressure of the first gas flow may be higher than the pressure of the gas flow at the input to the heat exchanger. Preferably, the pressure at the output of the compressor is higher than the second input flow pressure.

According to another aspect of the present invention there is provided 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; a compressor for compressing the gas from the first or second pressure to a third pressure; and means for combining a portion of gas at the second pressure with a portion of the gas having been passed through the compressor to form part of an outlet flow; wherein said outlet flow is at a lower pressure than the first pressure. Such a system may allow for excess heat to be generated and utilised elsewhere if required.

Preferably, the third pressure is higher than the second pressure. Optionally, the system may further comprise a pressure reducer. Preferably, the pressure reducer is adapted to receive the gas flow having been passed through the compressor prior to combination with said portion of gas at the second pressure. Preferably, the pressure reducer comprises a let-down valve. Optionally, the temperature of the outlet flow may be less than the temperature at the outlet of the compressor. So as to reducing freezing of components and/or for efficiency, the system preferably further comprises means for redistributing the heat from said higher temperature compressor outlet flow. Preferably, the means for redistributing the heat from said higher temperature compressor outlet flow is a defrost line. Optionally, the expander may drive the compressor, preferably directly. Preferably, the expander drives the compressor by way of a common shaft.

Preferably, the system further comprises a heat exchanger adapted to receive said gas at said second pressure. Preferably, the heat exchanger is arranged to provide heat exchange to ambient air. Preferably, the heat exchanger is arranged to provide cooling to a refrigeration load.

Preferably, for controllability and/or efficiency the system further comprises means for controlling the flow through the system. The means for controlling the flow through the system may comprise one or more valves. Preferably, the one or more valves may be selected from a group of valve types including at least the following: slam shut valve; two-way valve; or throttle valve. Preferably the system further comprises a sealable vessel containing system rotative components. Preferably the system rotative components include at least an output drive shaft of the expander and an input drive shaft of the compressor. The sealable vessel can enable the entire rotative system to operate in an environment at one of the system gas flow pressures, thus avoiding the requirement for rotating seals in communication with the external environment, therefore removing the risk of natural gas leakage and potential explosion.

Preferably the system further comprises a gas and/or magnetic bearing supporting the output drive shaft of the expander and/or the input drive shaft of the compressor. Gas and/or magnetic bearings can enable particularly low contamination of the gas in the expander and the compressor, in particular with respect to oil lubricant, as they require no oil lubricant.

Preferably the system further comprises a controller for activating the system when an ambient air temperature is below a pre-defined threshold. This can enable the system to operate selectively when the ambient environment makes it necessary, and otherwise assume another mode of operation such as in a conventional let-down gas expander. Optionally, the system may further comprise a recuperator for transferring heat from one portion of the gas to another. Optionally, the expander may comprise a turbine. Preferably, the turbine comprises a variable geometry turbine. This allows for a wide range of flow rates to be controlled efficiently by the system. According to another aspect of the present invention there is provided 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. By first expanding the gas and then compressing the gas the intermediate temperature of the gas at the second pressure is lower than if the gas is expanded directly to the third pressure. The gas is preferably natural gas. The expander is preferably a turbine. The combination of an expander and a compressor may also be referred to as a compander.

Optionally, the expander may drive the compressor, preferably directly. This can provide efficiency. As used herein "directly" preferably connotes without conversion to another energy form such as electricity, pressure or heat. Preferably the expander drives the compressor by way of a common shaft. Optionally a mechanical coupling may connect the expander and the compressor.

Preferably the system further comprises a heat exchanger for heating the gas at the second pressure. This can allow efficient heating of the gas. Preferably the heat exchanger is arranged to provide heat exchange to ambient air. The heat exchanger may be arranged to provide heat exchange to a secondary circuit for absorbtion of heat from ground or water or other ambient source or a waste heat source. The heat exchanger may be arranged to provide cooling to a refrigeration load. The heat exchanger may be arranged to provide heat exchange to ground, or water, or an ambient heat source, or a waste heat source. The system may further comprise a plurality of heat exchangers, with each of the heat exchangers arranged to provide heat from a different heat source. A secondary circuit may be arranged to transfer heat from a heat source to the or a heat exchanger. The cold gas at the outlet of the expander may transfer energy via a secondary circuit or a heat pipe to provide cooling to a refrigeration load.

Preferably the expander further drives an electrical generator. The electricity may be used to drive further electrical parts such as a fan and/or an electric defrosting heater. The electricity may be used for exportation to an electrical grid. Preferably 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. The further portion of gas may drive a further expander that optionally further drives an electrical generator. The further portion of gas may be expanded at a valve.

The system may provide sufficient excess heat in the compander outlet gas stream to be mixed with another stream (such as the further portion of gas) from a valve or an expander with an electrical output to produce a mixed gas stream at a suitable temperature for the downstream requirements.

Preferably 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. The further portion preferably drives the compressor, preferably by means of a tip turbine. The compressor preferably comprises a tip turbine. This can provide supplementary drive to the compressor.

Preferably 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. Preferably the further expander and the expander both drive the compressor. Preferably the system is arranged to conduct the further portion of gas from the outlet of the further expander to the outlet of the compressor. Optionally, the system may further comprise a recuperator for transferring heat from one portion of the gas to another. Preferably the recuperator is arranged to transfer heat from gas upstream of the expander to gas downstream of the heat exchanger. This can serve to reduce the gas temperature at the heat exchanger to a sufficiently low value to enable effective heat exchange with ambient air (or a secondary circuit for absorbtion of heat from ground or water or other ambient source or a waste heat source). This may be appropriate if the temperature drop and pressure drop across the expander are relatively small.

The recuperator may be arranged to transfer heat from gas downstream of the compressor to gas upstream of the expander. This can pre-heat the gas entering the expander. The recuperator can prevent the gas downstream of the expander from excessive cooling which can cause solid formation and frosting at the heat exchanger, leading to deterioration of the performance of and potentially damage to the system. Also, gas exiting the compressor can be provided at a high enough temperature for effective defrosting of the heat exchanger in case of solids formation occurring.

Preferably the system is arranged to conduct a portion of gas from the outlet of the compressor to the inlet of the heat exchanger. This can provide additional heating and prevent frosting of the heat exchanger. Different sections may be defrosted sequentially at different times. Preferably the system is arranged to conduct a portion of gas from the outlet of the compressor to the inlet of the compressor. This can enable adjustment of the duty position of the system by increasing compressor power when open.

Preferably the system further comprises a sealable vessel containing system rotative components. Preferably the system rotative components include at least an output drive shaft of the expander and an input drive shaft of the compressor. Preferably the system rotative components include the common shaft by way of which the expander drives the compressor. Preferably the sealable vessel contains the generator, expander and compressor.

Preferably the system further comprises a gas and/or magnetic bearing supporting the output drive shaft of the expander and/or the input drive shaft of the compressor.

Preferably the system further comprises a controller for activating the system when an ambient air temperature is below a pre-defined threshold. This can enable the system to operate selectively when the ambient environment makes it necessary, and otherwise assume another mode of operation such as in a conventional let-down gas expander. The system can assist when the ambient temperature (and in particular the temperature of the incoming gas which is typically at ground temperature) is too low to permit reducing pressure in a gas flow directly from the first to the third pressure, as in a conventional gas let-down expander. When the ambient temperature is high enough to permit reducing pressure in a gas flow directly from the first to the third pressure the system can be deactivated. The controller may also activate the system for a pre-determined period of the year, preferably during a period of the year that is expected to have an ambient air temperature below a threshold (for example in winter months). Alternatively, if the system is operated when an ambient air temperature is above a pre-defined threshold, then the generator is able to generate more electricity. In this example the system may further comprise a controller for exporting surplus electricity when an ambient air temperature is above a pre-defined threshold. Preferably the system further comprises a drying system as described below.

Features of the system may include:

• Turbine directly drives compressor

• For use in natural gas let-down of compressed natural gas

· Process gas is used as the operating fluid for the heat pump

According to a further aspect of the invention, there is provided apparatus for gas let-down comprising an expander driving a compressor. Preferably the expander comprises a turbine.

According to a further aspect of the invention, there is provided a gas let-down station comprising a system for reducing pressure in a gas flow as described above. According to a further aspect of the invention, there is provided a gas distribution network comprising a system for reducing pressure in a gas flow as described above.

According to a further aspect of the invention, there is provided a system for reducing pressure in a gas flow for a gas let-down system substantially as herein described and/or with reference to the accompanying drawings. The invention extends to methods and/or apparatus substantially as herein described with reference to the accompanying drawings.

Preferably, the term "downstream" is used herein to refer to a later point along a gas flow path in the direction of flow, and the term "upstream" is used herein to refer to an earlier point along the direction of flow. Any apparatus feature as described herein may also be provided as a method feature, and vice versa. As used herein, means plus function features may be expressed alternatively in terms of their corresponding structure. Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa. Furthermore, any, some and/or all features in one aspect can be applied to any, some and/or all features in any other aspect, in any appropriate combination.

It should also be appreciated that particular combinations of the various features described and defined in any aspects of the invention can be implemented and/or supplied and/or used independently. These and other aspects of the present invention will become apparent from the following exemplary embodiments that are described with reference to the following figures in which:

Figure 1 shows an embodiment of a system for reducing pressure in a gas flow; Figure 2a shows a further embodiment of a system for reducing pressure in a gas flow;

Figure 2b shows a further embodiment of a system for reducing pressure in a gas flow;

Figure 2c shows a further embodiment of a system for reducing pressure in a gas flow;

Figure 3 shows an embodiment of a system for reducing pressure in a gas flow with a low let-down pressure drop;

Figure 4 shows a further embodiment of a system for reducing pressure in a gas flow with a low let-down pressure drop;

Figure 5 shows an embodiment of a system for reducing pressure in a gas flow with a high let-down pressure drop;

Figure 6 shows an embodiment of a system for reducing pressure in a gas flow with an upstream separator;

Figures 7a - d show embodiments of a system for reducing pressure in a gas flow with an expander that is out of series with a compressor; Figure 8 shows an embodiment of a system for reducing pressure in a gas flow suitable for electricity generation;

Figure 9 shows a more detailed view of the system shown in Figure 8;

Figure 10 shows an embodiment of a system for reducing pressure in a gas flow configured to provide heat to a pressure reducer; and

Figure 1 1 shows a more detailed view of the system shown in Figure 10.

At a conventional gas let-down station gas (natural gas) pressure is reduced by expansion across a valve. Following expansion the outlet temperature can be too low to allow the gas to be reintroduced to the downstream pipe network, so the gas needs to be heated to offset the cooling effect from the expansion of gas. Conventionally gas is burned to provide this heat, with both an economic and an environmental cost. Water bath heating, as is conventionally used for heating expanded gas at let-down stations, uses significant quantities of gas and is expensive to run and maintain. Heating by burning gas is also environmentally damaging due to the CO₂ release from combustion.

It has been proposed to use the expansion energy to generate electricity and to use this electricity to run a vapour compression heat pump to heat the gas. This is an expensive and complex way to heat the gas.

A gas let-down system with an expander and combined heat pump that uses the expansion energy from a turbine to directly drive a compressor which acts as a heat pump is proposed. The intermediate pressure of the gas, downstream of the turbine and upstream of the compressor, is lower than the outlet pipeline pressure. At the same time the intermediate temperature of the gas is lower than it would be if the gas were brought directly to the outlet pipeline pressure. The serial combination of the expander and the compressor allows the turbine to work over a greater pressure ratio and increases the temperature drop of the gas. Ordinarily in gas let-down the outlet gas temperature is too high to allow effective heat exchange with the environment. Due to the small temperature difference, in the order of for example only a few degrees Celsius, the size of the heat exchanger (in the order of magnitude of acres) would be prohibitive. Heat exchange may not even be possible if the air temperature is too low. By the combination of the expander and the compressor in series, a greater temperature difference is available, for example 20°C or more. With the increased gas temperature drop, efficient heat exchange with the environment at the cold intermediate condition can heat the gas. The size of the heat exchanger can be in the order of tens of square metres rather than the prohibitively sized heat exchangers that would be required in conventional systems. Instead of requiring burning of gas to heat the gas, heat exchange with the environment can raise the gas temperature sufficiently.

This can achieve a number of things:

1 . Increase the temperature difference with ambient air (or other suitable ambient or waster heat sources) to allow smaller heat exchangers

2. Allow some pre-heat to be applied to the gas

3. Move the position on the psychrometric chart to reduce the propensity of the turbine to ice up (in particular control the pressure and temperature in order to decrease the relative humidity of gas passing through the turbine)

4. Provide heat to defrost external heat exchangers

The system harvests heat from the atmosphere (or other suitable ambient or waster heat sources) and uses this to pre-heat the gas, avoiding the need for gas combustion and water bath heaters. The combination of expander and compressor provides outlet gas at the required pressure and temperature for the downstream network. The system has aspects of both expander and heat pump, taking atmospheric heat and boosting it to a temperature suitable for the efficient heating of gas within the process to acceptable outlet temperatures.

Figure 1 shows a basic system 10 for reducing pressure in a gas flow 120. The gas flow 120 is directed first to an expander 102, typically a turbine (but other expanders such as a screw expander are possible), and subsequently to a compressor 104. The intermediate pressure of the intermediate gas flow 128, downstream of the turbine 102 and upstream of the compressor 104, is lower than the outlet pipeline pressure of the outlet gas flow 126. The intermediate temperature of the intermediate gas flow 128 is lower than if the gas pressure were at the outlet pipeline pressure. At the heat exchanger 108 atmospheric air provides heat to warm up the intermediate gas flow 128. The warmed intermediate gas flow 128 is directed to a compressor 104 that increases the gas pressure; downstream of the compressor 104 the system outlet gas flow 126 is at the outlet pipeline pressure. The outlet pipeline pressure of the outlet gas flow 126 is below the inlet pipeline pressure of the inlet gas flow 120; and the intermediate pressure of the intermediate gas flow 128 is below the outlet pipeline pressure of the outlet gas flow 126.

The compressor and turbine are relatively balanced with 1 -2MW shaft power transferred. In Figure 1 the heat exchanger 108 is arranged to warm up the intermediate gas flow 128 where atmospheric air provides heat. In an alternative, such an air source heat pump is replaced with a ground source heat pump, such that a ground source provides heat to warm up the intermediate gas flow 128. Other heat sources for warming up the intermediate gas flow 128 can be used as available and convenient.

Figure 2a shows a variant of the system shown in Figure 1 , where the cold gas at the outlet of the expander (the intermediate gas flow 128) is arranged to provide cooling to a refrigeration load by transferring a portion of the energy away from the cold gas. The transfer of energy for refrigeration purposes can be arranged via a heat exchanger 121 to where heat is absorbed from a secondary circuit 123 as shown in Figure 2a. Alternative means for the transfer of energy, such as a heat pipe for example may be used. The system can be configured to additionally generate electricity. The electrical generation is 100kW in an example, which provides enough electricity to operate fans for the system. An electric defrost system (in particular for low temperature portions of the gas flow) can also or alternatively be operated by the electricity. Systems configured to also be operable during summer months can convert a significant proportion of the turbine power to electrical power due to the reduced necessity to use the heat pump features of the system, with up to 1 MW able to be exported.

Figure 2b shows a system 20 for reducing pressure in a gas flow 120 having electrical generator 1 10. The turbine 102 drives both the compressor 104 and also the electrical generator 1 10. The electricity generated by the electrical generator 1 10 is used to power a fan 1 12 that assists the heat exchanger 108. Further, a pressure envelope 130 encloses the turbine 102, the compressor 104 and the generator 1 10. The pressure envelope 130 includes suitable connections for inlet and outlet of gas streams and also for electrical connection between the generator 1 10 and the electrical load. The pressure in the pressure envelope 130 is at the intermediate pressure of the intermediate gas flow 128. This avoids the necessity for shaft seals (on the expander shaft and the compressor shaft) across a pressure drop (to the external environment) and the risk of seal failure, and allows the use of gas bearings for the turbine shaft and compressor shaft. The use of gas or magnetic bearings is advantageous as oil contamination of the gas can be minimised. The pressure envelope 130 contains natural gas, same as the incoming and outgoing streams, and residual mass transport across the bearings does not cause contamination of the gas stream.

The systems disclosed above may include the following features:

• The combination of expander and heat pump using a single rotating element.

• Process gas used as the operating fluid for the heat pump.

• No net electrical output from the system; a small electrical power output from the generator (ca. 1 % of expander shaft power) can be used to run fans, with the remainder running the heat pump.

• The system can be hermetic with no seals to ambient, and can run on gas bearings, precluding the potential for leaks, seal failure or oil contamination of the gas.

• The system can be made available as a road transportable temporary unit, for use on site when an existing conventional let-down system with water bath heaters fails, so removing the requirement for a secondary stand-by unit to stand alongside unused. This is possible due to the system being configured to enable the heat exchanger to be appropriately sized for housing in such a unit yet being able to sufficiently increase the gas temperature intermediate the expander and compressor.

• The system can have remote diagnostic and control capability reducing or preventing site visits.

A further sub-system to reduce or virtually eliminate the frosting of the external heat exchanger is a drier that dries ambient atmospheric air before it comes into contact with the sub-zero heat exchange surfaces in order to heat the gas stream. A possible way to achieve this is to use a portion of the cold air exiting the external heat exchanger to pre-cool the air entering the heat exchanger, and dropping the temperature of the air entering the heat exchanger it to just above zero degrees to allow a large part of the condensate load to drop out. This can then be separated from the air stream. A system for drying air for a heat exchanger is described in more detail with reference to Figure 7 and 8.

Optionally, the system for reducing pressure in a gas flow may only be used if certain conditions are provided, for example the natural gas at the system inlet being below a certain threshold (or the ambient air temperature being below a certain threshold). For example, the system can be used only in winter, and in summer when the inlet gas temperature (and also the ambient air temperature) is relatively high the system can be switched into a summer mode with the gas being expanded directly to the outlet pipeline pressure (omitting an intermediate condition with lower pressure and temperature). At the higher temperature the gas is expanded directly to the outlet pipeline pressure and requires no heating as it remains above the minimum temperature required by the downstream system. Alternatively the system can be operated at all temperatures, and when the temperature is above a certain threshold surplus electricity can be generated.

Figure 2c shows a variant of the system shown in Figure 1 . A first portion 131 of the gas flow 120 bypasses the expander 102 and heat exchanger 108 and the compressor 104. A second portion 124 of the gas flow is directed to the expander 102 and heat exchanger 108 and compressor 104. The first portion 131 of the gas flows via a pressure reducer 133 (such as a valve or an expander), optionally with an associated electrical generator for generating an electrical output, and is reintroduced downstream of the compressor 104. The gas flow at the outlet of the compressor 104 system provides sufficient excess heat to be mixed with the first portion 131 of the gas to produce a mixed gas stream at a suitable temperature for the downstream requirements. The second portion 124 is expanded directly to the system outlet (target) pressure, and mixed with a warmer gas stream treated by the system shown in Figure 1 in order to ensure the resulting gas stream exiting the system is sufficiently heated. Figures 3 to 6 show further versions of the system for reducing pressure in a gas flow. For the heat exchanger a blast cooler type heat exchanger is used. A recuperator can provide supplementary heat exchange. An expander turbine drives the compressor. Additionally an electrical generator is driven by the turbine and produces electricity for fans that assist the heat exchange at the blast cooler.

Figures 3 and 4 show systems for reducing pressure in a gas flow with a relatively low let-down pressure drop, for example with a pressure drop from 44 bar at the system inlet to 38 bar at the system outlet. The intermediate pressure is for example 20 to 25 bar. The temperature drop is around 0.45 to 0.6 °C per bar pressure drop. Due to the relatively low let-down pressure drop, and assuming the expander has a suitably high pressure ratio (in order to ensure the gas temperature is sufficiently low at the heat exchanger), the compressor needs to provide relatively large compression in order to bring the gas back to the outlet pipeline pressure. Due to limited efficiency of the turbine, the turbine alone may not suffice to drive the compression in this case. Therefore the systems described with reference to Figures 3 and 4 provide supplemental compression drive, in addition to the drive provided to the compressor from the expander. Figure 3 shows a system 100 for reducing pressure in a gas flow 120. In this system 100 there is a relatively low let-down pressure drop. A first portion 122 of the gas flow bypasses the expander 102 and heat exchanger 108 and is reintroduced at the compressor 90. A second portion 124 of the gas flow is directed to the turbine expander 102 and compressor 90. The compressor 90 is a compressor with a tip turbine, and the first bypass gas flow portion 122 drives the compression of the second heated gas flow portion 124 by means of the tip turbine. In the tip turbine the first bypass gas flow portion 122 accelerates the compressor blades and provides supplemental compression drive, in addition to the drive provided to the compressor 90 from the expander 102. A recuperator 106 allows transfer of some of the heat from the gas upstream of the expander 102 to the gas downstream of the heat exchanger 108. This can reduce the gas temperature at the heat exchanger to a sufficiently low value to enable effective heat exchange. At the heat exchanger 108 atmospheric air provides heat to warm up the intermediate gas flow 128. The warmed gas is fed into a compressor 90 and is compressed and combined with the first bypass gas flow portion 122 to form the system outlet gas flow 126 at a lower outlet pipeline pressure. The turbine 102 drives both the compressor 90 and also an electrical generator 1 10. The electricity generated by the electrical generator 1 10 is used to power a fan 1 12 that assists the heat exchanger 108.

Figure 4 shows a further system 200 for reducing pressure in a gas flow 120. In this system 200 also there is a relatively low let-down pressure drop. Two expanders 102 202 are combined. A first portion 222 of the gas flow is expanded in a supplementary expander 202 directly to the outlet pipeline pressure and reintroduced downstream of the compressor 104. A second portion 224 of the gas flow is directed to the turbine expander 102 and compressor 104. The supplementary expander 202 contributes supplemental drive to the drive provided from the heat pump expander 102 in order to provide sufficient drive to the compressor 104. This can enable a sufficiently low intermediate pressure (and hence sufficiently low gas temperature at the heat exchanger) for efficient heat exchange with ambient atmosphere air.

In the systems in Figures 3 and 4 the recuperator 106 is arranged to transfer heat from the incoming gas flow 120 upstream of the expander 102 to the intermediate gas flow 128, downstream of the heat exchanger 108 and upstream of the compressor 104. The purpose of the recuperator 106 is to reduce the gas temperature at the heat exchanger to a sufficiently low value to enable effective heat exchange with ambient air or ambient air which has been dried by cooling.

For the recuperator 106 to provide efficient transfer of heat between different gas stream portions, the temperature difference between the different gas stream portions should be sufficiently large, typically at least 5 or 10 °C. In the systems with a relatively low let-down pressure drop described with reference to Figures 3 and 4, the temperature difference between the incoming gas flow 120 and the system outlet gas flow 126 can be relatively small and insufficient for providing heat to the inlet side, in which case the recuperator arrangement as shown in Figures 3 and 4 and as described above can be appropriate.

Figure 5 shows a further system 300 for reducing pressure in a gas flow 120. In this system 300 there is a relatively high let-down pressure drop, for example with a pressure drop from 33 bar at the system inlet to 17 bar at the system outlet. The intermediate pressure is for example 9 to 12 bar. The temperature drop is around 0.45 to 0.6 °C per bar pressure drop.

A recuperator 302 allows transfer of some of the heat from the gas downstream of the compressor 104 to the gas upstream of the expander 102. This can provide pre-warming of the gas entering the expander 102. The gas flow 120 is directed to the turbine expander 102 and compressor 104.

Downstream of the compressor 104 a recirculation gas flow 322 is separated from an outlet gas flow 320. The outlet gas flow 320 passes to the recuperator 302 before progressing to the main outlet gas flow 126. The recirculation gas flow 322 is split into a bypass line 324 and a defrost line 326. The flow in the defrost line 326 is reintroduced upstream of the heat exchanger 108. Because the flow in the defrost line 326 is relatively warm it can heat the heat exchanger 108 and thus avoid frosting in the heat exchanger 108. The flow of the bypass line 324 is reintroduced downstream of the heat exchanger 108 and upstream of the compressor 104 and can be used to adjust the compressor duty.

Flow controllers 304 control the flow rate in the bypass line 324 and defrost line 326 based on (for example) temperature sensing or flow speed sensing. The flow controllers 304 can include an actuated valve.

In Figure 5 typical temperatures are indicated for the different gas flows. Only one defrost line 326 is shown, but more may be included in the system 300. The defrost line 326 can defrost both internal and external heat exchanger surfaces. The duty of the heat exchanger 108 (blast cooler) in Figure 5 is approximately 1 .1 MW. The duty of the recuperator 302 in Figure 5 is approximately 1 .35MW.

The recirculation gas flow 322 shown in Figure 5 can be used with all system variants.

Figure 6 shows a further system 400 for reducing pressure in a gas flow 120. In this system 400 a separator 402 is included upstream of the recuperator 302. The separator 402 separates the gas flow 120 into a liquid stream 424 and a substantially dry gas stream 422. The liquid stream 424 can include a gas and/or solid component. The substantially dry gas stream 422 passes to the recuperator 302 same as the gas flow 120 in the system 300. Actuated valves 406 control the flow in the liquid stream 424 and the gas stream 422. If necessary a heat addition 404 to the liquid stream 424 can be provided, for example by exchange with heated compressor outlet gas, or by mixing with a hot stream (with suitable restrictors).

In Figure 6 typical temperatures are indicated for the different gas flows. The defrost line 326 can defrost both internal and external heat exchanger surfaces. Only one defrost line 326 is shown, but more may be included in the system 400. The duty of the heat exchanger 108 (blast cooler) in Figure 6 is approximately 1 .1 MW. The duty of the recuperator 302 in Figure 6 is approximately 1 .35MW.

The separator 402 shown in Figure 6 can be used with all system variants. If the separator 402 is upstream of a pre-heater (where a pre-heater is used) such as the recuperator 302 shown in Figure 6, then on entering the pre-heater the flow has very low or no liquid loading (as the liquid stream has been removed in the separator), and is a saturated gas. The heating process in the pre-heater (recuperator 302 in Figure 6) moves the gas away from its saturation line, drying it and therefore reducing the tendency to frost downstream of the expander 102. If the separator 402 is downstream of the pre-heater, then liquid is evaporated in the pre-heater which means that the liquid stream that is removed in the separator is reduced; also, the gas exiting from the separator would be saturated, or closer to its saturation line on entering the expander 102, so more frosting would occur downstream of the expander 102. Hence locating the separator 402 upstream of the pre-heater is preferred.

In the systems in Figures 5 and 6 the recuperator 302 is arranged to transfer heat from the system outlet gas flow, downstream of the compressor 104, to the incoming gas stream upstream of the expander 102. In essence, this can transfer some of the heat gained from the heat exchange with ambient atmospheric air to the low pressure, low temperature portions of the gas flow in order to change the gas conditions such that condensation and especially ice formation is prevented. In some circumstances the recuperator 302 may not be necessary and can be omitted.

In a further intermediate pressure version of the system the features of a supplementary expander 202 as described with reference to Figure 4 and a recuperator 302 arrangement as described with reference to Figures 5 and 6 are combined.

As can be seen from the above arrangements, the gas pressures at gas let-down stations can vary considerably depending on a variety of factors and the systems disclosed above are adaptable to the differing pressures and can be configured as discussed to efficiently reduce the pressure of a gas to a desired outlet gas pressure.

Sub-zero expander outlet

The sublimation of water vapour to ice is a non-equilibrium process so does not occur quickly enough for ice crystals to appear in the expander turbine in a volume or size to cause significant problems. Ice crystals may form in the gas downstream of the turbine, and can clog downstream equipment such as heat exchangers. The system can have defrost units for this eventuality.

Liquid formation can occur within the turbine and this is generally hydrocarbon matter that does not freeze at the temperatures considered here. Condensation may occur in the turbine wheel and not in the nozzles so erosion is avoided. It may be necessary to heat the expander shroud/body.

Out-of-series Configurations Figure 7a shows a further system 700 for reducing pressure in a gas flow 120. The gas flow 120 is directed first to an expander 102 in a similar manner as described above. The expander 102 drives a compressor 104. In one example, the expander 102 comprises a turbine.

Following expansion by the expander 102, the intermediate gas flow 128 (at a lower pressure to the gas flow 120) is then directed to a heat exchanger 108. At the heat exchanger 108 atmospheric air provides heat to warm up the intermediate gas flow 128. The warmed intermediate gas flow 128 is then directed to a location 702 downstream of the compressor 104, where it may mix with the gas flow exiting the compressor 104. The compressor may receive a portion of the inlet gas flow 120 as shown in Figure 7b. Alternatively, the compressor may receive a portion of the warmed intermediate gas flow 128 from downstream of the heat exchanger 108, as shown in Figure 7c.

In the system 700, the expander 102 and compressor 104 are not in series as previously described with reference to other variants of the system 10, as at least a portion of the intermediate gas flow 128 bypasses the compressor 104. This provides the advantage that the compressor 104 causes a greater increase in temperature on the gas flowing through the compressor 104 than in other variants of the system 10 as a result of the reduced mass flow through the compressor 104.

The lower temperature intermediate gas flow 128 is mixed with the compressor 104 outlet flow; such an arrangement allows for a compressor 104 outlet flow having a higher temperature than is required for the final outlet gas flow 126. This excess heat may be redistributed and used to defrost other parts of the system as is described below with reference to Figures 8-1 1 . As a result of this arrangement, the gas flow at the outlet of the compressor 104 can more effectively defrost components of the system 700 (such as the heat exchanger 108, for example) without the need to incorporate further components such as a recuperator into the system 700; this is because the gas flow at the outlet of the compressor 104 is at a sufficiently high temperature. In many situations, it is desirable to not use a recuperator due to the typically large size and high capital cost of such a device. Additionally, this arrangement may provide a reduced temperature of the gas flow at the outlet of the expander 102, improving the efficiency of the heat exchanger 108 and thereby allowing a reduction in the size of heat exchanger 108 used in the system.

Figure 7d shows a simplified version of the system 700 shown in Figure 7a without a heat exchanger 108. In this version of the system 700, the intermediate gas flow 128 may be heated by ambient air surrounding the system 700. Figure 8 shows a variant of the system 800 shown in Figure 7c in which the warmed intermediate gas flow 128 is separated into two lines. A first heat exchanger outlet line 802 provides the inlet flow to the compressor 104. A second heat exchanger outlet line 804 bypasses the compressor 104 and mixes with the compressor output.

Downstream of the compressor 104 a defrost line 326 is separated from the gas flow exiting the compressor 104 at a junction 806. This junction 806 is positioned upstream of the compressor bypass junction 702 as the gas is at a higher temperature at this point. The defrost line 326 can be used to supply heat to the heat exchanger 108 so as to avoid solid formation and frosting at the heat exchanger. This may be afforded by the heat exchanger 108 comprising two separate parts which are alternated: an operational part through which the intermediate gas flow 128 is passed; and a defrosting part for a heated gas (such as from the defrost line 326) to provide heat to the heat exchanger 108. Figure 8 illustrates heat being provided by the defrost line 326 to the heat exchanger 108 via a junction in the inlet to the heat exchanger 108, in practice flow controllers 304 may be used to ensure that only one stream (only the intermediate gas flow 128 or gas flowing through the defrost line 326) passes through the heat exchanger 108 at any time.

Downstream of the junction 806 where the defrost line 326 is separated from the compressor outlet gas flow, the second line heat exchanger outlet line is reintroduced to the remaining portion of the compressor outlet gas flow, producing a mixed gas stream. In one example, the gas flow at the outlet of the compressor 104 system provides sufficient excess heat such that the mixed gas stream is at a suitable temperature for the downstream requirements.

As previously described, flow controllers 304 are provided to control the rate of the flow through the compressor 104 and defrost line 326 based on (for example) temperature sensing or flow rate sensing. The flow controllers 304 can be used to adjust the compressor 104 duty. The flow controllers 304 may be controlled such that gas flow through the compressor 104 is minimised, when, for example, there are no requirements to provide heat to the heat exchanger 108 or downstream components, such as might be the case in relatively warm ambient conditions.

The expander 102 may drive both the compressor 90 and an electrical generator 1 10. This generated electricity may be used for exportation to an electrical grid or provide power to electrical components locally to the let-down valve.

Figure 9 shows an embodiment of the system 800 in more detail. For reasons of clarity, the electrical generator 1 10 is not shown in this figure. In this embodiment, two heat exchangers 108 are used in parallel, with the defrost line 326 being split so as to supply heat to each heat exchanger 108. Flow controllers 304c allow differing amounts of flow to go to each heat exchanger 108. Figure 9 also shows more detail regarding flow controllers 304. Different types of flow controllers 304 may be provided for differing uses. One or more slam shut valves 304a are provided so that the gas flow through the system 800 may be quickly shut down in the event of an emergency. In the system 800, a slam shut valve 304a is arranged to shut off the inlet gas flow 120. Throttle valves 304b are provided to control the rate of the flow through the various lines of the system 800. In the system shown in Figure 9, two-way valves 304c are provided so that the defrost line 326 may be set as input to the heat exchanger 108 to defrost the heat exchanger 108, rather than the intermediate gas flow 128 being the inlet flow to the heat exchanger 108 (as in normal operation). The inlet gas flow 120 to the expander 102 may be shut off (for example using slam shut valve 304a). It will be appreciated that many different arrangements of the flow controllers 304, 304b, 304c may be used to control the flow through the system 800. Figure 10 shows another variant of the systems 900 shown in Figures 7a-c. This variant of the system is designed to be attached around a pressure reducer 906. Typically, the pressure reducer 906 would comprise a let-down valve (and may be pre-existing in a system to which the present system is retrofitted - either on a permanent or temporary basis) or alternatively may comprise an expander. A first portion of the inlet gas flow bypasses the expander 102 and heat exchanger 108 and the compressor 104 and flows via the pressure reducer 906. A second portion of the inlet gas flow 120 is separated into an expander inlet flow 902 and a compressor inlet flow 904. The compressor inlet flow 904 flows via the compressor 104 and is reintroduced into the first portion of the inlet gas flow at a location 908 upstream of the pressure reducer 906. The gas flow at the outlet of the compressor 104 is sufficiently heated such that the mixed flow entering the pressure reducer 906 is sufficient to prevent the freezing of the pressure reducer 906. The expander inlet flow 902 flows via the expander 102 and the heat exchanger 108 and is reintroduced at a location 702 downstream of the pressure reducer 906. The expander 102 reduces the temperature of the expander inlet flow 902 sufficiently such that the flow can be efficiently warmed by the heat exchanger 108. The warmed intermediate gas flow 128 is then at a sufficient temperature such that the outlet gas flow 126 downstream of the pressure reducer 906 is at a suitable temperature for the downstream requirements.

As previously described, flow controllers 304 are provided to control the rate of the flow through the expander 102, compressor 104 and defrost line 326. The flow controllers 304 can be controlled to allow gas to only flow through the pressure reducer 906, by blocking the flow of gas through all other lines. The system 900 may thereby be held in reserve and used only when the temperatures at the pressure reducer 906 or downstream components become low enough that there is a risk of components freezing. The system also comprises a defrost line 326 as previously described. The defrost line 326 originates at a junction 806 upstream (i.e. closer to the compressor output) to the junction 908 of the bypass line as the gas flow is at a higher temperature at this point. The system 900 is particularly useful for very cold ambient conditions, for example, or any other situation in which freezing of any pressure reducer 906 in a gas let-down system is an impediment to such a system functioning effectively. It will be appreciated that the pressure reducer 906 described above may optionally comprise a system 10 for reducing pressure in a gas flow as described above. It will also be appreciated the system 900 may be configured for electricity generation, when the expander 102 is configured to drive an electrical generator 1 10, or to supply heat to the inlet gas flow 120, using a recuperator as previously described or by other means. Figure 1 1 shows an embodiment of the system 900 in more detail. In this embodiment, two heat exchangers 108 are used in parallel, with the defrost line 326 being split so as to supply heat to each heat exchanger 108. More details about the flow controllers 304 are also shown, as previously described with reference to Figure 9. The pressure reducer 906 is shown as comprising a set of slam shut and throttle valves 304a, 304b and a heat exchanger 108, which may heat the gas entering the pressure reducer 906. It will be appreciated that this configuration of the pressure reducer 906 is exemplary, and many different arrangements may be used to reduce the pressure of the gas. Variable Geometry Turbine

The expander 102 may comprise a variable geometry turbine. A variable geometry turbine comprises movable vanes which may be controlled by means of one or more actuators to modify the amount of gas flow through the turbine. Variable geometry turbines allow operation across a greater pressure ratio and improvement of efficiency across a range of flow rates compared to fixed turbines which may be designed for a specific flow rate.

The use of variable geometry turbines thereby allows expansion of the operating range of the system without necessarily compromising on efficiency. Any of the systems described above with reference to Figures 1 to 1 1 could incorporate a variable geometry turbine so as to be used with a variable gas flow without the need to use a pressure reducer within such a system. A pressure reducer, such as a conventional let-down station valve, may otherwise be required to effectively reduce the pressure of the gas when the gas flow is outside of the operating range of the system. Additionally, the use of a variable geometry turbine may reduce the requirements on a recuperator 106, 302 to pre-heat the inlet gas flow 120 (as described in the systems 100, 200, 300, 400 with reference to Figures 3- 6) or eliminate the need for a recuperator 106, 302 entirely. In a further example, two systems as previously described where the expander comprises a variable geometry turbine may be used in parallel to increase the efficiency across a broad flow range.

It will be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Reference numerals appearing in the claims are by way of illustration only and shall have no limiting effect on the scope of the claims.

Claims

1 . A system for reducing pressure in a gas flow for a gas let-down system comprising:

an expander driven by a first gas input;

a heat exchanger, wherein an output of the expander is input to the heat exchanger; and

a compressor driven by a second gas input;

wherein a portion of the heat exchanger output is combined with gas having passed through said compressor, to form an outlet flow;

such that the pressure in the outlet flow being lower than the pressure of the first gas input.

2. A system according to Claim 1 wherein the outlet flow comprises a portion of the compressor output.

3. A system according to Claim 1 or 2, wherein the second gas input comprises a portion of the first gas input.

4. A system according to Claim 1 or 2, wherein the second gas input comprises a portion of the heat exchanger output.

5. A system according to any preceding claim, wherein the system further comprises a pressure reducer arranged to receive a portion of an output from the compressor.

6. A system according to Claim 5, wherein a portion of the first input is directed to an inlet of the pressure reducer.

7. A system according to Claim 6 wherein said portion of the first gas input is directed to bypass the expander and the compressor.

8. A system according to any of claims 5 to 7 wherein the pressure reducer is upstream to where the portion of the heat exchanger output is combined with a portion of an output from the compressor.

9. A system according to any preceding claim wherein the system is arranged to direct a portion of the compressor output to supply heat to the heat exchanger.

10. A system according to claim 9 wherein the portion of the compressor output directed to supply heat to the heat exchanger is from a junction upstream to where the portion of the heat exchanger output is mixed with a portion of an output from the compressor.

1 1 . A system according to claim 9 or 10 when dependent on any of claims 5 to 7 wherein the portion of the compressor output directed to supply heat to the heat exchanger is from a junction upstream of the pressure reducer.

12. A system according to any preceding claim wherein the pressure of the first gas flow is higher than the pressure of the gas flow at the input to the heat exchanger.

13. A system according to Claim 12 wherein the pressure at the output of the compressor is higher than the second input flow pressure.

14. 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;

a compressor for compressing the gas from the first or second pressure to a third pressure; and

means for combining a portion of gas at the second pressure with a portion of the gas having been passed through the compressor to form part of an outlet flow;

wherein said outlet flow is at a lower pressure than the first pressure.

15. A system according to Claim 14 wherein the third pressure is higher than the second pressure.

16. A system according to Claims 14 to 15 further comprising a pressure reducer.

17. A system according to Claim 16 wherein the pressure reducer is adapted to receive the gas flow having been passed through the compressor prior to combination with said portion of gas at the second pressure.

18. A system according to Claim 16 or 17 wherein the pressure reducer comprises a let-down valve.

19. A system according to any preceding claim, wherein the temperature of the outlet flow is less than the temperature at the outlet of the compressor.

20. A system according to Claim 19 further comprising means for redistributing the heat from said higher temperature compressor outlet flow.

21 . A system according to Claim 20 wherein the means for redistributing the heat from said higher temperature compressor outlet flow is a defrost line.

22. A system according to any preceding claim, wherein the expander drives the compressor, preferably directly.

23. A system according to Claim 22 wherein the expander drives the compressor by way of a common shaft.

24. A system according to any of Claims 14 to 23 further comprising

exchanger adapted to receive said gas at said second pressure.

25. A system according to any one of Claims 1 to 13 or Claim 24, wherein the heat exchanger is arranged to provide heat exchange to ambient air.

26. A system according to Claim 25, wherein the heat exchanger is arranged to provide cooling to a refrigeration load.

27. A system according to one of Claims 1 to 13 or Claim 24, wherein the heat exchanger is arranged to provide heat exchange to ground, or water, or an ambient heat source, or a waste heat source.

28. A system according to any preceding claim, further comprising a plurality of heat exchangers, with each of the heat exchangers arranged to provide heat from a different heat source.

29. A system according to any preceding claim, wherein a secondary circuit is arranged to transfer heat from a heat source to the or a heat exchanger.

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

31 . A system according to any preceding claim, wherein the system further comprises means for controlling the flow through the system.

32. A system according to claim 31 wherein the means for controlling the flow through the system comprises one or more valves.

33. A system according to claim 32 wherein the one or more valves are selected from at least the following: slam shut valve; two-way valve; or throttle valve.

34. A system according to any preceding claim further comprising a sealable vessel containing system rotative components.

35. A system according to Claim 34, 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.

36. A system according to Claim 34 or 35 further comprising a gas bearing supporting the output drive shaft of the expander and/or the input drive shaft of the compressor.

37. A system according to Claim 34 or 35 further comprising a magnetic bearing supporting the output drive shaft of the expander and/or the input drive shaft of the compressor.

38. A system according to any preceding claim further comprising a controller for activating the system when a system inlet gas temperature is below a predefined threshold.

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

40. A system according to claim 39, wherein the turbine comprises a variable geometry turbine.

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

42. A gas let-down station comprising a system according to any preceding claim.

43. A gas distribution network comprising a system according to any preceding claim.